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Timothy Huygens
of propane on Pt and Pt3Ga catalystsComputational study of the catalytic dehydrogenation
Academic year 2014-2015Faculty of Engineering and ArchitectureChairman: Prof. dr. ir. Guy MarinDepartment of Chemical Engineering and Technical Chemistry
Master of Science in Chemical EngineeringMaster's dissertation submitted in order to obtain the academic degree of
Counsellor: Stephanie SaerensSupervisors: Prof. dr. Marie-Françoise Reyniers, Dr. ir. Maarten Sabbe
Timothy Huygens
of propane on Pt and Pt3Ga catalystsComputational study of the catalytic dehydrogenation
Academic year 2014-2015Faculty of Engineering and ArchitectureChairman: Prof. dr. ir. Guy MarinDepartment of Chemical Engineering and Technical Chemistry
Master of Science in Chemical EngineeringMaster's dissertation submitted in order to obtain the academic degree of
Counsellor: Stephanie SaerensSupervisors: Prof. dr. Marie-Françoise Reyniers, Dr. ir. Maarten Sabbe
FACULTY OF ENGINEERING AND ARCHITECTURE
Department of Chemical Engineering and Technical Chemistry Laboratory for Chemical Technology
Director: Prof. Dr. Ir. Guy B. Marin
Laboratory for Chemical Technology
Declaration concerning the accessibility of the master thesis Undersigned, Timothy Huygens Graduated from Ghent University, academic year 2014-2015 and is author of the master thesis with title: Computational study of the catalytic dehydrogenation of propane on Pt and Pt3Ga catalysts The author(s) gives (give) permission to make this master dissertation available for consultation and to copy parts of this master dissertation for personal use. In the case of any other use, the copyright terms have to be respected, in particular with regard to the obligation to state expressly the source when quoting results from this master dissertation.
Laboratory for Chemical Technology • Technologiepark 914, B-9052 Gent • www.lct.ugent.be Secretariat : T +32 (0)9 33 11 756 • F +32 (0)9 33 11 759 • Petra.Vereecken@UGent.be
Acknowledgements The last five years as chemical engineering student have rushed past and now graduation is
already coming up. However, no graduation is possible without the final act: writing your
master thesis. During that last year, you put some much work and effort into this work so you
can be proud on the work that you summit at the end of May.
First of all, I want to thank prof. dr. ir. Guy B. Marin and prof. dr. Marie-Françoise Reyniers
for the opportunity to cooperate on the topic of “Design of Pt-based bimetallic catalysts for the
catalytic dehydrogenation of propane” and finish my Chemical Engineering studies at the
Laboratory of Chemical Technology, in the University of Ghent.
Special thanks goes to my coach ir. Stephanie Saerens for her yearlong dedication and support.
She was available to answer all my questions, even in the weekends, and without her, this work
should not be what it is today. Further, I would like to thank dr. ir. Maarten Sabbe for providing
me insights in the complicated matter, during our meetings and discussions.
Verder ben ik dankbaar voor de korte, maar leuke ontspanningsmomenten in ons thesislokaal.
Ook wil ik graag mijn familie bedanken voor de steun tijdens al mijn examens en ook mijn
thesis. Ik weet nog altijd niet of al die kaarsen geholpen hebben, maar ze kunnen zeker en vast
geen kwaad. Tot slot, wil ik mijn vriendin Eva bedanken. Samen hebben we vaak recht
tegenover elkaar gewerkt aan onze thesis en het is ons gelukt om deze mooi af te ronden. Nu
beginnen we aan een nieuw hoofdstuk in ons leven.
Timothy Huygens
May 2015
Computational study of the catalytic dehydrogenation of propane on Pt and Pt3Ga catalysts Timothy Huygens Supervisors: prof. dr. M.-F. Reyniers
dr. ir Maarten Sabbe Counsellor: ir. Stephanie Saerens Master’s dissertation submitted to obtain the academic degree of Master of Science in Chemical Engineering Department of Chemical Engineering and Technical Chemistry Chairman: prof. dr. ir. Guy Marin Faculty of Engineering and Architecture Academic year 2014-2015
Summary
Catalytic dehydrogenation of propane towards propylene provides an on purpose alternative for propylene production via steam cracking. Pt-based catalysts for light alkane dehydrogenation have been extensively studied but bimetallic catalysts exhibit more promising activity and selectivity towards propylene for this reaction. For a novel Pt-Ga/Mg(Ga)(Al)Ox catalyst developed earlier, with excellent catalytic properties, the most probable surface composition is the Pt3Ga composition. In this work, ab initio calculations are conducted with an optPBE vdW-DF functional to unravel the propane dehydrogenation characteristics. Primarily, the role of Ga in Pt-Ga catalysts is investigated on the Pt3Ga(111) catalyst model. The calculations on the Pt3Ga(111) model show a potential higher catalytic activity, increased selectivity towards propylene and a greater deactivation resistance with respect to Pt(111), confirming the experimental results on Pt-Ga/Mg(Ga)(Al)Ox. Furthermore, a cokes deactivated Pt catalyst model is proposed to determine the influence of the cokes on propane dehydrogenation on nearby empty Pt atoms. When the cokes enclose the intermediates, the catalytic activity drops significantly and propane dehydrogenation is unlikely to occur. However, the steric effects due the cokes are short-ranged and the catalytic activity is partially retained further away from the cokes. Near the graphene ribbon, a higher selectivity to propylene is found. Finally, a model for the stepped Pt sites is investigated to determine their role in the propane dehydrogenation characteristics. The step sites show an upward shift in catalytic activity with respect to the Pt terrace surface.
Keywords Catalytic propane dehydrogenation, propylene, ab initio calculations, Pt-Ga catalyst, cokes
deactivated Pt, undercoordinated Pt
Computational study of the catalytic dehydrogenation of propane on Pt and Pt3Ga catalysts
Timothy Huygens Supervisors: prof. dr. M.-F. Reyniers, dr. ir. Maarten Sabbe, ir. Stephanie Saerens
Abstract Catalytic dehydrogenation of propane
towards propylene provides an on purpose alternative for propylene production via steam cracking. Pt-based catalysts for light alkane dehydrogenation have been extensively studied but bimetallic catalysts exhibit more promising activity and selectivity towards propylene for this reaction. For a novel Pt-Ga/Mg(Ga)(Al)Ox catalyst developed earlier [1, 2], with excellent catalytic properties, the most probable surface composition is the Pt3Ga composition. [3] In this work, ab initio calculations are conducted with an optPBE vdW-DF functional to unravel the propane dehydrogenation characteristics. Primarily, the role of Ga in Pt-Ga catalysts is investigated on the Pt3Ga(111) catalyst model. The calculations on the Pt3Ga(111) model show a potential higher catalytic activity, increased selectivity towards propylene and a greater deactivation resistance with respect to Pt(111), confirming the experimental results on Pt-Ga/Mg(Ga)(Al)Ox. Furthermore, a cokes deactivated Pt catalyst model is proposed to determine the influence of the cokes on propane dehydrogenation on nearby empty Pt atoms. When the cokes enclose the intermediates, the catalytic activity drops significantly and propane dehydrogenation is unlikely to occur. However, the steric effects due the cokes are short-ranged and the catalytic activity is partially retained further away from the cokes. Near the graphene ribbon, a higher selectivity to propylene is found. Finally, a model for the stepped Pt sites is investigated to determine their role in the propane dehydrogenation characteristics. The step sites show an upward shift in catalytic activity with respect to the Pt terrace surface.
Keywords Catalytic propane dehydrogenation, propylene, ab initio calculations, Pt-Ga catalyst, cokes deactivated Pt, undercoordinated Pt
I. INTRODUCTION
Recently, the catalytic dehydrogenation of light
paraffins (C2-C3) has emerged as promising technology for on purpose production of light olefins, independent of the olefin production via steam cracking. The focus of this thesis is on the catalytic dehydrogenation of propane towards propylene. Propane dehydrogenation is an endothermic reaction that is limited by chemical equilibrium and the equilibrium constant is rather low for smaller carbon chains such as ethane and propane, so high temperatures in presence of a catalyst are required. Platinum metal catalysts are an adequate choice as
Timothy Huygens is with the Chemical Engineering Department, University (UGent), Ghent, Belgium. (timothy.huygens@ugent.be)
catalyst for light alkane dehydrogenation as they exhibit a high activity for the dehydrogenation steps. However, those catalysts lack a high selectivity towards olefins, as many side reactions occur on the surface. Eventually these side reactions initiate coke formation, leading to unwanted deactivation of the catalyst. [4] To improve the catalytic properties of platinum-based catalysts, the metal phase is alloyed with an additional metal such as Ga, In, Sn.
II. COMPUTATIONAL METHODOLOGY
Periodic DFT calculations are carried out with the Vienna Ab Initio Simulation Package (VASP). A non-local van der Waals-density functional (optPBE vdW-DF) is employed in all calculations. The description of the atoms is conducted with Projected Augmented Wavefunction (PAW) pseudopotentials and a plane wave energy cutoff of 400 eV is employed.
A four-layers slab with 4×2 unit cell is employed to represent the Pt(111), Pt3Ga(111) and Gr/Pt(111) catalyst model. A Monkhorst-Pack grid of 3×5×1 is employed for the Brillion zone sampling. For the representation of Pt(211), a four-layers slab with 3×2 unit cell is employed. The Brillouin zone is sampled with a k-mesh of 5×5×1. The ground state geometries are obtained under strict convergence criteria of 10-8 eV/atom and 0.015 eV/Å.
To determine the transition states, the dimer method is employed in conjugation with the Nudged Elastic Band (NEB) method, to locate the saddle point on the potential energy surface (PES) along the reaction coordinate. The geometries of the intermediates and transition states are validated using vibrational analyses.
III. RESULTS To evaluate the propane dehydrogenation
characteristics on various catalyst models, reaction energies and barriers are calculated. Based on these parameters, energy profiles relative to gaseous propane are constructed. In order to satisfy the conservation of mass, the total energy of a dehydrogenated adsorbate is the sum of the electronic energy of the adsorbate and the dehydrogenated hydrogen atom(s).
A. Propane dehydrogenation kinetics on Pt(111) catalyst model
To determine the propane dehydrogenation characteristics on pure Pt catalysts as a reference for more advanced models, a 4×2 catalyst model of the most abundant Pt(111) phase is constructed. The considered elementary steps are illustrated in Figure 1.
Figure 1. Elementary steps involved in propane dehydrogenation to propylene and deep dehydrogenation.
The proposed reaction network is divided in three main sections: propane dehydrogenation to propylene, deep dehydrogenation of C3H6-species and hydrogenolysis of C3
intermediates. The elementary steps in the first section are employed to quantify the catalyst activity for propane dehydrogenation towards propylene.
In the second section, the deep dehydrogenation of the surface intermediates towards coke precursors is investigated. The selectivity towards these species is an indication for the catalyst deactivation kinetics.
In the third section, the hydrogenolysis reactions of the C3Hx-species (x=6-8) are considered. These reactions form C1- and C2-adsorbates on the surface. These species can eventually lead to gaseous side products such as methane, ethane and ethylene, which affect the selectivity towards gaseous propylene.
For all three sections, the thermodynamics and kinetics of propane dehydrogenation of the reactions involved are evaluated. The propane dehydrogenation reaction towards propylene can occur via 1-propyl and via 2-propyl but the reaction path via 2-propyl is preferred as it has a lower reaction barrier (12 kJ/mol) with respect to 1-propyl. However, as the Gibbs free energies at 873 K of these species and the reaction barriers of the elementary step are similar, it is expected that both reaction paths contribute to the formation of propylene. Both 1-propyl and 2-propyl preferentially dehydrogenate further to propylene, as it is the thermodynamically most stable C3H6-species. The resulted electronic activation energy is similar for both reactions.
Deep dehydrogenation leads to the thermodynamically most stable species, i.e. 1-propylidyne on the Pt(111) surface. Multiple reaction paths towards 1-propylidyne are possible if isomerization reactions are included. However, it is shown that the isomerization reactions are highly activated and unlikely to occur. The most probable path to coke precursors is via propane →1-propyl →1-propylidene →1-propylidyne, as the final step is kinetically favored. However, this is solely based on the electronic energy (enthalpic contributions).
Figure 2. Gibbs free energy diagram at 900 K for the propane dehydrogenation to propylene (···) and propane deep dehydrogenation to 1-propylidyne ( ̶ ̶ ) on Pt(111).
The entropic contributions are included by determining the Gibbs free energy of the intermediates and transition states. The entropic contributions are larger for gaseous species than for adsorbates, so based on the Gibbs free diagram at 900 K, gaseous propylene is 11 kJ/mol more stable than 1-propylidyne, see Figure 2.
Hydrogenolysis competes with dehydrogenation reactions, but C-C cleavage leads to the formation of less stable species with respect to the dehydrogenated products expect for the cracking of 2-propylidene. However, all hydrogenolysis reactions are higher activated than the competing dehydrogenation reactions. Therefore, it can be concluded that these reactions are to occur less for the considered species on the Pt(111) surface.
Microkinetic simulation of the reaction network shows a high catalytic activity for propylene formation at short (0.1 s) and intermediate (10 s) time scale. The most abundant surface species is initially 1-propylidyne, but at longer simulation time this species is converted to CH3C≡Pt and HC≡Pt as these species are thermodynamically favored.
B. Propane dehydrogenation kinetics on Pt3Ga(111) as a model for Pt-Ga catalysts
The most interesting bimetallic catalyst are Ga-promoted Pt catalysts. The recent experimental studies on Pt-Ga catalyst (on hydrotalcite support) during propane dehydrogenation have already shown an increase in catalytic activity and selectivity towards propylene with respect to an unmodified Pt catalyst on the same support. [2] In previous work, the PtxGay alloy at the catalytic surface of the bimetallic surface is determined by comparing frequencies of CO adsorption experiments with the results of Pt-Ga catalyst models with various Pt/Ga ratios. The best candidate of the different studied catalyst models is the Pt3Ga bulk alloy, with the lowest Ga content. [3]
Hence, to assess the role of Ga on propane dehydrogenation thermodynamics and kinetics on Pt-Ga catalysts, a 4×2 Pt3Ga(111) unit cell is constructed. Propane dehydrogenation towards propylene consists of two elementary steps and these can occur via two reaction pathways (via 1-propyl and via 2-propyl).
2-propyl
1-propylidene
10
CH2=CH-CH3,phys
CH2=CH-CH3(g)
13
_CH-CH-CH3 + H
PtPt2
_
Pt
= CH2-C-CH3 + H
Pt
_
Pt
_
Pt2
=
2-propenyl1-propenyl
C-CH2-CH3 + H
Pt3
_
Pt
≡
14 1517 20 21
4 5
= _CH-CH2-CH3 + H
Pt2 Pt2-propylidene
CH3-C-CH3 + H
Pt2
=
Pt
_
7 8
CH3-CH-CH3 + H
Pt Pt
_ _CH2-CH2-CH3+ H
Pt Pt
_ _
1-propyl
1 2
CH3-CH2-CH3,phys
CH3-CH2-CH3(g)Gaseous propane
0
Physisorbed propane
CH2-CH-CH3 + H
Pt
__
Pt
_
PtPropylene
11 12
1-propylidyne
Physisorbed propylene
Gaseous propylene
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25
75
125
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Gib
bs f
ree
ener
gy (k
J/m
ol)
Propane (g)
Propane, phys
1-propyl + H*
Propylene,phys + 2H*
Propylene +2H*
Propylene (g)+ 2H*
TS1
TS5
1-propylidene + 2H*
TS17TS4
1-propylidyne + 3H*
Reactant Product
1-propylidyne + 3/2 H2 (g)
Propylene (g) + H2 (g)
Figure 3. Relative electronic energy profile for the propane dehydrogenation towards propylene on Pt(111) (black) and Pt3Ga(111) (green).
On Pt3Ga(111), both 1-propyl are 2-propyl are more stabilized than on Pt(111). As 2-propyl is thermodynamically more stable than 1-propyl, the reaction pathway via 2-propyl selected to define the catalytic activity on Pt3Ga(111).
Furthermore, adsorbed propylene bonds stronger on the Pt3Ga(111) surface and the calculated reaction barriers are lower than on Pt(111), while the propylene desorption barrier remains constant, see Figure 3. It can be concluded that catalytic activity is increased on Pt3Ga(111). In contrast, Sn alloying lowers the catalytic activity. Sn is frequently used as Pt catalyst modifier for alkane dehydrogenation. [5]
In addition, the deep dehydrogenation of C3H6-species is investigated on Pt3Ga(111). The formation of these species is an indication for the deactivation of the catalyst. Consistent with Pt(111), 1-propylidyne is thermodynamically the most favored species on the Pt3Ga(111) surface. To quantify the selectivity to the coke precursors, the reaction energies and barriers of reaction path via 1-propyl and 1-propylidene towards 1-propylidyne are determined, see Figure 4.
This reaction path has higher reaction barriers on Pt3Ga(111) than on Pt(111) and consequently 1-propylidyne is not as kinetically favored as on Pt(111). Furthermore, the electronic energy difference between propylene and 1-propylidyne on Pt3Ga(111) is much smaller than on Pt(111). As it can be assumed that the entropic contributions for adsorbed species such as 1-propylidyne are similar on Pt3Ga(111) as on Pt(111), the relative Gibbs free energy of 1-propylidyne on Pt3Ga(111) with respect to Pt(111) is determined by the difference in electronic energy on Pt(111) and Pt3Ga(111). Based on this reasoning, it is expected that 1-propylidyne is less likely to be formed on Pt3Ga(111) than on Pt(111). This indicates that fewer coke precursors will be formed and it can be concluded that Ga reduces the formation of cokes on the surface and increases the stability of the catalyst with respect to Pt(111).
Finally, the hydrogenolysis of propane is studied on Pt3Ga(111) as this reaction forms C1- and C2-adsorbates, which are critical for the formation of gaseous side products such as ethane, ethylene and methane.
Figure 4. Relative electronic energy profile for the propane dehydrogenation to propylene (···) and propane deep dehydrogenation to 1-propylidyne ( ̶ ̶ ) on Pt3Ga(111).
However, this reaction is highly activated (191 kJ/mol), 4 kJ/mol more than on Pt(111) while the electronic activation energies for the competing dehydrogenation reactions are lower on Pt3Ga(111).
Hence, the formation of gaseous side products via this way is unlikely. This result is supported by the experimental results that show that the propylene selectivity is above 98% on Pt-Ga/ Mg(Ga)(Al)Ox for the total time on stream. [2]
C. Propane dehydrogenation kinetics on graphene ribbon covered Pt(111) as a model for cokes deactivated Pt catalysts
The coke formation on the surface of the Pt catalysts leads inherently to the deactivation of the catalyst surface. Furthermore, the effect of cokes deactivated Pt on nearby Pt atoms is investigated in terms of catalytic activity and further deactivation of the catalyst. [6] Graphene is selected as representation of the coke on the cokes deactivated Pt catalyst based on TEM observations. [7] The catalyst model Gr/Pt(111) is primarily constructed in a 4×2 unit cell, in which half of the supercell is covered with a continuous 1-D graphene ribbon. The catalytic activity on this catalyst model is poor as steric effects of the graphene ribbon strongly destabilize the adsorbates, see Figure 5. It is postulated that dehydrogenation reactions are improbable to occur on this catalyst model. Apart from the poor activity, an improved selectivity towards propylene with respect to deactivation reactions is observed.
Thermodynamically, 1-propylidyne is favored on the catalyst surface and an exothermic reaction energy is observed for its formation, but all preceding intermediates are more unstable than on clean Pt(111). The electronic energy difference between gaseous propylene and 1-propylidyne is smaller. Additionally, the approximation is made that the entropic contributions are similar on Pt(111) and Gr/Pt(111). This leads to the conclusion that the Gibbs free energy of 1-propylidyne is higher on Gr/Pt(111) than on Pt(111), indicating that nearby the graphene ribbon, deactivation is reduced.
-100
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0
25
50
75
100
125
150
Rel
ativ
een
ergy
(kJ/
mol
)
Propane (g)
Propane, phys2-propyl + H*
Propylene,phys + 2H*
Propylene +2H*
Propylene (g) + H2 (g)
TS2 TS7
Reactant Product
Propylene (g)+ 2H*
-125
-75
-25
25
75
125
Rea
ltive
ene
rgy
(kJ/
mol
)
Reactant Product
Propane (g)
Propane, phys2-propyl + H*
Propylene,phys + 2H*
Propylene + 2H*
Propylene (g) + H2 (g)
TS1 TS2 TS7Propylene (g) + 2H*
1-propyl + H*
TS4
1-propylidene+ 2H*
1-propylidyne + 3H*
1-propylidyne + 3/2 H2 (g)
TS17
Figure 5. Comparison of reaction path towards propylene via 2-propyl on clean Pt(111) (black), 4×2 Gr/Pt(111) (blue) and 5×2 Gr/Pt(111) (purple), based on relative electronic energy.
Furthermore, the obtained reaction energy of propane hydrogenolysis is extremely high (265 kJ/mol) and the hydrogenolysis reaction barrier must be even higher, so this reaction is improbable.
It is clear that the steric effects in the 4×2 unit cell are too large due the graphene ribbon inclusion, so a 5×2 unit cell is constructed on which intermediates can adsorb further away from the graphene ribbon. This model restores most of its activity compared with the smaller unit cell, see Figure 5. The strength of the steric interactions is reduced as they have a small range, however repulsion by the graphene ribbon still affects the intermediates, but on a smaller scale. For the considered reaction path, the catalytic activity remains lower than on the clean Pt(111) catalyst model.
D. Propane dehydrogenation kinetics on Pt(211) as a model for undercoordinated sites of Pt catalysts
The initial coke formation and origin of gaseous side products can be assigned to the reactivity of undercoordinated sites such as steps and edges of the Pt catalyst. [7] As model for the undercoordinated sites on Pt catalysts, a stepped Pt(211) catalyst model is constructed. For this model, a 3×2 unit cell is employed to obtain a coverage of 0.17 ML, which is higher than the reference coverage on Pt(111) of 0.13 ML. Furthermore, the hydrocarbon intermediates are optimized in adsorption sites located near the step and the reaction path towards propylene via 2-propyl is investigated and compared with literature.
Both thermodynamically and kinetically, the steps are preferred as adsorption sites due to their higher reactivity than the terrace plane (Pt(111)), see Figure 6. However, it should be noted that by strengthening the propylene bonds with the surface, a higher reaction barrier is observed for propylene desorption, indicating that propylene is less likely to desorb on Pt(211) than on Pt(111). Furthermore, Yang et al. propose that step sites are more sensitive for deep dehydrogenation and hydrogenolysis reactions. [8] Deep dehydrogenation steps are thermodynamically and kinetically favored until formation of propyne. Consecutively, these deeply dehydrogenated species have low reaction barriers for hydrogenolysis, leading to C1 and C2-species on the catalyst surface.
Figure 6. Relatieve electronic energy profile for propylene reaction path via 2-propyl on (black) Pt(111) and (red) Pt(211). (respectively coverage of 0.13 ML and 0.17 ML).
IV. CONCLUSIONS Ga alloying of Pt catalysts shows superior catalytic
properties over alloying with Sn. With respect to the reference, the Pt3Ga catalyst model shows an increased catalytic activity for propane dehydrogenation. Furthermore, it is shown that the formation of coke precursors is reduced, indicating a higher deactivation resistance. The hydrogenolysis of propane is unlikely and higher selectivity towards propylene is obtained on this Pt-Ga catalyst.
On the cokes deactivated Pt catalyst, the propane dehydrogenation does not occur near the graphene ribbon due to steric effects. Beyond the short-ranged steric effects, the catalytic activity is retained, but it is lower compared to clean Pt(111) due to the repulsion by the graphene ribbon. Near the graphene ribbon, the selectivity towards coke precursors and hydrogenolysis is reduced.
Undercoordinated sites on the Pt catalysts are thermodynamically and kinetically preferred as sites for propane dehydrogenation towards propylene with respect to the terrace Pt surface.
REFERENCES 1. Sun, P.P., et al., Journal of Catalysis, 2010.
274(2): p. 192-199. 2. Siddiqi, G., et al., Journal of Catalysis, 2010.
274(2): p. 200-206. 3. Saerens, S., in Department of Chemical
Engineering and Technical Chemistry. 2014, University of Ghent: Ghent.
4. Vora, B.V., Topics in Catalysis, 2012. 55(19-20): p. 1297-1308.
5. Yang, M.L., et al., Acs Catalysis, 2012. 2(6): p. 1247-1258.
6. Vu, B., et al., Korean Journal of Chemical Engineering, 2011. 28(2): p. 383-387.
7. Peng, Z., et al., Journal of Catalysis, 2012. 286: p. 22-29.
8. Yang, M.L., et al., Physical Chemistry Chemical Physics, 2011. 13(8): p. 3257-3267.
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25
75
125
175
225
Rel
ativ
e en
ergy
(kJ/
mol
)
ProductReactant
Propane (g)
Propane, phys 2-propyl + H*
Propylene,phys + 2H
Propylene + 2H*
TS2
TS7
Propylene,phys + H2(g)
Propylene (g) + H2(g)
-100
-50
0
50
100
150
Rel
ativ
e en
ergy
(kJ/
mol
)
Reactant Product
Propane (g)
Propane, phys
2-propyl + H*
Propylene,phys + 2H*
Propylene +2H*
Propylene (g) + 2H*
TS2
Propylene (g) + H2(g)
TS7
Table of Contents i
Table of Contents Table of Contents ........................................................................................................................ i
List of symbols .......................................................................................................................... ix
Acronyms .............................................................................................................................. ix
Roman symbols ...................................................................................................................... x
Greek symbols ....................................................................................................................... xi
Chapter 1 Introduction ................................................................................................................ 1
1.1 Catalytic propane dehydrogenation ............................................................................. 1
1.2 Recent developments in Pt-based catalysts ................................................................. 3
1.2.1 The novel Pt-Ga/Mg(Ga)(Al)Ox catalyst .............................................................. 4
1.2.2 Cokes formation and deactivation on Pt-based catalysts ..................................... 5
1.3 Goal of this work ......................................................................................................... 5
1.4 Structure of this work .................................................................................................. 6
1.5 References .................................................................................................................... 6
Chapter 2 Literature review ........................................................................................................ 8
2.1 Catalysts for propane dehydrogenation reaction ......................................................... 8
2.1.1 Catalysis in general ............................................................................................... 8
2.1.2 Pure platinum catalysts ....................................................................................... 10
2.1.3 Bimetallic Pt catalysts ........................................................................................ 11
2.1.4 Bimetallic Sn-Pt catalysts ................................................................................... 13
ii Table of Contents
2.1.4.1 Light alkane dehydrogenation .................................................................... 15
2.1.5 Bimetallic Ga-Pt catalysts .................................................................................. 16
2.1.5.1 Light alkane dehydrogenation .................................................................... 17
2.1.5.2 Determination of active phase of Pt-Ga catalysts ....................................... 19
2.2 Coke formation during propane dehydrogenation..................................................... 20
2.2.1 Coke formation on pure Pt catalysts .................................................................. 20
2.2.2 Coke formation on bimetallic Sn-Pt catalysts .................................................... 23
2.2.3 Coke formation on bimetallic Ga-Pt catalysts ................................................... 24
2.3 Interaction between active metal phase and support ................................................. 25
2.3.1 Active metal phase ............................................................................................. 26
2.3.2 Catalyst support.................................................................................................. 29
2.3.3 Active metal phase and support interaction ....................................................... 31
2.4 Conclusions ............................................................................................................... 32
2.5 References ................................................................................................................. 35
Chapter 3 Computational methodology ................................................................................... 39
3.1 Catalyst models ......................................................................................................... 39
3.1.1 Adaptations of the ideal catalyst model ............................................................. 41
3.1.2 Catalyst model used in this work ....................................................................... 43
3.2 Periodic ab initio calculations ................................................................................... 44
3.3 Computational framework used in this work ............................................................ 47
3.3.1 Vienna Ab initio Simulation Package ................................................................ 47
3.3.2 Geometry optimizations ..................................................................................... 49
3.3.3 Transition state calculation techniques .............................................................. 50
3.3.3.1 Nudged Elastic Band (NEB) method .......................................................... 51
3.3.3.2 Dimer method ............................................................................................. 52
3.3.4 Frequency calculations ....................................................................................... 53
Table of Contents iii
3.3.4.1 Dimmins.pl script ........................................................................................ 54
3.4 References .................................................................................................................. 54
Chapter 4 Propane dehydrogenation kinetics on Pt(111) catalyst model ................................. 57
4.1 Catalyst model ........................................................................................................... 57
4.1.1 Adsorption site nomenclature ............................................................................. 59
4.1.2 Determination of the degree of coverage ........................................................... 60
4.1.3 Selection of the DFT functional ......................................................................... 60
4.2 Adsorption ................................................................................................................. 61
4.2.1 Alkanes ............................................................................................................... 61
4.2.1.1 Propane ........................................................................................................ 62
4.2.1.2 Ethane .......................................................................................................... 63
4.2.1.3 Methane ....................................................................................................... 65
4.2.2 Alkenes ............................................................................................................... 66
4.2.2.1 Propylene ..................................................................................................... 66
4.2.2.2 Ethylene ....................................................................................................... 69
4.3 Thermodynamics ....................................................................................................... 71
4.3.1 Propane dehydrogenation to propylene .............................................................. 75
4.3.2 Deep dehydrogenation of propylene and other C3H6 species ............................. 79
4.3.3 Hydrogenolysis of C3 intermediates ................................................................... 81
4.3.3.1 Formation of gaseous side products ............................................................ 83
4.3.4 Overall thermodynamics..................................................................................... 84
4.4 Kinetics ...................................................................................................................... 85
4.4.1 Propane dehydrogenation to propylene .............................................................. 86
4.4.2 Deep dehydrogenation of propylene and other C3H6-species ............................ 90
4.4.3 Hydrogenolysis of C3 intermediates ................................................................... 93
4.4.3.1 Formation of gaseous side products ............................................................ 94
4.5 Microkinetic modelling .............................................................................................. 96
iv Table of Contents
4.5.1 Determination of the thermodynamic and kinetic parameters ........................... 97
4.5.2 Construction of the microkinetic and reactor model .......................................... 97
4.5.3 Results of the simulation .................................................................................... 98
4.5.3.1 Effect of varying the simulated time .......................................................... 99
4.5.3.2 Effect of varying temperature ................................................................... 102
4.5.3.3 Effect of varying propylene pressure ........................................................ 105
4.5.3.4 Effect of varying side product pressure .................................................... 107
4.6 Conclusions ............................................................................................................. 109
4.7 References ............................................................................................................... 111
Chapter 5 Propane dehydrogenation kinetics on Pt3Ga(111) as a model for Pt-Ga catalysts 114
5.1 Catalyst model ......................................................................................................... 115
5.1.1 Adsorption site nomenclature .......................................................................... 116
5.1.2 Determination of the degree of coverage ......................................................... 117
5.2 Adsorption ............................................................................................................... 117
5.2.1 Propane............................................................................................................. 117
5.2.2 Propylene ......................................................................................................... 118
5.3 Thermodynamics ..................................................................................................... 119
5.3.1 Propane dehydrogenation to propylene............................................................ 121
5.3.2 Deep dehydrogenation of propylene ................................................................ 124
5.3.3 Hydrogenolysis of C3 intermediates ................................................................ 126
5.4 Kinetics .................................................................................................................... 127
5.4.1 Propane dehydrogenation to propylene............................................................ 127
5.4.2 Deep dehydrogenation of propylene ................................................................ 129
5.4.3 Hydrogenolysis of C3 intermediates ................................................................. 131
5.5 Conclusions ............................................................................................................. 132
5.6 References ............................................................................................................... 133
Table of Contents v
Chapter 6 Propane dehydrogenation kinetics on graphene ribbon covered Pt(111) as a model
for cokes deactivated Pt catalysts ........................................................................................... 123
6.1 Catalyst model ......................................................................................................... 124
6.2 Adsorption ............................................................................................................... 127
6.2.1 Propane ............................................................................................................. 128
6.2.2 Propylene .......................................................................................................... 128
6.3 Thermodynamics ..................................................................................................... 129
6.3.1 Propane dehydrogenation to propylene ............................................................ 132
6.3.2 Deep dehydrogenation of propylene................................................................. 135
6.3.3 Hydrogenolysis of C3-intermediates ................................................................. 137
6.4 Kinetics .................................................................................................................... 137
6.4.1 Propane dehydrogenation to propylene ............................................................ 138
6.4.2 Deep dehydrogenation of propylene................................................................. 140
6.5 The range and strength of cokes influence on propane dehydrogenation ................ 142
6.5.1 Thermodynamics .............................................................................................. 142
6.5.2 Kinetics ............................................................................................................. 144
6.6 Conclusions .............................................................................................................. 146
6.7 References ................................................................................................................ 146
Chapter 7 Propane dehydrogenation kinetics on Pt(211) catalyst model ............................... 159
7.1 Catalyst model ......................................................................................................... 159
7.1.1 Adsorption site nomenclature ........................................................................... 160
7.1.2 Determination of the degree of coverage ......................................................... 161
7.2 Adsorption ............................................................................................................... 161
7.2.1 Propane ............................................................................................................. 161
7.2.2 Propylene .......................................................................................................... 162
7.3 Thermodynamics ..................................................................................................... 163
vi Table of Contents
7.3.1 Propane dehydrogenation to propylene............................................................ 165
7.4 Kinetics .................................................................................................................... 167
7.5 Conclusions ............................................................................................................. 169
7.6 References ............................................................................................................... 170
Chapter 8 Conclusions and prospects .................................................................................... 171
Future work ........................................................................................................................ 176
References .......................................................................................................................... 176
Appendix A Examples of INCAR files .................................................................................. 178
A.1 INCAR file for strict geometry optimization .......................................................... 178
A.2 INCAR files for transition state optimization ......................................................... 179
A.2.1 NEB optimization ............................................................................................ 179
A.2.2 Dimer optimization .......................................................................................... 180
A.3 INCAR file for frequency calculations ................................................................... 181
Appendix B Optimized geometries on the Pt(111) catalyst model ........................................ 182
B.1 Reaction network ..................................................................................................... 183
B.2 Optimized geometries.............................................................................................. 184
B.2.1 Gasphase species .............................................................................................. 184
B.2.2 Hydrogen .......................................................................................................... 186
B.2.3 (Deeply) dehydrogenated intermediates .......................................................... 187
B.2.4 Dissociation products ....................................................................................... 189
B.2.5 C1 and C2 hydrocarbon intermediates .............................................................. 190
B.2.6 Transition states ............................................................................................... 191
Appendix C Thermodynamic and kinetic parameters of the elementary steps for the Pt(111)
catalyst model ........................................................................................................................ 195
C.1 Thermodynamic parameters .................................................................................... 195
C.2 Kinetic parameters ................................................................................................... 196
Table of Contents vii
Appendix D Optimized geometries on the Pt3Ga(111) catalyst model .................................. 198
D.1 Reaction network ..................................................................................................... 199
D.2 Optimized geometries .............................................................................................. 200
D.2.1 Gasphase species .............................................................................................. 200
D.2.2 Hydrogen .......................................................................................................... 200
D.2.3 (Deeply) dehydrogenated intermediates ........................................................... 200
D.2.4 Dissociation products ....................................................................................... 202
D.2.5 Transition states ................................................................................................ 202
Appendix E Optimized geometries on the graphene ribbon covered Pt(111) catalyst model 204
E.1 Reaction network ..................................................................................................... 205
E.2 Optimized geometries in the 4×2 unit cell ............................................................... 206
E.2.1 Gasphase species .............................................................................................. 206
E.2.2 Hydrogen .......................................................................................................... 207
E.2.3 (Deeply) dehydrogenated intermediates ........................................................... 207
E.2.4 Dissociation products ....................................................................................... 209
E.2.5 Transition states ................................................................................................ 209
E.3 Optimized geometries in the 5×2 unit cell ............................................................... 210
E.3.1 Gasphase species .............................................................................................. 210
E.3.2 Hydrogen .......................................................................................................... 211
E.3.3 Dehydrogenated species ................................................................................... 211
E.3.4 Transition states ................................................................................................ 212
Appendix F Optimized geometries on the Pt(211) catalyst model ......................................... 213
F.1 Reaction network ..................................................................................................... 214
F.2 Optimized geometries .............................................................................................. 214
F.2.1 Gasphase species .............................................................................................. 214
F.2.2 Hydrogen .......................................................................................................... 215
viii Table of Contents
F.2.3 Dehydrogenated intermediates ......................................................................... 215
F.2.4 Transition states ............................................................................................... 216
List of symbols ix
List of symbols
Acronyms
ALISS Alkali ion scattering spectroscopy BEEF Bayesian Error Estimation Functional BET Brunauer–Emmett–Teller DFT Density functional theory DP Deposition-precipitation EDX Energy-dispersive X-ray spectroscopy EXAFS Extended X-ray adsorption fine structure fcc Face-centered cubic crystal structure or hollow adsorption site FFT Fast Fourier transformation FTIR Fourier transform infrared spectroscopy GGA Generalized gradient approximation hcp Hollow adsorption site HREELS High-resolution electron energy loss spectroscopy HRTEM High-resolution transmission electron microscopy IMP Incipient wetness impregnation IRAS Infrared adsorption spectroscopy IUPAC International Union of Pure and Applied Chemistry LDA Local density approximation LEED Low energy electron diffraction MA Mesoporous alumina MAS NMR Magic angle spinning nuclear magnetic resonance MD Molecular dynamics MEP Minimum energy path MO Molecular orbital ML Monolayer NEB Nudged Elastic Band PAW Projector-Augmented Wave PBE Perdew-Burke-Ernzerhof functional PES Potential energy surface
x List of symbols
PSSA Pseudo stationary state approximation PW91 Perdew-Wang 91 functional RAIRS Reflection-absorption infrared spectroscopy SMSI Strong metal-support interactions SOMC Surface organometallic chemistry STEM Scanning transmission electron microscopy TAP Temporal Analysis of Products TEM Transmission electron microscopy TEOM Tapered element oscillating microbalance TGA Thermogravimetric analysis TOF Turn-over frequency TPD Temperature programmed desorption TPO Temperature programmed oxidation TPR Temperature programmed reduction TST Transition state theory VASP Vienna Ab initio Simulation Package vdW van der Waals vdW-DF van der Waals density functional XANES X-ray absorption near edge structure XPS X-ray photoelectron spectroscopy XPD X-ray photoelectron diffraction XRD X-ray diffraction ZPE Zero point energy
Roman symbols
A Pre-exponential factor (for 1st order reaction) s-1 as Asymmetrical vibration mode - Ct Concentration of active sites mol kgcat
-1
Ea Activation energy J mol -1 Ead,ads Electronic energy of the adsorbed species J mol -1
Ead,gas Electronic energy of the adsorbate species in the gas phase J mol -1 𝐸𝐸C1𝐻𝐻𝑧𝑧/surface Electronic energy of the C1Hz-species on the surface J mol -1 𝐸𝐸C2𝐻𝐻6(g) Electronic energy of gaseous ethane J mol -1 𝐸𝐸C2𝐻𝐻𝑦𝑦/surface Electronic energy of the C2Hy-species on the surface J mol -1 EC2𝐻𝐻𝑦𝑦 Relative electronic energy of the C2Hy-species J mol -1 𝐸𝐸C3𝐻𝐻8(g) Electronic energy of gaseous propane J mol -1 EC3𝐻𝐻𝑥𝑥 Relative electronic energy of the C3Hx-species J mol -1 𝐸𝐸C3𝐻𝐻𝑥𝑥/surface Electronic energy of the C3Hx-species on the surface J mol -1 𝐸𝐸C3𝐻𝐻𝑥𝑥−1/surface Electronic energy of the C3Hx-1-species on the surface J mol -1 𝐸𝐸C3Hy+z/surface Electronic energy of the C3Hy+z-species on the surface J mol -1
List of symbols xi
𝐸𝐸H/surface Electronic energy of the adsorbed hydrogen J mol -1 Eslab or Esurface Electronic energy of the clean catalyst surface J mol -1 Ereact,ads Electronic energy of the adsorbed reactant(s) J mol -1 Eprod,ads Electronic energy of the adsorbed reaction product(s) J mol -1 ETS,ads Electronic energy of the transition J mol -1 F Molecular flux s-1 Fi Molar flow of component i mol s-1
𝐹𝐹𝑖𝑖NEB Artificial nudged elastic band force on NEB image i N 𝐹𝐹𝑖𝑖⊥ Perpendicular force of NEB image i N 𝐹𝐹𝑖𝑖
S∥ String force of NEB image i N k Rate coefficient s-1 kB Boltzmann constant J K-1 nt Number of active sites per m² m-2
s Symmetrical vibration mode - s0 Sticking coefficient - W Catalyst mass kg
Greek symbols
Г Special gamma point to sample the Brillouin zone - γ Vibrational scissoring mode - ∆ǂ𝐸𝐸 Electronic reaction barrier J mol -1 ∆adsE(ad) Electronic adsorbate adsorption energy J mol -1 ∆Er Electronic reaction energy J mol -1 δ Vibrational deformation mode - ν Vibrational stretching mode - τ Vibrational twisting mode - τ�𝑖𝑖 Tangent between NEB image i and image i+1 - ω Vibrational wagging mode -
Chapter 1: Introduction 1
Chapter 1 Introduction
1.1 Catalytic propane dehydrogenation
Recently, the catalytic dehydrogenation of light paraffins (C2-C3) has emerged as promising
technology for on-purpose production of light olefins. This work focuses on the production of
propylene (C3) by catalytic propane dehydrogenation. The global propylene demand for
polymer grade propylene is expected to double in 2030 with respect to 2005. [1] Nowadays, the
main production route to ethylene and propylene is steam cracking of ethane or naphtha.
However, the main product of this route is ethylene and the yield of co-product propylene is
limited to 13-17 wt%, independent of the feedstock. [2] Innovation has led to new on-purpose
production routes towards propylene without interference with steam cracker production, such
as methanol-to-olefins (MTO) [3] and propane dehydrogenation (PDH) [4]. Additionally, the
latter production route is applicable for exploitation of shale gas (which consists of a large
proportion of propane). [5]
Dehydrogenation processes of paraffins are chemical reactions in which alkanes are converted
into the corresponding alkenes by double bond formation. The resulting release of hydrogen
gas is considered as valuable side product. Dehydrogenation of propane (C3H8) leads to the
formation of propylene (C3H6) and hydrogen (H2).
C3H8 → C3H6 + H2 (1)
Paraffin dehydrogenation chemistry is strongly endothermic and its conversion is limited by
chemical equilibrium. According to Le Châtelier’s principle, higher conversion is obtainable at
either higher temperatures or lower pressures. [6] To obtain 40% conversion at 1 atmospheric
2 Chapter 1: Introduction
pressure absolute, the dehydrogenation of propane requires a temperature of at least about 853
K. [2] At these temperatures, side reactions such as hydrogenolysis and coke formation are
unavoidable. The use of a catalyst will increase the overall conversion of the selective
dehydrogenation of propane at similar reaction temperatures. [7] Furthermore, a catalyst can
also improve selectivity by attenuating the side reactions.
Since the late 1930’s, paraffin catalytic dehydrogenation for the production of olefins has been
commercialized. During World War II, chromia-alumina catalysts have been used for butane
dehydrogenation towards butenes, which were then converted to yield high-octane fuel. [7]
However, chromia-alumina catalysts are less frequently used as the regenerated catalyst
contains Cr(IV), which poses a significant health and environmental risk as it is a well-known
carcinogenic compound. [8]
In the 1940’s, Haensel proved that noble Pt catalysts have a high activity for the
dehydrogenation of paraffins to the corresponding olefins. [9] Bloch developed Pt-based
catalysts in the 1960’s that were used successfully in the production of biodegradable detergents
based on dehydrogenation of heavy paraffins. These Pt-based catalysts excelled at heavy
paraffin dehydrogenation as they exhibited high activity and stability with a minimum of
cracking. [10] Based on the promising results of heavy paraffin dehydrogenation on Pt-based
catalysts, it was expected that the extrapolation to light paraffins would be straightforward.
However, the required temperature for dehydrogenation of light paraffins is much higher than
for the dehydrogenation of heavy paraffins. As mentioned above, paraffin dehydrogenation is
equilibrium limited and smaller equilibrium coefficients are obtained for paraffin
dehydrogenation of smaller saturated hydrocarbons. Therefore, there is a larger driving force
for the dehydrogenation of heavy paraffins than of light paraffins. To obtain similar conversion
for the dehydrogenation of light paraffins as heavy paraffins, a higher temperature is required,
as illustrated in Figure 1-1.
Chapter 1: Introduction 3
Figure 1-1. Temperatures required to achieve 10 and 40% equilibrium conversion of C2–C15 n-paraffins at 1 atm. [7] The equilibrium conversions are based on the equilibrium constant of the corresponding paraffin dehydrogenation.
The requirement of higher temperatures during light paraffin dehydrogenation causes
acceleration of side reactions and catalyst deactivation. Suppression of these reactions can be
achieved by a careful catalyst, selection, e.g. a well-chosen Pt-based catalyst. Therefore,
catalyst development for the dehydrogenation of light paraffins has led to new, promising
catalysts.
1.2 Recent developments in Pt-based catalysts
Pure platinum is a well-known active metal phase for dehydrogenation reactions of light
alkanes. Theoretical studies, performed by Yang et al. [11, 12] and Valcarcel et al. [13]
proposed a reaction network for the propane dehydrogenation reaction, see Figure 1-2.
However, platinum suffers from low olefin selectivity and fast deactivation due to coke
accumulation. By promoting Pt with modifiers such as Sn or Ga, coke formation on the active
phase can be reduced, resulting in a higher olefin selectivity of the catalyst. [14, 15] The
catalytic properties of Pt-Sn catalysts are intensively studied both experimentally and
theoretically. However, recent studies have highlighted the promising role of Ga on the catalytic
properties of Pt-based catalysts during light paraffin dehydrogenation. Furthermore, a new Pt-
Ga catalyst has been developed for paraffin dehydrogenation reactions at the Laboratory for
Chemical Technology. [16]
4 Chapter 1: Introduction
Figure 1-2: Elementary dehydrogenation steps involved in propane dehydrogenation on Pt(111). Each row consists of C3Hx-species with one less hydrogen until C3H3. The dehydrogenated hydrogens are not shown. Employed color code: C (grey), H (white) and Pt (blue). [12]
1.2.1 The novel Pt-Ga/Mg(Ga)(Al)Ox catalyst
A new hydrotalcite-supported Pt-Ga catalyst has been developed for alkane dehydrogenation
reactions. The novel Pt-Ga/Mg(Ga)(Al)Ox catalyst is active for light paraffin dehydrogenation
and remains highly selective for the formation of the corresponding olefin. [14] Furthermore,
experiments have shown that the catalyst is less prone to surface carbon formation and exhibits
higher stability during regeneration cycles than Pt/HT. Ab initio techniques aid to determine
the surface composition of the Pt-Ga catalyst since modern spectroscopic techniques have been
applied to the catalyst and they do not provide a decisive answer. Based on a comparison of
calculated C-O stretch frequencies of adsorbed CO with experimental FTIR frequencies, Pt3Ga
is proposed as most probable surface composition of the Pt-Ga/Mg(Ga)(Al)Ox catalyst. [17]
The novel Pt-Ga/Mg(Ga)(Al)Ox catalyst shows superior qualities for the propane
dehydrogenation reaction compared to the pure platinum catalyst. However, the origin of the
increased selectivity and activity of the promoted catalyst is unclear.
Chapter 1: Introduction 5
1.2.2 Cokes formation and deactivation on Pt-based catalysts
Pt-based catalysts are prone to deep dehydrogenation reactions, which can eventually lead to
coke deposits on the surface. However, the coke formation cannot be directly coupled to
deactivation of Pt-based catalysts as only a small part of the formed coke deposits on the active
metal phase, while the majority is formed on the support or spills over onto the support from
the active phase. [18, 19] Furthermore, studies have shown that undercoordinated sites such as
edges and steps are preferred for initiating the coke formation on Pt-based catalysts. [20]
Characterization of the coke deposits could lead to further understanding of the coke formation
mechanism and enabling the minimization of coke formation. Recent TEM studies have shown
that cokes formed during propane dehydrogenation has a graphitic nature. [21] However, the
exact effect of cokes formation on the reaction thermodynamics and kinetics remains elusive.
1.3 Goal of this work
The goal of this master thesis is to obtain fundamental insight into the propane dehydrogenation
mechanism on Pt-based catalysts using optPBE vdw-DF DFT calculations. The focus is on the
determination of the activity (propylene formation rate), selectivity towards gaseous side
products (methane, ethane and ethylene) and towards coke precursors (tendency to form
carbonaceous species) of the catalyst.
First, a reaction network on Pt(111) is constructed based on the articles of Yang et al. [12] and
Valcarcel et al. [13], including all elementary steps to describe the propane dehydrogenation
reaction, the side product formation and coke formation. This Pt(111) catalyst model is
considered as a reference model and its results can be validated with literature.
Thereafter, three other catalyst models are investigated: a Pt-Ga catalyst model representing the
novel Pt-Ga/Mg(Ga)(Al)Ox catalyst, a cokes deactivated Pt catalyst model and a step edge Pt
catalyst model, representing the undercoordinated Pt atoms where coke formation could
preferably occur. With these models it is possible to study the effect on the reaction
thermodynamics and kinetics of, respectively, Ga-alloying, catalyst deactivation by coke
formation, and structure sensitivity. The focus is on the quantification of the activity and
selectivity towards propylene.
6 Chapter 1: Introduction
1.4 Structure of this work
In Chapter 2, a detailed literature study is performed on Pt-based catalysts for light alkane
dehydrogenation. The main topics covered are pure, Sn-modified, Ga-modified Pt catalysts,
and their catalytic properties. Furthermore, the coke formation and deactivation of Pt-based
catalysts are discussed. A section is dedicated to the interaction between the active metal phase
and the support. Chapter 3 consists of an overview of the computational methodology used for
the DFT study.
In Chapter 4, the considered intermediates and transition states are calculated in a DFT
framework on a pure Pt catalyst model (the Pt(111) model). The results are validated with
literature and are used to determine propane dehydrogenation thermodynamics and kinetics.
Furthermore, a kinetic study using a microkinetic model is conducted to obtain insight into the
surface coverages and the rate-determining step.
In Chapter 5, a Pt-Ga catalyst model (the Pt3Ga(111) model) is used to determine the role of Ga
alloying on the thermodynamics and kinetics of propane dehydrogenation. Chapter 6 makes use
of a cokes deactivated Pt catalyst model to quantify the influence of cokes on propane
dehydrogenation characteristics. Chapter 7 discusses the last catalyst model, the
undercoordinated sites of the pure Pt catalyst. In previous three chapters, the catalytic activity
and selectivity towards propylene are determined with respect to the reference catalyst model
of Chapter 4.
Finally, in Chapter 8, the overall conclusions of this work are stated and an outlook on the future
is given.
1.5 References
1. Mackenzie, W., Global propylene long-term outlook 2H 2014. September 2014. 2. Vora, B.V., Development of Dehydrogenation Catalysts and Processes. Topics in
Catalysis, 2012. 55(19-20): p. 1297-1308. 3. Ren, T., M.K. Patel, and K. Blok, Steam cracking and methane to olefins: energy use,
CO 2 emissions and production costs. Energy, 2008. 33(5): p. 817-833. 4. Rahimi, N. and R. Karimzadeh, Catalytic cracking of hydrocarbons over modified ZSM-
5 zeolites to produce light olefins: A review. Applied Catalysis A: General, 2011. 398(1–2): p. 1-17.
5. Bruijnincx, P.C.A. and B.M. Weckhuysen, Shale Gas Revolution: An Opportunity for the Production of Biobased Chemicals? Angewandte Chemie International Edition, 2013. 52(46): p. 11980-11987.
Chapter 1: Introduction 7
6. Bond, G.C., Metal-Catalysed Reactions of Hydrocarbons. Fundamental and applied Catalysis. Vol. XXI. 2005: Springer. 666.
7. Bhasin, M.M., et al., Dehydrogenation and oxydehydrogenation of paraffins to olefins. Applied Catalysis a-General, 2001. 221(1-2): p. 397-419.
8. Davis, R.J., R.H. Griffith, and J.D.F. Marsh, The physical properties of chromia-alumina catalysts. Advances in Catalysis, 1957. 9: p. 155-168.
9. Haensel, V., Hydrocarbon Conversion Process with Subsequent Reforming of Selected Hydrocarbon Fractions. US Patent, 1959.
10. Bloch, H.S., UOP discloses new way to make linear alkylbenzene. Oil Gas J 79–81, 1967(US Patent 3,448,165,).
11. Yang, M.L., et al., Density functional study of the chemisorption of C-1, C-2 and C-3 intermediates in propane dissociation on Pt(111). Journal of Molecular Catalysis a-Chemical, 2010. 321(1-2): p. 42-49.
12. Yang, M.L., et al., DFT study of propane dehydrogenation on Pt catalyst: effects of step sites. Physical Chemistry Chemical Physics, 2011. 13(8): p. 3257-3267.
13. Valcarcel, A., et al., Theoretical study of dehydrogenation and isomerisation reactions of propylene on Pt(111). Journal of Catalysis, 2006. 241(1): p. 115-122.
14. Siddiqi, G., et al., Catalyst performance of novel Pt/Mg(Ga)(Al)O catalysts for alkane dehydrogenation. Journal of Catalysis, 2010. 274(2): p. 200-206.
15. Galvita, V., et al., Ethane dehydrogenation on Pt/Mg(Al)O and PtSn/Mg(Al)O catalysts. Journal of Catalysis, 2010. 271(2): p. 209-219.
16. Sun, P.P., et al., Synthesis and characterization of a new catalyst Pt/Mg(Ga)(Al)O for alkane dehydrogenation. Journal of Catalysis, 2010. 274(2): p. 192-199.
17. Saerens, S., Determination of the active phase of a Pt/Mg(Ga)AlOx catalyst of the catalytic dehydrogenation of propane, in Department of Chemical Engineering and Technical Chemistry. 2014, University of Ghent: Ghent.
18. Vu, B., et al., Electronic density enrichment of Pt catalysts by coke in the propane dehydrogenation. Korean Journal of Chemical Engineering, 2011. 28(2): p. 383-387.
19. Vu, B.K., et al., Location and structure of coke generated over Pt–Sn/Al2O3 in propane dehydrogenation. Journal of Industrial and Engineering Chemistry, 2011. 17(1): p. 71-76.
20. Peng, Z., et al., High-resolution in situ and ex situ TEM studies on graphene formation and growth on Pt nanoparticles. Journal of Catalysis, 2012. 286: p. 22-29.
21. Preto, I., Graphene formation and oxidation on Pt/Mg(Ga)AlOx dehydrogenation catalysts, in Department of Chemical Engineering and Technical Chemistry. 2015, University of Ghent: Ghent.
Chapter 2: Literature review 8
Chapter 2 Literature review
2.1 Catalysts for propane dehydrogenation reaction
2.1.1 Catalysis in general
The International Union of Pure and Applied Chemistry (IUPAC) has defined a catalyst as “a
substance that increases the rate of a reaction without modifying the overall standard Gibbs
energy change in the reaction”. [1] The chemical process of increasing the reaction rate is called
catalysis, and the catalyst is both a reactant and a product of the reaction, i.e., the catalyst is
restored after each catalytic act. Furthermore, the catalyst does not influence the
thermodynamical equilibrium composition after the cessation of the reaction. [2] Catalysts only
enhance the kinetics of the occurring reactions, not the thermodynamics and the chemical
equilibrium since it influences both the forward and reverse reaction rate. While catalysts are
not consumed during the reaction, several side reactions such as inhibition and deactivation can
influence the catalyst chemical and mechanical properties.
By means of catalysis, other reaction mechanisms with different transition states are
energetically more favorable, but often have a higher level of complexity. Nonetheless,
catalysis is widely employed in both research and industrial applications as it improves two
important characteristics of the overall reaction mechanism: activity and selectivity. The most
straightforward principle is the activity of the catalyst as the catalyst increases the reaction rate
with multiple orders of magnitude. Often the highest activity can be correlated to the interaction
between substrate and catalyst, which should neither be too strong, so the substrate remains
available for reaction, nor too weak so the catalyst enhances the reaction rate. The catalyst can
9 Chapter 2: Literature review
also improve the selectivity towards a desired product by increasing exclusively its production
rate. Activity and selectivity can play a major role to select a catalyst.
While several types of catalysis are available, the focus in this work will be on heterogeneous
catalysis. Here, the catalyst consists of a different phase, mostly solid, that differs from the
reactants phase. Its main advantage is that no chemical separation is needed to separate the gas
or liquid reaction products from the solid-state catalyst. The reactions occur on the catalyst
surface and typically consist of multiple steps: adsorption, reaction and desorption, often
accompanied with diffusion towards, outwards and on the catalyst surface and dissociation of
the adsorbate. The first chemical reaction step is adsorption where a bond is formed between
the solid catalyst, the adsorbent, and reactants, the adsorbates. Two main types of adsorption
are possible: physisorption, which is generally a weak bond formation between adsorbent and
adsorbate based on long-range van der Waals forces and chemisorption, where stronger bonds
are formed due to the overlap of orbitals between the adsorbent and adsorbate. The latter is
necessary to enhance the further catalytic reactions on the surface. The adsorbed molecules can
diffuse on the surface, react to products and eventually desorb from the surface. [3-5]
In this work, the focus lays on the dehydrogenation of paraffins and more specific that of
propane. Platinum, an archetype catalyst for this reaction, has a high activity but suffers from
low olefin selectivity and coke formation, which leads to catalyst deactivation. Paraffin
dehydrogenation is an endothermic reaction that is limited by chemical equilibrium and as
illustrated in Figure 2-1, the equilibrium constant is rather low for smaller carbon chains.
Consequently, to obtain higher conversion either higher temperatures or lower pressures are
necessary, according to Le Chatelier’s principle. [6, 7]
Promoting Pt catalysts with metals such as As, Bi, Pb, Ga, Ge and Sn will weaken the platinum-
olefin interaction without influencing the platinum-paraffin interaction. Hence, the paraffin
dehydrogenation rate will remain constant while the dehydrogenation rate of the desired olefins
decreases and the olefin desorption barrier lowers. This can result in an overall increase of
olefin production rate. The above-mentioned modifiers will also restrict the formation of coke
precursors, this way decreasing the coke deposit on the catalyst active sites. [8, 9]
Chapter 2: Literature review 10
Figure 2-1: Equilibrium constants for n-paraffin dehydrogenation at 500 °C [6]
In the following sections, platinum and its modified bimetallic catalysts employed in
dehydrogenation reactions are discussed. Apart from the pure platinum catalyst, the Sn-
promoted and Ga-promoted Pt catalysts are discussed in particular. Pt-Sn catalysts have already
been researched extensively and the effects of Sn-promoting are well known. This could be
useful to link to Ga-promoting, since recent studies have shown that Ga-promoted catalysts
have improved catalytic properties. [10, 11] The catalyst characteristics either found
experimentally and/or computationally are evaluated.
2.1.2 Pure platinum catalysts
Platinum is a noble metal and its bulk phase has a face-centered cubic (fcc) structure with a
coordination number of twelve, a packing factor of 0.74 and a lattice distance of 3.92 Å. [12]
The most stable surface cut of the fcc unit cell is the (111) plane. [13] Each atom on the (111)
surface has a coordination number of nine, so there are three broken bonds per surface atom.
Three broken bonds is the minimal amount possible, making the (111) plane the most abundant
surface plane in Pt particles. However, the surface of an actual solid-state catalyst does not
solely consist of a single crystal plane. Multiple irregularities can be encountered such as edges,
steps and kinks. These irregularities will have a higher activity due to the geometrical and
electronic effect of the higher undercoordination.
Pure platinum catalyst has frequently been utilized to determine experimentally the activity,
selectivity towards products and deactivation during paraffin dehydrogenation. [14-19] The
most employed experimental methods to investigate the surface chemistry on Pt catalysts are
temperature programmed desorption (TPD), infrared adsorption spectroscopy (IRAS) and
11 Chapter 2: Literature review
transmission electron microscopy (TEM). Since many data are available for pure Pt catalysts,
the Pt catalysts can be used as a reference case in this study.
The platinum metal catalyst is very active for paraffin dehydrogenation; however, it lacks a
high selectivity towards olefins, as many side reactions occur on the surface such as coke
formation, leading to unwanted deactivation. It is known that the formation of coke on the
catalyst surface results from both consecutive dehydrogenation and hydrogenolysis of adsorbed
carbon species. Zaera et al. conducted TPD experiments with propylene on Pt(111) to unravel
the thermal chemistry and hence, propose a set of consecutive reactions of propylene that lead
to coke formation. When adsorbed alone, propylene thermal activation leads mainly to
molecular desorption and dehydrogenation reactions. The latter reaction leads to the formation
of propylidyne (Pt3≡C-CH2-CH3), which is the trigger for surface coke formation as consecutive
dehydrogenation steps form surface carbon. In addition, a small amount of propane is formed
through self-hydrogenation with hydrogen, formed in the dehydrogenations steps. Zaera et al.
propose that coke formation reactions demand larger ensembles of active metal atoms to
accommodate the formed hydrogens in consecutive dehydrogenation reactions and to be able
to cleave the bond in the adsorbed species during hydrogenolysis. [17, 18]
Coke formation can be reduced by alloying the platinum catalysts with other elements as it
distorts the larger ensembles of active metal atoms and hereby increases the olefin selectivity
by reducing the coke formation. Furthermore, promoting metals can increase the catalyst
activity. Two major steps in the dehydrogenation reaction are the dissociative adsorption of
saturated hydrocarbons and the desorption of the olefin product. The bimetallic catalyst can
attenuate the paraffin adsorption and the olefin desorption, increasing the rate of olefin
formation. [10, 15]
Catalyst models based on pure platinum catalysts are often employed in a computational
approach or in a combined experiment-and-theory study to correlate the above-mentioned
experimental properties. However, as the modified platinum catalyst excels in multiple
properties in comparison with pure platinum catalyst, the latter are used to obtain fundamental
insights or as reference with respect to the modified Pt catalysts. [14, 20-25]
2.1.3 Bimetallic Pt catalysts
The drawbacks of pure platinum catalysts are the poor olefin selectivity and the high
deactivation due to the coke formation on the surface. The main role of platinum modifiers is
to circumvent these drawbacks in the light of paraffin dehydrogenation. [8] Bimetallic surfaces
Chapter 2: Literature review 12
exhibit catalytic properties different from their pure counterparts. The bimetallics local
electronic and geometric structure influences their catalytic properties and this way enhances
the desired characteristics of the dehydrogenation reaction.
It is rather difficult to pinpoint the surface structure and properties after modification with
another metal, as spectroscopic techniques such as X-ray diffraction (XRD), X-ray
photoelectron spectroscopy (XPS) and TEM give limited results in the sub-nanometer range.
Three major effects can possibly occur when altering the catalyst metal composition. First, the
electronic structure of the surface is changed due to the bond formation with the modifier metal
as it can act as a ligand and induce an electronic effect. The alloying metal can directly influence
the geometric structure through different metal-metal bonding distance, as so inducing
additional strain in the catalyst. Moreover, it is possible that the surface atoms rearrange
themselves, but this depends heavily on the composition of the catalyst and reaction conditions.
Therefore, the configuration of the bimetallic alloy has to be determined: bulk alloys, where the
heteroatoms are present in the bulk and surface layers and surface alloys, where the heteroatoms
are only present in the surface layers, as illustrated in Figure 2-2.
Figure 2-2: (left) four layered model of a Pt3M/Pt(111) surface alloy. (right) four layered model of a Pt3M bulk alloy. Pt atoms are indicated in blue while the hetero atoms M are red (left) and green (right). [26]
This is not a static given, but rather dynamical depending on the reaction conditions. The
bimetallic alloy always strives towards the lowest Gibbs free energy and in this case, this
corresponds to the lowest surface energy of the exposed surface. Two rearrangement
phenomena can occur: segregation and antisegregation. In segregation, the heteroatoms
segregate to the surface while Pt atoms migrate to the bulk, while in antisegregation the reverse
will happen. The rearrangement of the surface atoms can alter the surface reactivity in several
ways as illustrated in Figure 2-3. [27, 28]
13 Chapter 2: Literature review
Figure 2-3: Geometric factors determining the adsorption and dissociation of molecules on bimetallic surfaces. [27]
The site blocking refers to the blocking of adsorption sites by the heteroatoms as these atoms
can only weakly interact with adsorbate molecules. The ensemble effect occurs when an atom
is removed or another heteroatom is incorporated in the ensemble or adsorption site and this
hereby alters the adsorption properties. Furthermore, the template effect induces a shape change
of the adsorption site. At last, there is the coordination effect, which inhibits reactions that need
a minimum number of adjacent adsorption sites. These rearrangement effects induce an
additional electronic effect, so it is often impossible to discriminate between the different
contributions of geometric and electronic effects.
Platinum can be alloyed with both transition metals e.g., Ag, Co, Fe and non-transition metals
e.g. Ga, In, Sn. The main effect that is observed for alloys with non-transition metals is site
blockage, while this does not occur with metals alloyed with transition metals. [27] In the
following sections, bimetallic Pt catalysts are outlined. The focus of this work is on Ga-
promoted Pt catalysts, but since Sn-promoted Pt catalyst are already extensively researched,
these could offer a useful guideline for the study of the relative new Ga-promoted Pt catalysts.
2.1.4 Bimetallic Sn-Pt catalysts
Sn-promoted Pt catalysts exhibit excellent characteristics for light paraffin dehydrogenation
since both high reaction rates and high selectivity towards olefins are observed. Surface science
Chapter 2: Literature review 14
studies of Pt-Sn chemistry have provided insight in the catalyst properties. However, the lack
of detailed knowledge on the surface composition and structure has made it difficult to correlate
the surface structure with the superior catalytic properties. [29]
The role of Sn is to improve activity, selectivity and stability of catalyst particles. It is expected
that the addition of Sn neutralizes acidity of supports, interacts electronically with Pt and
reduces the ensemble effect that favors coke formation. Several models are proposed for Pt-Sn
interaction, as illustrated in Figure 2-4. When increasing the Pt+Sn load, Sn/Pt ratio or reduction
temperature, the system shifts towards the formation of Pt-Sn alloys compared to formation of
Sn clusters. In this work, it is assumed that Pt-Sn alloys have formed on the catalyst surface.
[30]
Figure 2-4: Models of Pt–Sn interaction. Pt-Sn alloys are formed under higher Pt+Sn loading, higher Sn/PT ratio and increased reduction temperature [30]
High-resolution transmission electron microscopy (HRTEM), alkali ion scattering
spectroscopy (ALISS) and X-ray photoelectron diffraction (XPD) are employed to shed a light
on the formation of the Pt-Sn phase. The exact mechanisms contributing to the formation of Pt-
Sn alloys are still unknown. [15, 29]
In the case of promoting platinum metal particles with Sn, several ordered surface alloys can
be distinguished as illustrated in Figure 2-5. At low Sn content, Pt3Sn or PtSn are the most
probable ordered alloys at dehydrogenation reaction conditions (~800 K), but other alloys such
as Pt2Sn3 and PtSn2 can be formed under certain reaction conditions as well. These compositions
of Pt-Sn alloys can be produced accurately, using controlled deposition methods. These alloys
are further utilized to determine the most stable and the most catalytic active alloy under certain
reaction conditions.
It is challenging to determine the composition of the active phase of the Pt-Sn alloy since the
reaction conditions can alter the surface composition. The role of Sn on the catalytic properties
of Pt-Sn catalyst is correlated to the oxidation state of tin, however, this state is difficult to
determine without sophisticated analytical techniques. Possibilities are extended X-ray
Pt+Sn loading ↑ Sn/Pt ↑ Reduction T ↑
Pt+Sn loading ↑↑ Sn/Pt ↑↑ Reduction T ↑↑
15 Chapter 2: Literature review
adsorption fine structure (EXAFS) and XPS, but even these techniques have limits
characterizing the catalyst surface certainly if operated under reaction (in situ).
Figure 2-5: Binary alloy phase diagram of Pt-Sn [31]
2.1.4.1 Light alkane dehydrogenation
Galvita et al. investigated the effect of Sn-modified Pt catalysts during ethane dehydrogenation.
In most studies an alumina support (Al2O3) is used, however Galvita et al. made use of a
calcined hydrotalcite (Mg(Al)Ox) support during the experiments. The introduction of Sn on
the dispersed Pt-particles by impregnation leads to geometrical and electronic effects,
enhancing the catalyst activity and selectivity. The addition of Sn reduces the size of Pt
ensembles on the surface (geometrical effect), hereby reducing the hydrogenolysis reaction rate
as it requires sufficient large Pt clusters to cleave the carbon-carbon bond in the adsorbed
species. This leads also to a higher selectivity towards ethylene. The electronic effects of Sn-
promoting enhance the dissociative adsorption of ethane on the catalyst surface, improving the
catalyst activity and selectivity towards ethylene. [15]
Analogous conclusions were made for propane dehydrogenation on Pt-Sn catalysts, supported
on SBA-15, alumina [14] and ZSM-5 [10]. The geometrical effect of Sn inclusion in the Pt
particles decreases the size of the active Pt particles, hence reducing coke formation and other
side reactions. Furthermore, sticking coefficient measurements, low energy electron diffraction
(LEED) and TPD have proven that these Pt-Sn surface alloys induce a weaker chemisorption
bond between propylene and Pt (electronic effect). [32]
Chapter 2: Literature review 16
As limited theoretical work is performed to determine a detailed reaction mechanism of propane
dehydrogenation on supported Pt-Sn catalysts, the fundamental understanding of the role of Sn
is limited. Yang et al. conducted ab initio density functional theory (DFT) calculations of
propane dehydrogenation on pure Pt and Pt-Sn catalysts. [24, 33] In the latter, they used five
models with different Sn to Pt ratios to represent the Pt-Sn catalysts: Pt3Sn/Pt(111), Pt3Sn(111),
Pt2Sn/Pt(111), Pt2Sn(111) and PtSn2(111). The first and third are surface alloys where only the
top layer is alloyed with Sn. The Pt2Sn alloys are theoretical alloys and cannot be found in the
Pt-Sn phase diagram (see Figure 2-5). [31] The descriptors for the catalyst activity are defined
by the activation energies of the dehydrogenation steps from propane to propylene. This
sequence consists out of two major elementary steps: dissociative propane adsorption to propyl
and dehydrogenation of propyl to propylene. The selectivity descriptor is determined as the
difference in activation energy between dehydrogenation and desorption of propylene.
Alloying Pt with Sn will significantly lower the reaction rate of propane dehydrogenation and
the variation in the activation energies depends strongly on the change in the binding strength
of H in the geometry of the transition state. Analyzing the competition between
dehydrogenation and C-C scission leads to the conclusion that the cracking is kinetically
hindered because of the much higher energy barriers. [33] The deposition of Sn makes it
difficult to achieve a large ensemble, which is essential to activate cracking reactions because
sufficient large ensembles are required to accommodate detached fragments. The introduction
of Sn lowers the desorption barrier for propylene to the gas phase and simultaneously increases
the energy barrier for propylene dehydrogenation. As the Sn content increases, the selectivity
toward propylene desorption is significantly improved. Considering the trade-off between the
catalytic activity and the selectivity, the Pt3Sn bulk alloy is proposed as the best candidate of
those five models for propane dehydrogenation. [21, 33]
2.1.5 Bimetallic Ga-Pt catalysts
Recent studies have shown that alloying Pt catalysts with Ga leads to promising catalyst
properties such as reduced coke formation and deactivation of the catalyst during light alkane
dehydrogenation. [34] Furthermore, a novel synthesis method is proposed by Sun et al. for
incorporating Ga in the Pt catalyst particles, which facilitates the use of Ga as modifier
element. [35]
Sun et al. describe a new approach for preparing Ga-promoted Pt particles in the context of light
alkane dehydrogenation. For the novel Pt-Ga/Mg(Ga)(Al)Ox catalyst, the modifying element,
17 Chapter 2: Literature review
Ga, was introduced by transference from the support, a calcined Mg(Ga)(Al)Ox hydrotalcite.
First, Pt nanoparticles were dispersed onto the calcined Mg(Ga)(Al)Ox starting from an
organometallic precursor and secondly the catalyst precursor is reduced. The formation of Pt-
Ga alloy particles is dependent on reduction temperature. Reduction at 723 K or lower produces
mainly metallic Pt particles, while at reduction temperatures of 773–873 K, PtxGay alloys were
observed by EXAFS and STEM-EDX results. It is proposed that at high reduction temperatures,
hydrogen formed on the surface of the metal particles spills over onto the support where they
reduce Ga3+ cations to Ga, which then interact with the supported Pt to form PtxGay alloys with
different compositions, as illustrated in Figure 2-6. [35, 36]
Unfortunately, the exact composition of these alloys remains elusive as the average particle size
is in the order of a few nanometers and even sophisticated spectrometry methods such as TEM
and STEM fall short to determine the exact stoichiometry. To discover the unknown
composition of the Pt-Ga alloys, other methods can be employed and these will be discussed
further on in this chapter.
Figure 2-6: Proposed mechanism for migration of Pt alloy formation during synthesis and reaction of Pt/Mg(Ga)(Al)Ox catalysts. [35]
2.1.5.1 Light alkane dehydrogenation
In Figure 2-7, the results of the catalyst activity (propylene formation rate) and the propylene
selectivity as function of time on stream are illustrated. The activity of the Pt-Ga/Mg(Ga)(Al)Ox
catalyst is initially more than two times higher than the pure Pt/Mg(Al)Ox catalyst. While both
suffer from activity loss as function of time on stream, the former catalyst is more stable as it
loses only 30% of its activity after two hours on stream with respect to 40% activity loss on
Pt/Mg(Al)Ox (see Figure 2-7a). Furthermore, the experiments show that the propylene
selectivity is enhanced for the Pt-Ga/Mg(Ga)(Al)Ox catalyst as it remains 99% the entire time
Chapter 2: Literature review 18
on stream. In contrast, the Pt/Mg(Al)Ox catalyst has initially a low propylene selectivity of 78%,
but increases to 87% after one hour on stream (see Figure 2-7b).
Figure 2-7: (a) Comparison of activity of Pt/Mg(Ga)(Al)O (Ga/Pt = 2.86) and Pt/Mg(Al)O and (b) selectivity for propane dehydrogenation. Reaction temperature of 873 K, 20 vol.% C3H8 in feed, H2/C3H8 = 1.25, with balance He for total flowrate of 60 ml/min. [11]
Siddiqi et al. determined the optimal ratio of Ga to Pt in a Pt-Ga/Mg(Ga)(Al)Ox catalyst by
performing experiments with Pt-Ga catalysts with various Ga/Pt ratios. As illustrated in
Figure 2-8, a trade-off has to be made between catalyst activity and propylene selectivity. The
rate of propylene formation has a distinct maximum of 64 µmol/s/gcat at a Ga/Pt ratio of two
and the rate sharply decreases for different Ga/Pt ratios (see Figure 2-8a). The propylene
selectivity initially increases sharply until a maximum of 99% at Ga/Pt of 5.4 and further
decreases monotonically (see Figure 2-8b). The approximated optimal Ga/Pt ratio is 5.4, when
the propylene selectivity is maximal and the rate of formation is 50 µmol/s/gcat.
Figure 2-8: Effect of Ga/Pt ratio on Pt/Mg(Ga)(Al)O catalysts for C3H8
dehydrogenation (a) activity and (b) selectivity, feed composition of H2/C3H8 = 1.25 and all data points are after 120 min time on stream. Reaction temperature of 873 K, 20 vol.% C3H8 in feed, H2/C3H8 = 1.25, with balance He for total flowrate of 60 ml/min. [11]
19 Chapter 2: Literature review
2.1.5.2 Determination of active phase of Pt-Ga catalysts
Almost no theoretical work is performed on propane dehydrogenation on supported Pt-Ga
catalysts, and consequently the fundamental understanding of the role of Ga as a promotor is
slight. In previous work, the active phase of the Pt-Ga/Mg(Ga)(Al)Ox catalyst is investigated
by conducting ab initio DFT calculations on various Pt-Ga alloys to determine an appropriate
catalyst model that describes the surface composition of the novel catalyst. [37]
As illustrated in Figure 2-9, the following Pt-Ga compositions are taken into account in
ascending order: Pt3Ga, Pt2Ga, Pt5Ga3, PtGa and Pt2Ga3. These are all the alloys up to 0.6 mole
fraction of Ga and stable above reduction temperatures of 773 K. On the Pt-Ga/Mg(Ga)(Al)Ox
catalyst, CO adsorption experiments are performed in unison with frequency calculations of
adsorbed CO on each catalyst model, accounting for different coverages and segregation. The
experimental results indicate a small shift in CO frequencies (~5 cm-1) for increasing Ga
content. Compared to performed DFT calculations on the candidate structures, this small shift
indicates the formation of the Pt-Ga alloy with the lowest Ga content, which is Pt3Ga in this
study. Other catalyst models with low Ga-content, which can be correlated with the small shift,
are e.g. a surface alloy Pt3Ga/Pt. [37]
Figure 2-9: The phase diagram of the Pt–Ga binary system as reassessed with first-principles calculations by Wang et al. [38]
The role of gallium on increased activity and selectivity during propane dehydrogenation
remains elusive. The focus on this work is to provide a fundamental insight in the reaction
Chapter 2: Literature review 20
mechanism on this novel Pt-Ga/Mg(Ga)(Al)Ox catalyst. Based on previous studies, the Pt3Ga
alloy will be chosen as a model for the Pt-Ga/Mg(Ga)(Al)Ox catalyst. [37]
2.2 Coke formation during propane dehydrogenation
During propane dehydrogenation, side reactions such as cracking to lighter hydrocarbons,
skeletal isomerization and aromatization lead to formation of coke deposit on the surface of a
platinum catalyst, causing rapid deactivation and lower yields. [39] Characterization of the coke
deposits could lead to further understanding of the coke formation mechanism and enabling the
minimization of coke formation. This section focuses on the characterization of the coke and
its formation rate, which is essential for understanding the mechanism of coke formation as
well as the mechanism of catalyst deactivation.
2.2.1 Coke formation on pure Pt catalysts
Coke formation on supported Pt catalysts depends on the selected reaction temperature. At
lower temperature regimes, coke formation can be contributed to condensation and
rearrangement steps. However, at high temperature, also hydrogen transfer and
dehydrogenation steps are observed. The higher temperature regime is a more realistic reaction
condition in light alkane dehydrogenation reactions since higher conversions are obtained at
these conditions. [40]
The coke mobility has been studied with temperature programmed oxidation (TPO) techniques
and results show that coke can be formed on the Pt metal, Pt-support boundary or on the support.
These studies have shown that reaction conditions influence the rate of coke formation and the
nature of the coke deposited on different sites are essential for the understanding of the
mechanism of coke formation as well as the mechanism of catalyst deactivation. [41] TPO
studies show that different type of cokes can be found on the supported catalyst. The coke
formed on those different sites could have other effects on the catalyst deactivation. Larsson et
al. proposed a model where only a small part of the formed coke was responsible for catalyst
deactivation, while the major part of the coke was formed regardless of the gas composition but
did not contribute to the deactivation. [41] It is also observed that the coke formation rate
consists out of two time regimes; initially the coking rate is high, but eventually it decreases to
a lower constant rate. These regimes can be contributed to the formation of different types of
coke deposits. Essentially, there is constant coke formation during both regimes, which can be
21 Chapter 2: Literature review
contributed to coke deposits on the support. However, the initial high coke formation rate is
due to the formation of coke precursors on the active phase. The coke precursors migrate to the
boundary between the platinum and the support and when most of these sites are occupied, the
coke formation rate is greatly reduced. These coke deposits are responsible for the deactivation
of the active sites on the metal as it hinders the transport of coke precursors to the support,
which thus increases the coke accumulation on the platinum. [42]
Vu et al. investigated the nature of coke generated in propane dehydrogenation. Two supported
Pt catalysts were synthesized: each with 3% Pt and the first on mesoporous alumina and the
other on SBA-15 (mesoporous silica). In this study, Vu et al. characterized the coke structure
of both spent catalysts with both XRD and the sophisticated magic angle spinning nuclear
magnetic resonance (13C-CP/MAS NMR) technique. The XRD results show that the nature of
the coke exhibits a pregraphite-like carbon structure. Further characterization with NMR leads
to the determination of the pregraphite-like carbon structure as a distinct aromatic structure
while no detectable presence of aliphatic coke was found, independently of the different acidity
of the support. Further investigation was conducted with the Fourier transform infrared
spectroscopy (FTIR) and the XPS techniques, which confirmed the polyaromatic structure of
the deposited coke. It was also observed that the electron-rich state of the polyaromatic ring in
the coke donates electrons to the Pt active metal, inducing a higher electron density of Pt metals
as illustrated in Figure 2-10. This causes a shift in binding energy towards lower values. [43]
Figure 2-10: A schematic diagram for coke-Pt interaction and electron transfer [43]
The most favorable adsorption site for coke formation and possible coke spillover are not yet
discussed. Peng et al. performed in situ and ex situ HRTEM studies on pure platinum catalysts,
supported on MgO. Various average metal particle diameters (1.4 nm, 3.5 nm and 6.1 nm) were
Chapter 2: Literature review 22
investigated. The employed carbon source for the coke deposits was a mixture of ethylene and
hydrogen under light alkane dehydrogenation conditions. The TEM observations confirmed
that the formed coke species are a type of graphene and their formation initiates at highly
preferred low-coordination sites such as step sites on the surface of the metal particles. The
growth of the graphene layers depends on the size and shape of the catalyst particles. In these
studies, particles with particle diameter larger than 6 nm are covered with multiple layers of
piled graphene, the number of layers depending on the hydrocarbon exposure time. In the region
with particle diameters of 2-6 nm, the formation of graphene nanotubes are observed and at
even smaller particle diameters (< 2 nm), a graphene sheet is formed, which migrates towards
the support. The size dependency of graphene growth is attributed to the accommodation of
strain energy generated in the graphene layers and the minimization of overall free energy in
the growth process. The graphene growth models are illustrated in Figure 2-11. However, this
classification based on the average particle diameter is ambiguous, as other factors clearly
influence the graphene growth such as the shape of the particle. [44]
Figure 2-11: Schematic illustration of graphene layer growth on Pt particles of increasing size: (left) envelopment of Pt particles by graphene for particles greater than ∼6 nm in diameter; (middle) formation of graphene nanotubes on Pt particles of 2–6 nm; (right) formation of graphene sheets and their migration to the support for Pt particles less than 2 nm in diameter. [44]
Additionally, Larsson et al. reported the effect of hydrogen on suppressing coke formation.
Hydrogen is active in preventing coke formation but does not remove any significant amount
of coke already formed. On the one hand, the coke formed in dedicated coking experiments,
which means higher temperature and longer time on stream, probably has been more deeply
dehydrogenated and strongly bonded to the catalyst. During the reaction, on the other hand, the
formation of coke precursors is suppressed by hydrogen. This explains why hydrogen helps
maintain the catalytic activity. [41]
23 Chapter 2: Literature review
2.2.2 Coke formation on bimetallic Sn-Pt catalysts
Li et al. and Vu et al. both determined the influence of alloying the Pt catalyst with Sn on the
coke characteristics and its formation rate. Both employ alumina supported Pt-Sn catalysts in
their research but Vu et al. focused more on the location and structure of coke generated during
the propane dehydrogenation reaction, while Li et al. determine different coke formation
mechanisms. [41, 42, 45]
Vu et al. prepared different Pt-Sn catalysts with a constant 1% Pt content, while varying the Sn
content between 0 and 1.67%. Based on a thermogravimetric analysis (TGA), the best coke
tolerance is found with the 1% Pt-1.67% Sn catalyst, which has the slowest deactivation rate
and lowest coke content. Furthermore, Vu et al. determined, based on XRD and XPS analyses,
that the coke on the Pt-Sn/Al2O3 catalyst has the same pregraphite-like carbon structure as on
the Pt/Al2O3 catalyst. Consequently, the addition of Sn has no influence on coke structure. The
TPO technique allows determining the coke location on the supported bimetallic catalyst. Two
types of coke are identified on the spent catalysts and can be assigned to coke on the metal
(lower TPO temperature) and the support (higher TPO temperature). For increasing Sn contents,
the TPO profiles shift to higher temperatures, indicating that Sn alters the electronic properties
of the Pt surface and weakens the propylene-platinum bond. Therefore, the Sn addition
accelerates the coke spillover from the catalytic phase to the support, decreasing the coke
fraction on the metal and hence ensuring a longer stability and activity of the catalyst. [45]
Li et al. confirmed the results of the formation of two types of coke on the alumina supported
Pt-Sn catalyst and assigned them to coke on the metal and the support. Furthermore, Li et al.
varied the reaction conditions, especially the gas composition (propane and/or hydrogen), to
determine its influence on the coke formation. Based on results with various H/C ratios, Li et
al. conclude that the coke formed on the metal has aliphatic hydrocarbon characteristics,
containing more hydrogen than that formed on the support, which has an aromatic
characteristic. The proposed model of coke formation is divided in two parts, one for each type
of coke formed.
The rate of coke formation on the metal is weakly dependent on the propylene and hydrogen
pressures but increases simultaneously with the propane pressure, while the rate of coke
formation on the support increases concurrently with the propane and propylene pressures and
decreases with the hydrogen pressure. In the following paragraph, the focus will be on the coke
formation on the active metal phase only.
Chapter 2: Literature review 24
Li et al. propose the following reaction mechanism for coke formation on active metal phase of
supported Pt-Sn catalysts based on a kinetic analysis:
Reaction mechanism 1: Coke formation on active metal phase of supported Pt-Sn catalyst [42]
𝐶𝐶3𝐻𝐻8(𝑔𝑔) + 2 ∗ → 𝐶𝐶3𝐻𝐻7 ∗ +𝐻𝐻 ∗ (1)
𝐶𝐶3𝐻𝐻7 ∗ + ∗ → 𝐶𝐶3𝐻𝐻6 ∗ +𝐻𝐻 ∗ (2)
𝐶𝐶3𝐻𝐻6 ∗ → 𝐶𝐶3𝐻𝐻6(𝑔𝑔) + ∗ (3)
2𝐶𝐶3𝐻𝐻6 ∗ → 𝐶𝐶6𝐻𝐻12 ∗ + ∗ (4)
2𝐻𝐻 ∗ ⇌ 𝐻𝐻2(𝑔𝑔) + 2 ∗ (5)
The reaction between the two strong adsorbed C3H6* molecules, which are formed by
dehydrogenation of propane, is identified as the kinetic relevant step (4) for the coke formation
on the Pt surfaces. It is noted here that in this model the hydrogen pressure has little effect on
the rate of coking on the metal, but it changes significantly the hydrogen content in the coke.
At higher hydrogen partial pressure, the coke is less dense and compact due to a higher H/C
ratio in the coke itself. [46] However, this contradicts the observations of Larsson et al. which
report a stronger dependence on hydrogen pressure, namely the reduction of coke precursor
formation, together with less dense coke due the presence of hydrogen in the feed.
A large portion of the precursors migrates to the acid sites of the alumina and is involved in the
coke formation on the support. In addition, the propylene in the gas phase can also adsorb on
the alumina support and form coke through complex reaction mechanisms. [42]
2.2.3 Coke formation on bimetallic Ga-Pt catalysts
The characterization of coke is less researched for supported Pt catalysts alloyed with Ga.
Siddiqi et al. studied the catalyst performance of Pt-Ga catalysts, supported on calcined
hydrotalcite for both ethane and propane dehydrogenation. Here the coke formation and catalyst
deactivation were also reported. The presence of Ga compared to Sn reduces the deactivation
and the amount of coke deposited during both ethane and propane dehydrogenation. The ratio
of deposited carbon to surface Pt atoms are much greater than unity in all cases, but suddenly
drops from 47 to 15 at Ga/Pt ratio of 5.4, as illustrated in Figure 2-12. The large carbon to
surface Pt atoms ratio suggests that most of the accumulated coke is present on the support and
that only a small fraction remains on the surface of the metal particles, in agreement with the
model for Pt-Sn catalysts. It is hypothesized that Ga affects the surface, similar to that of Sn,
by reducing the desorption barrier of the desired alkene product. [11, 42]
25 Chapter 2: Literature review
Figure 2-12: Carbon formation after 120 min of propane dehydrogenation at different Ga/Pt ratios, Reaction temperature of 873 K, 20 vol.% C3H8 in feed, H2/C3H8 = 1.25, with balance He for total flowrate of 60 ml/min. [11]
Additionally, Siddiqi et al. determined the effect of the extra methyl moiety of propane with
respect to ethane on the coke formation and the deactivation. The overall amount of coke
generated during ethane dehydrogenation is greater than during propane dehydrogenation,
while the deactivation was larger in the latter. Therefore, this confirms additionally that the total
coke amount cannot be directly linked to deactivation. However, it is expected that the mobility
of carbonaceous species is lower for such species originating from propane than those
originating from ethane, resulting in larger coke buildup on the active phase. [11]
Peng et al. suggested that the suppression of coke formation on bimetallic Pt catalysts, modified
with Ga, Sn, In, etc. may be assigned to the presence of these elements at the undercoordinated
sites. [44]
2.3 Interaction between active metal phase and support
Platinum is a highly active catalytic element and is not required in large quantities to catalyze
the reaction when it is dispersed on a high surface-area support. The high dispersion is also
necessary to achieve high selectivity towards dehydrogenation relative to undesirable side
reactions. [8] In the following section, the active metal phase properties of the pure and
modified platinum catalyst are discussed with focus on their dispersion and particle structure.
Furthermore, several high surface-area supports with high Pt dispersion inducement are
evaluated. Finally, the link is made between the two sections and their interaction will be
elaborated on.
Chapter 2: Literature review 26
2.3.1 Active metal phase
The degree of dispersion and thus the particle size depends on various factors such as the
amount of platinum precursors, the type of the support and the utilized impregnation method.
As platinum particles exhibit a high catalytic activity, small particles are favored since they
have a higher area-to-volume ratio with respect to larger particles and less bulk platinum atoms
are present. There are different particle structures and shapes for transition metal particles in
relation to their length scale regimes, as illustrated in Figure 2-13. Each structure and shape
contributes differently to the chemical reactivity of the metal particle.
Figure 2-13: Schematic overview of structure-stability regimes of transition metal particles [47]
It is not possible to distinguish between interior and exterior surface atoms in metal particles
with a size less than 1 nm. Such small clusters often have a low energy barrier for
reconstructing, especially when in contact with adsorbing molecules and atoms. They react
typically as a sole molecule. Their reactivity can be directly related to their orbital structure,
which varies strongly with number of atoms. These small metal clusters can be distributed on
a microporous support and are highly reactive. Between one and three nm, the number of
surface atoms increases over the number of interior atoms. The shape of these particles is often
that of an ideal Platonic or Archimedian structure, composed of similar regular polyhedral, as
illustrated in Figure 2-14. [47] At the upper boundary of this particle size regime, the ratio of
surface-to-bulk atoms decreases beneath unity. The surface of larger particles is determined by
the termination of the most stable bulk structure. However, in the size regime 3-10 nm, the
interior atoms in the bulk may not yet have a structure similar to the corresponding most stable
bulk structure. Apart from the different length regimes, the formation of step sites can be
27 Chapter 2: Literature review
relevant to explain the structure dependence of the catalytic activity. These are observed in the
size domain of 2-20 nm. [47]
It is expected that alloying with a transition metal such as Sn and Ga influences the average
particle size of Pt catalysts. Galvita et al synthesized and characterized a Sn-promoted Pt
supported on calcined hydrotalcite (Mg(Al)Ox) and compared it with a Pt/Mg(Al)Ox catalyst as
reference. The Pt/Mg(Al)Ox catalyst has a calcined support with a BET (Brunauer–Emmett–
Teller) surface of 200 m²/g and H2-chemisorption experiment resulted in a dispersion of 84%,
which means an average Pt particle diameter of 1.35 nm. This is confirmed by TEM analysis
results which lead to a particle diameter of 1.0-1.5 nm. [15, 48] The alloying of Pt with Sn did
not lead to a loss in the surface area of the calcined support, but since the majority of the
observed particles were so small (~1.5 nm) and the Sn-to-Pt ratio was 0.23, diffraction patterns
were inconclusive to determine the effect of Sn on the dispersion. However, bimetallic Pt-Sn
particles were observed based on HRTEM and fast Fourier transformation (FFT) analysis,
indicating that for small Sn-to-Pt ratios, Sn has no distinct effect on the dispersion. [15]
Other researchers such as Zhu et al. [49] and Zhang et al. [10] prepared Pt-Sn catalysts with
various Sn-to-Pt ratios and determined a more profound effect of Sn on the average particle
size. Zhu et al. developed a new one pot, surfactant-free, synthetic route based on the surface
organometallic chemistry (SOMC) concept for the synthesis of Sn surface-enriched Pt-Sn
nanoparticles, with Sn-to-Pt ratios between 0 and 2. XRD and TEM analyses were performed
to determine the average particle size and both conclude that tin alloying leads to smaller
particle sizes e.g. for a Sn-to-Pt ratio of 1/12, the average particle size is 3.4 nm, while for a
ratio of 1, the particle size is decreased to 2.0 nm. [49] Zhang et al. prepared Pt-Sn/ZSM-5
catalysts with higher Sn-to-Pt ratios and confirmed the trend observed by Zhu et al. However,
when the concentration of Sn is excessive, Sn0 species are formed, of which the formation is
proposed as the reason for the reduced metal dispersion.
Sun et al. prepared and characterized Pt-Ga catalysts, supported on calcined hydrotalcite with
various Ga-to-Pt ratios. The dispersion of the Pt particles on calcined hydrotalcite-like supports
is determined by H2-chemisorption. Pure Pt supported on Mg(Al)Ox (support without Ga) has
a dispersion of 84%, corresponding to an average particle size of 1.35 nm. The dispersion of Pt
on Mg(Ga)(Al)Ox (support with Ga) tends to decrease with increasing Ga/Pt ratio from about
78% to 52%, corresponding to an increase in average Pt particle size from 1.5 to 2.2 nm.
However, the observed decrease in dispersion with increasing Ga-to-Pt ratio is not contributed
Chapter 2: Literature review 28
to increase of Ga atoms, but is likely due to the decrease in the concentration of Al cations at
the surface of the support, which have been observed to facilitate the dispersion of Pt. [35]
As was earlier illustrated in Figure 2-13, modified Pt catalysts can be classified based on their
average particle size in either ‘Platonic and Archimedian structures’ (1-3 nm, see also
Figure 2-14) or ‘surface structures, that can deviate from their most stable bulk configuration’
(3-10 nm).
Figure 2-14: (a) Archimedian cuboctahedron, (b) Archimedian decahedron and (c) Platonic icosahedron. [47]
Kumar et al. performed experiments on pure Pt catalysts, supported on SBA-15, to determine
the effect of the particle size on propane dehydrogenation characteristics. In this study, two
catalysts are synthesized, using incipient wetness impregnation (IMP) and deposition-
precipitation (DP) techniques. This results respectively in a catalyst with an average particle
size of 3 nm (small) and an average particle size of 21 nm (large). The results from tapered
element oscillating microbalance (TEOM) experiments show that small Pt particles are very
active for C–C bond activation, namely cracking reactions, while relatively large Pt particles
are more selective for C–H bond activation, namely dehydrogenation reactions. Consequently,
selectivity to propylene depends on the activity ratio between C–H and C–C bond activation,
confirming that propane dehydrogenation is structure sensitive. [50, 51]
Furthermore, Yang et al. performed combined experiments and DFT calculations on larger
particles with different surface geometries, as the surface of large particles cannot be described
as an extension of the most stable bulk geometry. Two Pt catalysts with different particle shapes
and close particle sizes, namely, octahedron (12.0 nm) and cube (11.5 nm), have been used to
test the catalytic performances. The octahedral particles are dominated by Pt (111) facets and
the cubic particles are surrounded by Pt (100) facets. As indicated by the measured turnover
29 Chapter 2: Literature review
frequency (TOF) for propylene formation and selectivity toward propylene, the cubic particles
exhibit a higher catalytic activity for propane dehydrogenation but a lower selectivity for
propylene than the octahedral ones. In order to explain the experimental observations, the
differences in activation energies for the dehydrogenation process from propane to propylene
between Pt (111) and Pt (100) are compared. Based on DFT calculations, the activation energies
for dehydrogenation and cracking on Pt (100) are lower than those on Pt (111), which indicates
that cubic particles exhibits a higher catalytic activity than the octahedral particles. However,
the desired product, propylene, may undergo further dehydrogenation readily on Pt (100),
leading to a low selectivity toward propylene. [25]
2.3.2 Catalyst support
Properties that are desired for support materials are high platinum dispersion inducement, as
small particles are the most active, a high surface area and thermal stability. Another important
support aspect is a low pore diffusional resistance. Without the low diffusional resistance, the
catalytic reaction rate is limited by the intraparticle mass transfer rate, so both activity and
selectivity are lowered. [8] Among many high surface area materials, alumina is the support of
choice. Alumina (Al2O3) has excellent thermal stability and mechanical strength under
processing, transport and catalyst regeneration conditions. However, the most important reason
why alumina is chosen as support material is its superior capability of maintaining a high degree
of platinum dispersion. However, also other high surface support materials are proposed and
employed in various studies such as SBA-15, ZSM-5, SAPO-34, ZnAl2O4, mesoporous alumina
(MA) and calcined hydrotalcite Mg(Al)Ox. [15, 51-54]
Apart from the selected support, the employed impregnation method plays a key role in the
characteristics of the active metal phase. Therefore, if a comparison of different supports is
conducted, other preparation conditions are kept constant such as the impregnation method.
Zhang et al. prepared a set of different bimetallic Pt-Sn catalysts on four supports: γ-Al2O3,
MA, ZSM-5, SBA-15 (in order of increasing BET surface), all prepared with the co-
impregnation method with constant loading of Pt (0.5 wt.%) and Sn (1.0 wt.%). In the case of
first three supports, the metallic Pt-Sn particles are distributed homogeneously with an average
particle sizes of 16.7 (γ-Al2O3), 15.5 (MA) and 11.6 nm (ZSM-5), indicating that the dispersion
increases with the BET surface of the support. However, the SBA-15 sample has the highest
specific surface, but here an average particle size of 26.4 nm is observed. This can be explained
based on the weak interaction between active phase and support. These weak interactions can
Chapter 2: Literature review 30
facilitate migration of metallic particles during the catalyst preparation, leading to
conglomeration of particles. [54]
Zhang et al. propose also that the support influences the stability of the promoting metal tin, as
it can be present in different oxidation states with different catalytic properties. When this
promoter exists in a reduced state (Sn0), it is expected to poison the catalyst; while it exists in
an oxidized state (Sn4+ or Sn2+), it acts as a promoter. Therefore, it is deduced that the SBA-15
support cannot effectively stabilize the oxidative state of tin species, explaining the decrease in
catalytic activity. The acidity of the support is another important parameter, as it is proposed
that it aids in the high dispersion of the metal particles. However high support acidity is
disastrous as side reactions are mainly catalyzed by strong acid centers, which is expected in
ZSM-5 supported catalysts. [10, 54]
Furthermore, Zhang et al. determined the performance of the catalysts during propane
dehydrogenation, as illustrated in Figure 2-15. The poorest catalytic activity and activity loss is
described by the Pt-Sn/SBA-15 catalyst, as it exhibits weak interaction between support and
the metal particles, leading to agglomeration of metal particles and unstable Sn promotors.
However, the selectivity towards propylene is high since almost no side reactions occur. The
PtSn/ZSM-5 catalyst shows the highest catalyst activity, but its high acidity facilitates side
reactions such as C-C scission, reducing its propylene selectivity. A trade-off has to be made
between activity and selectivity; the most promising supports with the highest propylene yield
are γ-Al2O3 and MA.
Figure 2-15: (a) Conversion and (b) selectivity as function of time for the different catalysts: (1) Pt-Sn/ZSM-5; (2) Pt-Sn/γ-Al2O3; (3) Pt-Sn/MA; (4) Pt-Sn/SBA-15. Reaction conditions: 590 °C, H2/C3 = 0.25 (molar ratio), m(cat) = 1.0 g, WHSV = 3.0 h−1. [54]
31 Chapter 2: Literature review
Recent studies have shown that calcined hydrotalcite is a promising support for light alkane
dehydrogenation as it results in high dispersion of the modified platinum particles and maintains
a high thermal stability under light alkane dehydrogenation reaction conditions. [15, 35, 55] As
hydrotalcite support is a double-layered Mg-Al mixed oxide, the presence and strength of basic
or acid sites depends on the Mg-to-Al ratio. [56, 57] Furthermore, hydrotalcite-derived supports
can be utilized to promote platinum catalysts with other elements (e.g., Zn, Ga, In), which
replace the Al in the hydrotalcite support. These promoters are able to form an alloy with Pt
under certain conditions, hereby enhancing its catalytic properties.
2.3.3 Active metal phase and support interaction
Metal-support interactions are widespread phenomena that differ in strength. This is influenced
by three key properties of the interacting metal-support: energetics of metal particles, geometric
properties and electronic properties.
The interactions are often divided based on their strength, varying from weak to strong metal-
support interactions. Strong metal-support interactions (SMSI) are the most promising kind as
they have the most influence on the active metal phase. The general examples for the SMSI
phenomenon are Group VIII metals such as Pt, Ir, Os and Pd, dispersed on titania support
(TiO2). This strong effect can alter both the morphology of the active phase and the support. In
the case of Pt clusters on titania, the metal particles take on a more pillbox-like morphology as
opposed to the rounded globular morphology in the absence of SMSI. Moreover, the titania
support is transformed to Ti4O7 in the presence of Pt clusters. The appearance of SMSI strongly
depends on the reduction temperature, as at low reduction temperature these morphology
alterations are not observed.
In the case of a bimetallic active phase, the interaction with support becomes more important,
especially as the support strength differs between the two metals. This difference in support
interactions could lead to phase separation of the two metals. When this reconstruction of the
surface occurs, the catalytic activity can change dramatically and it is important to consider this
phenomenon. [58, 59]
Ren-Yuan et al. found evidence of strong metal-support interaction in alumina-supported
platinum catalysts. Based on temperature programmed reduction (TPR) experiments, it is
concluded that a Pt-Al alloy is formed after the reduction of the alumina supported Pt catalysts.
However, at higher reduction temperature, the alumina support reduces further and dilutes the
Pt-Al alloy with Al atoms, indicating that selecting the appropriate reduction temperature is key
Chapter 2: Literature review 32
in the formation of desired alloys. It is important to note that interactions are reversible and can
be destroyed by reoxidation, which could be the key for understanding the regeneration process
of deactivated catalysts. [60]
As cokes formation on the active phase remains unavoidable, regeneration under oxidizing
atmosphere is often necessary in industrial applications. However, two deactivation modes of
the metal particle can occur under reaction conditions, depending on the active metal-support
interactions. Pt catalyst supported on oxides such as Al2O3, Si2O3, Ti2O3 etc. show sintering
which results in loss of small catalyst particles and a larger average particle diameter. It is also
possible that reactions with the support itself leads to the formation of MPtOx (M=Mg, Ce),
which reduces the amount of catalytic exposed active Pt particles and consequently a loss in
catalytic activity. To reactivate these metal particles often expensive H2-reduction and/or oxy-
chlorination are required [61]
Li et al. proposed a well-defined cuboctahedral MgAl2O4 spinel support, which is able to
stabilize platinum particles in the range of 1-3 nm and does not show deactivation modes under
oxidizing conditions. This support material has two dominant {100} and {111} facets and in
the cuboctahedral shape, six {100} facets are isolated by eight {111} facets. Both spectrometric
analysis and DFT calculations are employed to unravel the stabilization mechanism of Pt
particles on this spinel support. The HRTEM analysis shows that fresh prepared catalyst has
small Pt particles on both above-mentioned facets, however during aging, the small Pt particles
on the {100} surfaces are sintered and unstable, while the high dispersion is maintained on the
{111} surfaces. Due to the small {111} surfaces and their isolation in the cuboctahedral
structure, the supported Pt particles are limited in size. DFT calculations are conducted on Pt
depositions on Mg/O-terminated {100} facet and O/(Mg-Al-Mg) terminated {111} facet
dynamically for 10 ps at 800 °C. These experiments revealed that Pt(111) is attracted by O/(Mg-
Al-Mg) terminated {111} facet and grows epitaxial in a lattice-matching way on the support,
while Pt(100) is repelled by Mg/O-terminated {100} facet, confirming the spectrometric results.
2.4 Conclusions
Light alkane dehydrogenation is an endothermic reaction that is limited by chemical
equilibrium and the equilibrium constant is rather low for smaller carbon chains such as ethane
and propane. Consequently, to obtain higher conversion either higher temperatures or lower
pressures are necessary. Platinum metal catalysts are an adequate choice as catalyst for light
33 Chapter 2: Literature review
alkane dehydrogenation as they exhibit a high activity for the dehydrogenation steps. However,
those catalysts lack a high selectivity towards olefins, as many side reactions occur on the
surface. Eventually these side reactions initiate coke formation, leading to unwanted
deactivation of the catalyst. To improve the catalytic properties of platinum-based catalysts, the
metal phase is alloyed with an additional metal such as Ga, In, Sn. Primarily, the alloying leads
to an increase in catalyst activity. The rate of olefin formation is higher for bimetallic catalyst
as the alloying metal attenuates the paraffin adsorption and the olefin desorption, two major
steps in the dehydrogenation reaction mechanism. Secondly, the coke formation is reduced by
alloying the platinum catalysts with other elements as it distorts the larger ensembles of active
metal atoms and hereby increases the olefin selectivity by reducing the coke formation.
The two most interesting bimetallic catalysts are Ga-promoted Pt and Sn-promoted Pt catalysts.
The first is employed as catalyst model throughout this thesis, but little research is conducted
on this catalyst type, while the properties of the second catalyst are already heavily investigated
and can be employed as a guide. The introduction of Sn lowers the desorption barrier for
propylene to the gas phase and simultaneously increases the energy barrier for propylene
dehydrogenation. The deposition of Sn makes it difficult to achieve a large ensemble of Pt
atoms, which is essential to activate cracking reactions because sufficient large ensembles are
required to accommodate detached fragments. As the Sn content increases with respect to the
Pt content, the selectivity toward propylene desorption is significantly improved. Considering
the trade-off between the catalytic activity and the selectivity, the Pt3Sn bulk alloy is proposed
as the best candidate for propane dehydrogenation.
The recent experimental studies on Ga-promoted Pt catalyst during light alkane
dehydrogenation have already shown an increase in catalytic activity and propylene selectivity
with respect to an unmodified Pt catalyst on the same support. The PtxGay alloy at the catalytic
surface of the bimetallic surface is determined by comparing frequencies of CO adsorption
experiments with the results of Pt-Ga catalyst models with various Pt/Ga ratios. The best
candidate of the different studied catalyst models is the Pt3Ga bulk alloy, with the lowest Ga
content. This model will be further used to correlate experimental observed catalytic properties
with those based on DFT calculations.
As coke formation on the catalyst surface is an unavoidable, it is rudimental to find the origin,
nature and location of the coke and the mechanism that leads to the deactivation of the catalyst.
At high temperature, TPO results shown three possible locations where cokes can be formed:
the Pt metal itself, Pt-support boundary or on the support. A different type of cokes can be
Chapter 2: Literature review 34
formed on each location and models are proposed that only a small part of the coke is
responsible for the deactivation of the catalyst while the larger part is formed on the support
directly or spills over from the catalyst surface. The nature of the coke is a pregraphite-like
carbon structure such as graphene and the coke formation is mainly initiated at low-
coordination sites such as step sites. However, the final structure of the coke and its spillover
characteristics is highly dependent on the particle size of the catalyst.
The effect of alloying of the Pt with Sn accelerates the coke spillover from the catalytic phase
to the support, decreasing the coke fraction on the metal and hence ensuring a longer stability
and activity of the catalyst. Several models are also proposed to predict the coke formation on
Pt-Sn catalysts and the dependency of the hydrogen effect on the formation rate. Few
observations are made concerning the coke formation on Pt-Ga catalysts. However, it is
proposed that the presence of Ga acts similar as Sn and reduces the deactivation and the amount
of coke deposited during light alkane dehydrogenation.
Besides alloying the Pt metal particles with other metals, the active metal phase structure and
the support type can contribute to the overall activity and selectivity of the catalyst as the
dehydrogenation reactions are expected to be structure sensitive and support-metal phase
interaction can enhance the catalytic properties. Platinum is a highly active catalytic and
expensive element, so high dispersion of the metal particles is desired property. Various factors
such as the amount of platinum precursors, the type of the support and the utilized impregnation
method can be altered to achieve a higher degree of dispersion. Various researchers have
established that the alloying of the Pt with either Sn or Ga has a small negative to no influence
on the average particle size.
The selection of an appropriate support is based on the desired properties such as high platinum
dispersion inducement, as small particles are the most active, a high surface area and thermal
stability. Among many high surface area materials, alumina is the support of choice in industrial
applications. However multiple other high surface materials such as SBA-15, ZSM-5, SAPO-
34, ZnAl2O4, (MA) and calcined hydrotalcite Mg(Al)Ox are proposed to stabilize the active
metal phase and replace alumina as support.
Furthermore, the metal phase-support interactions and the acidity of the support also play a
major role. When the metal phase-support interaction is too weak, the Pt cannot be stabilized
and conglomerate to larger particles. If the acidity of the support is high, a higher overall
catalytic activity is observed, however poor selectivity towards propylene is obtained due to
35 Chapter 2: Literature review
side reactions occurring on the support surface. Another interesting support is calcined
hydrotalcite Mg(Al)Ox as it induces a high degree of dispersion and can impregnate other metals
e.g., Zn, Ga, In by replacing the Al in the hydrotalcite layers.
Finally, deactivated catalysts have to remain stable under oxidizing regeneration conditions as
often the morphology of the active metal phase is altered during this process since sintering or
the formation of unreactive MPtOx can occur.
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58. Tauster, S., Strong metal-support interactions. Accounts of Chemical Research, 1987. 20(11): p. 389-394.
59. Spencer, M.S., Models of strong metal-support interaction (SMSI) in Pt on TiO2 catalysts. Journal of Catalysis, 1985. 93(2): p. 216-223.
Chapter 2: Literature review 38
60. Ren-Yuam, T., W. Rong-An, and L. Li-Wu, Evidence of strong metal-support interaction in alumina-supported platinum catalysts. Applied catalysis, 1984. 10(2): p. 163-172.
61. Li, W.Z., et al., Stable platinum nanoparticles on specific MgAl2O4 spinel facets at high temperatures in oxidizing atmospheres. Nature Communications, 2013. 4: p. 8.
Chapter 3: Computational methodology 39
Chapter 3 Computational methodology In the last three decades, quantum chemistry has become a valid tool in the study of atoms and
molecules and, increasingly, in modeling complex systems such as in biology, chemistry and
materials science. The current ab initio techniques are generally based on the computational
solution of the electronic Schrodinger equation. For given positions of a collection of atomic
nuclei and the total number of electrons in the system the electronic energy and electron density
are calculated. The ability to obtain appropriate solutions to the electronic Schrodinger equation
within the desired convergence for systems containing tens, or even hundreds, of atoms has
revolutionized the ability of theoretical chemistry to address important problems in a wide range
of disciplines. [1]
Nowadays, computational chemistry is able to provide adequate results complementing
experimentally obtained data, which have the disadvantage of being labor intensive and time
consuming to obtain. Computational methods also add another advantage with respect to
experimental methods since they can describe molecular behavior on a smaller scale than the
available experimental tools.
This chapter firstly introduces the available computational methods to represent catalyst
models. Secondly, the density functional theory techniques and the applied framework of this
work are described.
3.1 Catalyst models
Industrial dehydrogenation processes are generally based on a heterogeneous noble metal
catalyst on a support. The active sites for catalysis are located on the surface of the metal
40 Chapter 3: Computational methodology
particles. An appropriate catalyst model is necessary to obtain relevant results from ab initio
calculations. Due to the computational limitations, it is not possible to describe every atom in
the metal particles, so three main approaches are proposed to model the catalyst on nanoscale:
cluster approaches, embedded cluster approaches and periodic models, as visualized in Figure
3-1. Each method has their own merits and flaws, and the choice depends mainly on the
industrial context of the catalyst, but constraints on computational resources often result in the
selection of the computationally less demanding method. [2]
The cluster approach (Figure 3-1, left) represents the surface by a finite number of atoms. This
type of model is the most basic and is computationally the fastest. Its accuracy however is
variable, depending on the absolute size of the catalyst and the chosen bond saturation at the
cluster periphery. In the case of a metal catalyst with a diameter of few nm this approach is
inadequate due to the delocalized nature of metal bonding. [3]
Embedding techniques (Figure 3-1, middle) improve upon the cluster approach by introducing
electrostatic, steric and possibly elastic constraints on the cluster imposed by its environment.
As a result, this will reduce the periphery effects. The degree of detail in the description of the
environment and cluster interactions determines the level of accuracy and performance of
different embedding methods. For example, an accurate reproduction of the electrostatic field
in the region of interest is crucial for the study of the majority of solid metal oxide catalysts.
[4]
Periodic methods (Figure 3-1, right) place the catalyst geometries in periodic boundary
conditions and usually employ a full quantum chemical treatment for the total system. These
approaches have been widely used in studies of catalysis on metal and semiconductor surfaces
where the delocalized electron states are of importance and edge effects become negligible. As
periodic images are employed, the size of the unit cell is constrained; on the one side, a well-
chosen size is required to model correctly adsorbate coverage or less periodic effects e.g. point
defects. On the other side, larger unit cells will heavily increase the computational cost. [2, 4]
Chapter 3: Computational methodology 41
Figure 3-1: Different catalyst models for a fcc(111) surface: (left) cluster model, (middle) embedded cluster approach, and (right) periodic slab model [2]
The approaches mentioned above are easily implemented for an ideal description of the catalyst,
but the representation deviates from the actual catalyst as these models exclude surface defects,
multifaceted surfaces to account for structural sensitivity and support interaction influencing
the catalytic phase and enacting possible spillover from the metal catalyst to the support.
3.1.1 Adaptations of the ideal catalyst model
Non-idealities such as surface defects, structural sensitivity and support interactions can be
implemented into a more sophisticated and representable catalyst model, however including
one or multiple of these complexities will give rise to higher computational effort. For this
reason, only the most relevant one is incorporated in the catalyst model.
Figure 3-2: Modeling of more complex surfaces: facet edges, steps and three-phase boundary, shown for an fcc metal: (left) zig-zag slab structure; (middle) high-index surface, here the (211) surface; (right) modeling of the particle-support interface: infinite metal rod on support. [2]
42 Chapter 3: Computational methodology
In the case of structural sensitivity, all the involved multifaceted surfaces have to be calculated.
Multiple calculations for differently indexed surfaces have to be performed, hereby increasing
steeply the computational cost. Furthermore, it is also necessary to model the undercoordinated
sites e.g. edges, steps, kinks and terraces in a representable vicinity of the catalyst particle as
they often have an essential influence on the catalyst properties, e.g. B5-sites of Ru catalysts.
[5] In both cluster and embedded cluster approaches, the structural sensitivity can be addressed
straightforwardly by devising models that represent the undercoordinated sites of the catalyst
particle. However, the construction of finite clusters, whether or not embedded, has to be done
with care, as studies on cluster size convergence are necessary. [6]
In the periodic slab model, it is harder to incorporate structural sensitivity since the surface unit
cell is periodically extended and a zig-zag structure, where the edge, kink and/or step are
exposed, is used to model the structure (i.e. facet edge, Figure 3-2, left). A flaw of these zig-
zag structures is that some undercoordinated faces are in an unnatural local environment.
Another option to model these undercoordinated surfaces is through implementation of high
index surfaces, which are arrays of the desired step or kink, separated by terraces with specific
width (i.e. step edge, Figure 3-2, middle). It is important to remark that properties of the system
calculated by these two models could deviate due to interference of the periodicity of the
approach, as certain edge effects are artificially induced on each other. This flaw could be
avoided by increasing the size of the unit cell, but this is disastrous for the needed computational
effort.
Interaction between the catalyst particle (the active phase) and the support phase (Figure 3-2,
right) can strongly alter the catalytic properties of the active phase as the support can induce
charge transfer or strain effects on the catalyst itself. When modeling the catalyst-support
interaction, it is important to note that its properties can depend on the molecular coverage, as
this influences the catalyst geometry relaxation and metal-support strength. [7] Another
phenomenon that will occur is spillover i.e. adsorbate diffusion between the support and the
particle. In the vast majority of computational studies, these support effects are neglected, while
they can contribute to specifics of the catalyst activity. [2, 8]
To incorporate support effects, it is essential to model simultaneously the catalyst and the
support. Due to the high computational cost, the support effects are often neglected when
modeling a supported catalyst, but several methods are available that integrate the support
effect. Two models are highlighted: the first is based on the periodic film model, where periodic
layers of metal catalyst are deposited on the periodically modeled support (Figure 3-3, left).
Chapter 3: Computational methodology 43
The second is the cluster-on-periodic-support approach, in which finite metal clusters are
deposited on a periodic modeled cluster (Figure 3-3, right). As the first model utilizes the
smallest unit cell, it is computationally more attractive, but the results can be influenced by
incorrect charge transfer between periodic images or false induced strain between the catalyst
and support. The other model has the merit that relaxation of the catalyst is not hindered.
Another merit of cluster-on-periodic-support model is that also adsorption on the support phase
or catalyst-support interface can be accounted for. [9, 10]
Figure 3-3: (left) Periodic film model: V2O5−TiO2 slabs. Side view of a (001) V2O5 monolayer supported on (001) TiO2 anatase (weak interaction with support).[9] (right) Cluster-on-periodic-support model: The structures of anatase TiO2 (001)-supported Pt with bond distances are in Å. [10]
The main difficulty with incorporating structure sensitive and metal-support effects is that you
have to do it fully; there is no middle ground where abstraction can be made of certain effects.
Thus, it is essential to have a clear understanding of the catalyst structure and its composition
under reaction conditions. In situ methods, such as in situ X-ray absorption near edge structure
(XANES), extended X-ray absorption fine structure (EXAFS) and X-ray diffraction techniques
(XRD) provide valuable information of catalysts when particles have the size of a few
nanometers. While the development of in situ characterization techniques of nanocatalysts
continues, it is impossible to obtain useful results on small particles (size <2 nm) and
consequently a well-educated guess of the catalyst model has to be proposed. [11, 12]
3.1.2 Catalyst model used in this work
In this work, the periodic slab approach will be used in first instance to model the catalyst
surface and determine the reaction network. Later on, the model can be extended or altered to
incorporate the support and multifaceted effects. The configurations of each model will be
discussed further in this work.
44 Chapter 3: Computational methodology
3.2 Periodic ab initio calculations
Ab initio quantum calculations are based on so-called “first principles” as no input from
experimental or other sources are employed to calculate molecular geometries and properties.
At its basis stand the postulates of quantum mechanics and other laws of physics and chemistry
to define the framework of these calculations. Centrally the Schrödinger equation, which
represents the most stable electronic structure of the nuclei-electron system, is solved.
In its exact form, the electronic Schrödinger equation is a many-body problem of which the
computational complexity grows exponentially with the number of electrons, and hence, a brute
force solution is time-consuming and inefficient. Several approximating equations can be
applied to the electronic Schrödinger equation to reduce calculation time. Two major
approaches are utilized to solve this equation: Molecular orbital (MO) method and Density
functional theory (DFT) method. The first method is generally based on the Hartree-Fock
theory: a wave function method with a mean field approach, neglecting the correlation between
electrons. This method produces reasonable results for many properties, but is incapable of
providing a robust description of the electronic energy and the electron correlation that is
essential to describe systems with delocalized electrons. Post Hartree-Fock techniques improve
the robustness of the electronic energy calculation, but are lagging for molecular systems with
high amount of electrons as this sharply increases the computational cost of ab initio
calculations. [1, 13]
Nowadays, first-principles calculations on heterogeneous metal catalysts are entirely based on
the Kohn-Sham DFT methods. This method employs the optimization of three-dimensional
electron density to solve the electronic Schrödinger equation in contrast to Hartree-Fock’s high
dimensional wave function. The electron density uniquely determines the Hamiltonian operator
and thus the properties of the system as stated by the first Hohenberg-Kohn theorem. Hence,
the ground state energy can be described with a unique functional as function of the electron
density. This universal functional, independent of the system at hand, is the key of the DFT
methods. The explicit form cannot be determined entirely because description of the electron-
electron correlation cannot be extracted directly and hence DFT methods employ the
description of an exchange-correlation functional to approximate this. The Jacob’s ladder, as
illustrated in Figure 3-4, summarizes different types of functional with increasing complexity
and computational cost. [2, 14, 15]
Chapter 3: Computational methodology 45
Figure 3-4: Jacob's ladder of density-functional approximations (after Perdew). [2]
Basic DFT functionals employ semi-local exchange correlations such as local-density (LDA)
or generalized gradient approximations (GGA). [16, 17] Both have a relative low computational
cost and still yield decent results of covalent bonds and geometric structure. Two major flaws
can be attributed to these types of functionals: artificial electron delocalization and
underestimation of long-range interactions e.g. van der Waals (vdW) interactions. The first can
be attributed to an incomplete cancellation of repulsive Coulomb self-interactions by the
approximate exchange energy calculated by the selected functional. The second arises from the
nature of these more basic functionals as they only consider local electron density and neglect
the non-local effects like dispersion interactions.
Initially, the Perdew-Burke-Ernzerhof (PBE) functional, belonging to the GGA functional
category, is employed for the ab initio calculations in this work. [17] The corresponding results
are expected to be mostly inaccurate due to mentioned shortcomings, especially when applied
in heterogeneous catalysis. The results obtained by PBE are used as initial geometry in a next
calculation using a more advanced functional and hence reducing the computational cost of this
progressive functional. Promising DFT functionals are the hybrid and advanced non-local
functionals. In this work, it was opted to select a vdw-DF functional, the optPBE-vdW
functional. [18, 19] Generally, vdw-DF functionals introduce long-range dispersion in
approximate exchange-correlation functionals. The exact form of this non-local correlation
energy is the key parameter that determines the excellent properties of this type of functional.
In the optPBE-vdW functional, the exchange functional is optimized with respect to the
46 Chapter 3: Computational methodology
correlation part of PBE-types and the long-range dispersion part of vdW-DF functionals to
describe accurately the bonding energies in the S22 dataset. [20, 21] However, the vdW-DF
functional has some flaws e.g. it gives falsified results if hydrogen bonds are present in the
molecular system as it overestimates the bonding strength of hydrogen.
The accuracy of RPBE, a PBE type of functional is in range of 20-30 kJ/mol for DFT
calculations. [22] No direct accuracy results are available for covalent bindings for DFT
calculations with the vdw-DF functional. Still, it is reported that especially for vdW interactions
the accuracy is lower than 15 kJ/mol. [21] Literature states that vdW-DF functionals can give
overbinding of chemisorbed species on a metal surface. However, a new Bayesian error
estimation (BEEF) vdw-DF functional is proposed that better fits various datasets and correctly
describes the binding of chemisorbed species. [23] However, none of these tested benchmarks
consisted of chemisorption on transition metal catalyst models. As verification of this
statement, the calculated adsorption energies of propane (physisorbed) and propylene on
Pt(111) (4×2 unit cell) using optPBE and BEEF vdW-DF will be compared to experiment. [24]
It is reported that the BEEF functional predicts less strong overbinding compared to optPBE
vdw-DF. As seen in Table 3-1, the BEEF vdw-DF functional indeed gives better predictions
for adsorption energies. However, it should be noted that these are solely preliminary results as
different coverage are employed in the DFT study than experimentally.
Table 3-1. Comparison of propylene/propane adsorption strength for two different functionals and experiment.
ΔEads
(kJ/mol) Experimental data
(0.2 ML) [24] OptPBE vdW-DF
(0.13 ML) BEEF vdW-DF
(0.13 ML) Propylene chemisorption -68 -134 -105
Propane physisorption - -43 -33
Recent literature reports that the selected optPBE vdW-DF is adequate to predict the adsorption
strength of benzene on transition metals, indicating that this functional indeed satisfy for
propane dehydrogenation on Pt-based catalysts. [25] However, to reliable asses the
performance of optPBE vdW-DF functional for propane dehydrogenation further
benchmarking is needed.
Chapter 3: Computational methodology 47
3.3 Computational framework used in this work
3.3.1 Vienna Ab initio Simulation Package
The calculations from this work are executed with the Vienna Ab initio Simulation Package
(VASP 5.3.3). [26-29] VASP is a complex package for performing ab initio quantum
mechanical and molecular dynamics (MD) simulations using pseudopotentials and a plane-
wave basis set. In this work, it is opted for the projector-augmented wave (PAW)
pseudopotentials method with a plane-wave cutoff of 400 eV. [30, 31] To start up a calculation
in this framework four input files have to be configured: INCAR file, KPOINTS file, POSCAR
file and POTCAR file. Their meaning and configurations are discussed below. [32]
• INCAR file: The main input file of VASP. It determines the calculation type and the
corresponding settings, and contains a relatively large number of parameters. Several
parameters can be kept on their standard value, but some need to be tuned to obtain the
desired result. The first is the setting for the selection of DFT functional (the ‘GGA’ tag).
The PBE functional corresponds to the GGA=PE, while the optPBE-vdW functional
corresponds to GGA=OR.
The determination of the minimal energy of the potential energy surface (PES) can be
achieved by two main algorithms: the quasi-Newton RMM-DIIS algorithm and the
conjugate gradient algorithm. The IBRION tag determines how atomic position are updated
and moved in each iteration. In the case of relaxation problems close to the minimum, the
quasi-Newton RMM-DIIS algorithm (IBRION=1) is recommended and when the minimum
is still far away, the conjugate gradient algorithm (IBRION=2) should be selected. Other
IBRION values are available, but these are for other optimization problems e.g. transition
state and frequency analysis. The working of these algorithms can be tuned by a scaling
constant for the forces (POTIM tag in the INCAR file). Standard value is 0.5 for relaxation
problems, decreasing this value will lead to smaller position changes each iteration.
Another parameter is the EDIFF tag in VASP, which represents the electronic energy
convergence criterion. This parameter is set to 10-7 for loose geometry optimizations and
10-8 for strict geometry optimizations. It is obliged to obtain strict geometry optimizations,
as they are required for frequency calculations. Otherwise, imaginary frequencies cannot be
excluded.
For the forces, there is also a convergence criterion (EDIFFG tag in the INCAR file) with a
threshold of 0.05 eV/Å for loose geometry optimizations and 0.015 eV/Å for strict geometry
48 Chapter 3: Computational methodology
optimizations. The relaxation will be converged if both the energy and forces criteria are
met. Examples of INCAR files for geometry, frequency and transition state calculations can
be found in Appendix A.
• KPOINTS file: The file KPOINTS must contain the k-point coordinates and weights or the
mesh size for creating the k-point grid. The formulation of a so-called Monkhorst-Pack grid
is obligatory as integrals in real space over the periodic system are replaced by integrals
over the first Brillouin zone in reciprocal space, by virtue of Bloch's theorem to reduce
computational time. [33]
• POSCAR file: This file can be split into two sections: the lattice parameters and the initial
atom positions. The lattice parameters contain the unit cell shape, size and the total number
of each element present. A vacuum layer is introduced in the z-direction (c-coordinate)
where gas phase molecules can adsorb. It is attempted to keep the height of this vacuum
layer constant on 12 Å. In the z-direction between subsequent unit cells, an intermediate
artificial dipole layer is used to correct for the dipole interaction between the slabs. In the
lower part of the POSCAR file, the initial atomic positions are noted. Each line stands for
one atom and contains three numbers and three letters. The numbers represent the atomic
position in the unit cell of that specific atom, noted in Cartesian coordinates or fractional
with respect to the lattice parameters. The letters (T or F) denote on the fact that atoms need
relaxation (T) or are kept fixed (F) in the x-, y- or z-direction. In this work, slab
configurations utilize four atomic layers where the bottom two are kept fixed and the upper
two can relax. The bottom layers represent the bulk layers of the catalyst.
• POTCAR file: This file is a collection of information about each atomic species present in
the calculation, merged with the PAW pseudopotentials needed for the calculation.
When VASP calculations are completed, each time several output files are generated. The most
relevant ones are the CONTCAR file, the OUTCAR file and the standard output (stdout) file:
• CONTCAR file: This file has the same format as the initial POSCAR file and contains the
atomic positions of each atom after the last ionic step is converged in VASP. As this file
has the same format as POSCAR files, they can easily be converted and utilized as an initial
position for more accurate calculations.
Chapter 3: Computational methodology 49
• OUTCAR file: Detailed file that summarizes extensively all information obtained during
the calculations.
• Stdout file: During the calculations, data are collected in this file of every iteration step.
More information can be retrieved about convergence speed and details on each sequence.
At the bottom of this file, either errors or the desired quantities of the converged result can
be retrieved.
By combining several output and input files, it is possible to determine all kinds of desired
properties of a system. The desired properties are the most stable geometries, the reaction
energies and reaction barriers. These properties can be attained by geometry optimizations of
intermediates on the surface and transition state calculations.
3.3.2 Geometry optimizations
In this section, the practical execution of the VASP geometry optimizations are described. All
computational settings are implemented according to the strict convergence criteria as discussed
in previous paragraphs. In this work, the reaction network of propane dehydrogenation will be
examined and discussed in detail. Here an overview of the practical approach will be described,
while in later chapters the results are discussed. The most stable slab geometries of the desired
PtxGay catalyst were calculated in previous work. [34] The following four catalyst models are
employed in this work i.e. Pt(111), Pt3Ga(111), Gr/Pt(111) and Pt(211). On these slabs,
adsorbate molecules or intermediates in the propane dehydrogenation network are modelled.
Initial geometries and most probable adsorption sites are based on the article of Yang et al. [35]
As first iteration, the calculations are conducted with the PBE functional and the resulted
geometries are utilized as initial condition for the consecutive iteration with the vdW-DF
functional.
In dehydrogenation reactions, adsorbed hydrogen is formed on the catalyst surface. Due the low
hydrogen coverage on the surface and repulsion between the adsorbate and hydrogen, it is
assumed that hydrogen diffuses away from the adsorbate. This is implemented in DFT
calculations by optimizing the adsorbate and the hydrogen in a separate unit cell. For this
reason, the most stable geometry of hydrogen needs to be found. The hydrogen atom is placed
on several possible adsorption sites in the empty unit cell and the most stable hydrogen
geometry is employed in further calculations. Simultaneously, the dehydrogenated adsorbate is
50 Chapter 3: Computational methodology
optimized on different adsorption sites and the several optimized possibilities are compared
based on the adsorbate adsorption energy ΔadsE(ad) conform following formula:
∆𝑎𝑎𝑎𝑎𝑎𝑎𝐸𝐸(𝑎𝑎𝑎𝑎) = 𝐸𝐸𝑎𝑎𝑎𝑎,𝑎𝑎𝑎𝑎𝑎𝑎 − 𝐸𝐸𝑎𝑎𝑎𝑎,𝑔𝑔𝑎𝑎𝑎𝑎 − 𝐸𝐸𝑎𝑎𝑠𝑠𝑎𝑎𝑠𝑠 (1)
where Ead,ads is the electronic energy of the dehydrogenated species inside the catalyst unit cell,
Ead,gas is the electronic energy of the adsorbate species in the gas phase and Eslab is the energy
of the clean catalyst surface. The electronic energy of the adsorbate species in the gas phase are
calculated in a 20×20×20 ų unit cell and a special gamma point Г is used to sample the
Brillouin zone. These most stable geometries will be used in further calculations.
When the most stable geometries of several multiple adsorbed species are calculated, reaction
energies can be determined. When adsorbed species are used to determine the reaction energy,
it is essential that both reactant and product have the same atoms present in the unit cell. The
reaction energy ΔEr is determined based on the following formula:
∆𝐸𝐸𝑟𝑟 = 𝐸𝐸prod,ads − 𝐸𝐸react,ads (2)
Where Eprod,ads is the electronic energy of the adsorbed reaction product(s) and Ereact,ads is the
electronic energy of the adsorbed reactant(s). The type of reaction such as dehydrogenation,
isomerization or dissociation depends on the reacting molecules.
For the calculation of the reaction barrier, it is essential to model the transition state between
the reactant and the product. The employed computational techniques are discussed in the
following section.
3.3.3 Transition state calculation techniques
To determine the transition state of a reaction, it is necessary to find a saddle point along the
reaction coordinate on the potential energy surface between the reactant and reaction product
with only one imaginary frequency. As finding transition states in a PES is computationally
demanding, the computational strategy will be twofold. First, the Nudged Elastic Band (NEB)
method will be employed to optimize several intermediate images along the reaction path. The
most promising intermediate image will be used in the dimer method to search the nearby saddle
point and to converge towards the desired transition state. In the following sections, these
methods and their configurations will be discussed.
Chapter 3: Computational methodology 51
3.3.3.1 Nudged Elastic Band (NEB) method
The NEB is a method to find a minimum energy path (MEP) between a pair of stable states. In
the context of reaction rates, this reactant-product pair has an initial (reactant) and a final state
(product), both of which are local minima on the potential energy surface. The MEP has the
property that any point on the path is at an energy minimum in all directions perpendicular to
the path. This path passes through at least one first-order saddle point. The NEB method utilizes
a string of images (geometric configurations of the system) to describe a reaction pathway.
These configurations are initially created linearly between the initial and final state.
Additionally, the neighboring images are connected by spring forces to ensure equal spacing
along the reaction path, during the calculation. The artificial nudged elastic band force 𝐹𝐹𝑖𝑖NEB is
equal to the spring force 𝐹𝐹𝑖𝑖S∥, along the tangent τ�𝑖𝑖, and the perpendicular force due to the
potential 𝐹𝐹𝑖𝑖⊥, as illustrated in Figure 3-5. Upon convergence of the NEB method, the images
describe the reaction mechanism along the MEP, with a resolution determined by the number
of images. [36-38]
Figure 3-5: Two components make up the nudged elastic band force FNEB: the spring force 𝑭𝑭𝒊𝒊
𝐒𝐒∥, along the tangent 𝛕𝛕�𝒊𝒊, and the perpendicular force due to the potential 𝑭𝑭𝒊𝒊⊥. The unprojected force due to the potential Fi is also shown for completeness. [36]
This technique results in a MEP diagram between the initial and final state and while infinitely
increasing the number of images will eventually lead to the accurate pinpointing of the
transition state, this is a computational impossibility. So this technique is rather used in
52 Chapter 3: Computational methodology
combination with the dimer method to optimize the transition state. From the optimized images
in the NEB method, the energetic most unstable image (highest electronic energy) will be used
as an initial guess in the dimer method.
In VASP, the NEB method is implemented as follows: first, a set of images are generated
between two optimized states by using their converged CONTCAR and stdout files. In general,
a number of ten images will be generated, including the initial and final state. The corresponding
VASP function will generate a number of POSCAR files, with initial geometries between the
converged CONTCAR geometries. Furthermore, a special INCAR file has to be loaded (see
Appendix A). Additional parameters have to be configured such as the energy of the initial state
and final state. Those have to be filled respectively in the EFIRST- and ELAST-tag. To obtain
faster convergence, loose convergence criteria are used. The KPOINTS and POTCAR file can
be used from either initial or final state. The electronic energy results of the MEP can be
obtained by applying the corresponding script (TS_Extract.sh) which gives the energies of the
images in the MEP.
3.3.3.2 Dimer method
The dimer method uses a minimum-mode following algorithm to climb to a saddle point,
starting from any initial configuration. The method is designed for finding saddle points without
knowledge of the final configuration of the transition state. The method only makes use of first
derivatives of the potential energy, which makes it computationally more interesting and is
therefore applicable in situations where second derivatives are too costly or too tedious to
evaluate, e.g. in this work where plane wave based density functional theory calculations are
utilized. The method makes use of two replicas -dimer- of the system, spaced apart by a finite
small distance. The dimer alters the force in such a way that the original system converges to a
saddle point rather than a minimum. [39, 40] In this work, the NEB method is used prior to the
dimer calculations to obtain an initial geometry, closer to the transition state geometry and so
further reducing the computational cost.
To obtain the transition state in VASP, the dimer method is employed as follows: primarily two
intermediate images from the converged NEB method are transferred: the electronic most
unstable image (highest electronic energy) and the previous image. The former’s CONTCAR
file is converted to a new POSCAR as the initial transition state geometry. Both images are
used to generate a MODECAR file, which gives an initial direction along the dimer to state the
coordinates that are likely involved in the reaction. If this file is not supplied, the dimer method
Chapter 3: Computational methodology 53
will random generate a direction, costing extra computational power. The KPOINTS and
POTCAR file from either the initial or the final state can be used, while a special version of the
INCAR with adaptations is used (see Appendix A). The convergence criteria are set on strict
for this method.
Apart from the general CONTCAR, OUTCAR and stdout output files, a converged dimer
calculation also generates another important file named DIMCAR. This file is also updated
each iteration step. Six values are monitored but force, torques and especially curvature are the
most important parameters. To correctly obtain convergence, the force has to decrease and the
curvature should be negative each iteration.
If the dimer method converges strictly, the transition state geometry and its electronic energy
are available and it will be possible to calculate the electronic reaction barrier based on the
following formula:
∆ǂ𝐸𝐸 = 𝐸𝐸TS,ads − 𝐸𝐸react,ads (3)
where ETS,ads is the electronic energy of the transition state as obtained from the dimer
calculation and Ereact,ads is the electronic energy of the adsorbed reactant(s).
3.3.4 Frequency calculations After the geometry of an adsorbate is optimized under strict convergence criteria, consecutively
a frequency calculation can be conducted. Resulted frequencies of the normal modes should all
be real in the case of adsorbates and all except one in the case of transition states. For transition
states, the normal mode corresponding to the imaginary frequency should coincide with the
direction of the reaction. The strict geometry optimizations are essential to avoid additional
imaginary frequencies, which are indications that the most stable geometry is not reached. Even
if additional imaginary frequencies are found, these can be solved by applying the dimmins.pl
script, which will be discussed in section 3.3.4.1.
In agreement with the strict geometry optimizations, largely the same computational settings
are employed for frequency calculations, unless noted differently. For the consecutive
frequency calculation, the same types of input files are used with often some alternations. The
KPOINTS and POTCAR file can be transferred from the strict geometry calculations. As
POSCAR file, the resulted CONTCAR of the strict geometry optimization has to be used as
initial starting point in the frequency calculations. The electronic settings of the INCAR are
kept the same as for the strict geometry optimization. However, the IBRION-tag is set on a
54 Chapter 3: Computational methodology
value of 5, which corresponds to frequency calculations. Additionally, the POTIM-tag is
decreased to 0.015 to ensure that only the vibrations are captured during the calculations, which
can be seen as small deviations from the most stable geometry.
The results from the frequency calculations can be used as an indication of the quality of the
found most stable geometry because the occurrence of an imaginary frequency -or multiple in
the case of transition state- points out that there is a more stable geometry in direction of the
imaginary normal mode. Moreover, the calculated normal modes can be linked with
experiments based on vibrational techniques such as reflection-absorption infrared
spectroscopy (RAIRS).
3.3.4.1 Dimmins.pl script In the case that frequency calculations resulted in an additional imaginary frequency, the
dimmins.pl script should be applied. The following procedure in cooperation with this script
will lead to the most stable geometry with all real frequencies in the case of adsorbates. First,
the movements of the imaginary frequency are copied from the OUTCAR to a new MODECAR
file. Next, the dimmins.pl script is applied together with the POSCAR-file from the frequency
calculations and the newly generated MODECAR file. Additionally, a certain distance can set
in the dimmins.pl script, which corresponds to which extent the atoms are moved along the
direction of the imaginary frequency. As a default, this distance is equal to 0.1 Å. This script
constructs directories: ‘min1’ and ‘min2’, where the adsorbate is moved over the inserted
distance according to the imaginary mode, but in opposite directions. Then, the geometries of
the ‘min1’ and ‘min2’ directories are optimized in accordance with the strict convergence
criteria as the most stable geometry of the two is employed for an additional frequency
calculation. If an imaginary frequency remains, the procedure is repeated, eventually with an
enlarged distance until the most stable geometry is effectively reached.
3.4 References
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3. Murzin, D. and D.Y. Murzin, Nanocatalysis 2006. 2006: Research Signpost. 4. Catlow, C.R.A., et al., Computational approaches to the determination of active site
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7. Valero, M.C., P. Raybaud, and P. Sautet, Interplay between molecular adsorption and metal-support interaction for small supported metal clusters: CO and C2H4 adsorption on Pd-4/gamma-Al2O3. Journal of Catalysis, 2007. 247(2): p. 339-355.
8. Marshall, S.T. and J.W. Medlin, Surface-level mechanistic studies of adsorbate–adsorbate interactions in heterogeneous catalysis by metals. Surface Science Reports, 2011. 66(5): p. 173-184.
9. Alexopoulos, K., et al., Theoretical Study of the Effect of (001) TiO2 Anatase Support on V2O5. The Journal of Physical Chemistry C, 2010. 114(7): p. 3115-3130.
10. Wanbayor, R. and V. Ruangpornvisuti, A periodic DFT study on binding of Pd, Pt and Au on the anatase TiO2 (001) surface and adsorption of CO on the TiO2 surface-supported Pd, Pt and Au. Applied Surface Science, 2012. 258(7): p. 3298-3301.
11. Stierle, A. and A.M. Molenbroek, Novel in situ probes for nanocatalysis. Mrs Bulletin, 2007. 32(12): p. 1001-1005.
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13. Amusia, M.Y., A.Z. Msezane, and V.R. Shaginyan, Density functional theory versus the Hartree-Fock method: Comparative assessment. Physica Scripta, 2003. 68(6): p. C133-C140.
14. Perdew, J.P. and K. Schmidt. Jacob's ladder of density functional approximations for the exchange-correlation energy. in AIP Conference Proceedings. 2001. Iop Institute Of Physics Publishing Ltd.
15. Perdew, J.P., et al., Prescription for the design and selection of density functional approximations: More constraint satisfaction with fewer fits. The Journal of chemical physics, 2005. 123(6): p. 062201.
16. Kohn, W. and L.J. Sham, Self-Consistent Equations Including Exchange and Correlation Effects. Physical Review, 1965. 140(4A): p. A1133-A1138.
17. Perdew, J.P., K. Burke, and M. Ernzerhof, Generalized Gradient Approximation Made Simple. Physical Review Letters, 1996. 77(18): p. 3865-3868.
18. Dion, M., et al., Van der Waals density functional for general geometries. Physical Review Letters, 2004. 92(24): p. 4.
19. Klimes, J., D.R. Bowler, and A. Michaelides, Van der Waals density functionals applied to solids. Physical Review B, 2011. 83(19): p. 13.
20. Klimes, J., D.R. Bowler, and A. Michaelides, Chemical accuracy for the van der Waals density functional. Journal of Physics-Condensed Matter, 2010. 22(2).
21. Lee, K., et al., Higher-accuracy van der Waals density functional. Physical Review B, 2010. 82(8): p. 081101.
22. Hammer, B., L.B. Hansen, and J.K. Norskov, Improved adsorption energetics within density-functional theory using revised Perdew-Burke-Ernzerhof functionals. Physical Review B, 1999. 59(11): p. 7413-7421.
23. Wellendorff, J., et al., Density functionals for surface science: Exchange-correlation model development with Bayesian error estimation. Physical Review B, 2012. 85(23): p. 235149.
56 Chapter 3: Computational methodology
24. Tsai, Y.-L., C. Xu, and B.E. Koel, Chemisorption of ethylene, propylene and isobutylene on ordered Sn/Pt (111) surface alloys. Surface science, 1997. 385(1): p. 37-59.
25. Matos, J., H. Yildirim, and A. Kara, Insight into the Effect of Long Range Interactions for the Adsorption of Benzene on Transition Metal (110) Surfaces. The Journal of Physical Chemistry C, 2015. 119(4): p. 1886-1897.
26. Kresse, G. and J. Hafner, Ab initio molecular dynamics for liquid metals. Physical Review B, 1993. 47(1): p. 558.
27. Kresse, G. and J. Hafner, Ab initio molecular-dynamics simulation of the liquid-metal–amorphous-semiconductor transition in germanium. Physical Review B, 1994. 49(20): p. 14251.
28. Kresse, G. and J. Furthmüller, Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set. Computational Materials Science, 1996. 6(1): p. 15-50.
29. Kresse, G. and J. Furthmüller, Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set. Physical Review B, 1996. 54(16): p. 11169.
30. Sun, G.Y., et al., Performance of the Vienna ab initio simulation package (VASP) in chemical applications. Journal of Molecular Structure-Theochem, 2003. 624: p. 37-45.
31. Blöchl, P.E., Projector augmented-wave method. Physical Review B, 1994. 50(24): p. 17953.
32. Kresse G., M.M., Furthmüller J, VASP the GUIDE. 2014. 33. Monkhorst, H.J. and J.D. Pack, Special points for brillouin-zone integrations. Physical
Review B, 1976. 13(12): p. 5188-5192. 34. Saerens, S., Determination of the active phase of a Pt/Mg(Ga)AlOx catalyst of the
catalytic dehydrogenation of propane, in Department of Chemical Engineering and Technical Chemistry. 2014, University of Ghent: Ghent.
35. Yang, M.L., et al., First-Principles Calculations of Propane Dehydrogenation over PtSn Catalysts. Acs Catalysis, 2012. 2(6): p. 1247-1258.
36. Sheppard, D., R. Terrell, and G. Henkelman, Optimization methods for finding minimum energy paths. Journal of Chemical Physics, 2008. 128(13): p. 10.
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38. Henkelman, G., G. Jóhannesson, and H. Jónsson, Methods for finding saddle points and minimum energy paths, in Theoretical Methods in Condensed Phase Chemistry. 2002, Springer. p. 269-302.
39. Pedersen, A., S.F. Hafstein, and H. Jónsson, Efficient Sampling of Saddle Points with the Minimum-Mode Following Method. SIAM Journal on Scientific Computing, 2011. 33(2): p. 633-652.
40. Henkelman, G. and H. Jónsson, A dimer method for finding saddle points on high dimensional potential surfaces using only first derivatives. The Journal of Chemical Physics, 1999. 111(15): p. 7010-7022.
Chapter 4: Propane dehydrogenation kinetics on Pt(111) catalyst model 57
Chapter 4 Propane dehydrogenation kinetics on Pt(111) catalyst model In this chapter, ab initio calculations are employed to unravel the reaction mechanism of
propane dehydrogenation on a Pt(111) catalyst model. The most stable geometries of the
adsorbed intermediates are calculated to determine the thermodynamics. Furthermore,
transition states of the selected reaction paths are optimized to obtain information on the
kinetics. Eventually, these data are used to perform a kinetic study to obtain coverages of
adsorbed species and dominant paths. The Pt(111) catalyst model is interesting to consider as a
reference for more advanced catalyst models, which are studied in further chapters. This model
can be used as benchmark for the applied method since it can be compared with available
literature on light alkane dehydrogenation on pure Pt catalysts.
4.1 Catalyst model
The bulk phase of platinum has a face-centered cubic (fcc) structure with a coordination number
of twelve, a packing factor of 0.74 and a lattice distance of 3.92 Å. [1] The most stable surface
cut of the fcc unit cell is the (111) plane. Each atom on the (111) surface has a coordination
number of nine, so there are three broken bonds per surface atom. [2] Three broken bonds is
the minimal amount possible, making the (111) plane the most abundant surface plane in Pt
particles and the most validate candidate for this highly coordinated catalyst model.
The periodic slab approach is employed as method to model the metal surface geometries. As
discussed in section 3.1, this approach is the most suitable for metal catalysts as it can describe
the delocalized nature of the metallic bonding. At the basis of this method stands the selection
58 Chapter 4: Propane dehydrogenation kinetics on Pt(111) catalyst model
of the unit cell, of which the periodic extension describes the infinite metal catalyst surface.
The determination of the optimal size of the unit cell is essential as larger cells demand high
computational effort, while smaller unit cells can incorrectly describe the coverages of the
adsorbates.
Figure 4-1: Isometric representation of the 4×2 unit cell of the Pt(111) catalyst model
In this work, a 4×2 unit cell is used to represent the Pt(111) catalyst model. The choice of this
4×2 unit cell will be explained further in section 4.1.2. In this 4×2 unit cell, four Pt atoms are
taken in the a-direction, two Pt atoms in the b-direction and four Pt atoms in the c-direction.
The a-, b- and c-directions are the lattice vectors of the unit cell, which can be correlated with
the x-, y- and z-direction of the Cartesian coordinate system. In the c-direction, the bottom two
layers are kept fixed as they represent the bulk atoms of the catalyst model, while the upper two
layers represent the surface of the catalyst model and are allowed to relax. A vacuum layer of
12 Å and an artificial dipole layer are constructed to avoid periodic interactions between unit
cells in the c-direction. The dimensions of the unit cell are 11.29×5.64×18.91 ų. For geometry
optimization of the unit cell, strict convergence settings are employed conform Chapter 3.3.1.
The non-local vdW-DF functional is utilized during the strict optimizations. An isometric view
is illustrated in Figure 4-1.
b
a
c
Chapter 4: Propane dehydrogenation kinetics on Pt(111) catalyst model 59
From the optimized geometry of the unit cell, a lattice parameter of 3.99 Å is determined, which
is an acceptable overestimation from the reported value of 3.92 Å. [1] In literature, catalyst
models have mostly four layers in the c-direction for similar DFT studies. [3-8] As verification
of the four-layer model, the adsorption energy of propylene, an essential intermediate during
propane dehydrogenation, is calculated with a four-layer model (bottom two are fixed) and a
six-layer model (bottom four are fixed). An energetic difference of -9 kJ/mol is observed with
respect to the four-layer model (propylene binds 9 kJ/mol stronger on the six-layered model).
This deviation (6.5%) is too great to be within a precision of 5%. However, the deviation is
acceptable for this work, as including two additional bulk layers greatly increase the
computational cost.
4.1.1 Adsorption site nomenclature
On this catalyst model, four types of adsorption sites are possible: top, fcc, hcp and bridge sites.
The top sites are defined as adsorption on a single surface atom of the catalyst. On top of these
single atom sites, the adsorbate species is placed. Bridge sites involve two adjacent surface
atoms. Between those atoms, the adsorbate species is placed. Both the fcc and hcp sites use
three adjacent atoms in the surface layer. Between these three atoms, a hollow site is present
where the adsorbate species is placed. The difference between the fcc and hcp sites is made
based on the second layer underneath the surface layer. When there are another three atoms
beneath the three surface atoms, this site is defined as an fcc site. Otherwise, when there is only
one atom beneath the three surface atoms, this site is defined as an hcp site. This various
adsorption site on the catalyst model are illustrated in Figure 4-2.
Figure 4-2: Adsorption sites on Pt(111) catalyst model: top ●, bridge █ , three-folded hcp ▲ and fcc ▼.
b
a
60 Chapter 4: Propane dehydrogenation kinetics on Pt(111) catalyst model
In more advanced catalyst models, this nomenclature for adsorption sites is too common, as
multiple sites of the same type are possible. Each site has other characteristics because the atoms
in the layers underneath differ between various sites. The identification of each site is simplified
by incorporating the atoms in the layers underneath in the site naming, until each site can be
distinguished from each other. When a more sophisticated nomenclature is needed, this will be
pointed out.
4.1.2 Determination of the degree of coverage
When catalyst models are employed, it is essential to define the coverage of the adsorbate
species on the surface. To describe the degree of coverage, equation (1) is employed. In the
case of the Pt(111) catalyst model, eight surface atoms are present in the unit cell, so when one
adsorbate species is on the surface, a coverage of 0.13 ML is obtained for that species.
θx =
# of adsorbate molecule X# of surface atoms in the unit cell
(1)
By selecting a Pt(111) 4×2 unit cell, a large flexibility in degrees of coverage is obtained.
However, it is essential to validate this selection with respect to literature. The unit cell selection
for Pt(111) is based on two main reasons: the catalyst model extension towards Pt3Ga and the
monolayer coverage of propylene, determined experimentally. In the next chapter, this catalyst
model will be extended to Pt3Ga to study the effect of gallium alloying. To do this, every fourth
Pt atom has to be replaced with a Ga atom and as periodic slab approach model is employed,
only multiples of four are possible as upper surface layer. A 2×2 or a 4×2 unit cell are the best
candidates, as the computational cost is too great with larger unit cells e.g. 6×2 unit cell. During
our calculation, it is preferable to remain under the monolayer coverage of propylene and this
influences the final decision. TPD analysis has shown that the monolayer of chemisorbed
propylene is 0.20 ML, so based on the equation above, this means one propylene molecule for
five surface atoms. [9, 10] Consequently, the 4×2 unit cell remains valid for this work.
4.1.3 Selection of the DFT functional
In literature, PBE is a widely used functional for DFT calculations. However, in this work, this
functional is solely employed to optimize the initial geometries, as it does not address non-local
interactions such as van der Waals forces. Hence, it was opted to select a vdw-DF functional,
the optPBE-vdW functional, to optimize the final geometries. This non-local vdW-DF
Chapter 4: Propane dehydrogenation kinetics on Pt(111) catalyst model 61
functional introduces long-range dispersion in the approximate exchange-correlation
functionals.
4.2 Adsorption
In this section, the adsorption of reactants and products that appear in the gasphase during
propane dehydrogenation are discussed. Logically, the main products are propane, propylene
and hydrogen. Additionally, ethane, ethylene and methane are also included as they can be
formed through cracking of adsorbed intermediates. Essentially, it is tried to correlate the results
of this work with experimental aspects of the adsorption of hydrocarbons on the surface of
platinum, preferably on Pt(111). Throughout this section, the following equation is employed
to describe the adsorption energy of each compound.
∆ads𝐸𝐸(ad) = 𝐸𝐸ad,ads − 𝐸𝐸ad,gas − 𝐸𝐸surface (2)
Vibrational spectrometry such as RAIRS is a useful tool to describe the adsorbate based on its
vibrational modes. However, it is essential to compose a nomenclature of the possible
vibrational modes. The abbreviation of a vibrational mode consists out of three parts: the type
of vibrational mode denoted by a Greek letter, the direction of the movement (asymmetrical or
symmetrical, denotes as subindex) and the moiety on which the vibrational mode acts, denoted
between brackets. The types of vibration modes are deformation (δ), rocking (ρ), scissoring (γ),
stretching (ν), twisting (τ) and wagging (ω). The subindices indicate that the vibration mode is
symmetrical (s) or asymmetrical (as).
4.2.1 Alkanes
Alkanes are chemically saturated hydrocarbons that show a low bond breaking activity even on
transition metal surfaces. In the absence of bond breaking, these molecules prefer to adsorb
with their carbon chain parallel to the surface and their heat of adsorption increases by ~9
kJ/mol per -CH2-group. Thus at low temperatures and on relatively chemically inactive low
Miller-index metal surfaces the molecules adsorb intact and in a flat-lying configuration. [11]
This phenomenon is called physisorption as the alkane keeps drifting over the Pt surface, as has
only a weak bond with surface. Molecular beams techniques are employed at low temperatures,
under the alkane desorption temperature, to obtain information on the physisorbed geometries
and adsorption energies.
62 Chapter 4: Propane dehydrogenation kinetics on Pt(111) catalyst model
4.2.1.1 Propane
The chemisorption of propane on Pt surface has been a subject of various studies, both
experimentally and theoretically. As propane is a saturated hydrocarbon with no unpaired
electrons, it will not dissociatively adsorb on the Pt surface by binding with the surface atoms.
Consequently, a low adsorption energy is expected. McMaster et al. measured the molecular
adsorption dynamics of propane on Pt(110) and Pt(111) at 95 K using molecular beams. They
conducted their studies on clean and propane covered surfaces to describe the coverage effect
on the adsorption characteristics of propane. They reported that the propane oriented itself
parallel to the metal surface in the case of a monolayer. The molecular beam experiments show
that adsorbed propane facilitates trapping of physisorbed propane on the surface as the
adsorption probability increases linearly with propane coverage. Experimentally, the saturated
monolayer coverage is 3·1014 molecules/cm² and a monolayer of Pt atoms consists of 1.5·1015
molecules/cm²; hence the adsorption energies determined at a coverage of 0.2 ML. [12, 13]
Furthermore, both Nykänen et al. [14] and Yang et al. [3] have conducted a DFT study that
included the physisorption of propane on Pt(111). The resulted adsorption energies can be found
in Table 4-1.
Table 4-1. Adsorption energies of physisorbed propane on Pt(111) surface, determined both experimentally as theoretically. Nykänen et al. employed a vdW-DF functional, while Yang et al. used of a PBE functional. This work uses an optPBE vdW-DF functional.
ΔEads
(kJ/mol) Experimental data (0.2 ML) [13, 15]
Nykänen et al. (0.13 ML) [14]
Nykänen et al. (0.25 ML) [14]
Yang et al. (0.11 ML)[3]
This work
(0.13 ML)
Pt(111) -41 ~ -44 -33 -36 -6 -43
Due the use of a GGA functional, Yang et al. underestimate the energy of propane physisorption
and evaluate it as nearly thermoneutral. Their calculations did not include the non-local vdW
interactions, which are essential to describe the physisorbed propane on the Pt(111) surface.
The results of this work and of Nykänen et al. are comparable with the experimental results.
The discrepancy between the results of Nykänen et al. and this work for the same coverage can
be contributed to slightly different functionals are employed (vdW-DF by Nykänen and optPBE
vdW-DF in this work). The geometry of the adsorbed propane is identical to the optimized
structure of gasphase propane, which suggests that the interaction between propane and surface
is non-covalent, as expected for physisorption. From the results of Nykänen, a weaker
adsorption is observed at low coverages, indicating that there is an attractive interaction
between the physisorbed propane molecules. These results confirm the molecular beams
experiments of McMaster et al., which show a higher adsorption probability at higher coverage.
Chapter 4: Propane dehydrogenation kinetics on Pt(111) catalyst model 63
Chesters et al. studied the RAIRS spectra of monolayer and multilayer propane adsorbed on
Pt(111) at 95 K. The (sub-)monolayer coverage is achieved by dosing 1 Langmuir on a clean
Pt(111) surface. [16] The frequencies they obtained of physisorbed propane are compared with
the calculated frequencies of physisorbed propane in this work. As a low propane coverage is
employed in the DFT calculations, only the monolayer frequencies of propane are evaluated,
see Table 4-2.
Table 4-2. Comparison of vibrational modes of physisorbed propane between experiments and this work. Experiments made use of monolayer coverage of propane, while in this work a coverage of 0.13 ML propane is used.
Vibrational modes νas (CH3) νas (CH3) νas (CH2) δas (CH3)
Experimental (cm-1) [16] 2949 2937 2915 1454
This work (cm-1) 2997 2985 2930 1455
Chesters et al. observes four vibrational modes: three asymmetrical stretchings and one
asymmetrical deformation of the methyl groups. While compared with the experiments the
asymmetrical stretching of both methyl and methylene group are shifted respectively with 48
cm-1 and 15 cm-1, while for the deformation of the methyl group, no shift is observed. Three
imaginary frequencies are reported for the physisorbed propane. In general, physisorbed species
have a higher degree of freedom than chemisorbed species. These imaginary frequencies are
assigned to translations and external rotation of the molecule above the surface. It is not possible
to alter these vibrational modes to become real. When kinetic parameters are determined, these
frequencies can be modified accordingly. This modification is discussed in section 4.5.1. [17]
However, at higher temperatures, dissociative chemisorption of propane into 1-propyl or 2-
propyl and hydrogen (in the case of C-H bond breaking) or methyl and ethyl (in the case of C-
C bond breaking) occurs. These reactions are respectively dehydrogenation reactions and
dissociation reaction and play a major role in propane dehydrogenation. Their description falls
under the thermodynamics section of this chapter, see section 4.3.
4.2.1.2 Ethane
Ethane is a saturated C2 hydrocarbon, which can be formed during propane dehydrogenation as
a product of C-C bond breaking. As it is similar to propane, it is expected that ethane will
physisorb on the Pt surface, conform the physisorption of propane. Only a weak bond is formed
with the surface at a higher distance from the surface than chemisorbed species. Arumainaygam
et al. investigated the dynamics of the ethane adsorption with molecular beams on clean and
64 Chapter 4: Propane dehydrogenation kinetics on Pt(111) catalyst model
ethane-covered Pt(111) at 95 K. The geometry of ethane in a monolayer, adsorbed on Pt(111),
is parallel to the metal surface. Conform to the molecular beam results on propane adsorption,
the ethane sticking probability increases with a higher ethane coverage. The adsorption energy
is experimentally determined at coverage range of 0.1 – 0.4 ML. [15, 18, 19] Various DFT
studies have been conducted on ethane dehydrogenation and hydrogenolysis, but did not
consider ethane physisorption as a reaction step prior further reactions. [20, 21] However, from
the comparison of this work with the experimental data, it can be concluded that the molecular
adsorption energy of ethane can be predicted adequately.
Table 4-3. Adsorption energies of physisorbed ethane on Pt(111) surface, determined both experimentally as theoretically. This work uses an optPBE vdW-DF functional.
ΔEads (kJ/mol) Experimental data (0.1 – 0.4 ML) [15, 18, 19]
This work
(0.13 ML) Propane physisorption -32 ~ -33 -31
Chesters et al. studied the RAIRS spectra of monolayer and multilayer ethane adsorbed on
Pt(111) at 95 K. [16] The (sub-)monolayer coverage is achieved by dosing 1 Langmuir on a
clean Pt(111) surface. The frequencies they obtained of physisorbed ethane are compared with
the calculated frequencies of physisorbed ethane in this work. As a low ethane coverage is
achieved during DFT calculations, only the monolayer frequencies of ethane are evaluated, see
Table 4-4.
Table 4-4. Comparison of vibrational modes of physisorbed ethane between experiments and this work. Experiments made use of monolayer coverage of propane, while in this work a coverage of 0.13 ML propane is used.
Vibrational modes νas (CH3) νs (CH3) δas (CH3)
Experimental (cm-1) [16] 2958 2853 1457
This work (cm-1) 2969 2921 1468
Chesters et al. reported three vibration modes: a symmetrical and asymmetrical stretching of
the methyl groups and an asymmetrical deformation of the methyl groups. A shift compared to
experiment is observed of 11 cm-1 in the case for the asymmetrical modes, while a larger shift
is found for the symmetrical stretching of 68 cm-1. Again, three imaginary frequencies are
observed during the vibrational analysis. However, the allocation of these frequencies is
discussed previously for propane.
Chapter 4: Propane dehydrogenation kinetics on Pt(111) catalyst model 65
At higher temperatures, thermal dissociation of ethane occurs, providing enough energy to
dehydrogenate to adsorbed ethyl or dissociate to two adsorbed methyl groups. The first reaction
is accounted for in the thermodynamics section of this chapter, see section 4.3.
4.2.1.3 Methane
Methane is a saturated compound that can be formed from C-C bond breaking of intermediates
during propane dehydrogenation. As it has no unpaired electrons, it is expected that methane
will physisorb on the Pt surface, conform the previous discussed physisorptions. Apart from
ethane adsorption, Arumainaygam et al. also investigated the dynamics of the methane
adsorption with molecular beams on clean Pt(111) at 100 K. From these experiments, the
adsorption energy of physisorbed methane could be determined at low coverages. [22] Further,
adsorption also determined by Cushing et al. at intermediates coverages in the range of 0.1 –
0.4 ML with respect to the surface atoms of clean Pt(111). DFT studies have been conducted
by Qi et al. [23] and Zhang et al. [24] on the adsorption and dissociation of methane on Pt
surfaces, which included the physisorption of methane, prior to the C-H bond breaking.
Table 4-5. Adsorption energies of physisorbed methane on Pt(111) surface, determined both experimentally as theoretically. Qi et al. employed a PBE functional, while Zhang et al. used a PW91 functional. This work uses an optPBE vdW-DF functional.
ΔEads (kJ/mol) Experimental data (0.1 – 0.4 ML) [15]
Qi et al. (0.25 ML) [23]
Zhang et al. (0.25 ML) [24]
This work
(0.13 ML) Pt(111) -16 -3 -4 -20
As both DFT studies do not account for van der Waals interactions, the physisorption energy
of methane is underestimated. In contrast, the result of this work has the same order of
magnitude as the experimental results, but still overestimates the methane adsorption energy up
to 5 kJ/mol. Physisorbed methane has three imaginary frequencies. However, the allocation of
these frequencies is discussed previously for propane and ethane.
Fuhrman et al. have shown that methane dissociates thermally at 120 K, when molecular
beaming methane at kinetic energies between 30 and 80 kJ/mol. Hence, methane adsorbs on
the Pt(111) surface as a methyl and a hydrogen. At temperatures of 300 K, the adsorption of
methane is observed to lead directly to deeply dehydrogenated methylidyne (CH) as surface
species. [25, 26] The first dehydrogenation step of methane will be discussed further in the
thermodynamics chapter, see section 4.3.
66 Chapter 4: Propane dehydrogenation kinetics on Pt(111) catalyst model
In general, it is proposed that the heat of adsorption of alkanes increases by ~9 kJ/mol per -CH2-
group. [11] The alkane adsorption energies, calculated based of DFT theory, increase by 11
kJ/mol, which is in accordance with the experimental observation.
4.2.2 Alkenes
Because of the availability of π-electrons that can readily be donated to the metal, the bonding
of alkenes to the metal surface is much stronger than the bonding of alkanes. Furthermore, the
absorbed alkene undergoes molecular rearrangements depending on temperature and the metal
surface structure. Additionally, the metal substrate may restructure under the influence of
alkene adsorption to optimize bonding to the adsorbed species. For propane dehydrogenation,
propylene and ethylene are considered as alkene compounds on the surface.
4.2.2.1 Propylene
Adsorbed propylene plays a major role in propane dehydrogenation. When adsorbed on the
surface, several reaction ways are possible: hydrogenation to propane, desorption to gaseous
propylene as desired product and dehydrogenation to deep dehydrogenated species such as
propenyl. To promote propane dehydrogenation, the desorption of propylene should be favored
compared to the side reactions, and hence the binding of propylene with the surface should be
weakened.
Zaera et al. characterized the adsorption of propylene on Pt(111) single-crystal surfaces by
infrared spectroscopy (RAIRS). The uptake of propylene was studied at 90 K to avoid further
thermal decomposition. As function of the coverage, four adsorption modes of propylene are
observed.
Figure 4-3: Adsorption modes of propylene on Pt(111): propylene di-σ V-shape (a), propylene di-σ (b) and propylene π (c) White, H, grey, C and light blue, Pt atoms.
At the lowest propylene dosages, a di-σ bonded adsorbate with a V-shape is seen, as the central
carbon atom is preferentially bonded with the platinum surface. Above half saturation of the
Chapter 4: Propane dehydrogenation kinetics on Pt(111) catalyst model 67
monolayer, propylene rearranges itself toward a more horizontal double bond and vertical
methyl group. This orientation leads to better packing of the propylene molecules on the
surface. Above the monolayer saturation, a second layer of weakly π-bonded propylene is
observed. In this layer, propylene orients itself parallel to the surface and interacts weakly with
the surface via π-bonding. At high propylene (> 2.0 L exposure), a solid film with randomly
oriented propylene is formed. [27, 28] In this work, di-σ adsorbed propylene is studied.
Valcarcel et al. performed DFT calculations on propylene dehydrogenation and isomerization.
A part of the study was to conduct a frequency analysis on di-σ adsorbed propylene and deep
dehydrogenated species to verify the origin of certain surface species during propylene
adsorption. They performed their analysis with PW91 as functional. [7, 8] To validate the
geometry of di-σ adsorbed propylene, the calculated frequencies are compared with the RAIRS
spectra, conducted at 90 K by Zaera et al. and the frequency analysis of Valcarcel et al. in
Table 4-6 .
Table 4-6. Comparison of propylene vibration modes between RAIRS and DFT based frequency analysis of the di-σ adsorption mode. Valcarcel et al. used a PW91 for the di-σ propylene optimization, while this work uses an optPBE vdW-DF functional.
Vibrational mode (cm-1)
Zaera et al. [27] Valcarcel et al. [7] This work
0.4 – 0.8 L exposure 0.25 ML 0.13 ML
ρ(CH3) 1015 1007 1017
τ(CH2) 1037 1030 1036
ν(C-CH3) 1088 1092 1081
ω(CH2) 1260 1161 1162
δ(CH) 1309 1296 1305
δs(CH3) 1375 1337 1358
γ(CH2) 1437 1400 1408
νs(CH3) - - 2902
νs(CH2) 2830 2982 2951
2δas(CH3),
νs(CH3) 2860 2933 -
ν(CH) 2883 2986 2972
νas(CH2) 2915 3013 3022
The di-σ adsorption mode of propylene appears on the platinum surface at low exposures, so
the observed RAIRS spectrum at low exposure is compared with the theoretical calculated
frequency analysis of the di-σ adsorption mode of Valcarcel et al. and this work. The vibrational
modes of both theoretical models are similar and each can be linked to identical modes. In the
68 Chapter 4: Propane dehydrogenation kinetics on Pt(111) catalyst model
theoretical frequency analysis, all possible vibrational modes are calculated in contrast to the
RAIRS spectra where only the strongest and most IR-active frequencies are observed. Still,
almost all vibrational modes can be coupled with those based on DFT calculations. Some
discrepancy arises in the 2800-3100 cm-1 range as the C-H stretching modes are strongly
interfered by anharmonicity. Moreover, the experimental coverage and periodicity has a strong
influence on the orientation of the methyl group and its environment. These factors cause the
change in the frequencies. [27] In the di-σ configuration, the double C=C is stretched to 1.5 Å
with respect to the C=C bond length of 1.34 Å of gasphase propylene. This elongated bonding
is similar to the bonding length of gasphase propane, indicating the rehybridisation of these two
C atoms. Another indicator for this phenomenon is the loss of planarity; the methyl group
orientates itself away from the surface and the angle between the planar double bond plane and
the methyl group is ~21°. The rehybridisation of double bonds C atoms points out the formation
of covalent bonds with the platinum surface.
Various DFT studies have been conducted on the adsorption of propylene, especially on the di-
σ mode. All calculated adsorption energies are tabulated in Table 4-7 and when compared to
the experimental values, a clear overestimation is observed, especially for this work. The
explanation is two-fold. Firstly, the selected vdW-DF functional has a tendency of overbinding
adsorbed species on the surface, overestimating their adsorption energies. Compared to other
functionals, e.g. PW91 and PBE, vdw-DF overbinds propylene. Secondly, Zaera et al. proposed
that the adsorption energy based on TPD experimental data may be influenced by thermal
decomposition of propylene before desorption at 230-250 K. The hydrogen, which is released
due the thermal decomposition, remains on the surface and weakens the propylene binding with
the surface, leading to the observation of a lower adsorption energy. [28] Furthermore, also a
difference is noted between the work of Nykänen et al. and this work, even though both use a
vdW-DF functional. However, Nykänen et al. used other computational settings for his
functional. Nykänen et al. observed that the adsorption energy increases at higher coverage,
indicating the occurrence of small repulsion between adsorbed propylene. [14]
Table 4-7. Adsorption energy of di-σ adsorbed propylene, determined both experimentally and theoretically. Valcarcel et al., Yang et al. and Nykänen et al. used respectively a PW91, a PBE and a vdW-DF functional. This work uses an optPBE vdW-DF functional.
ΔEads
(kJ/mol) Experimental data (0.2 ML) [29, 30]
Valcarcel et al. (0.13 ML) [8]
Yang et al. (0.25 ML)[3]
Nykänen et al. (0.13 ML) [14]
This work (0.13 ML)
Pt(111) -51 ~ -68 −87 -90 -80 -134
Chapter 4: Propane dehydrogenation kinetics on Pt(111) catalyst model 69
Furthermore, the physisorption of propylene is considered on the Pt(111) surface. This
adsorption mode can act as a precursor state prior to the di-σ adsorption mode. In contrast with
di-σ propylene, the double bond is not lost during this mode. The observed C=C length is 1.34
Å, which is identical to that of gasphase propylene. The geometry of this mode is differently as
the planarity remains and the double bond length is the same as in gasphase propylene.
However, the geometry does not orientate itself parallel, but with an angle of 42° between the
surface and propylene due the hindrance of the methyl group. In this work, the optimized
geometry of physisorbed has a remaining root mean square (RMS) of 0.026 with respect to the
imposed strict ionic convergence criterion of 0.015. However, the resulted RMS is sufficient
low for this work. Furthermore, three imaginary frequencies are obtained for the physisorbed
propylene. Still, these can be treated accordingly for the determination of kinetic parameters.
4.2.2.2 Ethylene
As C-C cleavage reactions occur on the catalyst surface, ethylene is one of the most likely side
product to be formed. Furthermore, adsorbed ethylene is a central component to describe the
hydrogenation of ethylene and dehydrogenation of ethane and various DFT and experimental
studies have dedicated to it. [20, 29-33] Two distinct adsorption modes of ethylene are
experimentally observed: di-σ and π-adsorption mode. The majority of ethylene adsorbs in the
di-σ mode, as it is more stable than the π mode. High-resolution electron energy loss
spectroscopy (HREELS) analyses have shown that the di-σ mode solely occurs at low coverage,
while the minor adsorption mode π-ethylene generally is observed at higher coverages.
TPD experiments around 100 K have shown that for low coverage the adsorption energy of di-
σ ethylene is around -70 kJ/mol, however all of the DFT studies overestimate this value, even
beneath ethylene monolayer coverage of 0.25. Discrepancies can occur when comparing these
values as energies deduced from TPD correspond to a kinetically controlled out-of-equilibrium
process, which does not necessarily yield a correct adsorption energy, whereas DFT reports
adsorption energies based on electronic energies at zero K. It is proposed in literature that at
100 K ethylene already dehydrogenates to more stable species, clouding the detected adsorption
energy. [32]
In this work, di-σ adsorbed ethylene is studied, as it is generally more stable on the Pt (111)
surface. In this mode, the C=C bond of ethylene is elongated, conform propylene. The observed
C=C bond length is 1.5 Å, which is more similar to the length of single bonded C atoms (1.53
70 Chapter 4: Propane dehydrogenation kinetics on Pt(111) catalyst model
Å) than double bonded C atoms (1.34 Å). Furthermore, rehybridisation of the C atoms occurs
as covalent bonds are formed with the surface.
Table 4-8. Adsorption energy of di-σ adsorbed ethylene determined both experimentally and theoretically. Essen et al. employed a PW91 functional, while Watwe et al. used a RPBE functional. This work employs an optPBE vdW-DF functional.
ΔEads
(kJ/mol) Experimental data
(0.2- 0.25 ML)[9, 32, 33] Watwe et al.
(0.25 ML) [21] Essen et al.
(0.11 ML) [32] This work
(0.13 ML) Pt(111) -71 ~ -73 -117 -100 -131
Apart of the di-σ adsorbed ethylene, the physisorbed ethylene is studied. During physisorption,
ethylene retains its double bond as no elongation with respect to gaseous ethylene is observed.
However, it should be noted that physisorbed ethylene is not converged under the strict ionic
criterion. The remaining RMS is 0.026 and as this mode is metastable, four imaginary vibration
modes are obtained during the frequency analysis. The lowest frequency (7 cm-1) is attributed
to lattice vibration, while the upper three are allocated to translation and external rotation of
ethylene. These frequencies are treated prior the kinetic simulation.
Chapter 4: Propane dehydrogenation kinetics on Pt(111) catalyst model 71
4.3 Thermodynamics
In this work, the following intermediates are included to describe the reaction network of
propane dehydrogenation on the Pt(111) surface: C3Hx (x=5-8), C2Hy (y=3-6), CHz (z=1-4) and
atomic hydrogen. For each component, the most stable geometry on the Pt(111) catalyst model
is calculated and a frequency analysis is conducted to evaluate its stability. The considered
species are listed in Table 4-9 and a streamlined version of the proposed reaction network is
shown in Figure 4-4. The optimal geometries can be found in Appendix B, together with a
visualized reaction network based on those optimal geometries.
Table 4-9. Adsorbed hydrocarbon species on the Pt(111) surface. * indicates with how many C-Pt bonds the species is adsorbed to the surface.
C3Hx C2Hy CHz
CH3-CH2-CH3 Propane,phys CH3-CH3 Ethane,phys CH4 Methane,phys
CH3-CH2-CH2* 1-Propyl CH3-CH2
* Ethyl CH3* Methyl
CH3-CH*-CH3 2-Propyl CH2*-CH2
* Ethylene CH2* Methylidene
CH3-CH*-CH2* Propylene CH2=CH2 Ethylene,phys CH2
* Methylidyne
CH3-CH=CH2 Propylene,phys CH3-CH** Ethylidene
CH3-CH2-CH** 1-Propylidene CH3-C*** Ethylidyne
CH3-CH**-CH2 2-Propylidene
CH3-CH2-C*** 1-Propylidyne
CH3-CH*-CH** 1-Propenyl
CH3-C**-CH2* 2-Propenyl
72 Chapter 4: Propane dehydrogenation kinetics on Pt(111) catalyst model
Figure 4-4: Reaction netw
ork for propane dehydrogenation on Pt(111). T
he consecutive reactions of the methyl and ethyl species are given in Figure 4-6
2-propyl
1-propylidene
10
CH
2 =CH
-CH
3,phys
CH
2 =CH
-CH
3 (g)
13
CH
2 + CH
-CH
3
=Pt2
=Pt2_
CH
-CH
-CH
3+ HPt
Pt2
_Pt
=
Methyleneand ethylidene
CH
2 -C-C
H3
+ HPt _
Pt _
Pt2
=2-propenyl
CH
3 -C + C
H3
Pt3
≡
Pt _
CH
+ CH
2 -CH
3
Pt3Pt
≡
_
1-propenyl
C-C
H2 -C
H3
+ H
Pt3
_
Pt
≡
1415
1617
1819
2021
Methyl and ethyl
CH
2 + CH
2 -CH
3
Pt2Pt
=
_
45
6
=
_
CH
-CH
2 -CH
3+ H
Pt2Pt
CH
3 -CH
+ CH
3
Pt2Pt
=
_
2-propylidene
CH
3 -C-C
H3
+ H
Pt2
=
Pt _
78
9
CH
3 + CH
2 CH
3
PtPt
_
_C
H3 -C
H-C
H3
+ H
PtPt
_
_
CH
2 -CH
2 -CH
3 + H
PtPt
_
_
1-propyl
12
3
CH
3 -CH
2 -CH
3,phys
CH
3 -CH
2 -CH
3 (g)Gaseous
propane
0
Physisorbed propane
Ethylideneand m
ethyl
CH
2 -CH
-CH
3+ HPt _
_Pt
_PtPropylene
1112
Methylidene
and ethyl
Methylidene
and ethyl1-propylidyne
Physisorbed propylene
Gaseouspropylene
Ethylidyneand m
ethyl
Chapter 4: Propane dehydrogenation kinetics on Pt(111) catalyst model 73
The reaction network will be divided in three sections: propane dehydrogenation to propylene
(4.3.1), deep dehydrogenation of propylene (4.3.2) and hydrogenolysis of C3 intermediates
(4.3.3). In each section, reaction energies are determined and the stability of each intermediate
is evaluated. The first section can be interpreted to quantify the activation of the catalyst, while
section two and three quantify the selectivity towards propylene with respect to deep
dehydrogenation reactions and cracking reactions that can lead to cokes formation. In the
second section, the relative energy of adsorbed propylene and deep dehydrogenated species are
compared, as the latter species are coke precursors and indicate the catalyst sensitivity to
deactivation. In the third section, hydrogenolysis or dissociation of the C-C bond are discussed.
These reactions lead to smaller hydrocarbons on the surface and eventually gaseous side
products such as methane, ethane and ethylene. Their formation indicates a decrease of
propylene selectivity as propane is converted to other gaseous species.
Three main reaction types can be distinguished: dehydrogenation, isomerization and
hydrogenolysis. In the first type, the breaking of C-H plays a major role, while in the last, the
breaking of a C-C bond is considered. The location of the fragment(s) is essential to describe
their reaction energy. In the case of dehydrogenation, this fragment is always an adsorbed
hydrogen. In this work, it is assumed that the fragmented hydrogen diffuses away from the
adsorbate due to repulsion between hydrogen and adsorbate. This assumption is valid because
the calculations are done at low H coverages, and at propane dehydrogenation temperatures of
873 K, hydrogen is mobile on the surface. [34] Furthermore, this validation can be extended to
dissociation reactions, as there is also repulsion observed between the C1 and C2 fragments. In
addition, the product C-fragments will be calculated in a separate unit cell. In this work, this
assumption is implemented as follows. The product adsorbate and the atomic hydrogen are both
optimized in separate unit cell and equation (3) is used to calculate the reaction energy for
dehydrogenation reactions, while for the hydrogenolysis reactions equation (4) is employed and
both hydrocarbon fragments are optimized in a separate unit cell. For other reactions such as
isomerization and physisorption, equation (5) is used as the reaction is treated in one unit cell.
ΔE𝑟𝑟 = 𝐸𝐸C3𝐻𝐻𝑥𝑥−1/surface + 𝐸𝐸H/surface − 𝐸𝐸C3𝐻𝐻𝑥𝑥/surface − 𝐸𝐸surface (3)
ΔE𝑟𝑟 = 𝐸𝐸C2𝐻𝐻𝑦𝑦/surface + 𝐸𝐸C1𝐻𝐻𝑧𝑧/surface − 𝐸𝐸C3𝐻𝐻𝑦𝑦+𝑧𝑧/surface − 𝐸𝐸surface (4)
ΔE𝑟𝑟 = 𝐸𝐸𝑝𝑝𝑟𝑟𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝/surface − 𝐸𝐸reactants/surface (5)
As for each dehydrogenation step, the hydrogen is optimized in separate unit cell, it is essential
to find its most stable adsorption site on the surface. As hydrogen adsorption is dissociative
74 Chapter 4: Propane dehydrogenation kinetics on Pt(111) catalyst model
adsorption, a small adaptation is made to equation (2) to ensure that the conservation laws are
respected. Equation (6) is applied as follows and can be extended to other dissociative
adsorption reactions.
∆ads𝐸𝐸(H2) = 2𝐸𝐸H,ads − 𝐸𝐸ad,gas − 2𝐸𝐸surface (6)
Various researchers have studied the adsorption of hydrogen gas on Pt(111) surfaces with either
molecular beams or TPD analysis. They report energies all in the same order of magnitude;
however, the adsorption energy of hydrogen depends strongly on the hydrogen coverage on the
surface. The hydrogen adsorption strength decreases for increasing hydrogen coverage on
Pt(111). [35-37] The experimental allocation of the most stable adsorption site of hydrogen is
difficult as different sources contradict each other. This discrepancy can be explained based on
the employment of the harmonic oscillator to allocate the vibration modes during vibration
analysis and to whether or not the zero point energy (ZPE) effects are included. Bădescu et al.
performed a combined experiment and theory study on hydrogen adsorption at both low and
high hydrogen coverage. At low coverage (θ < 0.7), the three-folded fcc site is most stable
according to their theoretical calculations, while the top site is 2.5 kJ/mol less stable. [38] At
monolayer coverage, top site becomes more stable than the fcc site, but only if the zero point
energy correction is excluded. [39] Over the complete coverage range, the hcp site is less stable
than the other adsorption sites.
Yang et al. determined hydrogen adsorption energies on a 2×2 unit cell with PBE as functional.
However, the adsorption energy was calculated with respect to atomic hydrogen in the
gasphase. [3] In Table 4-10, the adsorption energies are listed; the results of this work are
modified based on the formula used by Yang et al. to compare with their results (see right
column Table 10). The adsorption energy of each site falls into the experimental range of
values. However, the most stable adsorption site is top site at low coverage (θ =0.13), while
Yang et al. and Bădescu et al. both determined the hollow fcc site as most stable under these
conditions. When comparing both DFT studies, it can be observed that the vdW-DF functional
stabilizes the three-folded adsorption sites with ~89 kJ/mol and the top site with ~97 kJ/mol,
which can be explained based on van der Waals interactions. For further calculations, the
adsorption energy of hydrogen on the top site is employed to determine the reaction energies
of dehydrogenation reactions, as formulated in equation (3).
Chapter 4: Propane dehydrogenation kinetics on Pt(111) catalyst model 75
Table 4-10. Hydrogen adsorption energies on Pt(111), determined experimentally and theoretically. Yang et al. used a PBE functional while this work uses an optPBE vdW-DF functional.
ΔEads
(kJ/mol) Experimental data
(0 – 0.55 ML) [35-37] This worka
(0.13 ML) Yang et al.b
(0.25 ML) [3] This workb
(0.13 ML)
On top -73 ~ -80 -77 -257 -354
Fcc -73 ~ -80 -71 -263 -351
Hcp -59 -256 -346 (a) Energies calculated according to equation (6). (b) Adsorption energy is determined with respect to atomic hydrogen in the
gasphase.
4.3.1 Propane dehydrogenation to propylene
The dehydrogenation of propane to propylene consists at least of two elementary steps on the
platinum surface. Additionally, the physisorption of propane is included prior to the propane
activation on the surface. This initial activation of propane is the first elementary step and
dehydrogenates at either the methyl or methylene group, respectively generating 1-propyl or 2-
propyl. The most stable adsorption of these species is located on top site of a platinum atom.
Consequently, these species form propylene via β-dehydrogenation. Propylene orientates itself
most stable over a bridge site. Each step generates also a detached hydrogen. Its location plays
a major to determine the kinetics as the activation energy is strongly influenced by the (de-
)stabilization due to this hydrogen. However, for the thermodynamics, it is assumed that
hydrogen is highly mobile on the surface and diffuses away from the product hydrocarbon.
Both 1-propyl and 2-propyl can also form respectively 1-propylidene and 2-propylidene via α-
dehydrogenation. An additional hydrogen is detached from the carbon atom that is already
bonded with the surface. The bridge adsorption site is the most stable for both these species.
Apart from dehydrogenation reactions, isomerization reactions are also included between 1-
propyl and 2-propyl, 1-propylidene and propylene and 2-propylidene and propylene. The
reaction energies determined in this work are compared with the DFT studies of Yang et al. [4]
and Valcarcel et al. [7] Yang et al. report all reaction energies studied in this work, while
Valcarcel investigated all reactions originating from propylene. Here, the formation of 1-propyl
and 2-propyl are treated as hydrogenation reactions with respect to propylene, but by altering
the sign of the reaction energy, they can be treated as the reaction energies of the respective
dehydrogenation reactions.
76 Chapter 4: Propane dehydrogenation kinetics on Pt(111) catalyst model
Table 4-11. Comparison between reaction energies for propane dehydrogenation towards propylene of different DFT studies. Yang et al. [3] used a PBE functional while Valcarcel et al. [7] employed a PW91 functional. This work uses an optPBE vdW-DF functional.
ΔEr (kJ/mol) Yang et al.
(0.25 ML)
Valcarcel et
al. (0.25 ML)
This work
(0.13 ML) # Surface reaction
0 Propane(g) → Propane,phys Physisorption -2 - -43
1 Propane,phys → 1-propyl + H Adsorption -7 - -7
2 Propane,phys → 2-propyl + H Adsorption -6 - -14
4 1-propyl → 1-propylidene + H Dehydrogenation 3 1 7
5 1-propyl → propylene + H Dehydrogenation -22 -18 -22
7 2-propyl → propylene + H Dehydrogenation -23 -31 -15
8 2-propyl → 2-propylidene + H Dehydrogenation 7 - 15
10 1-propyl → 2-propyl Isomerization 1 13 -7
11 1-propylidene → propylene Isomerization -25 -19 -29
12 2-propylidene → propylene Isomerization -30 - -30
13a Propylene→ propylene,phys Desorption 90† 87† 92
13b Propylene,phys →Propylene (g) Desorption - - 43 † Reaction energy is determined with respect to propylene in the gasphase, as physisorbed propylene is not studied.
Aside from the adsorption of propane and the desorption of propylene, it can be observed that
the order of magnitude of the results of the DFT studies are similar. However, because van der
Waals interactions are incorporated in our calculations, some discrepancies are noticed with the
literature. The discussion of the difference in propane adsorption and propylene desorption can
be found in 4.2.2.1 and 4.2.1.1 respectively.
In the first elementary step, Yang et al. observe 1-propyl as more stable intermediate compared
to 2-propyl with an energetic difference of one kJ/mol. However, in this work, 2-propyl is more
stabilized than 1-propyl and a larger energy difference of 7 kJ/mol beneficial for 2-propyl is
reported. In the case of 1-propyl, Yang et al. report that the ethyl moiety orientates itself
perpendicular with respect to the surface, while d(Pt-C) is 2.1 Å and the d(C-C) is 1.52 Å and
1.54 Å. [3] Compared with this work, this geometry of 1-propyl is identical. However, an
additional geometry with the ethyl moiety parallel to the surface is calculated, see Figure 4-5.
The resulted electronic energy is a fraction lower (~2 kJ/mol). However, the electronic energy
only takes into account enthalpic contributions, so to allocate the most stable geometry of 1-
propyl, it is essential to evaluate the entropic factors.
Chapter 4: Propane dehydrogenation kinetics on Pt(111) catalyst model 77
Figure 4-5: Isometric representation of two possible ethyl moiety orientation of 1-propyl: ethyl moiety perpendicular to the Pt surface (left) and ethyl moiety parallel to the surface (right). Following color code us used: H (white), C (gray) and Pt (blue).
Hence, the Gibbs free energy is determined based on the vibrational analysis of both geometries
at 873 K. This calculation is based on the partition functions. Additionally, the imaginary
frequencies and frequencies assigned to lattice movement are neglected to determine the Gibbs
free energy. Based on those results, the geometry with ethyl moiety perpendicular to the surface
(Figure 4-5, left) is ~2 kJ/mol more stable than with ethyl moiety parallel to the surface
(Figure 4-5, right). Hence, the geometry with ethyl moiety perpendicular to the surface is
employed in further calculations. This geometry is stabilized due to entropic contributions, as
the ethyl moiety is farther away for the surface. Still, as the Gibbs free energy difference is so
small, both species are expected to from on the surface, certainly at low coverages. At higher
coverage, the 1-propyl with the upwards ethyl group is more favorable as it blocks fewer
adsorption sites.
The geometries of 2-propyl in this work and in the work of Yang are almost identical, only the
distance between Pt and the bonded C is slightly shorter in the case of this work (1.52 Å) with
respect to Yang et al. (1.54 Å). The small geometric differences can be explained based on van
der Waals interactions. An energetic difference of -8 kJ/mol in favor of this work was found.
This stabilization can be explained based on the lower coverage (0.13 ML) employed in this
work, which leads to less interaction between adsorbates. It is expected that this species will be
hindered at higher coverage as the two methyl groups of 2-propyl partially block two nearby
atoms.
In this work, 2-propyl is more stable than 1-propyl based on their electronic energy. However,
this energy calculation neglects zero point energy (ZPE) and entropic contributions, so for the
most stable geometries of 1-propyl and 2-propyl, a frequency analysis is conducted to determine
78 Chapter 4: Propane dehydrogenation kinetics on Pt(111) catalyst model
the Gibbs free energy at 873 K. From this calculation, it can be concluded that 1-propyl is more
stable than 2-propyl with a difference of -9 kJ/mol. The entropic effects of the ethyl moiety of
1-propyl cause additional stabilization with respect to the two methyl groups of 2-propyl.
However, as the difference in Gibbs free energy is still quite low, both pathways will be
important at propane dehydrogenation temperatures.
For the second elementary step, the β-dehydrogenation is most favorable, but has to compete
with α-dehydrogenation, that occurs simultaneously. Both literature and this work reports that
α-dehydrogenation generate less stable species than β-dehydrogenation. Propylene is the most
stable C3H6-species on the surface, with 1-propylidene as second and 2-propylidene as third.
When comparing the stable geometries in this work with those of Yang et al, it can be concluded
that propylene is identical, while 1-propylidene and 2-propylidene slightly differ at the upper
C-C bonds. For 1-propylidene, the upper C-C bond of the ethyl moiety is elongated from 1.53
Å to 1.54 Å, while both C-C bonds of 2-propylidene are stretched from 1.52 Å to 1.53 Å. This
small elongation is contributed to the van der Waals interactions between the intermediates and
the surface. As with 1-propyl, the location of the ethyl moiety of 1-propylidene is important to
determine the global minimum of this species. Hence, two configurations are optimized: with
the ethyl moiety perpendicular to the surface (same as Yang et al.) and parallel to the surface.
The resulted electronic energy differs slightly in favor of the first geometry. However, a
conclusion is made based on the Gibbs free energy calculation at 873 K. The most stable
geometry remains the first with less than 1 kJ/mol difference. Therefore, it is expected that both
species will be present on the surface at low coverage, where the first geometry is preferential
at higher coverage due to the direction of the ethyl moiety.
The propane dehydrogenation to propylene can be treated as a catalytic cycle via two pathways:
via 1-propyl (reactions: 0 – 1 – 5 – 13a – 13b) and 2-propyl (reactions: 0 – 2 – 7 – 13a – 13b).
Additionally, hydrogen desorption is included to complete these loops. Both reaction paths are
compared with the gasphase reaction based on DFT study and microcalorimetry experiments,
see Table 4-12.
The global reaction energy of each path is identical with a value of 139 kJ/mol. To validate this
energy, it has to be compared with experimental microcalorimetry data. In literature, no
information is found on the microcalorimetry of propane dehydrogenation. However, the
reaction energy can be interpolated from microcalorimetry data, obtained for ethane and
isobutene dehydrogenation. The reaction energies are respectively 157 kJ/mol [40, 41] and 110
Chapter 4: Propane dehydrogenation kinetics on Pt(111) catalyst model 79
kJ/mol [42], which means an interpolated value of 134 kJ/mol is obtained for propane
dehydrogenation. This estimate is in accordance with the calculated values of reaction energies
in this work. Based on the thermodynamical data, both reaction pathways will occur on the
surface.
Table 4-12. Comparison of DFT calculated reaction energies (kJ/mol) to form propylene via two reaction paths on the Pt(111) surface and the gasphase reaction.
Elementary step Via 1-propyl Via 2-propyl Gasphase
Propane(g) → Propane,phys -43 -43 -
Propane,phys → 1-propyl + H -7 - -
Propane,phys → 2-propyl + H - -14 -
H → ½ H2(g) 38 38 -
1-propyl → propylene + H -22 - -
2-propyl → propylene + H - -15 -
H → ½ H2(g) 38 38 -
Propylene→ propylene,phys 92 92 -
Propylene,phys → Propylene (g) 43 43 -
Propane(g) → Propylene (g) + H2(g) 139 139 139
4.3.2 Deep dehydrogenation of propylene and other C3H6 species
The deep dehydrogenation, especially the dehydrogenation of propylene, is essential to describe
the selectivity towards coke precursors. In this work, three deeply dehydrogenated species are
investigated: 1-propylidyne, 1-propenyl and 2-propenyl. 1-propylidyne is formed via α-
dehydrogenation of 1-propylidene, while β-dehydrogenation of 1-propylidene leads to the
formation of 1-propenyl. Propylene forms 1-propenyl and 2-propenyl, respectively via
dehydrogenation of the methylene and methylidyne groups. 2-propenyl is also formed via β-
dehydrogenation of 2-propylidene. 1-propylidyne prefers adsorption on a three-folded fcc site
and orientates its ethyl moiety perpendicular to the surface. The adsorption orientation of 1-
propenyl and 2-propenyl are similar, the carbon atoms that misses two hydrogen adsorbs on a
bridge site, while the carbon with a single-detached hydrogen prefers the top position. It can be
observed that all adsorbed hydrocarbons on Pt(111) are sp3-bound.
In Table 4-13, the reaction energies determined in this work are compared with those of DFT
studies performed by Yang et al. [4] and Valcarcel et al. [7]. While the same trend appears for
all three DFT studies, it can be observed that the reaction energies of this work shift with ~10
80 Chapter 4: Propane dehydrogenation kinetics on Pt(111) catalyst model
kJ/mol with respect to Yang et al. Van der Waals interactions cause the deeply dehydrogenated
species to be less stable on the surface. This work proposes 1-propylidyne as the
thermodynamic most stable species on the surface, which is confirmed by literature.
Table 4-13. Comparison between reaction energies for deep dehydrogenation of propylene on Pt(111) of different DFT studies. All considered reactions are dehydrogenation reactions. The employed functionals are the same as in Table 4-11.
ΔEr (kJ/mol) Yang et al.
(0.25 ML) [4]
Valcarcel et al.
(0.25 ML) [7]
This work
(0.13 ML) # Surface reaction
14 Propylene → 1-propenyl + H 6 -6 20
15 Propylene → 2-propenyl + H -1 -2 7
17 1-propylidene → 1-propylidyne + H -76 -77 -67
20 1-propylidene → 1-propenyl + H -18 -25 -10
21 2-propylidene → 2-propenyl + H -32 - -24
Zaera et al. performed infrared analyses on propylene adsorption at higher temperature than 95
K. At 275 K, the RAIRS spectra show a new species on the surface, which is identified as 1-
propylidyne through vibrational analysis. This thermal conversion takes place between 230 and
270 K, indicating that this species in the most stable on surface. However, propylene cannot
directly react to 1-propylidyne. The shortest path is a mechanism of two elementary steps. The
best guess is an initial isomerization (1,2 hydrogen shift) from propylene to 1-propylidene,
followed by α-dehydrogenation to 1-propylidyne. Still, no direct experimental evidence is
found of this mechanism, so other two steps or even three steps mechanism are possible. [27]
In previous DFT studies, even further dehydrogenated species are determined on the catalyst
surface: 1-propenylidene, propyne and propynyl. [4, 7] Both 1-propenylidene and propyne have
four bounds with the Pt surface. 1-propenylidene is formed via dehydrogenation of 1-propenyl
or 1-propylidyne and its outer carbon atoms adsorbs on a fcc site, while the center carbon atom
prefers the top site. Propyne has two carbon atoms that are double bonded on the Pt surface and
they both prefer the bridge site. Theoretically, it is also possible that this intermediate desorbs
as gaseous propyne. Both four-bonded species can dehydrogenate to form propynyl. This
species has five bonds with surface and orientates itself in a fcc site (three-bonded C atom) and
a bridge site (two-bonded C atom). Their reaction energies can be found in Table 4-14 and are
subsequently discussed.
Chapter 4: Propane dehydrogenation kinetics on Pt(111) catalyst model 81
Table 4-14. Additional reactions of deep dehydrogenation of propylene and their reaction energies, reported in literature. All considered reactions are dehydrogenation reactions Yang et al. used a PBE functional while Valcarcel employed a PW91 functional.
ΔEr (kJ/mol) Yang et al.
(0.25 ML)[4]
Valcarcel et al. (0.25 ML)[7] Surface reaction
1-propylidyne → 1-propenylidene + H 25 22
1-propenyl → 1-propenylidene + H -32 -30
1-propenyl → propyne + H -14 -11
2-propenyl → propyne + H -7 -25
1-propenylidene → propynyl + H 72 83
Propyne → propynyl + H 54 64
Dehydrogenation of 1-propylidyne forms a less stable species so, this adsorbate will probably
not be formed. While 1-propenyl and 2-propenyl both form more stable species, themselves are
not favored in the dehydrogenation process. Eventually those four-bonded species can form
propynyl, which is the most unstable species on the surface as both reaction energies are
strongly endothermic. These deeply dehydrogenated species are expected to be less involved in
the overall dehydrogenation reaction as 1-propylidyne is the most stable C3-species on the
surface. For this reason, these intermediates will not be added to the reaction network in this
work.
4.3.3 Hydrogenolysis of C3 intermediates
The scission reactions of C-C, C=C and C≡C bonds in the gasphase are unfavorable as the bond
energies are respectively 350, 620 and 835 kJ/mol at 298 K. [43] However, literature reports
that the metal surface weakens the C-C bond of hydrocarbon intermediates on the surface and
scission reactions occur more easily. [44, 45] Via these reactions, smaller adsorbates (C1 and
C2 hydrocarbons) are formed. They either hydrogenate and desorb from the surface as alkanes
such as methane and ethane or dehydrogenate further, leading to coke precursors and adsorbed
carbon. The inhibition of these steps can greatly improve the selectivity to propylene, as no
other gaseous hydrocarbon species can be formed. Experiments have shown that during propane
dehydrogenation on Pt catalysts, before steady state, the selectivity towards propylene is low
(< 80 %) and even at steady state the propylene selectivity is only 87 %. Experimentally, the
82 Chapter 4: Propane dehydrogenation kinetics on Pt(111) catalyst model
selectivity is determined with respect the other gaseous hydrocarbon products. During the
complete time on stream, hydrogenolysis of C3 intermediates occurs on pure Pt catalysts. [46]
In this work, the scission reaction of each intermediate up to the C3H6-species is included in the
reaction mechanism. The reaction energies are compared with those of Yang et al. [3] in
Table 4-15. Yang et al. did not report how the reaction energies of the dissociation reaction are
determined. It is assumed that the dissociated species are optimized in separate unit cells as
done in this work.
Table 4-15. Comparison of reaction energies for hydrogenolysis during propane dehydrogenation on Pt(111) between different DFT studies. Yang et al. employ a PBE functional, while this work uses an optPBE vdW-DF functional.
ΔEr (kJ/mol) Yang et al.
(0.25 ML) [3]
This work
(0.13 ML) # Surface reaction
3 Propane,phys→ methyl + ethyl Dissociation 11 -5
6 1-propyl → methylidene + ethyl Dissociation 36 22
9 2-propyl → methyl + ethylidene Dissociation 27 21
16 Propylene → methylidene + ethylidene Dissociation 62 57
18 1-propylidene → methylidyne + ethyl Dissociation -22 -21
19 2-propylidene → methyl + ethylidyne Dissociation -55 -62
- 1-propylidyne → C + ethyl Dissociation 105 -
In general, the calculated reaction energies of Yang et al. and in this work for these reactions
are quite similar. It is observed that in general the reaction energies are lower in this work with
respect to Yang et al. The dissociation of 1-propyl, 2-propyl and propylene are
thermodynamically unfavorable (positive reaction energy). In contrast, propane hydrogenolysis
leads to more stable species (methyl and ethyl), which are both important for the formation of
gaseous side products. However, the formation of 1-propyl and 2-propyl remains
thermodynamically more favorable with respect to the dissociation products. The
hydrogenolysis of 1-propylidene and 2-propylidene forms stronger adsorbed species with
respect to the reactants, but the 1-propylidene and 2-propylidene are thermodynamically less
favored than propylene. Furthermore, it is interesting to look into the dissociation of 1-
propylidyne, which as thermodynamic most favorable product could be important as
intermediate for dissociated species. However, Yang et al. report strong endothermic reaction
energy of 105 kJ/mol for this reaction. Based on this reasoning, this reaction is not likely occur
Chapter 4: Propane dehydrogenation kinetics on Pt(111) catalyst model 83
and has not been incorporated in this work. The most realistic paths toward dissociated species
are via 1-propylidene and 2-propylidene.
4.3.3.1 Formation of gaseous side products
The cleavage of the C-C bonds results in the formation of C1 and C2 hydrocarbons on the
surface. Further reaction of these species leads either to adsorbed coke precursors or to the
desorption of smaller alkanes and alkenes, which strongly influences the selectivity for
propylene. The formation of methane, ethane and ethylene are included in the reaction
mechanism and compared with DFT studies from literature, see Figure 4-5 and Table 16. [20,
32, 47] The optimal geometries of these species can be found in Appendix B.
Figure 4-6: Reaction network for formation of gaseous side products on Pt(111).
The poor description of the physisorbed state of the alkanes is due the negligence of van der
Waals interactions in the other studies. Adsorbed ethyl prefers to dehydrogenate to ethylene
instead of to hydrogenate to ethane in both studies. However it should be noted that the
desorption energy of ethylene is high (131 kJ/mol) with respect to ethane (27 kJ/mol). However,
it should be noted that solely the enthalpic factors are included in the electronic energy.
CH2CH3
Pt
_
CH3-CH3,phys
CH4(g)
CH3-CH3(g)
CH2-CH2 + H
+ H-Pt
CH2=CH2,phys CH2=CH2(g)
Pt
_ CH4,physCH3 + H
Pt
_Pt
_
Pt
_
Pt
_
Methyl Physisorbed methane Gaseous methane
Ethyl
Physisorbed ethane Gaseous ethane
Ethylene Physisorbed ethylene Gaseous ethylene
A1 A2
B1
C1
B2
C2 C3
84 Chapter 4: Propane dehydrogenation kinetics on Pt(111) catalyst model
Table 4-16. Comparison between reaction energies for side product formation on Pt(111) of different DFT studies. The employed functionals and coverages in literature are noted underneath the table. This work uses an optPBE vdW-DF functional.
ΔEr (kJ/mol) Literature
[20, 32, 47]
This work
(0.13 ML) # Surface reaction
A1 Methyl + H → Methane,physisorbed Hydrogenation -21a -15
A2 Methane,physisorbed → Methane(g) Desorption 0 a 20
B1 Ethyl + H → Ethane,physisorbed Hydrogenation 43b
-4
B2 Ethane,physisorbed → Ethane(g) Desorption 31
C1 Ethyl → Ethylene + H Dehydrogenation -30b -13
C2 Ethylene → Ethylene,physisorbed Desorption 100c 90
C3 Ethylene,physisorbed →Ethylene(g) Desorption 35c 41 (a) A coverage of 0.11 ML is achieved and RPBE is used as functional. In addition, the ZPE is added to the reaction energy.
[47] (b) A coverage of 0.25 ML is achieved and RPBE is used as functional. [20] (c) A coverage of 0.11 ML is achieved and
PW91 is used as functional. [32]
4.3.4 Overall thermodynamics
To evaluate relative electronic energies of the various C3Hx-species an energy profile is
constructed with the energy of each intermediate with gaseous propane as reference. To satisfy
the conservation laws, it is essential that the mass of every species is identical. In the case of
dehydrogenated species, the relative energy is a combination of the energy of the
dehydrogenated species and the energy of the dehydrogenated hydrogen(s), conform equation
(7) (see below). The constructed energy profile is illustrated in Figure 4-7.
EC3𝐻𝐻𝑥𝑥 = 𝐸𝐸C3𝐻𝐻𝑥𝑥/surface + (8 − 𝑥𝑥)(𝐸𝐸H/surface − 𝐸𝐸surface)
− (𝐸𝐸C3𝐻𝐻8(g) + 𝐸𝐸surface) (7)
The activation of propane leads to both 1-propyl and 2-propyl, as the Gibbs free energies of
these species are similar. These species dehydrogenate preferentially further to propylene, as it
is the thermodynamically the most stable C3H6-species. Still propylidyne is the most energetic
stable species on the Pt(111) surface, so it is expected that the most likely reaction will be this
formation reaction and if isomerization reactions are excluded and Gibbs free energy is taken
into account as discussed in section 4.3.1, the most probable path is via propane →1-propyl
→1-propylidene →1-propylidyne.
Chapter 4: Propane dehydrogenation kinetics on Pt(111) catalyst model 85
Figure 4-7: Energy profile of adsorbed C3Hx (x= 5–8) species on the Pt(111) surface. Energies are determined relative to gaseous propane. In addition, the dissociated species are shown (red).
Hydrogenolysis reactions lead to the formation of endothermic products, except for the cracking
of 1-propylidene and 2-propylidene. Cracking of 2-propylidene leads to the formation of
species more stable than propylene; through this cracking reaction, C1 and C2 hydrocarbons are
formed on the surface. However, the description of the kinetics is necessary to evaluate the
dominant reaction paths and, eventually, the distribution of the intermediates on the surface.
4.4 Kinetics
In this work, the kinetic descriptors of our reactions are determined based on the transition
states, in this case the electronic activation energies. The geometry of the transition state has to
be found along the reaction coordinate. These structures are so-called first order saddle points
on the potential energy surface (PES) of the considered species. Hence, the transition state is
energetic minimum on the PES, in all directions except the reaction coordinate. In vibrational
analysis of transition states, this corresponds to one imaginary frequency assigned along the
reaction coordinate. Furthermore, the Hammond–Leffler postulate is applied to determine the
structure of the transition state. This postulate predicts a transition state that resembles the
-125
-105
-85
-65
-45
-25
-5
15R
elat
ive
ener
gy (k
J/m
ol)
C3H8 (g) C3H8* C3H7
* + H* C3H5* + 3H*
Propane (g)
Propane, phys
2-propyl
Propylene,phys
Propylene
1-propenyl1-propyl 1-propylidene
2-propylidene
2-propenyl
1-propylidyne
Ethyl & methyl
Ethyl & methylidene
Ethyl idene & methyl
Ethylidene & methylidene
Ethyl & methylidyne
Ethylidyne & methyl
C3H6* + 2H*
86 Chapter 4: Propane dehydrogenation kinetics on Pt(111) catalyst model
products (late) for endothermic reaction and transition state that resembles the reactants (early)
for exothermic reactions. [48]
If both the optimal geometry of transition state and reactants are obtained, the electronic
activation energy Δ‡E can be determined, as shown in equation (8). This value can be used as
an indication of the activation barrier. In equation (8), the forward electronic activation energy
of an elementary step is calculated, while to determine the reverse electronic activation energy,
either the reaction energy has to be added or the electronic activation energy has to be calculated
with respect to the electronic energy of the products.
∆ǂ𝐸𝐸 = 𝐸𝐸TS,ads − 𝐸𝐸react,ads (8)
To describe the kinetics of the reaction network, the network will be divided in three sections:
propane dehydrogenation to propylene (4.4.1), deep dehydrogenation of propylene (4.4.2) and
C-C cleavage of C3 intermediates (4.4.3). In each section, the stability of transition states is
evaluated and the electronic activation energy is determined for each reaction. The first section
can be interpreted to quantify the activity of the catalyst, while section two and three quantify
the selectivity towards propylene with respect to side reactions that lead to coke precursors.
Three main reaction types can be distinguished: dehydrogenation, isomerization and
dissociation. In the first and second, the breaking of C-H plays a major role, while in the last,
the breaking of a C-C bond is considered.
As the transition state geometries are calculated with the NEB method, an initial and final state
has to be given. The final state of dehydrogenation reaction has to be optimized with the
hydrogen nearby the intermediate in the same unit cell. The location of the adsorbed hydrogen
is either on top a Pt site or in a fcc site. The obtained geometries of the transition states can be
found in Appendix B.
4.4.1 Propane dehydrogenation to propylene
As discussed previously, the dehydrogenation of propane towards propylene consists of two
elementary steps: the activation of propane to either 1-propyl or 2-propyl via respectively
dehydrogenation of the methyl or methylene group and β-dehydrogenation of 1-propyl or 2-
propyl leads to propylene. In this work, before the first elementary step, the physisorption of
propane has been taken into account. However, for this type of reaction it is assumed that the
electronic activation energy is zero.
Chapter 4: Propane dehydrogenation kinetics on Pt(111) catalyst model 87
Table 4-17. Comparison of the electronic activation energies between DFT studies for propane dehydrogenation towards propylene on Pt(111). Yang et al. used a PBE functional, while this work uses an optPBE vdW-DF functional.
† Reaction 11: an additional imaginary frequency has been observed. Reaction 12: Electronic energy of transition state is
determined based on the NEB calculation.
The electronic activation energies in this work are compared with the DFT study of Yang et al.
In general, the obtained activation barriers show a rather similar trend. However, for all
dehydrogenation reactions, an increase is observed of ~13 kJ/mol, except for the reaction path
towards propylene via 2-propyl. The results of Yang et al. and the reaction path via 1-propyl of
this work have shown that the two elementary steps for propylene formation have similar
reaction barriers. In contrast, the reaction barriers of the reaction path via 2-propyl are dissimilar
as the second reaction barrier is 12 kJ/mol higher than the first.
The location of detached hydrogen is the key parameter to describe the geometry of the
transition state. The transition state geometries of both DFT studies are similar. The distance
between the carbon and hydrogen atom along the reaction coordinate differs only ~0.1 Å.
Additionally, the detached hydrogen is located either on the same or on a nearby Pt atom.
Δ‡E (kJ/mol) Yang et al.
(0.25 ML) [4]
This work
(0.13 ML) # Surface reaction
0 Propane(g) → Propane,phys Physisorption 0 0
1 Propane,phys → 1-propyl + H Adsorption 67 80
2 Propane,phys → 2-propyl + H Adsorption 68 68
4 1-propyl → 1-propylidene + H Dehydrogenation 70 83
5 1-propyl → propylene + H Dehydrogenation 68 79
7 2-propyl → propylene + H Dehydrogenation 61 84
8 2-propyl → 2-propylidene + H Dehydrogenation 81 93
10 1-propyl → 2-propyl Isomerization - 203
11 1-propylidene → propylene Isomerization - 154†
12 2-propylidene → propylene Isomerization - 168†
13a Propylene→ propylene,phys Chemisorption 90 92
13b Propylene,phys →Propylene (g) Physisorption - 43
88 Chapter 4: Propane dehydrogenation kinetics on Pt(111) catalyst model
Figure 4-8: Relative energy profile of the propane dehydrogenation towards propylene. The energies are determined relative to gaseous propane. Dehydrogenated hydrogen(s) are optimized in separate unit cells.
Based on the energy profile, shown in Figure 4-8, the reaction path via 2-propyl to form
propylene is the least activated. However, in reality, both reaction paths will occur on the
surface, as both the reaction and activation energies are similar for the two elementary steps.
The activation energies for the second elementary step of both reaction paths are higher than in
the first, indicating this may be the rate-determining step for the formation of propylene.
However, this is solely based on enthalpic contributions. When entropic factors are included, it
is expected that the first elementary step is rate determining as the entropy decreases during
adsorption since gaseous propane loses part of its degree of freedom.
The formation of 1-propylidene and 2-propylidene is disadvantaged as the activation energy of
the α-dehydrogenation is higher than of the β-dehydrogenization. This is illustrated in a relative
energy profile, see Figure 4-9. Hence, it is observed that propylene thermodynamically and
kinetically is preferred with respect to 1-propylidene and 2-propylidene. From those two
species, the formation of 2-propylidene is the highest activated.
-100
-50
0
50
100
150
Rel
attiv
een
ergy
(kJ/
mol
)
Reactant Product
Propane (g)
Propane, phys
1-propyl + H*
Propylene,phys + 2H*
Propylene +2H*
Propylene (g) + 2H*
TS1TS5
TS2
2-propyl + H*
TS7
Chapter 4: Propane dehydrogenation kinetics on Pt(111) catalyst model 89
Figure 4-9: Relative energy profile of the propane dehydrogenation towards C3H6-species. The energies are determined relative to gaseous propane. Dehydrogenated hydrogen(s) are optimized in separate unit cells.
Furthermore, it is shown that the electronic activation energies of the isomerization reactions
are difficult to obtain within in the strict convergence criteria. Only the electronic activation
energy of the isomerization of 1-propylidene to propylene is obtained. However, for the
isomerization between 2-propylidene and propylene, the electronic energy of the transition state
is determined based on the NEB calculation. Furthermore, the transition state of the
isomerization between 1-propylidene and propylene has two imaginary frequencies. The
observed reaction barriers are in the range of 150 to 200 kJ/mol, indicating that these reactions
are unlikely to occur; certainly, as other paths have much smaller reaction barriers. Chen et al.
performed a DFT analysis on ethane dehydrogenation. Here, the isomerization reactions were
included and the obtained reaction barriers were at least 200 kJ/mol or higher, confirming our
assumption that isomerization reactions are unlikely to occur on the platinum surface. [20]
It is assumed that the adsorption of propylene is not an activated process. However, for
propylene to desorb, it has to overcome the reaction energy of desorption. In this case, the
reaction barrier of desorption is 135 kJ/mol, which is much higher than the determined barrier
of 90 kJ/mol by Yang et al. This discrepancy is discussed in 4.2.2.1.This value is essential to
determine the descriptor of selectivity towards propylene.
-80
-60
-40
-20
0
20
40
60
Rel
attiv
e en
ergy
(kJ/
mol
)
Reactant Product
Propane (g)
Propane, phys
1-propyl + H*
Propylene +2H*
TS1 TS6
TS2
2-propyl + H*
TS7
TS4
2-propylidene +2H*
1-propylidene +2H*
90 Chapter 4: Propane dehydrogenation kinetics on Pt(111) catalyst model
4.4.2 Deep dehydrogenation of propylene and other C3H6-species
The deep dehydrogenation, especially the dehydrogenation of propylene, is essential to describe
the selectivity towards propylene with respect to formation of coke precursors. The
dehydrogenation reactions towards 1-propenyl, 2-propenyl and 1-propylidyne are studied in
this work.
Table 4-18. Comparison of the electronic activation energies between DFT studies for propane dehydrogenation towards propylene. Yang et al. used a PBE functional, while in this work, an optPBE vdW-DF functional is employed.
Δ‡E (kJ/mol) Yang et al.
(0.25 ML) [4]
This work
(0.13 ML) # Surface reaction
14 Propylene → 1-propenyl + H Dehydrogenation 73 91
15 Propylene → 2-propenyl + H Dehydrogenation 74 81
17 1-propylidene → 1-propylidyne + H Dehydrogenation 22 45†
20 1-propylidene → 1-propenyl + H Dehydrogenation 60 72
21 2-propylidene → 2-propenyl + H Dehydrogenation 52 66 † The optimized transition state has one additional imaginary frequency.
For all reactions, an increase of electronic activation energy, varying between 7 and 23 kJ/mol,
is observed with respect to the results of Yang et al. However, based on previous results, an
increase of at least 10 kJ/mol is expected based on the reaction energies in 4.3.2. Therefore, the
discrepancy is the largest for reaction 15, as a smaller increase is observed. Based on the
Brønsted–Evans–Polanyi principle, it is expected that the formation of 2-propenyl would have
a lower barrier than 1-propenyl as 2-propenyl is more stable than 1-propenyl, contradicting the
results of Yang et al. that observe a similar barrier for both reactions.
Deep dehydrogenation plays a major role in the selectivity of propane dehydrogenation with
respect to deactivation reactions. As 1-propylidyne is the most stable on the surface, its
formation can be related to the surface coke formation and the deactivation of the catalyst.
Hence, a selectivity descriptor is proposed as the difference between the activation energy of
propylene desorption and propylene dehydrogenation. In literature, the activation energy of
propylene dehydrogenation with the lowest barrier is taken, mostly the dehydrogenation to 2-
propenyl. [4] However, in this work, the activation energy for deactivation is determined based
on all deeply dehydrogenated species and not only those formed out of propylene. Hence, 1-
propylidyne is the most stable species on the surface and its formation has a low reaction barrier
via 1-propylidene dehydrogenation. Instead of comparing two reactions, two reaction paths are
Chapter 4: Propane dehydrogenation kinetics on Pt(111) catalyst model 91
used to define the selectivity descriptor in this work. Each reaction path consists out of two
elementary steps: for the desired product propylene, this is first the dehydrogenation of 1-propyl
to propylene and consequently the desorption of propylene, while for deep dehydrogenation,
this is consecutive α-dehydrogenation of 1-propyl to 1-propylidene and to 1-propylidyne.
To visualize the selectivity descriptor an energy profile, relative to gaseous propane, for both
considered reaction paths is constructed, see Figure 4-9. The mass conservation laws are
satisfied, so for the determination of relative energies of the dehydrogenated species, the energy
of dehydrogenated hydrogens is added, conform equation (7). In the final step, all adsorbates
desorb. In the case of 1-propylidyne, this is solely the adsorbed hydrogen atoms.
Figure 4-10: Relative energy profile for the propane dehydrogenation towards propylene (···) and propane deep dehydrogenation towards 1-propylidyne ( ̶ ̶ ) on Pt(111). The energies are determined relative to gaseous propane.
The dehydrogenation reaction to 1-propylidyne has the lowest barrier and its formation is
thermodynamically the most preferable. Hence, it is proposed that the most likely reaction
pathway in the network is propane → 1-propyl → 1-propylidene → 1-propylidyne. However,
it is necessary to note that both 1-propyl and 1-propylidene are both less stable and have higher
activation barriers with respect to 2-propyl and propylene. Still, the activation barriers of the
first elementary steps are more similar than those for deep dehydrogenation, indicating that the
most likely reaction is indeed the dehydrogenation towards 1-propylidyne. The activation
-125
-75
-25
25
75
125
Reactant Product
Propane (g)
Propane, phys 1-propyl + H*
Propylene,phys + 2H*
Propylene +2H*
Propylene (g) + 2H*
TS1
TS5
Propylene (g) + H2(g)
1-propylidyne + 3/2 H2(g)
1-propylidyne + 3H*
TS4
1-propylidene +2H*
TS17
92 Chapter 4: Propane dehydrogenation kinetics on Pt(111) catalyst model
energies for the first elementary step are similar, however for the second step, deep
dehydrogenation (45 kJ/mol) is less activated than desorption (135 kJ/mol).
However, it is important to note that the solely the enthalpic contributions are accounted for.
Otherwise, solely 1-propylidyne would be formed and would rapidly deactivate the catalyst. To
include the entropic contributions, a Gibbs free energy diagram for the considered reactions
path is constructed. The Gibbs free energy is determined at 900 K, and the entropy and enthalpy
for each intermediates and transition state is calculated based on partition functions and
vibrational analysis.
Figure 4-11: Gibbs free energy diagram at 900 K for the propane dehydrogenation towards propylene (···) and propane deep dehydrogenation to 1-propylidyne ( ̶ ̶ ) on Pt(111). The energies are determined relative to gaseous propane.
The entropic contribution causes the first step to have the highest activation energy, as the
entropy loss is the greatest on adsorption of gaseous species. On the surface, the intermediates
are less influenced by the entropic changes. However, in contrast with the electronic energy,
the desorption of propylene is favored with respect to dehydrogenation of 1-propylidyne as
during desorption, propylene obtains a higher degree of freedom. At the final step, all hydrogens
-75
-25
25
75
125
175
Gib
bs fr
ee e
nerg
y (k
J/m
ol)
Propane (g)
Propane, phys
1-propyl + H*
Propylene,phys + 2H*
Propylene +2H*
Propylene (g)+ 2H*
TS1
TS5
1-propylidene *H2+
TS17TS4
1-propylidyne + 3H*
Reactant Product
1-propylidyne + 3/2 H2 (g)
Propylene + H2 (g)
Chapter 4: Propane dehydrogenation kinetics on Pt(111) catalyst model 93
desorb and it is observed that the difference in Gibbs free energy is 11 kJ/mol in favor of
gaseous propylene.
4.4.3 Hydrogenolysis of C3 intermediates
Hydrogenolysis reactions lead to smaller hydrocarbons, which form either surface cokes or
gaseous side products. Determining the kinetic parameters of these reactions is essential to
describe propylene selectivity with respect to other gaseous products. However, the dissociation
reactions included in this reaction network are endothermic except for the dissociation of 1-
propylidene and 2-propylidene. The breaking of C-C leads to higher reactive barriers than for
C-H bond breaking. Wang et al. propose that the reaction barrier depends not only on the
thermodynamics, but also additionally on the bond polarity. Under similar conditions, breaking
of C-H bonds occurs before C-C bonds, as the difference in bond polarity between C-C and C-
H enhance the breaking of C-H bonds. [49]
Table 4-19. Comparison of the electronic activation energies between DFT studies for hydrogenolysis of C3-intermediates. All considered reactions are dissociation reactions. Yang et al. used a PBE functional, while this work uses an optPBE vdW-DF functional.
Δ‡E (kJ/mol) Yang et al.
(0.25 ML) [4]
This work
(0.13 ML) Surface reaction
3 Propane,phys→ methyl + ethyl 235 187
6 1-propyl → methylidene + ethyl 163 168†
9 2-propyl → methyl + ethylidene 175 185
16 Propylene → methylidene + ethylidene 193 214
18 1-propylidene → methylidyne + ethyl 114 119
19 2-propylidene → methyl + ethylidyne 126 138
- 1-propylidyne → C + ethyl 184 - † The optimized transition state has one additional imaginary frequency.
For all reactions, except propane dissociation, an increase of electronic activation energy is
observed. The propane dissociation differs so much because physisorbed propane is used a
reference species. This species is much more stabilized in this work, with respect to Yang et al.
because the vdw-DF functional has been applied. However, similar results are obtained if the
gaseous propane is used as reference. This means that the reaction energy for propane
physisorption, as reported in Table 4-11, needs to be subtracted. This results in an electronic
activation energy of 237 kJ/mol and 230 kJ/mol, respectively for Yang et al. and this work. The
dissociation of propylene is unlikely as both reaction energy as its barrier is the highest, while
94 Chapter 4: Propane dehydrogenation kinetics on Pt(111) catalyst model
other reactions such as dehydrogenation and desorption are more favorable. In addition, the
dissociation reaction barriers for 1-propylidene and 2-propylidene are higher than the barrier
for the dehydrogenation reactions of these species, while those have the lowest dissociation
reaction barriers. This indicates that cracking reactions are not likely to occur on flat Pt(111)
surfaces and these reactions solely occur with deeper dehydrogenated species or on other
adsorption sites, such as steps or edges.
4.4.3.1 Formation of gaseous side products
Gaseous side products such as ethylene, ethane and methane can be formed through reaction of
C1- and C2-hydrocarbons on the surface, as proposed in Figure 4-6. Various DFT studies have
reported the reaction barriers of these reactions. [20, 32, 47]
Table 4-20. Comparison between electronic activation energies for side product formation of different DFT studies. The employed functionals and coverages in literature are noted underneath the table. This work uses an optPBE vdW-DF functional.
Δ‡E (kJ/mol) Literature
[20, 32, 47]
This work
(0.13 ML) # Surface reaction
A1 Methyl + H → Methane,physisorbed Hydrogenation 69a 68
A2 Methane,physisorbed → Methane(g) Desorption 0 a 20
B1 Ethyl + H → Ethane,physisorbed Hydrogenation 98b 68
B2 Ethane,physisorbed → Ethane(g) Desorption - 31
C1 Ethyl → Ethylene + H Dehydrogenation 78b 93
C2 Ethylene → Ethylene,physisorbed Desorption 100c 90
C3 Ethylene,physisorbed →Ethylene(g) Desorption 35c 41 (a) A coverage of 0.11 ML is achieved and RPBE is used as functional. In addition, the ZPE is added to the reaction energy.
[47] (b) A coverage of 0.25 ML is achieved and RPBE is used as functional. [20] (c) A coverage of 0.11 ML is achieved and
PW91 is used as functional. [32]
The electronic activation energy for the hydrogenation of methyl is for both studies equivalent.
Furthermore, the description of the methane physisorption fails as van der Waals interactions
are excluded in literature. The electronic activation energy for methyl dehydrogenation is
smaller than the hydrogenation reaction of 1-propyl and 2-propyl. The electronic activation
energies of 1-propyl and 2-propyl hydrogenation are respectively 87 kJ/mol and 82 kJ/mol.
Furthermore, physisorbed methane is more stable than adsorbed methyl and hydrogen (-15
kJ/mol). Hence, it is expected that methyl hydrogenation occurs more frequently than methane
Chapter 4: Propane dehydrogenation kinetics on Pt(111) catalyst model 95
dehydrogenation. However, it should be noted that dehydrogenation is second order reaction
and depends on the hydrogen coverage.
The hydrogenation step of ethyl towards ethane and the consecutive desorption step are reported
together with one electronic activation energy, which represents the C-H formation as this step
is more activated than desorption. The electronic activation energy for ethyl hydrogenation is
lower than described in literature, while the ethyl dehydrogenation is less activated than
literature. However, it should be noted that in literature different functionals and coverage are
employed. To determine the kinetically favored reaction path, a reaction profile (see
Figure 4-12) of C2Hy-species on the surface is constructed based on equation (9) (see below).
EC2𝐻𝐻𝑦𝑦 = 𝐸𝐸C2𝐻𝐻𝑦𝑦/surface + (6 − 𝑥𝑥)(𝐸𝐸H/surface − 𝐸𝐸surface)
− (𝐸𝐸C2𝐻𝐻6(g) + 𝐸𝐸surface) (9)
Figure 4-12. Relative energy profile of ethane dehydrogenation towards ethylene on Pt(111). The electronic energy is calculated relative to gaseous ethane. The dehydrogenated hydrogens are optimized in separate unit cells.
Since both ethane and ethylene are not supplied as feed, the only way to enter this mechanism
is through the formation of ethyl through cracking. In this network, ethyl dehydrogenation
towards ethylene competes with ethyl hydrogenation towards ethane. On the surface, di-σ
adsorbed ethylene is the thermodynamic product as it is more stable than physisorbed ethane.
However, physisorbed ethane is the kinetically preferred product as it is kinetically favored
-60
-40
-20
0
20
40
60
80
100
Rel
ativ
e en
errg
y (k
J/m
ol)
Ethane (g)
Ethane,physEthyl + H*
Ethylene + 2H*
Ethylene + 2H*
Ethylene (g) + 2H*
TSB1
TSC1
Reactant AlkeneAlkane
96 Chapter 4: Propane dehydrogenation kinetics on Pt(111) catalyst model
with respect to adsorbed ethylene. This energy profile accounts solely for enthalpic
contributions and for the formation of gaseous species, entropy plays a major role as driving
force. To determine the most probable gaseous side product, the Gibbs free energy is
determined for gaseous ethane and gaseous ethylene plus gaseous H2, to satisfy the conservation
laws. The selected temperature is 900 K, which is a typical reaction temperature for propane
dehydrogenation [46]. At this temperature, gaseous ethane has a lower Gibbs free energy than
ethylene and gaseous H2. The Gibbs free energy difference is 10 kJ/mol. This indicates that
gaseous ethane is as the thermodynamically preferred C2 side product. However, it is noted that
the hydrogenation step also depends on hydrogen coverage, which is rather low in most
dehydrogenation processes.
To determine the Gibbs free energy driving force for the formation of small gaseous
hydrocarbons from gaseous propane, the Gibbs free energy of gaseous methane and ethane (as
preferential gaseous C2 hydrocarbon) at 900 K is compared with gaseous propane. However, to
satisfy the carbon and hydrogen balance, the Gibbs free energy of gaseous ethane and methane
are calculated together with respect to gaseous propane and gaseous H2. Gaseous methane and
ethane are together thermodynamically more stable as the resulted Gibbs free energy difference
is -17 kJ/mol. This indicates that these species will be formed despite the high reaction barrier
for propane hydrogenolysis.
As gaseous propylene is the desired product, the Gibbs free energy of this species should be
lower than those of the most probable gaseous side products ethane and methane. The Gibbs
free energy of gaseous propylene and H2 is 59 kJ/mol more stable than gaseous propane, as
illustrated in Figure 4-11. As the Gibbs free energy is lower for gaseous propylene than gaseous
methane and ethane, the formation of gaseous propylene is favored.
4.5 Microkinetic modelling
In this section, a microkinetic simulation of propane dehydrogenation will be performed. First,
all kinetic and thermodynamic parameters are calculated based on the electronic geometry and
transition state optimizations. Furthermore, the kinetic model is constructed based on the
elementary steps in the reaction network of Pt(111) as discussed previously, see section 4.3
(Figure 4-4 and Figure 4-6). Finally, the model is implemented in a computer code and
simulations with the constructed kinetic model are performed. In literature, Sui et al. have also
Chapter 4: Propane dehydrogenation kinetics on Pt(111) catalyst model 97
performed microkinetic simulations on a similar Pt catalyst surface. [50] However, they solely
focused on the dominant dehydrogenation path towards propylene while in this work all side
reactions and deep dehydrogenation reactions have also been considered.
4.5.1 Determination of the thermodynamic and kinetic parameters
Vibrational frequencies are necessary in order to calculate thermodynamic and kinetic
properties at finite temperatures. Vibrational analyses have been performed, as mentioned
before, on strictly optimized geometries to avoid spurious imaginary frequencies.
After the calculation of vibrational frequencies, vibrational partition functions, zero-point
energies and thermal corrections to the enthalpy and entropy can be derived from statistical
thermodynamic equations. [51] From these values, kinetic and thermodynamic parameters are
calculated as function of temperature.
In this work, all species are assumed to be immobile on the surface, which is mainly for
adsorbed hydrogen and the physisorbed species a strong approximation at the high propane
dehydrogenation temperature (~873 K). Consequently, for surface species only vibrational
contributions have been taken into account. Free rotation and translation have been considered
explicitly only for gas phase species. All thermodynamic and kinetic parameters (pre-
exponential A and activation energy Ea) have been determined at a temperature of 900 K. The
actual values of the calculated parameters can be found in Appendix C.
4.5.2 Construction of the microkinetic and reactor model
A microkinetic model can be constructed using adsorption and desorption steps and the
elementary reaction steps (dehydrogenation, dissociation and isomerization) on the surface,
based on the kinetics and thermodynamics calculated from DFT. Adsorption rate coefficients
are described as a product of the incident molecular flux, F, and the initial sticking coefficient
s0 (see equation 10). It is assumed that adsorption is non-activated for all species, and
consequently the sticking coefficient is approximately equal to that on a clean surface. This is
considered to be equal to one for all adsorbates. The rate coefficients for desorption are obtained
using the thermodynamic equilibrium coefficient as determined by the DFT calculations (see
equation 11).
𝑘𝑘𝑎𝑎𝑝𝑝𝑝𝑝 =𝑠𝑠0 ∙ 𝐹𝐹𝑝𝑝
=𝑠𝑠0
𝑛𝑛𝑝𝑝�2𝜋𝜋 ∙ 𝑚𝑚 ∙ 𝑘𝑘𝐵𝐵 ∙ 𝑇𝑇 (10)
98 Chapter 4: Propane dehydrogenation kinetics on Pt(111) catalyst model
𝑘𝑘𝑝𝑝𝑑𝑑𝑝𝑝 =
𝑘𝑘𝑎𝑎𝑝𝑝𝑝𝑝𝐾𝐾𝑑𝑑𝑒𝑒
(11)
In equation 10, F is the incident flux, p is the pressure in bar, nt is the number of active sites per
m2 (equal to 1.66×1019 m-2), m is the molecular mass of the adsorbate molecule in kg, kB is the
Boltzmann constant and T is the temperature.
An Arrhenius expression is used to describe the surface rate coefficients (k in s-1), using the
activation energies (Ea) and pre-exponential factors (A):
𝑘𝑘 = 𝐴𝐴 exp �-𝐸𝐸𝑎𝑎𝑅𝑅𝑇𝑇
� (12)
The microkinetic simulation will be performed using an in-house developed python code. First,
the reaction rates of all elementary reaction steps are written using basic rate laws. Production
rates of all intermediate species are also formulated. Furthermore, a site balance is constructed
to make sure that the sum of all fractional surface coverages amounts to one. Finally, the
turnover frequency for all gaseous species is calculated. The turnover frequency (TOF) of all
gaseous species is obtained as the molecules consumed/formed per active site per second (s-1).
For example, the production rate of the surface intermediate 2-propylidene (CH3)2C=Pt2 (s-1) is
given by:
𝑅𝑅(CH3)2C=Pt2 =d𝜃𝜃(CH3)2C=Pt2
d𝑡𝑡= 𝑘𝑘for,8𝜃𝜃2-propyl𝜃𝜃free − 𝑘𝑘for,12𝜃𝜃(CH3)2C=Pt2
− 𝑘𝑘for,19𝜃𝜃(CH3)2C=Pt2𝜃𝜃free − 𝑘𝑘for,21𝜃𝜃(CH3)2C=Pt2𝜃𝜃free
(13)
For reactants and products, the production rate is increased with an adsorption minus a
desorption term. The site balance comprises all surface species:
𝜃𝜃free + ��𝜃𝜃CiHj
8
𝑗𝑗=1
3
𝑖𝑖=0
= 1 (14)
The pseudo-steady-state surface concentrations are obtained transiently by solving the
differential equations of equation (13), using the LSODA integration routines from the
ODEPACK Fortran library, as implemented in the Python package SciPy.
4.5.3 Results of the simulation
In this section the simulation results of the microkinetic model will be discussed. First, the time
of the simulation is varied in order to study the different regimes of the propane
Chapter 4: Propane dehydrogenation kinetics on Pt(111) catalyst model 99
dehydrogenation reaction. The most interesting regime will be selected for further discussion
on the effect of varying temperature and pressures.
4.5.3.1 Effect of varying the simulated time
Three distinct simulated times will be discussed: one simulation over a very short integration
time interval of 0.1 seconds, one over an intermediate time of 10 seconds and one over a very
long time of 109 seconds. All simulations have been performed at a fixed reactant pressure (H2:
0.1 bar and C3H8: 0.3 bar) and fixed temperature (typical dehydrogenation temperature of 873
K) without an initial product pressure. These reactant pressures have been chosen based on
literature for typical reaction conditions for a comparable Pt catalyst. [52]
4.5.3.1.1 Simulated time of 0.1 seconds
In first instance, the shortest integration time of 0.1 seconds is considered (with intermediate
results printed out every 0.001 seconds). By selecting this short time scale it is possible to
capture the transient behavior of the reaction as can also be observed in a TAP reactor
(Temporal Analysis of Products).
The result for the turnover frequency (TOF) can be found in Figure 4-13 (blue). The TOF is the
highest for this small simulated time. This indicates that there is a high initial activity on the
Pt(111) catalyst.
Figure 4-13. Turnover frequency (TOF) in s-1 of H2 and C3H6 for different integrated times. Blue: 0.1s (left axis), red: 10s (left axis), green: 109s (right axis).
0.0E+00
2.0E-09
4.0E-09
6.0E-09
8.0E-09
1.0E-08
1.2E-08
1.4E-08
950
1000
1050
1100
1150
1200
H2 C3H6
TOF
[s-1
] for
109
sim
ulat
ion
tim
e onl
y
TOF
[s-1
]
Species
0.1 s 0.1 s
10 s
0.1 s 0.1 s
10 s
109 s
109 s
100 Chapter 4: Propane dehydrogenation kinetics on Pt(111) catalyst model
Furthermore, the coverage of the catalyst surface can also be examined, see Figure 4-14 (blue).
For all simulations it is assumed that the initial coverage on the surface equals zero for all
species. 1-propylidyne is dominantly present on the surface for the short simulated time. Only
a very small amount of hydrogen covers the surface. The whole surface is not yet covered
(approximately 80 %) and still a large amount of free sites is available on the surface.
Figure 4-14. Coverage of the surface for different simulated times. The last column gives the total occupancy of the catalytic surface. Blue: 0.1s, red: 10s, green: 109s. The coverage of all other species on the surface is equal to zero. For all simulations it is assumed that the initial coverage on the surface equals zero for all species.
The fact that there is still a large amount of free sites on the surface ensures that the activity of
the catalyst is maintained in this regime (high TOF of C3H6) . Furthermore, it is noticed that
1-propylidyne is dominant on the surface. This could already be guessed based on the kinetic
parameters (see Appendix C2). Reaction 17 (the reaction that leads to the formation of
1-propylidyne) has the smallest activation energy of all elementary steps in the reaction network
(Ea of 14.9 kJ/mol). For this reason, 1-propylidyne is kinetically favored and is formed in the
beginning of the reaction, i.e. in the short-time integration regime.
4.5.3.1.2 Simulated time of 10 seconds
For the second regime, an average integration time of 10 seconds (with intermediate results
printed out every second) has been selected. The selection of this time scale makes it possible
to look at phenomena that occur past the initial transient regime while still maintaining high
catalytic activity.
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
H CH3CH-Pt2 1-propylidyne CH-Pt3 CH3C-Pt3 Occupied
Perc
enta
ge o
f the
surf
ace
Species
0.1 s10 s1000000000 s
Chapter 4: Propane dehydrogenation kinetics on Pt(111) catalyst model 101
The TOF (red, see Figure 4-13) is only smaller by 12 % than compared to the short-time
integration regime of 0.1s, but is still high enough to have sufficient conversion of propane to
propylene. The coverages (red, see Figure 4-14) show another interesting feature. The coverage
of 1-propylidyne has decreased compared to the short-time integration regime and small
coverages of CH≡Pt3 and CH3C≡Pt3 are observed. It is proposed that 1-propylidyne, although
it is the kinetically favored deeply dehydrogenated species, is converted into these
thermodynamically very stable C1 and C2 species. This is confirmed, for e.g. CH≡Pt3, by the
net reaction rates of the interesting elementary steps, see Table 4-21.
Table 4-21. Net reaction rates for reaction 17 and reaction 18 (see Figure 4-4) for different simulated times. Reaction 17: 1-propylidene → 1-propylidyne + H-Pt, reaction 18: 1-propylidene → CH≡Pt3 + CH3CH2-Pt.
Simulated time 0.1 s 10 s
net17 [s-1] -0.00619 -0.00535 net18 [s-1] 0.00472 0.00408
The net reaction rate of reaction 17 is negative, which means that 1-propylidyne is converted
back to 1-propylidene and part of this species is converted to CH≡Pt3 via reaction 18. At a
simulated time of 10s this effect is clearly visible in the coverages (red, see Figure 4-14). The
Gibbs free energies of CH≡Pt3 + CH3CH2-Pt and 1-propylidyne + H-Pt are rather similar (see
Table 4-22), but the thermodynamic driving force for CH≡Pt3 + CH3CH2-Pt is larger because
CH3CH2-Pt has the ability to react further via reaction B or reaction C (see Figure 4-4) and
eventually to desorb as respectively gaseous ethane or ethylene. Furthermore, it is also possible
that CH3CH2-Pt undergoes a C-C scission reaction to form thermodynamically very stable C1
species. However, these reactions have not been taken into account yet and will be considered
for future work.
Table 4-22. Gibbs free energy of 1-propylidyne + H-Pt and CH≡Pt3 + CH3CH2-Pt at 900 K (electronic energy + thermal contributions + entropy).
Gibbs free energy (900 K)
kJ/mol 1-propylidyne + H-Pt -23430 CH≡Pt3 + CH3CH2-Pt -23364
It is proposed that, in terms of deactivation, 1-propylidyne is the ‘fast’ coke precursor, while
CH≡Pt3 and CH3C≡Pt3 species are ‘slow’ coke precursors. This interesting result could not have
102 Chapter 4: Propane dehydrogenation kinetics on Pt(111) catalyst model
been obtained by solely looking at reaction energies and barriers and this stresses the
importance of microkinetic simulations.
4.5.3.1.3 Simulated time of 109 seconds
Finally, a very long integration time of 109 seconds (approximately 32 years, with intermediate
results printed out every 1000 seconds) has been selected. By selecting this long time scale it is
possible to further study the deactivation of the catalyst as it was pointed out in the previous
section that deactivation has some interesting features.
From Figure 4-13 (green, right axis) it is clear that the activity of the catalyst after 109 seconds
has deteriorated substantially. The TOF of C3H6 has decreased by 12 orders of magnitude
compared to the situation after 0.1 seconds integration. This is confirmed by looking at the
surface coverage (see Figure 4-14, green): no free sites are available for reaction (total
occupancy of 100 %).
The hypothesis that the initial high amount of 1-propylidyne is converted into CH≡Pt3 and
CH3C≡Pt3 species is confirmed by looking at the coverages. There is no 1-propylidyne left on
the surface and the whole surface is blocked by the C1 and C2 species. At a simulated time of
109 seconds the coverages have reached a quasi steady-state regime since all coverages have
converged except for those of CH≡Pt3 and CH3C≡Pt3 species, but these only increase slightly
since the coke formation at this point has already reached 99.99 %.
Consequently, the 109 seconds time regime is not very interesting because activity is very low
and all free sites are blocked. For further study, the equations will not be solved until (quasi)
steady-state has been reached and only the most interesting time scale regime will be regarded.
Steady state can only be reached after a very long integration time (i.e. >32 years) and it is not
very useful to go to these long time scales. In order to investigate the effect of varying
temperature and pressures, the 10 seconds simulated time will be implemented since in this
regime activity is still high, but also a substantial amount of 1-propylidyne has already been
converted to CH≡Pt3 and CH3C≡Pt3.
4.5.3.2 Effect of varying temperature
All simulations have been performed at a fixed reactant pressure (H2: 0.1 bar and C3H8: 0.3 bar).
Again, these reactant pressures have been chosen based on literature for typical reaction
conditions for a comparable Pt catalyst. [52] The temperature is varied between 400 °C and 700
Chapter 4: Propane dehydrogenation kinetics on Pt(111) catalyst model 103
°C with an interval of 25 °C (typical dehydrogenation temperature is 600 °C or 873 K). The
simulated time has been kept fixed at 10 seconds. The resulting TOFs can be found in
Figure 4-15 and Figure 4-16.
Figure 4-15. TOF of H2 and C3H6 in s-1 as function of temperature in °C for a simulated time of 10 seconds. The reactant pressures are fixed (C3H8/H2: 0.3/0.1 bar).
Figure 4-16. TOF of CH2, C2H4 and C2H6 in s-1 as function of temperature in °C for a simulated time of 10 seconds. The reactant pressures are fixed (C3H8/H2: 0.3/0.1 bar).
The TOF increases for C3H6 and H2 with increasing temperature (Figure 4-15). Propane
dehydrogenation is an endothermic reaction so this is the expected behavior. The highest TOF
is obtained at a temperature of approximately 675 °C. At higher temperatures, deep
dehydrogenation barriers can be overcome and the TOF decreases again. Also, desorption can
0
200
400
600
800
1000
1200
1400
1600
1800
2000
400 450 500 550 600 650 700
TOF
[s-1
]
T [°C]
C3H6
H2
0
0.002
0.004
0.006
0.008
0.01
0.012
0.014
0.016
400 450 500 550 600 650 700
TOF
[s-1
]
T [°C]
CH4C2H4C2H6
104 Chapter 4: Propane dehydrogenation kinetics on Pt(111) catalyst model
occur more easily at these high temperatures, leading to more free sites on the surface. Both
TOFs overlap because hydrogen and propylene are formed together in a 1:1 ratio for the
propane dehydrogenation reaction when the amount of side reactions is still limited.
However, at the temperature of 675 °C, the selectivity of the catalyst becomes worse. This can
be seen from Figure 4-16 where at this temperature the relative TOF of methane and ethylene
compared to the TOF of C3H6 becomes higher. However, the TOF of ethane remains unaltered
with increasing temperature. This is because at these conditions dehydrogenation reactions,
such as elementary reaction C, are preferred compared to hydrogenation reactions, such as
elementary reaction B. This is also clearly seen in the net reaction rates, see Table 4-23. At high
temperatures (600 and 700 °C) the net reaction rate of reaction C is higher than reaction B.
However, at low temperatures (400 °C) this is no longer the case. Furthermore, at low
temperatures, the C-C scission reactions (e.g. elementary step 3) do not occur on the surface
due to a high activation barrier.
Table 4-23. Net reaction rates of the elementary reactions (see Figure 4-4) on Pt(111) for fixed reactant pressure (H2: 0.1 bar and C3H8: 0.3 bar) and varying temperature.
Temperature [°C] net rate in s-1 400 °C 600 °C 700 °C net1 2.7485 528.8687 836.0576 net2 4.3533 501.4381 659.6029 net3 0.0000 0.0001 0.0009 net4 0.3040 69.8104 116.4396 net5 2.4444 459.0585 719.6181 net6 0.0000 0.0000 0.0000 net7 4.1487 467.7158 609.5194 net8 0.2046 33.7220 50.0828 net9 0.0000 0.0001 0.0005 net10 0.0000 -0.0001 -0.0001 net11 0.0000 0.0073 0.0335 net12 0.0000 0.0000 0.0000 net14 -0.3040 -69.8043 -116.4092 net15 -0.2046 -33.7194 -50.0703 net16 0.0000 0.0000 0.0000 net17 0.0000 -0.0053 -0.0169 net18 0.0000 0.0041 0.0138 net19 0.0000 0.0026 0.0125 net20 0.3040 69.8043 116.4092 net21 0.2046 33.7194 50.0703 netA 0.0000 0.0028 0.0140 netB 0.0000 0.0000 0.0000 netC 0.0000 0.0042 0.0147
Chapter 4: Propane dehydrogenation kinetics on Pt(111) catalyst model 105
Table 4-23 also makes clear that at high temperature (700 °C) the deep dehydrogenation
reactions become favored, e.g. reaction 18 and 19 relatively to reaction 1, which results in a
lower TOF for C3H6.
From Table 4-23 it is also observed that both reaction pathways via 1- and 2-propyl towards
propylene are followed and both are the important paths toward propylene formation. There is
no clear distinction between the two paths, which was also assumed in previous section
(see 4.4.1).
It is also interesting to study the coverage of the surface at different temperatures, see
Figure 4-17.
Figure 4-17. Coverage of the Pt(111) surface (fraction) of species at different temperatures. Blue: 400 °C, red: 600 °C and green: 700 °C.
It is again confirmed that at the very high temperature (700 °C) side reactions and deep
dehydrogenation reactions are favored and the conversion of 1-propylidyne to
thermodynamically more stable C1 and C2 species is earlier initiated. Furthermore, due to easier
desorption, more free sites become available.
4.5.3.3 Effect of varying propylene pressure
All simulations have been performed at a fixed reactant pressure (H2: 0.1 bar and C3H8: 0.3 bar)
and fixed temperature (600 °C). The simulated time has been kept fixed at 10 seconds. The
C3H6 product pressure has been varied from 0 to 1 bar in order to evaluate the effect of product
pressure on the TOF, with steps of 0.1 bar. The resulting TOFs can be found in Figure 4-18 and
Figure 4-19.
0.00.10.20.30.40.50.60.70.80.91.0
H CH3CH-Pt2 1-propylidyne CH-Pt3 CH3C-Pt3 Occupied
Frac
tion
of th
e sur
face
Species
400 °C600 °C700 °C
106 Chapter 4: Propane dehydrogenation kinetics on Pt(111) catalyst model
Figure 4-18. TOF of H2 and C3H6 in s-1 as function of C3H6 pressure in bar for a simulated time of 10 seconds. The reactant pressures and temperature are fixed (H2: 0.1 bar and C3H8: 0.3 bar, 600 °C).
Figure 4-19. TOF of CH2, C2H4 and C2H6 in s-1 as function of C3H6 pressure in bar for a simulated time of 10 seconds. The reactant pressures and temperature are fixed (H2: 0.1 bar and C3H8: 0.3 bar, 600 °C).
It can immediately be noticed that changing the pressure of C3H6 does not have a large influence
on the TOFs. Even at the highest evaluated pressure of C3H6 (1 bar), which is over three times
the pressure of propane, the TOF of C3H6 and H2 only decrease by 10 % compared to the 0 bar
C3H6 case. Also the TOFs of methane, ethylene and ethane remain unaffected. This justifies the
fact that no reactor model equations have been taken into account in the simulation, since
920
940
960
980
1000
1020
1040
1060
0.0 0.2 0.4 0.6 0.8 1.0
TOF
[s-1
]
p_C3H6 [bar]
C3H6H2
0
0.0005
0.001
0.0015
0.002
0.0025
0.003
0.0035
0.004
0.0045
0.0 0.2 0.4 0.6 0.8 1.0
TOF
[s-1
]
p_C3H6 [bar]
CH4C2H4C2H6
Chapter 4: Propane dehydrogenation kinetics on Pt(111) catalyst model 107
accumulation of product pressures in the gas phase does not significantly change the overall
turnover rate.
It is also remarkable that the inverse reaction, i.e. propylene hydrogenation, does not occur at
very high pressures of C3H6. However, hydrogenation only occurs at high pressures of both
C3H6 and H2 and at lower temperatures. It was validated that the applied kinetic model predicts
propylene hydrogenation with a simulated TOF of 0.05 s-1 for a pressure of 0.3 bar C3H8, 3 bar
H2, 3 bar C3H6 and a temperature of 300 °C.
The coverages and net rates remain mainly unaffected by a change in C3H6 pressure. The
dominant reaction path (via 1- and 2-propyl) is not changed with increasing C3H6 pressure since
the net rates scale with the TOF. The net rates of the elementary reactions leading to propylene,
i.e. reactions 1, 2, 5 and 7, are lower at high pressure of C3H6 (1 bar) compared to the 0 bar
case. This is also reflected in the values of the TOFs (Figure 4-18).
4.5.3.4 Effect of varying side product pressure
All simulations have been performed at a fixed reactant pressure (H2: 0.1 bar and C3H8: 0.3 bar)
and fixed temperature (600 °C). The simulated time has been kept fixed at 10 seconds. The
CH4, C2H4 and C2H6 side product pressures have been varied from 0 to 1 bar with steps of 0.2
bar. All pressures are simultaneously increased by the same amount as a general check on the
effect of the side products pressures on the TOF. The resulting TOFs can be found in
Figure 4-20 and Figure 4-21.
108 Chapter 4: Propane dehydrogenation kinetics on Pt(111) catalyst model
Figure 4-20. TOF of H2 and C3H6 in s-1 as function of side products pressure (CH4, C2H4 and C2H6) in bar for a simulated time of 10 seconds. The reactant pressures and temperature are fixed (H2: 0.1 bar and C3H8: 0.3 bar, 600 °C).
Figure 4-21. TOF of CH2, C2H4 and C2H6 in s-1 as function of side products pressure (CH4, C2H4 and C2H6) in bar for a simulated time of 10 seconds. The reactant pressures and temperature are fixed (H2: 0.1 bar and C3H8: 0.3 bar, 600 °C).
Changing the pressure of the side product does not have a large influence on the TOFs of C3H6
and H2. They only change in a range of 0.5 %, which is practically negligible.
There are some interesting features to see for the TOFs of CH4, C2H4 and C2H6. First of all, it
is noticed that the TOF of CH4 remains unaffected, which is surprising since the CH4 pressure
is raised substantially. However, it is well-known that methane activation is difficult, as also
1040
1040.5
1041
1041.5
1042
1042.5
1043
1043.5
1044
1044.5
1045
0.0 0.2 0.4 0.6 0.8 1.0
TOF
[s-1
]
p_SideProduct [bar]
C3H6H2
-0.15
-0.1
-0.05
0
0.05
0.1
0.15
0.0 0.2 0.4 0.6 0.8 1.0
TOF
[s-1
]
p_SideProduct [bar]
CH4C2H4C2H6
Chapter 4: Propane dehydrogenation kinetics on Pt(111) catalyst model 109
stated in literature by Vines et al., and this is also observed here. [47] Furthermore, it is also
surprising that with increasing C2H4/C2H6 pressure, the TOF of C2H4 increases. This is due to
the conversion of ethane into ethylene (i.e. ethane dehydrogenation), which is also confirmed
by the net rates at 1 bar side product pressure. At high side product pressure of 1 bar the net
reaction rate of reaction B (ethane formation from ethyl) equals -0.1157 s-1, while the net
reaction rate of reaction C (ethylene formation from ethyl) equals 0.1193 s-1. This confirms that
ethane adsorbs and subsequently dehydrogenates to form ethylene.
4.6 Conclusions
In this chapter, the propane dehydrogenation characteristics on pure Pt catalysts are
investigated. The abundant Pt(111) phase is selected to represent the pure Pt catalyst. A 4×2
unit cell is constructed for the DFT calculations on the Pt(111) surface. This catalyst model is
used as reference for more advanced catalyst models later in this work. The proposed reaction
network is divided in three sections: propane dehydrogenation to propylene, deep
dehydrogenation of propylene and hydrogenolysis of C3 intermediates. The first section consists
of elementary steps towards propylene and other C3H6-species. Furthermore, this first section
is employed to quantify the catalyst activity of pure platinum. In the second section, the deep
dehydrogenation towards coke precursors is investigated. The selectivity towards these species
is an indication for the catalyst deactivation. In the third section, the hydrogenolysis reactions
are taken into account. These reactions form C1 and C2 adsorbates on the surface. These species
eventually lead to gaseous side products such as methane, ethane and ethylene and affect the
selectivity towards propylene.
Both the thermodynamics and kinetics of propane dehydrogenation are evaluated for the
considered reactions. The propane dehydrogenation towards propylene occurs both via 1-propyl
and via 2-propyl, as the Gibbs free energies at 873 K of these species are similar. However the
reaction path via 2-propyl is preferred as it has a lower reaction barrier with respect to 1-propyl.
Both 1-propyl and 2-propyl dehydrogenate preferentially further to propylene, as it is the
thermodynamically most stable C3H6-species. The resulted electronic activation energy is
similar for both reactions.
Thermodynamically, 1-propylidyne is the most stable species on the Pt(111) surface, so it is
expected that the most likely reaction will be its formation reaction. Multiple reaction paths
110 Chapter 4: Propane dehydrogenation kinetics on Pt(111) catalyst model
towards 1-propylidyne are possible if isomerization reactions are included. However, it is
shown that the isomerization reactions are highly activated and unlikely to occur. The most
probable path is via propane →1-propyl →1-propylidene →1-propylidyne. However, this is
solely based on the electronic energy. The entropic contributions are included by determining
the Gibbs free energy of the intermediates and transition states. The entropic contributions are
greater for gaseous species than for adsorbates, so based on the Gibbs free diagram, gaseous
propylene is 11 kJ/mol more stable than 1-propylidyne.
Hydrogenolysis reactions compete with the dehydrogenation reactions and C-C bond breaking
leads to the formation of less stable products with respect to dehydrogenated species, except
for the cracking of 2-propylidene. However, cracking of 2-propylidene and other
hydrogenolysis reactions are higher activated than the competing dehydrogenation reactions.
Therefore, it can be concluded that these reactions are unlikely to occur for the considered
species on the Pt(111) surface.
The microkinetic simulation of propane dehydrogenation on Pt(111) is initially performed at
873 K and total reactant pressure of 0.4 bar (C3H8/H2: 3/1), while varying the simulated time
(0.1 s, 10 s and 109 s). At 0.1 s, the TOF for propylene and hydrogen gas is the highest
(1191 s-1) and the surface is 80 % covered with adsorbates, dominantly 1-propylidyne. At
intermediate simulated time (10 s), the TOF for propane dehydrogenation decreases with 12 %
with respect to 0.1 s simulation time. Furthermore, the coverage of 1-propylidyne starts to
decline and ethylidyne (CH3C≡Pt) and methylidyne (HC≡Pt) are formed. This indicates that 1-
propylidyne is kinetically preferred as adsorbates (fast coke precursors), while the other species
are thermodynamically favored (slow coke precursors). At the very high simulation time (109
s), which represents the deactivated catalyst regime, no catalyst activation for propane
dehydrogenation is reported and the surface is completely covered with slow precursors.
The intermediate simulated time of 10 s is further employed as it still shows high catalytic
activity, while 1-propylidyne is substantially converted to slow coke precursors. While varying
the simulation temperature, a maximum TOF for propylene dehydrogenation is observed at 948
K. However, at this temperature, formation of gaseous side products (methane and ethylene) is
also increased, lowering the selectivity towards gaseous propylene. In addition, the effect of
increasing initial propylene pressure is investigated. It is observed that for increasing propylene
pressure, the TOF for propylene decreases linearly, but the absolute effect is small as only a
10% decrease is reported at 1 bar pressure of propylene. Additionally, the effect of increasing
Chapter 4: Propane dehydrogenation kinetics on Pt(111) catalyst model 111
the pressure of all gaseous side products on the TOF for propylene is negligible. However, the
formation of gaseous ethylene increases as ethane is converted to ethylene on the catalyst
surface.
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30. Salmeron, M. and G.A. Somorjal, Desorption, decomposition, and deuterium-exchange reactions of unsaturated-hydrocarbons (ethylene, acetylene, propylene, and butenes) on the Pt(111) crystal-face. Journal of Physical Chemistry, 1982. 86(3): p. 341-350.
31. McCrea, K.R. and G.A. Somorjai, SFG-surface vibrational spectroscopy studies of structure sensitivity and insensitivity in catalytic reactions: cyclohexene dehydrogenation and ethylene hydrogenation on Pt(111) and Pt(100) crystal surfaces. Journal of Molecular Catalysis a-Chemical, 2000. 163(1-2): p. 43-53.
32. Essen, J.M., et al., Adsorption of ethene on Pt(111) and ordered PtxSn/Pt(111) surface alloys: A comparative HREELS and DFT investigation. Surface Science, 2007. 601(16): p. 3472-3480.
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34. Papoian, G., J.K. Nørskov, and R. Hoffmann, A comparative theoretical study of the hydrogen, methyl, and ethyl chemisorption on the Pt (111) surface. Journal of the American Chemical Society, 2000. 122(17): p. 4129-4144.
35. Godbey, D.J. and G.A. Somorjai, The adsorption and desorption of hydrogen and carbon-monoxide on bimetallic Re-Pt(111) surfaces. Surface Science, 1988. 204(3): p. 301-318.
36. Lu, K.E. and R.R. Rye, Flash desorption and equilibration of H2 and D2 on single-crystal surfaces of platinum. Surface Science, 1974. 45(2): p. 677-695.
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37. Salmeron, M., R.J. Gale, and G.A. Somorjai, Modulated molecular-beam study of the mechanism of the H2-D2 exchange-reaction on Pt(111) and Pt(332) crystal-surfaces. Journal of Chemical Physics, 1979. 70(6): p. 2807-2818.
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40. Shen, J.Y., et al., Microcalorimetric, infrared spectroscopic, and DFT studies of ethylene adsorption on Pt/SiO(2) and Pt-Sn/SiO(2) catalysts. Journal of Physical Chemistry B, 1999. 103(19): p. 3923-3934.
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45. Koestner, R.J., et al., Evidence for the formation of stable alkylidyne structures from C-3 and C-4 unsaturated-hydrocarbons adsorbed on the Pt(111) single-crystal surface. Surface Science, 1982. 116(1): p. 85-103.
46. Siddiqi, G., et al., Catalyst performance of novel Pt/Mg(Ga)(Al)O catalysts for alkane dehydrogenation. Journal of Catalysis, 2010. 274(2): p. 200-206.
47. Vines, F., et al., Methane Activation by Platinum: Critical Role of Edge and Corner Sites of Metal Nanoparticles. Chemistry-a European Journal, 2010. 16(22): p. 6530-6539.
48. Pechukas, P., Transition State Theory. Annual Review of Physical Chemistry, 1981. 32(1): p. 159-177.
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52. Bariås, O.A., A. Holmen, and E.A. Blekkan, Propane Dehydrogenation over Supported Pt and Pt–Sn Catalysts: Catalyst Preparation, Characterization, and Activity Measurements. Journal of Catalysis, 1996. 158(1): p. 1-12.
Chapter 5: Propane dehydrogenation kinetics on Pt3Ga(111) as a model for Pt-Ga catalysts 114
Chapter 5 Propane dehydrogenation kinetics on Pt3Ga(111) as a model for Pt-Ga catalysts Pure platinum catalysts have drawbacks as they show low olefin selectivity and short lifetime due
to deactivation of the catalyst. The addition of metal modifiers to the Pt catalyst enhances the
catalytic properties. Various transition and non-transition metals are employed as alloy element
for pure platinum. In literature, PtxSny-catalysts are intensively researched, both experimentally
and theoretically, to obtain a fundamental understanding of the role of Sn as modifier on the
dehydrogenation of light alkanes and especially propane. [1-9] In this work, the role of Ga addition
on the catalytic properties during propane dehydrogenation is investigated. An innovative
synthesis method facilitates the preparation of Pt-Ga catalysts on a hydrotalcite support
(Mg(Ga)(Al)Ox). [10] Experimental results have shown that under the same reaction conditions,
an increase in activity and selectivity towards propylene are observed with respect to pure platinum
catalysts. [11] Still, Pt-Sn catalysts can be employed as guideline because gallium and tin are both
transition metal close to metal-nonmetal gap in the periodic table.
To construct the catalyst model for Pt-Ga catalysts, the active phase of the Pt-Ga/Mg(Ga)(Al)Ox
catalyst is determined based on DFT calculations of PtxGay catalysts. The selection of alloys is
based on the Ga content (max. 60 mol%) and their stability under both reaction and oxidizing
conditions. Additionally, the vibration modes of adsorbed CO are investigated to couple the
calculated shift on the various alloys with respect to the pure Pt catalyst with the observed
experimental shift of ~5 cm-1. The Pt3Ga(111) catalyst model resulted in best agreement with
experiments. This catalyst model is employed to determine the propane dehydrogenation kinetics
and evaluate the effects of Ga on the catalytic properties.
115 Chapter 5: Propane dehydrogenation kinetics on Pt3Ga(111) as a model for Pt-Ga catalysts
5.1 Catalyst model
Bulk Pt3Ga has a similar fcc-like crystal structure as Pt. Based on the similarity, the high
coordinated terrace plane Pt3Ga(111) is the most abundant plane as it is the most stable plane of
the fcc structure. [12] In this work, it was opted to select a non-segregated bulk alloy of Pt3Ga(111)
as catalyst model. Saerens et al. observed the lowest surface tension in a non-segregated model in
comparison with segregated and anti-segregated models. [13] The Pt(111) catalyst model from
Chapter 4 is employed as basis for the construction of the Pt3Ga(111) catalyst model. The same
4×2 unit cell is used to represent the Pt3Ga(111) catalyst model. For every four Pt atoms, a Ga
atom replaces one. The optimization is conducted with the same Monkhorst Pack grid (5×5×1) and
vacuum layer of 12 Å as in Chapter 4. Strict convergence criteria and the optPBE vdW-DF as
functional are selected. The optimized 4×2 unit cell is shown in Figure 5-1.
Figure 5-1: Isometric representation of the 4×2 unit cell of the Pt3Ga(111) catalyst model
The dimensions of the unit cell are 11.21×5.60×18.8 ų, which is slightly smaller than the Pt(111)
catalyst model because the atomic radius of Ga is smaller than of Pt. The lattice parameter for this
model is 3.96 Å, which is a small overestimation with the reported value of 3.90 Å in literature.
[12] However, the same discrepancy is observed with the Pt(111) catalyst model. It is expected
that the non-local vdW-DF functional slightly overestimates the lattice parameter of the catalyst
models.
c
a
b
Chapter 5: Propane dehydrogenation kinetics on Pt3Ga(111) as a model for Pt-Ga catalysts 116
5.1.1 Adsorption site nomenclature
Basically, the same four distinct adsorption sites are still possible (top, bridge, fcc, hcp) with
respect to the nomenclature of 4.1.1. However, the incorporation of Ga leads to multiple sites of
the same type. The characteristics of each site depend on the nearby atoms, so in the site naming,
the locations of gallium and platinum in the top (and if necessary second) layer are incorporated
in the name of site, similar to previous work. [13] The extended nomenclature is reported in Table
5-1, corresponding with the allocated adsorption sites, illustrated in Figure 5-2.
Figure 5-2: Adsorption sites on Pt3Ga(111) catalyst model: top ●, bridge █ , three-folded hcp ▲ and fcc ▼. The numbering of the sites corresponds to the site naming in Table 5-1.
Table 5-1. Adsorption site nomenclature for Pt3Ga(111) catalyst model.
Number Site Number Site Number Site
1 Pt-top 4 Pt2-bridge-Pt2Ga 7 Pt2Ga-fcc
2 Ga-top 5 Pt2-bridge-Pt3 8 Pt2Ga-hcp
3 PtGa-bridge-Pt3 6 Pt3-fcc 9 Pt3-hcp
Previous work has shown that hydrocarbon intermediates do not directly bond with gallium. This
results in a reduction of preferable adsorption sites per unit cell due the incorporation of Ga.
Furthermore, sites that exist of multiple atoms such as the hollow and bridge sites are more affected
by this Ga addition with respect to the single atom sites. In the context of propane dehydrogenation,
the geometrical effect of Ga will lead to less deeply dehydrogenated species as they bond on
multiple-atoms sites.
1 2
3 4
5 6 7
8 9 b
a
117 Chapter 5: Propane dehydrogenation kinetics on Pt3Ga(111) as a model for Pt-Ga catalysts
5.1.2 Determination of the degree of coverage
The degree of coverage as defined in 4.1.2, is used in this chapter as well. No distinction is made
between the Pt and Ga atoms in the surface layer. Therefore, the coverage is 0.13 ML in the case
of one adsorbate molecule.
5.2 Adsorption
In this section, the adsorption of the main gaseous reactants and products is discussed. These
compounds are propane, propylene and H2. Beside them, gaseous products such as ethane, ethylene
and methane are formed via cracking reaction of C3-species. However, these species are excluded.
The optimized geometries of the adsorbates can be found in Appendix D.
Most of the ab initio studies on modified Pt-based catalysts are conducted on Pt-Sn catalyst models
and none on Pt-Ga catalyst models. The Pt-Sn catalyst models for propane dehydrogenation are
used as guideline for Pt-Ga catalysts. The Pt3Sn catalyst model is the most interesting for this work
as it has the same alloy composition as the Pt3Ga catalyst model.
The adsorption energies are determined based on the equation formulated in 4.2.
5.2.1 Propane
The adsorption of propane is the first step of the reaction mechanism for propane dehydrogenation.
As a saturated compound, propane can adsorbed either via physisorption or dissociative
chemisorption. The first is the topic in this section as no internal bond breaking occurs in this
adsorption mode. Physisorbed propane acts as a precursor prior the dissociative chemisorption
step.
Conform the propane physisorption in Chapter 4, propane retains its gaseous structure. The C-C
bond lengths are 1.53 Å, which are identical to the bond lengths of gaseous propane. It is observed
that the physisorption height (shortest distance Pt-C) is 4.0 Å, which is 0.2 Å shorter with respect
to the observed height on Pt(111).
Nykänen et al. and Yang et al. observe that alloying with Sn increases the adsorption energies with
respect to pure Pt. A decrease of ~4 kJ/mol is calculated for both DFT studies, so Sn destabilizes
slightly the physisorbed species. Yang et al. also observe that the adsorption height increases due
the Sn impregnation. In contrast, the reverse is observed for Ga as modifier element. The
physisorption on Pt3Ga(111) is stabilized with 8 kJ/mol and, as previously mentioned, a shorter
Chapter 5: Propane dehydrogenation kinetics on Pt3Ga(111) as a model for Pt-Ga catalysts 118
adsorption height is found. These effects show that Ga addition stabilizes the physisorbed species
better than their Pt-Sn counterpart.
Table 5-2. Adsorption energies of propane on Pt(111), Pt3Sn (111) and Pt3Ga(111). Yang et al. and Nykänen et al. used a vdW-DF functional. This work uses an optPBE vdW-DF functional.
ΔEads
(kJ/mol) Nykänen et al. (0.13 ML) [4]
Nykänen et al. (0.25 ML) [4]
Yang et al. (0.08 ML) [7]
This work
(0.13 ML) Pt(111) -33 -36 -41 -43
Pt3Sn(111) -27 -33 -37 -
Pt3Ga(111) - - - -51
In this work, vibrational analysis of physisorbed propane resulted in two imaginary frequencies.
These are expected as physisorbed species have a higher degree of freedom with respect to
adsorbates. The vibration modes are assigned to translation and external rotation.
5.2.2 Propylene
As propylene is the main product, the description of propylene adsorption and desorption is
essential. At low propylene coverage (θ < 0.25 ML), di-σ propylene is the most stable adsorption
mode. Other adsorption modes are formed at higher propylene coverages. Π-adsorption mode is
an example of such a mode and is weakly bonded to the catalyst surface via its π-bond. [14]
In this work, solely the di-σ adsorption mode is investigated as this mode is more stable than π-
propylene. Conform 4.2.2.1, di-σ propylene loses its planar geometry upon adsorption and its C=C
bond is elongated from 1.34 Å in the gasphase to 1.49 Å adsorbed. This length is more similar to
C-C bond length of 1.53 Å. These effects indicate a covalent interaction between propylene and
the catalyst surface.
The adsorption energies of di-σ propylene are compared with the DFT studies of Nykänen et al.
and Yang et al, see Table 3. As discussed in Chapter 4, the discrepancy of propylene adsorption
energy on Pt(111) between this work and other studies can be based on the expected overbinding
of the employed optPBE vdW-DF functional. On Pt3Sn(111), propylene is less stabilized. The
resulted adsorption energies are increased with ~30 kJ/mol with respect to Pt(111). A deviation in
the energy difference is observed for the work of Yang et al. as both the Sn addition and different
coverages play a role. On Pt3Ga(111), propylene is slightly more stabilized. The absolute effect of
Ga (-4 kJ/mol) is smaller than the effect of Sn (30 kJ/mol)
119 Chapter 5: Propane dehydrogenation kinetics on Pt3Ga(111) as a model for Pt-Ga catalysts
Table 5-3. Adsorption energies of di-σ propylene on Pt(111), Pt3Sn (111) and Pt3Ga(111). Yang et al. and Nykänen et al. used respectively a PBE and vdW-DF functional. This work employs an optPBE vdW-DF functional.
ΔEads
(kJ/mol) Nykänen et al. (0.13 ML) [4]
Nykänen et al. (0.25 ML) [4] Yang et al. [7, 15] This work
(0.13 ML) Pt(111) -72 -80 -94 (0.25 ML) -131
Pt3Sn(111) -40 -52 -48 (0.08 ML) -
Pt3Ga(111) - - - -135
Beside di-σ adsorption mode, the physisorption of propylene above the catalyst surface is
investigated. This species is weakly bonded with the surface and is predominantly stabilized by
van der Waals interactions. The geometry of physisorbed propylene is similar on Pt(111) and
Pt3Ga(111), as it retains the structure of its gaseous form. However, a shorter adsorption height is
observed with respect to Pt(111), similar to propane physisorption. On Pt(111), the distance is 3.8
Å, while on Pt3Ga(111) the height is reduced to 3.3 Å.
The smaller adsorption height on Pt3Ga(111) indicates that propylene physisorbs stronger on the
Pt3Ga(111) model. This is confirmed by the calculated adsorption energies; on Pt3Ga(111) the
adsorption energy is -63 kJ/mol, while on Pt(111) a value of -43 is found for ΔadsE of physisorbed
propylene.
5.3 Thermodynamics
To describe propane dehydrogenation on Pt3Ga(111), the same reaction network as proposed in
4.3 is employed. However, some reactions are excluded as mainly the catalytic activity and
selectivity towards propylene with respect to deactivation reactions are focused on. Hence, the
main topics are the reaction pathways towards propylene and deep dehydrogenation of propylene
and other C3H6-species. Dissociation reactions (C-C scission reactions) are evaluated, initially
solely of propane as on Pt(111) these reactions are unfavorable. The reactions that lead to gaseous
side products such as ethane, ethylene and ethane are not considered
The considered species are listed in Table 5-4 and a streamlined version of the calculated reaction
network is shown in Figure 5-3. For each component, the most stable geometry on the Pt3Ga(111)
catalyst model is calculated and a frequency analysis is conducted to evaluate its stability.
Furthermore, the optimal geometries can be found in Appendix D, together with a visualized
reaction network based on those optimal geometries.
Chapter 5: Propane dehydrogenation kinetics on Pt3Ga(111) as a model for Pt-Ga catalysts 120
Table 5-4. Adsorbed hydrocarbon species on the Pt3Ga(111) surface in descending order of molecular weight. * indicates with how many C-Pt bonds the species is adsorbed to the surface.
CH3-CH2-CH3 Propane,phys CH3-CH2-CH2* 1-Propyl
CH3-CH*-CH3 2-Propyl CH3-CH*-CH2* Propylene
CH3-CH=CH2 Propylene,phys CH3-CH2-CH** 1-Propylidene
CH3-CH**-CH2 2-Propylidene CH3-CH2-C*** 1-Propylidyne
CH3-CH*-CH** 1-Propenyl CH3-C**-CH2* 2-Propenyl
CH3-CH2* Ethyl CH3* Methyl
Figure 5-3: Reaction network for propane dehydrogenation on Pt3Ga(111).
The reaction network will be divided in three sections: propane dehydrogenation to propylene
(5.3.1), deep dehydrogenation of propylene (5.3.2) and hydrogenolysis of C3 intermediates 5.3.3).
In each section, reaction energies are determined and the stability of each intermediate is evaluated,
2-propyl
1-propylidene
CH2=CH-CH3,phys
CH2=CH-CH3(g)
13
_CH-CH-CH3 + H
PtPt2
_
Pt
= CH2-C-CH3 + H
Pt
_
Pt
_
Pt2
=
2-propenyl1-propenyl
C-CH2-CH3 + H
Pt3
_
Pt
≡
14 1517 20 21
Methyl and ethyl
4 5
= _CH-CH2-CH3 + H
Pt2 Pt2-propylidene
CH3-C-CH3 + H
Pt2
=
Pt
_
7 8
CH3 + CH2CH3
Pt Pt
_ _CH3-CH-CH3 + H
Pt Pt
_ _CH2-CH2-CH3+ H
Pt Pt_ _
1-propyl
1 2 3
CH3-CH2-CH3,phys
CH3-CH2-CH3(g)Gaseous propane
0
Physisorbed propane
CH2-CH-CH3 + H
Pt
__
Pt
_
PtPropylene
11 12
1-propylidyne
Physisorbed propylene
Gaseous propylene
121 Chapter 5: Propane dehydrogenation kinetics on Pt3Ga(111) as a model for Pt-Ga catalysts
conform Chapter 4. The activity of the catalyst is investigated in section 5.3.1, while the selectivity
towards propylene with respect to the most stable coke precursors is discussed in section 5.3.2, as
propylene desorption is compared with deep dehydrogenation of propylene. Section 5.3.3
determines the selectivity to other side products, however on Pt(111) it was observed that these
reactions are kinetically unfavorable, so solely the dissociation of propane is selected as model
reaction for this section and will be discussed.
Three main reaction types are distinguished: dehydrogenation, isomerization and dissociation
reactions. The focus will be mainly on the dehydrogenation reactions as the C-H bond breaking
has the lowest reaction barriers on Pt(111) with respect to the isomerization and dissociation. Due
the low hydrogen coverage and the repulsion between the hydrocarbon intermediates and
fragmented hydrogen, it is expected the adsorbed hydrogen diffuses away from the hydrocarbon
intermediates after the dehydrogenation reactions. For the description of the thermodynamics, the
products of dehydrogenation reactions are optimized in different unit cells. The equations
proposed in 4.3 are employed to describe the reaction energies of the considered reactions.
The hydrogen in the separate unit cell orientates itself most favorable in the Pt3-hcp site with Ga
in the sublayer. The most stable adsorption site i.e. Pt3-hcp-Ga is determined with respect to other
considered adsorption sites. It is observed that hydrogen does not directly adsorb on a Ga atom so
it is expected that neighboring and sublayer Ga atoms induce an electronic effect on the Pt atoms,
thereby increasing the stabilization of hydrogen on the Pt3Ga(111) surface. Due these electronic
effects, the most stable adsorption site for hydrogen is different on Pt3Ga(111) than on the Pt(111)
catalyst model. The calculated adsorption energy for dissociative adsorption of H2 is -102 kJ/mol,
which is 25 kJ/mol more stable than on Pt(111).
5.3.1 Propane dehydrogenation to propylene
Two main reaction pathways are possible towards propylene: via 1-propyl and via 2-propyl. Each
path consists of two elementary steps (three if the physisorption of propane is included). The first
elementary step dehydrogenates a hydrogen at either the methyl or methylene group, respectively
generating 1-propyl or 2-propyl. The most stable adsorption of these species is located on top site
of a platinum atom. Consequently, these species form propylene via β-dehydrogenation in the
second elementary step. Propylene orientates itself most stable over a bridge site, which consists
of two Pt atoms. Aside from the β-dehydrogenation towards propylene, 1-propyl and 2-propyl can
also react further via α-dehydrogenation towards respectively 1-propylidene and 2-propylidene.
Both species adsorb both on a Pt2-bridge-Pt2Ga site.
Chapter 5: Propane dehydrogenation kinetics on Pt3Ga(111) as a model for Pt-Ga catalysts 122
Each step generates also a detached hydrogen. Its location plays a major role to determine the
kinetics as the activation energy is strongly influenced by the (de-)stabilization due to this
hydrogen. However, for the thermodynamics, it is assumed that hydrogen is highly mobile on the
surface.
The optimal geometries on Pt3Ga(111) are similar to those on Pt(111) so Ga induces no direct
geometrical changes in the intermediates, similar findings are reported for the Pt3Sn(111) model.
[7] It is also observed that the hydrocarbons cannot be stabilized by binding with Ga, as no Ga-C
bonds are formed. Vibrational analysis of the intermediates have shown that all intermediates,
except for the physisorbed species, have zero imaginary frequencies.
In literature, DFT studies report that the addition of Sn to Pt catalysts reduces the stability of
intermediates such as 1-propyl and 2-propyl. The same trend was already observed for adsorbed
propylene and propane. [7] However, the addition of Ga shows a different effect and stronger
binding with the surface is reported (see Table 5). For both reaction pathways, a similar reaction
energy is observed for the first elementary step on Pt(111) and Pt3Ga(111). With respect to Pt(111),
1-propyl and 2-propyl have a similar electronic energy on Pt3Ga(111), as the difference is only 2
kJ/mol in favor of 2-propyl, while on Pt(111) this is 7 kJ/mol. It is then also expected that both
reaction paths occur on the surface. However, the electronic effect due to Ga alloying is observed
in the second dehydrogenation step. On both catalyst models, propylene is thermodynamically
preferred, as it is the strongest adsorbate bonded with the surface. On Pt3Ga(111), the reaction
energies of the second elementary step are more exothermic with respect to Pt(111).
123 Chapter 5: Propane dehydrogenation kinetics on Pt3Ga(111) as a model for Pt-Ga catalysts
Table 5-5. Reaction energies of propane dehydrogenation towards propylene on Pt(111) and Pt3Ga(111). The energies are determined with an optPBE vdW-DF functional.
ΔEr (kJ/mol) This work (0.13 ML)
# Surface reaction Pt(111) Pt3Ga(111)
0 Propane(g) → Propane,phys Physisorption -43 -51
1 Propane,phys → 1-propyl + H Adsorption -7 -7
2 Propane,phys → 2-propyl + H Adsorption -14 -9
4 1-propyl → 1-propylidene + H Dehydrogenation 7 -4
5 1-propyl → propylene + H Dehydrogenation -22 -40
7 2-propyl → propylene + H Dehydrogenation -15 -38
8 2-propyl → 2-propylidene + H Dehydrogenation 16 6
10 1-propyl → 2-propyl Isomerization -7 -2
11 1-propylidene → propylene Isomerization -29 -36
12 2-propylidene → propylene Isomerization -32 -44
13a Propylene→ propylene,phys Desorption 85 71
13b Propylene,phys →Propylene (g) Desorption 49 63
Based on these intermediates, propylene is thermodynamically favored as can be seen in the
relative energy profile (see Figure 5-4). While the electronic energy difference is small, 2-propyl
is slightly favored as reaction intermediate towards propylene. With respect to Pt(111), all C3H6-
species are stronger bonded to the surface. Propylene is the most stable species and is 26 kJ/mol
stronger bonded to the Pt3Ga(111) surface than on Pt(111). Ga alloying affects 1-propylidene and
2-propylidene less in this catalyst model, as a decrease of respectively 19 kJ/mol and 12 kJ/mol in
stability is reported.
Chapter 5: Propane dehydrogenation kinetics on Pt3Ga(111) as a model for Pt-Ga catalysts 124
Figure 5-4: Energy profile of adsorbed C3Hx (x= 6–8) species on the Pt3Ga(111) surface at 0.13 ML. Energies are determined relative to gaseous propane. Dehydrogenated hydrogen is optimized in separate unit cell.
Conform Chapter 4, to validate the conducted calculations on propane dehydrogenation towards
propylene, the catalytic cycle of this reaction mechanism is evaluated. Hence, the electronic energy
difference for gaseous propane dehydrogenation has to be identical to total electronic energy
difference for the surface reactions on the catalyst. In the gasphase, the reaction energy is 139
kJ/mol. The catalytic reaction paths via 1-propyl or 2-propyl have both a total reaction energy of
138 kJ/mol. The 1 kJ/mol difference can be neglected due to rounding of the numbers and the
reaction pathways have the same total reaction energies.
5.3.2 Deep dehydrogenation of propylene
The deep dehydrogenation, especially the dehydrogenation of propylene, is essential to describe
the selectivity towards propylene with respect to deactivation of the catalyst during propane
dehydrogenation. Hence, the desorption reaction of propylene competes with deep
dehydrogenation reactions that form coke precursors and eventually deactivate the catalyst. Three
deeply dehydrogenated species are investigated: 1-propylidyne, 1-propenyl and 2-propenyl.
Formation of 1-propylidyne occurs via α-dehydrogenation of 1-propylidene, while 1-propenyl is
either formed via β-dehydrogenation of 1-propylidene or via dehydrogenation of the methylene
group of propylene. 2-propenyl is also formed via two paths: dehydrogenation of the methylidyne
-120
-100
-80
-60
-40
-20
0
20
40
60R
elat
ive
ener
gy (k
J/m
ol)
C3H8 (g) C3H8, phys C3H7* + H* C3H6
* + 2H* C3H6 (g) + 2H*
Propane (g)
Propane, phys
2-propyl
Propylene,phys
Propylene
Propylene (g)
1-propyl
1-propylidene
2-propylidene
C3H6, phys + 2H*
125 Chapter 5: Propane dehydrogenation kinetics on Pt3Ga(111) as a model for Pt-Ga catalysts
of propylene and via β-dehydrogenation of 2-propylidene. Reaction energies of these reaction can
be found in Table 6.
Table 5-6. Reaction energies of deep dehydrogenation reactions of C3H6-species on Pt(111) and Pt3Ga(111). The energies are determined with an optPBE vdW-DF functional.
ΔEr (kJ/mol) This work (0.13 ML)
# Surface reaction Pt(111) Pt3Ga(111)
14 Propylene → 1-propenyl + H Dehydrogenation 20 10
15 Propylene → 2-propenyl + H Dehydrogenation 7 5
17 1-propylidene → 1-propylidyne + H Dehydrogenation -67 -42
20 1-propylidene → 1-propenyl + H Dehydrogenation -10 -27
21 2-propylidene → 2-propenyl + H Dehydrogenation -24 -39
1-propylidyne, 1-propenyl and 2-propenyl all three adsorb on a hollow three-folded site. In the
case of 1-propylidyne, one carbon atom directly bond with three Pt atoms, while for 1-propenyl
and 2-propenyl, one carbon atom single bonds with one Pt atom and a second carbon atom bonds
with two Pt atoms. On Pt(111), the most stable adsorption site is in a fcc site. However, on
Pt3Ga(111), the most stable adsorption site is shifted to a hcp site with underneath a Ga atom. The
electronic effect induced by the sublayer Ga atom stabilizes the deeply dehydrogenated species.
With respect to each other, 1-propylidyne is the most stable C3H5-species, conform Chapter 4.
However, Ga alloying stabilizes 1-propylidyne less and an increase in reaction energy of 25 kJ/mol
is observed compared to the pure Pt(111) catalyst. In contrast, for the other two C3H5-species, a
decrease (of ~16 kJ/mol) in reaction energies is observed, consistent with the previous species (see
5.3.1). Thermodynamically, 1-propylidyne is still the most favorable species, however, the
electronic energy difference between 1-propylidyne and propylene is smaller on Pt3Ga(111) (-6
kJ/mol) with respect to Pt(111) (-38 kJ/mol). This smaller energy difference reduces the
thermodynamic driving force towards 1-propylidyne and indicates that the selectivity towards
propylene is improved with respect to the pure Pt catalyst model.
Chapter 5: Propane dehydrogenation kinetics on Pt3Ga(111) as a model for Pt-Ga catalysts 126
Figure 5-5: Energy profile of adsorbed C3Hx (x= 5–8) species on the Pt3Ga(111) surface at 0.13 ML. Energies are determined relative to gaseous propane. Dehydrogenated hydrogen is optimized in separate unit cell.
5.3.3 Hydrogenolysis of C3 intermediates
Cracking of C3-species leads to C1 and C2 species on the surface. Further surface reactions of these
species either lead to gaseous side products such as ethane, ethane, and ethylene or to coke
precursors and eventually cokes. These reactions mainly affect the selectivity towards propylene,
as other undesirable gaseous products are formed. On Pt(111), it was shown that hydrogenolysis
is solely favorable for deeply dehydrogenated species, as in general higher reaction barriers are
obtained for C-C cleavage than C-H bond breaking. On Pt3Ga(111), the dissociation of propane is
primarily evaluated as a model reaction for C-C bond breaking during propane dehydrogenation.
In this reaction, propane dissociates into ethyl and methyl. Both adsorb on top of a Pt atom. The
reaction products are optimized in the same unit cell. The optimized geometry on Pt3Ga(111) is
similar on Pt(111). The bond lengths are slightly elongated on this model, indicating weaker
adsorption on the surface. The calculated reaction energy is 14 kJ/mol, which is 19 kJ/mol larger
than on Pt(111), confirming the weaker adsorption. However, methyl and ethyl are optimized in
separate unit cells on Pt(111), eliminating repulsion between the dissociated products. Therefore,
no clear conclusions can be drawn for the difference between Pt(111) and Pt3Ga(111) in terms of
hydrogenolysis reaction energies.
-110
-90
-70
-50
-30
-10
10R
elat
ive
ener
gy (k
J/m
ol)
C3H8 (g) C3H8* C3H7
* + H* C3H6* + 2H* C3H5
* + 3H*
Propane (g)
Propane, phys
2-propyl
Propylene,phys
Propylene
1-propenyl
1-propyl
1-propylidene
2-propylidene
2-propenyl
1-propylidyne
127 Chapter 5: Propane dehydrogenation kinetics on Pt3Ga(111) as a model for Pt-Ga catalysts
5.4 Kinetics
To determine the kinetic descriptors for the considered reactions, the optimized geometry and
electronic energy of transition state are calculated. Each determined transition state is validated by
vibrational analysis, as every transition state should have one imaginary frequency of which the
mode is along the reaction coordinate. The resulted geometries can be found in Appendix D.
Yang et al. investigate the role of Sn on the propane dehydrogenation kinetics. The determination
of reaction barriers is focused on the elementary steps towards propylene, the dehydrogenation of
propylene and hydrogenolysis of dehydrogenated species. Pt(111) and various Pt-Sn catalyst
models are employed in the calculations, however the results on Pt(111) and Pt3Sn(111) are the
most interesting for this work. The resulted energy barriers on Pt3Sn(111) are all larger than on
Pt(111). Furthermore, the largest increase is reported for the deep dehydrogenation reactions
towards 1- and 2-propenyl. Yang et al. conclude that the addition of Sn lowers the catalytic activity
for propane dehydrogenation, while a higher propylene selectivity with respect to deeply
dehydrogenated species is observed. [7]
In the following sections, the quantification of the role of Ga on the catalytic activity (5.4.1) and
the selectivity towards propylene (5.4.2 and 5.4.3) is the point of focus. The selectivity towards
propylene is evaluated with respect to deeply dehydrogenated species (5.4.2) and gaseous side
products, formed via hydrogenolysis (5.4.3).
5.4.1 Propane dehydrogenation to propylene
As mentioned above, the focus lays on the role of Ga on the catalytic activity during propane
dehydrogenation towards propylene. Conform Chapter 4, it is assumed that the
adsorption/desorption steps are not activated, however during the desorption step, the desorption
energy must be overcome, so this value is set equal to the electronic activation energy for
desorption steps. For the first elementary step, the reaction barriers for 1- and 2-propyl formation
are calculated. As the electronic activation energies are identical, both reaction paths are likely to
occur. However, for the further determination of the activity, the reaction path via 2-propyl is
selected as on both Pt(111) and Pt3Ga(111) 2-propyl is electronically more stable than 1-propyl.
For the second elementary step, solely the β-dehydrogenation of 2-propyl is considered.
The calculated transition state energies (as shown in Table 5-7) are converged under strict criteria
and have one unique imaginary frequency with a vibrational mode along the reaction coordinate.
Chapter 5: Propane dehydrogenation kinetics on Pt3Ga(111) as a model for Pt-Ga catalysts 128
Table 5-7. Electronic activation energies for propane dehydrogenation towards propylene on Pt(111) and Pt3Ga(111). The energies are determined with an optPBE vdW-DF functional.
Δ‡E (kJ/mol) This work (0.13 ML)
# Surface reaction Pt(111) Pt3Ga(111)
0 Propane(g) → Propane,phys Physisorption 0 0
1 Propane,phys → 1-propyl + H Adsorption 80 65
2 Propane,phys → 2-propyl + H Adsorption 68 65
7 2-propyl → propylene + H Dehydrogenation 84 75
13a Propylene→ propylene,phys Desorption 85 71
13b Propylene,phys →Propylene (g) Desorption 49 63
To validate the role of Ga on the catalytic activity, the reaction path via 2-propyl towards propylene
is compared on Pt(111) and Pt3Ga(111). This is illustrated in Figure 5-6. For the energy profile,
all energies are determined relative to gaseous propane. To satisfy the conservation laws, the
energy of adsorbed hydrogen(s) is added in the case of dehydrogenated species. In the last steps,
all adsorbates (propylene and H2) desorb and the final energy should be the same for both catalyst
models, as the energies of those products are independent of the catalyst model.
All considered intermediates are stronger bonded on Pt3Ga(111) with respect to Pt(111).
Furthermore, lower reaction barriers are calculated on Pt3Ga(111), indicating that propane
dehydrogenation is less activated on Pt3Ga(111) than on Pt(111). The electronic energies of the
transition states downshift with at least 11 kJ/mol for these steps. The stabilizing effect of Ga is
the largest for di-σ propylene, as it is 27 kJ/mol more stable on Pt3Ga(111) than on Pt(111).
However, it is expected that the desorption energy of propylene is increased with the same amount,
but in contrast, the desorption energy for propylene to the gasphase is identical on both catalyst
models. It is the desorption energy of hydrogen that has become more endothermic on the
Pt3Ga(111) catalyst model compared to Pt(111).
Hence, the addition of Ga decreases the activation barriers of the elementary steps for propane
dehydrogenation towards propylene, without stronger propylene adsorption with respect to its
gaseous form. It can be concluded that Ga enhances the catalytic activity, as was proposed by
experiments by Siddiqi et al. [11]. In contrast, it was shown that alloying Pt with Sn lowered the
catalytic activity. [7] In terms of activity, alloying with Ga is a better choice in contrast to alloying
with Sn.
129 Chapter 5: Propane dehydrogenation kinetics on Pt3Ga(111) as a model for Pt-Ga catalysts
Figure 5-6 : Relative energy profile for the propane dehydrogenation towards propylene on Pt(111) (black) and Pt3Ga(111) (green). The energies are determined relative to gaseous propane.
5.4.2 Deep dehydrogenation of propylene
Deeply dehydrogenated species are considered coke precursors, as these species are strongly
bonded to the surface and further reactions lead to coke formation and eventually to deactivation
of the catalyst. To quantify the selectivity of propylene with respect to these coke precursors, two
reaction paths are compared. The first path consists of all elementary steps towards gaseous
propylene as discussed in previous section. 2-propyl is selected as intermediate for this reaction
path, as it is slightly more stable than 1-propyl on Pt3Ga(111). The second path consists of the
elementary steps towards the most stable deeply dehydrogenated species i.e. 1-propylidyne. The
reaction barrier is calculated for the following elementary steps: α-dehydrogenation of 1-propyl to
1-propylidene and the consecutive dehydrogenation of 1-proylidene to 1-propylidyne. The
activation energies for this path can be found in Table 8.
-100
-75
-50
-25
0
25
50
75
100
125
150
Rel
ativ
een
ergy
(kJ/
mol
)
Propane (g)
Propane, phys2-propyl + H*
Propylene,phys + 2H*
Propylene +2H*
Propylene (g) + H2 (g)
TS2 TS7
Reactant Product
Propylene (g)+ 2H*
Chapter 5: Propane dehydrogenation kinetics on Pt3Ga(111) as a model for Pt-Ga catalysts 130
Table 5-8. Electronic activation energies for deep dehydrogenation reaction pathway on Pt(111) and Pt3Ga(111). The energies are determined with an optPBE vdW-DF functional.
Δ‡E (kJ/mol) This work (0.13 ML)
# Surface reaction Pt(111) Pt3Ga(111)
4 1-propyl → 1-propylidene + H Dehydrogenation 83 94†
17 1-propylidene → 1-propylidene +H Dehydrogenation 45† 69† † On Pt(111), vibrational analysis resulted in an additional imaginary frequencies. On Pt3Ga(111), the electronic energies of the
transition states are based on the NEB results.
The resulted electronic activation energies are considerably larger on Pt3Ga(111) than on Pt(111),
indicating that this reaction path is less favored on Pt3Ga(111). However, to evaluate both reaction
paths, an energy profile is constructed, relative to gaseous propane (see Figure 5-7). To satisfy
conservation laws, the energies of adsorbed hydrogen(s) are added to the dehydrogenated species.
In the final steps, all adsorbates are desorbed, however as 1-propylidyne cannot desorb, solely its
dehydrogenated hydrogens are desorbed.
Figure 5-7: Relative energy profile for the propane dehydrogenation towards propylene (···) and propane deep dehydrogenation towards 1-propylidyne ( ̶ ̶ ) on Pt3Ga(111). The energies are determined relative to gaseous propane.
The competition between these reaction paths is centered at the desorption step and
dehydrogenation to 1-propylidyne. For the first elementary step, both intermediates are equally
-125
-75
-25
25
75
125
Rea
ltive
ene
rgy
(kJ/
mol
)
Reactant Product
Propane (g)
Propane, phys
2-propyl + H*
Propylene,phys + 2H*
Propylene + 2H*
Propylene (g) + H2 (g)
TS1 TS2 TS7 Propylene (g) + 2H*
1-propyl + H*
TS4
1-propylidene+ 2H*
1-propylidyne + 3H*
1-propylidyne + 3/2 H2 (g)
TS17
131 Chapter 5: Propane dehydrogenation kinetics on Pt3Ga(111) as a model for Pt-Ga catalysts
likely to be formed. In the second step, propylene is more likely to be formed as both its reaction
energy and barrier is lower than 1-propylidene. However, since 1-propylidyne is 6 kJ/ more stable
on the surface than propylene, this species is preferentially formed.
However, in this electronic energy profile (Figure 5-7), solely enthalpic contributions are taken
into account. As discussed in 4.3.2, the Gibbs free energy accounts for both the enthalpic and
entropic contributions. On Pt(111), the Gibbs free energy diagram at 900 K showed that gaseous
propylene was preferred with respect to 1-propylidyne because its entropic contributions in the
gaseous state are much larger than adsorbates such as 1-propylidyne due the higher degree of
freedom. Furthermore, the description of the Gibbs free energy on Pt(111) can be extended to
Pt3Ga(111). The Gibbs free energy of gaseous propylene is independent of the catalyst model,
however this is not the case for adsorbates such as 1-propylidyne. Since similar geometries were
obtained for the adsorbates on Pt3Ga(111) and Pt(111), it can be assumed that the entropic
contributions are also similar. Based on this reasoning, the Gibbs free energy of 1-propylidyne on
Pt3Ga(111) can be estimated based on the Gibbs free energy diagram on Pt(111). As the entropic
contributions are similar on both catalyst models, the electronic energy difference of 1-propylidyne
between Pt(111) and Pt3Ga(111) determines the Gibbs free energy of 1-propylidyne on
Pt3Ga(111). The electronic energy of 1-propylidyne is higher on Pt3Ga(111) than on Pt(111), so it
is expected that Gibbs free energy of 1-propylidyne is also higher. As the Gibbs free energy
difference between 1-propylidyne and gaseous propylene is higher on Pt3Ga(111), it can be
concluded that Pt3Ga(111) is more deactivation resistant than Pt(111) as it is less likely that coke
precursors are formed on Pt3Ga(111).
5.4.3 Hydrogenolysis of C3 intermediates
The cracking of the C-C bond of C3 intermediates leads to formation of smaller gaseous
hydrocarbons such as methane, ethane and ethylene. The description on the C-C cleavage is thus
essential for the propylene selectivity with respect to other gaseous side products. Siddiqi et al.
have reported that the selectivity towards propylene is above 98% on Pt-Ga/Mg(Ga)(Al)Ox for
total time on stream, while for Pt/Mg(Al)Ox the selectivity towards increases during the reaction,
but never pass a propylene selectivity of 87 %. [11] This indicates that pure Pt catalyst are more
prone to hydrogenolysis than Pt-Ga catalysts. Via the hydrogenolysis of propane, the formation of
these gaseous side products is evaluated. The resulted electronic activation energy is 191 kJ/mol,
which is 136 kJ/mol more than the dehydrogenation reactions of propane. Conform Pt(111), it is
expected that C-C cleavage is unfavorable with respect to the breaking of C-H bonds. The
Chapter 5: Propane dehydrogenation kinetics on Pt3Ga(111) as a model for Pt-Ga catalysts 132
considered dissociation reaction is even higher activated on Pt3Ga(111) than on Pt(111). This
confirms that on Pt3Ga(111) dissociation of propane is unlikely to occur and inherently inhibits
the formation of gaseous side products.
5.5 Conclusions
In this chapter, the characteristics of propane dehydrogenation on Pt-Ga catalysts is evaluated. In
a previous combined experiment-and-theory study, the Pt3Ga(111) surface cut is assigned as the
most probable surface composition of the novel Pt/Mg(Ga)(Al)Ox catalyst. Hence, a 4×2 unit cell
of is employed for the Pt3Ga(111) catalyst model. To determine the role of Ga on the activity and
propylene selectivity during propane dehydrogenation, our results are compared with literature on
pure Pt catalysts and literature on Pt3Sn catalysts.
Propane dehydrogenation towards propylene consists of two elementary steps and occurs via two
reaction pathways (via 1-propyl and via 2-propyl). The reaction barriers of the two elementary
steps towards propylene define the catalytic activity. Literature reports that on Pt3Sn(111), the
transition states are destabilized with respect to Pt(111) and hence higher reaction barriers are
observed. The addition of Sn lowers the catalytic activity for propane dehydrogenation. In contrast
on Pt3Ga(111), both 1-propyl are 2-propyl are more stabilized than on Pt(111) and as 2-propyl is
thermodynamically more stable than 1-propyl, the reaction pathway via 2 propyl is selected to
define the catalytic activity on Pt3Ga(111). Furthermore, adsorbed propylene bonds stronger on
the Pt3Ga(111) surface and the calculated reaction barriers for this reaction path are lower than on
Pt(111), while the propylene desorption energy remains constant (see Figure 5-6). It can be
concluded that catalytic activity is increased on Pt3Ga(111).
Furthermore, the deep dehydrogenation of C3H6-species is investigated. The formation of these
species are an indication for the deactivation of the catalysts and can be used as a descriptor for
the selectivity of the reaction. Conform Pt(111), 1-propylidyne is thermodynamically the most
favored species on the Pt3Ga(111) surface. To quantify the selectivity to the coke precursors, the
reaction path via 1-propyl and 1-propylidene towards 1-propylidyne is calculated. This reaction
path has higher reaction barriers on Pt3Ga(111) than on Pt(111), see Table 5-8 and 1-propylidyne
is not kinetically favored as on Pt(111) (see Figure 5-7). Furthermore, the electronic energy
difference between propylene and 1-propylidyne on Pt3Ga(111) is much smaller than on Pt(111).
It can be assumed that the entropic contributions for adsorbed species such as 1-propylidyne are
similar on Pt3Ga(111) as on Pt(111). Based on the Gibbs free energy, it is expected that 1-
133 Chapter 5: Propane dehydrogenation kinetics on Pt3Ga(111) as a model for Pt-Ga catalysts
propylidyne is less likely to be formed on Pt3Ga(111) than on Pt(111). This indicates that fewer
coke precursors will be formed and that Pt3Ga(111) has a higher stability than Pt(111).
Finally, the hydrogenolysis of propane is studied as this reaction forms C1 and C2 adsorbates, which
are critical for the formation of gaseous side products such as ethane, ethylene and methane.
However, this reaction is highly activated (191 kJ/mol) and the formation of gaseous side products
via this way is unlikely. This result is supported by the experimental results that show that the
propylene selectivity is above 98% for the total time on stream.
5.6 References
1. Feng, J., M. Zhang, and Y. Yang, Dehydrogenation of Propane on Pt or PtSn Catalysts with Al2O3 or SBA-15 Support. Chinese Journal of Chemical Engineering, 2014. 22(11–12): p. 1232-1236.
2. Koel, B.E., Structure, Characterization and Reactivity of Pt–Sn Surface Alloys, in Model Systems in Catalysis. 2010, Springer. p. 29-50.
3. Li, Q., et al., Kinetics of propane dehydrogenation over Pt–Sn/Al2O3 catalyst. Applied Catalysis A: General, 2011. 398(1–2): p. 18-26.
4. Nykanen, L. and K. Honkala, Selectivity in Propene Dehydrogenation on Pt and Pt3Sn Surfaces from First Principles. Acs Catalysis, 2013. 3(12): p. 3026-3030.
5. Nykänen, L. and K. Honkala, Density Functional Theory Study on Propane and Propene Adsorption on Pt(111) and PtSn Alloy Surfaces. The Journal of Physical Chemistry C, 2011. 115(19): p. 9578-9586.
6. Tsai, Y.L., C. Xu, and B.E. Koel, Chemisorption of ethylene, propylene and isobutylene on ordered Sn/Pt(111) surface alloys. Surface Science, 1997. 385(1): p. 37-59.
7. Yang, M.L., et al., First-Principles Calculations of Propane Dehydrogenation over PtSn Catalysts. Acs Catalysis, 2012. 2(6): p. 1247-1258.
8. Zhang, Y., et al., Propane dehydrogenation on PtSn/ZSM-5 catalyst: Effect of tin as a promoter. Catalysis Communications, 2006. 7(11): p. 860-866.
9. Zhu, H., et al., Sn surface-enriched Pt–Sn bimetallic nanoparticles as a selective and stable catalyst for propane dehydrogenation. Journal of Catalysis, 2014. 320(0): p. 52-62.
10. Sun, P.P., et al., Synthesis and characterization of a new catalyst Pt/Mg(Ga)(Al)O for alkane dehydrogenation. Journal of Catalysis, 2010. 274(2): p. 192-199.
11. Siddiqi, G., et al., Catalyst performance of novel Pt/Mg(Ga)(Al)O catalysts for alkane dehydrogenation. Journal of Catalysis, 2010. 274(2): p. 200-206.
12. Schweitzer, A., et al., Pt3Ga: Thermodynamics and nonstoichiometry. Zeitschrift für Naturforschung B, 2004. 59(9): p. 999-1005.
13. Saerens, S., Determination of the active phase of a Pt/Mg(Ga)AlOx catalyst of the catalytic dehydrogenation of propane, in Department of Chemical Engineering and Technical Chemistry. 2014, University of Ghent: Ghent.
14. Zaera, F. and D. Chrysostomou, Propylene on Pt(111) I. Characterization of surface species by infra-red spectroscopy. Surface Science, 2000. 457(1-2): p. 71-88.
15. Yang, M.L., et al., Density functional study of the chemisorption of C-1, C-2 and C-3 intermediates in propane dissociation on Pt(111). Journal of Molecular Catalysis a-Chemical, 2010. 321(1-2): p. 42-49.
Chapter 6: Propane dehydrogenation kinetics on graphene ribbon covered Pt(111) as a model for cokes deactivated Pt catalysts 134
Chapter 6 Propane dehydrogenation kinetics on graphene ribbon covered Pt(111) as a model for cokes deactivated Pt catalysts The coke formation on platinum catalyst plays a major role in the alternation of catalytic
properties of Pt catalysts during propane dehydrogenation. Initial coke formation starts rapidly
at highly undercoordinated sites such as steps and edges as those are prone to C-C cleavage.
When those sites are deactivated, the coke formation reaches it lower steady state rate as coke
formation continues on Pt(111) and the support. In literature, it is proposed that near cokes
deactivated Pt atoms, gaseous side products are less probable to be formed than on a clean Pt
catalyst. [1] Furthermore, the influence of cokes deactivated Pt atoms on propane
dehydrogenation characteristics in terms of catalytic activity and selectivity towards coke
precursors remains elusive.
To construct an appropriate catalyst model for the cokes deactivated Pt catalyst model, the
location and structure of the coke in the model has to represent the nature of the cokes,
determined experimentally. Literature has reported that only a small part of coke deposits is
formed on the active metal particles. The majority either is formed on the support or spills over
from the active metal phase to the support. The characterization of the coke deposits on the
platinum surface shows the aromatic nature of the cokes. Based on TEM experiments, an
interlayer space of the carbon deposits of 3.66 Å is found. This distance corresponds to the
interlayer distance of graphene. [2-5]
On the graphene covered Pt(111) catalyst model, the thermodynamics and kinetics of propane
dehydrogenation are calculated, with focus on the activity and selectivity during propane
Chapter 6: Propane dehydrogenation kinetics on graphene ribbon covered Pt(111) as a model 135 for cokes deactivated Pt catalysts
dehydrogenation. For the activity, the reaction barriers and energies of the most dominant path
towards the desired product propylene will be studied. For selectivity, the difference in reaction
barriers and energies between the most dominant path toward propylene and the most important
side reaction towards coke precursors will be investigated.
6.1 Catalyst model
Graphene is used as model compound to represent coke on the cokes deactivated Pt(111)
catalyst model. Furthermore, it was opted to start from the 4×2 Pt(111) catalyst model (see
Chapter 4.1). A graphene ribbon is constructed on a 2×2 island of Pt atoms. In the unit cell, the
graphene ribbon itself consist of eight carbon atoms and at its periphery, four hydrogen atoms
are added. The hydrogen atoms are added because during propane dehydrogenation hydrogen
is co-fed. Consequently, in these hydrogen-rich conditions, the graphene ribbon will most likely
be hydrogen-terminated. In the optimization, the strict convergence criteria and the non-local
optPBE vdW-DF functional is employed. The optimized unit cell is shown in Figure 6-1. The
catalyst model of the cokes deactivated Pt(111) catalyst model will be abbreviated as
Gr/Pt(111).
Figure 6-1: Isometric representation of the 4×2 unit cell of the cokes deactivated Pt(111) catalyst model
c
a
b
Chapter 6: Propane dehydrogenation kinetics on graphene ribbon covered Pt(111) as a model for cokes deactivated Pt catalysts 136
Hence, the dimensions of the unit cell are 11.29×5.64×18.91 ų, including a vacuum layer in
c-direction. This is the same as for the Pt (111) catalyst model without cokes (see Chapter 4).
The constructed graphene ribbon can be constructed inside the dimensions of the Pt(111)
catalyst model as the cokes atoms does not extend the unit cell in a- and b-direction. In the c-
direction, the vacuum layer of 12 Å and an artificial dipole layer suffice to avoid periodic
interactions in this direction.
The carbon atoms of the graphene bind by two on a Pt atom. This leads to Pt-C distances
between 2.15 Å and 2.45 Å. Still, the graphene layer keeps its planar geometry, so three
alternate C-C bonds are observed in an aromatic ring. The distances are 1.47 Å, 1.54 Å and 1.59
Å and are denoted in Angstrom below (see Figure 6-2). An extension of the unit cell in the a-
and b-direction leads to the visualization of a graphene ribbon, as illustrated in Figure 6-2. The
graphene ribbon alters the structure of the two top Pt layers of the unit cell. In the top layer, an
elongation of Pt-Pt bonds is observed of the Pt atoms beneath the graphene ribbon in a-
direction. This elongation leads to a bond length of 3.07 Å, while the bulk Pt-Pt length is 2.82
Å. As consequence, Pt-Pt bonds of the empty Pt atoms are contracted in the same direction from
2.82 to 2.67 Å. In the second Pt layer, the reverse is observed. The Pt atoms in the second layer
beneath the graphene ribbon contract with 0.4 Å, while the bonds of Pt atoms underneath the
empty Pt atoms are elongated with 0.4 Å. The graphene ribbon also affects the unit cell in the
c-direction. The Pt atoms on which the graphene is bonded, are 0.2 Å elevated with respect to
the empty Pt atoms.
Figure 6-2: Adsorption sites on 4×2 Gr/Pt(111) catalyst model: top ●, bridge █ , three-folded hcp ▲ and fcc ▼. Additionally, the three alternate C-C distances in an aromatic ring are shown in Angstrom.
1.47
1.59
1.53
b
a
Chapter 6: Propane dehydrogenation kinetics on graphene ribbon covered Pt(111) as a model 137 for cokes deactivated Pt catalysts
The 4×2 unit cell is one of the smallest supercells to investigate the effects on cokes on the
catalytic properties. Two rows of non-deactivated Pt atoms remain as empty sites. Again four
adsorption sites are distinguishable conform the nomenclature determined in 4.1.1. The cokes
equally affect the top sites of both rows, as they are equidistant to the coke. However, as it is
expected that nearby coke destabilizes the adsorbates, the bridge and hollow sites located
between these two rows are preferable. Those adsorption sites are the least affected by the
destabilization of the cokes.
Additionally, a 5×2 unit cell is constructed to determine the range and strength of the cokes
influence on propane dehydrogenation. Here, the 4×2 unit cell is extended with one row in the
a-direction and optimized under strict criteria and with vdW-DF as functional. The optimized
unit cell is shown in Figure 6-3. The dimensions of the enlarged unit cell are 14.11×5.64×18.91
ų. The structure of the optimized graphene ribbon is identical with respect to the 4×2 unit cell
as the same planarity and alternate C-C bond distances are observed.
Figure 6-3: Isometric representation of the 5×2 unit cell of the cokes deactivated Pt(111) catalyst model
Three rows of non-deactivated Pt atoms remain available as adsorption site, see Figure 6-4.
Adsorption occurs on four distinct adsorption sites conform the nomenclature determined in
4.1.1. Of the empty three Pt rows, the middle row top sites are more preferable than the outer
c
b
a
Chapter 6: Propane dehydrogenation kinetics on graphene ribbon covered Pt(111) as a model for cokes deactivated Pt catalysts 138
rows. The most preferable bridge and hollow sites are located on both sides of the middle empty
row. Here, the adsorption modes are the least affected by the repulsion of the adsorbed coke.
While the effect of cokes on the adsorbate is less in this extended unit cell, the required
computational cost is larger. Hence, it is opted to focus on a certain reaction path to evaluate
the properties observed on this unit cell.
Figure 6-4: Adsorption sites on 5×2 Gr/Pt(111) catalyst model: top ●, bridge █ , three-folded hcp ▲ and fcc ▼. Additionally, the three alternate C-C distances in an aromatic ring are shown in Angstrom.
The description of the degree of coverage is essential and should be unambiguous. Hence, it is
opted to maintain the definition for degree of coverage, as formulated in 4.1.2 (see equation
(1)). This means that the coverage is determined based on the number of Pt atoms in the top
layer, and not with respect to the free Pt atoms in the top layer. Hence, a coverage of 0.13 ML
is obtained for the 4×2 unit cell and a coverage of 0.10 ML for the 5×2 unit cell. However, it
should be remembered that in fact four of the Pt atoms in the top layer of the unit cell are
unattainable for reaction since they are covered with cokes.
In the following sections, the 4×2 unit cell is always employed, while the last section both unit
cells, 4×2 and 5×2 Gr/Pt(111), are employed to determine the range and strength of cokes
influence.
6.2 Adsorption
The focus in this section lays on the adsorption of the main reactants and products of propane
dehydrogenation. The considered gaseous species are propane, propylene and H2. However, the
b
a
1.48 1.54
1.59
Chapter 6: Propane dehydrogenation kinetics on graphene ribbon covered Pt(111) as a model 139 for cokes deactivated Pt catalysts
adsorption of propane, propylene and H2 on cokes deactivated Pt catalysts has not been
described before in literature. Instead, the adsorption of the compounds is compared to clean Pt
to assess the influence of cokes. The adsorption energies are calculated conform equation (2)
in 4.2. The optimized geometries are visualized in Appendix E.
6.2.1 Propane
The adsorption of propane plays a major role during propane dehydrogenation. Two modes of
propane adsorption can be observed: physisorption and dissociative chemisorption. The first is
discussed in this section, while the latter is discussed in the Thermodynamics section as it can
be treated as dehydrogenation step (see 6.3). As propane is a saturated compound, it has no
unpaired electron to directly interact with the surface, so solely physisorption occurs.
Physisorbed propane is stabilized by van der Waals interactions and this metastable species acts
as a precursor for further elementary steps in the propane dehydrogenation reaction mechanism.
Physisorbed propane orientates itself above the middle of the island of free Pt atoms. The
bottom C-C bond is parallel to the surface and the perpendicular distance to the Pt surface is
4.2 Å, which is similar to the 4.3 Å distance on Pt(111). Furthermore, the two C-C bonds have
a length of 1.53 Å, identical to the C-C bond length of gaseous propane.
The adsorption energy of physisorbed propane on Gr/Pt(111) is 10 kJ/mol lower than on
Pt(111). Rather the reverse was expected as repulsion by the graphene ribbon would increase
the adsorption energy. However, as the distance between the physisorbed species and the
graphene is more than 4 Å, it possible that van der Waals interactions stabilize the physisorbed
propane, while repulsion occurs closer to the graphene ribbon.
Table 6-1. Comparison between adsorption energies of propane on Pt(111) and Gr/Pt(111) (4×2 unit cell), calculated in this work at a coverage of 0.13 ML with optPBE vdW-DF as functional.
ΔEads (kJ/mol) Pt(111) Gr/Pt(111) Propane physisorption -43 -55
Vibrational analysis of physisorbed propane on Gr/Pt(111) has resulted in one imaginary
frequency. However, this can be attributed to the translation of the metastable physisorbed
species.
Chapter 6: Propane dehydrogenation kinetics on graphene ribbon covered Pt(111) as a model for cokes deactivated Pt catalysts 140
6.2.2 Propylene
Propylene has an unsaturated double bond so it can directly form covalent bonds with the Pt
surface during adsorption. Literature makes a distinction between four different adsorption
modes in function of the employed propylene coverage (see also 4.2.2.1). The two most stable
adsorption modes are di-σ propylene and π propylene. Literature indicates that the di-σ
adsorption mode is more stable than the π adsorption mode as di-σ propylene directly bonds
with Pt surface while π propylene is weakly bonded to surface. [6, 7] So solely the di-σ
adsorption mode is discussed out if these two. Physisorbed propylene is also a probable
adsorption mode of propylene. Both the di-σ and physisorbed adsorption modes are investigated
in this work as physisorbed propylene can act as a precursor for the di-σ adsorption mode.
The di-σ adsorption mode of propylene adsorbs on the bridge site, located on one row of the
empty Pt atoms. The methyl moiety orientates over the other empty row of Pt atoms, to reduce
the interaction with the graphene ribbon. Clearly, the elongation of the C=C bond of propylene
is observed as it is stretched from 1.36 Å (C=C bond) gaseous propylene to 1.51 Å, which is
similar to 1.53 Å of the C-C bond of gaseous propane. The physisorption mode of propylene
orientates itself above three empty Pt atoms, parallel to the surface. The shortest distance
between propylene and platinum surface is 4.5 Å. The C=C bond of propylene is not elongated
in this physisorption mode. The optimized geometries on Gr/Pt(111) and Pt(111) are similar as
the C-C and Pt-C length differ less than 5%.
Table 6-2. Comparison between adsorption energies of propylene on Pt(111) and Gr/Pt(111) (4×2 unit cell), calculated in this work at a coverage of 0.13 ML with optPBE vdW-DF as functional.
ΔEads (kJ/mol) Pt(111) Gr/Pt(111) Di-σ adsorption -135 -14
Physisorption -43 -47
While the adsorption energy for di-σ propylene is strongly negative on Pt(111), this is not the
case on Gr/Pt(111). Di-σ propylene is 120 kJ/mol less stable on Gr/Pt(111) due to steric effects
of the inclusion between the graphene ribbons. The physisorption of propylene is slightly more
stabilized on Gr/Pt(111) as the same was observed with physisorbed propane. The nearness of
graphene destabilizes the chemisorption of propylene, while it stabilizes the physisorption of
propylene due to long-range van der Waals interactions.
Chapter 6: Propane dehydrogenation kinetics on graphene ribbon covered Pt(111) as a model 141 for cokes deactivated Pt catalysts
6.3 Thermodynamics
The same reaction network as in Chapter 4 is employed to describe the propane
dehydrogenation on cokes deactivated Pt(111). However, as the focus lays on the catalyst
activity and selectivity towards propylene, certain intermediates are excluded in the calculations
such as the dissociation products and the cracked C1 and C2 hydrocarbons. The remaining
intermediates that are considered on Gr/Pt(111) are mainly C3Hx (x=5-8) species, ethyl, methyl
and atomic hydrogen. For each component, the most stable geometry on the Gr/Pt(111) catalyst
model is calculated and a frequency analysis is conducted to evaluate its stability. The
considered species are listed in Table 6-3 and a streamlined version of the calculated reaction
network is shown in Figure 6-5. The optimal geometries can be found in Appendix E, together
with a visualized reaction network based on those optimal geometries.
Table 6-3. Adsorbed hydrocarbon species on the Gr/Pt(111) (4×2 unit cell) surface in descending order of molecular weight. * indicates with how many C-Pt bonds the species is adsorbed to the surface.
CH3-CH2-CH3 Propane,phys CH3-CH2-CH2* 1-Propyl
CH3-CH*-CH3 2-Propyl CH3-CH*-CH2
* Propylene
CH3-CH=CH2 Propylene,phys CH3-CH2-CH** 1-Propylidene
CH3-CH**-CH2 2-Propylidene CH3-CH2-C*** 1-Propylidyne
CH3-CH*-CH** 1-Propenyl CH3-C**-CH2* 2-Propenyl
CH3-CH2* Ethyl CH3
* Methyl
Chapter 6: Propane dehydrogenation kinetics on graphene ribbon covered Pt(111) as a model for cokes deactivated Pt catalysts 142
Figure 6-5: Reaction network for propane dehydrogenation on Gr/Pt(111) (4×2 unit cell).
The reaction network will be divided in three sections: propane dehydrogenation to propylene
(6.3.1), deep dehydrogenation of propylene (6.3.2) and hydrogenolysis of C3 intermediates
(6.3.3). In each section, reaction energies are determined and the stability of each intermediate
is evaluated. The first section can be interpreted to quantify the activity of the catalyst, while
section two mainly quantifies the selectivity towards propylene with respect to deactivation of
the catalyst by cokes precursor formation. Section three determines the selectivity to other side
products, however on Pt (111) it was concluded that, these reactions are kinetically unfavorable,
so solely the dissociation of propane (C-C scission) is selected as model reaction for this section
and will be discussed.
Three main reaction types can be distinguished: dehydrogenation, isomerization and
dissociation reactions. The focus will be mainly on the dehydrogenation reactions as the C-H
2-propyl
1-propylidene
CH2=CH-CH3,phys
CH2=CH-CH3(g)
13
_CH-CH-CH3 + H
PtPt2
_
Pt
= CH2-C-CH3 + H
Pt
_
Pt
_
Pt2
=
2-propenyl1-propenyl
C-CH2-CH3 + H
Pt3
_
Pt
≡
14 1517 20 21
Methyl and ethyl
4 5
= _CH-CH2-CH3 + H
Pt2 Pt2-propylidene
CH3-C-CH3 + H
Pt2
=
Pt
_
7 8
CH3 + CH2CH3
Pt Pt
_ _CH3-CH-CH3 + H
Pt Pt_ _CH2-CH2-CH3+ H
Pt Pt
_ _
1-propyl
1 2 3
CH3-CH2-CH3,phys
CH3-CH2-CH3(g)Gaseous propane
0
Physisorbed propane
CH2-CH-CH3 + H
Pt
__
Pt
_
PtPropylene
11 12
1-propylidyne
Physisorbed propylene
Gaseous propylene
Chapter 6: Propane dehydrogenation kinetics on graphene ribbon covered Pt(111) as a model 143 for cokes deactivated Pt catalysts
bond breaking is the least activated on Pt(111), with respect to the isomerization and
dissociation. For the description of the thermodynamics, the products of dehydrogenation
reactions are optimized in different unit cells. It is assumed that the low hydrogen coverage and
the repulsion between the hydrocarbon intermediates and fragmented hydrogen lead to
diffusion of the hydrogen on the catalyst surface, away from the hydrocarbon intermediates.
The equations proposed in section 4.3 are employed to describe the reaction energies of the
considered reactions.
For the dehydrogenation reactions, the fragmented hydrogen is optimized in a separate unit cell.
It is essential to locate the most stable adsorption site for hydrogen on the cokes deactivated
Pt(111) catalyst. Four adsorption sites for hydrogen sites are studied: on top, bridged, hollow
fcc and hcp. These are compared with their respective adsorption energies on clean Pt(111). A
large upward shift in adsorption energies is observed for all adsorption modes compared to
clean Pt(111), while maintaining the same order of stability (top > bridge > fcc > hcp) (see
Table 4). Hydrogen in the bridge site is second most stable adsorption site, indicating that
hydrogen is stabilized if it is further away from the surface. The graphene ribbon alters the
geometry of on top hydrogen. While hydrogen on a Pt(111) top site has a perpendicular
orientation with respect to the surface, it bends away from the cokes on Gr/Pt(111). The optimal
geometries of hydrogen in fcc and hcp are similar to those of on clean Pt(111). For further
calculations, the adsorption energy of hydrogen on the top site is employed to calculate the
reaction energies of dehydrogenation reactions.
Table 6-4. Comparison of hydrogen adsorption energies on Pt(111) and Gr/Pt(111) (4×2 unit cell) with a coverage of 0.13 ML. This work uses an optPBE vdW-DF functional.
ΔEads (kJ/mol) On top Bridge Fcc Hcp Pt(111) -77 - -71 -59
Gr/Pt(111) -42 -37 -30 -23
6.3.1 Propane dehydrogenation to propylene
The dehydrogenation of propane to propylene consists at least of two elementary steps on the
platinum surface. Prior to those two elementary steps, the physisorption is included in our
reaction network. However, this step is already discussed in previous section (see 6.2.1). The
further reaction of physisorbed propane is the first elementary step, which leads to formation
of 1-propyl or 2-propyl by respectively dehydrogenation of the methyl or methylene group. The
Chapter 6: Propane dehydrogenation kinetics on graphene ribbon covered Pt(111) as a model for cokes deactivated Pt catalysts 144
second elementary step is the β-dehydrogenation of the C3H7-species into propylene. However,
the step competes with α-dehydrogenation of 1-propyl and 2-propyl, respectively leading to 1-
propylidene and 2-propylidene, which are undesirable intermediates on the surface. Each step
generates also a detached hydrogen. Its location plays a major role to determine the kinetics as
the activation energy is strongly influenced by the (de-)stabilization due to this hydrogen.
However, for the thermodynamics, it is assumed that hydrogen is highly mobile on the surface.
Table 6-5. Comparison between reaction energies for propane dehydrogenation towards propylene on clean Pt(111) and cokes deactivated Pt(111) (4×2 unit cell). A coverage of 0.13 ML is achieved. This work uses an optPBE vdW-DF functional.
ΔEr (kJ/mol)
Pt(111) Gr/Pt(111) # Surface reaction
0 Propane(g) → Propane,phys Physisorption -43 -55
1 Propane,phys → 1-propyl + H Adsorption -7 67
2 Propane,phys → 2-propyl + H Adsorption -14 120
4 1-propyl → 1-propylidene + H Dehydrogenation 7 91
5 1-propyl → propylene + H Dehydrogenation -22 71
7 2-propyl → propylene + H Dehydrogenation -15 20
8 2-propyl → 2-propylidene + H Dehydrogenation 15 18
13a Propylene→ propylene,phys Desorption 92 -33
13b Propylene,phys →Propylene (g) Desorption 43 47
In the Gr/Pt(111) catalyst model, both 1-propyl and 2-propyl adsorb on top of one of the four
empty Pt atoms. The optimized geometries of 1-propyl and 2-propyl are similar to that on
Pt(111) as the same bond lengths are observed on both catalyst models. However, it should be
noted that, while the hydrocarbon moiety remains the same, the Pt atom on which adsorption
occurs protrudes above the other Pt atoms of the top layer. The distance between that Pt atom
and the nearby Pt atoms in the underlying layer is 3.07 Å for 1-propyl and even 3.17 Å for 2-
propyl, confirming that the surface layers are further distorted by adsorption of hydrocarbon
species. 1-propyl bends away from the cokes and over the island of the four clean Pt atoms. The
ethyl moiety of 1-propyl is orientated perpendicular with the surface, to reduce the repulsion of
the nearby cokes. The methyl moieties of 2-propyl are orientated above two other empty Pt
atoms, however one methyl group is always near the graphene ribbon, see also Appendix E.
Chapter 6: Propane dehydrogenation kinetics on graphene ribbon covered Pt(111) as a model 145 for cokes deactivated Pt catalysts
The second elementary step produces propylene, 1-proylidene and 2-propylidene. Propylene
orientates itself on the bridge site, parallel to the graphene ribbon. The bonds in propylene are
identical to those on clean Pt(111), however the two Pt atoms on which propylene is bonded
protrude above the other Pt atoms of the top layer. The methyl moiety of propylene is orientated
away from the graphene ribbon. 1-propylidene adsorbs on the bridge site, parallel with the
graphene ribbon. The ethyl moiety is orientated perpendicular with the surface and above the
remaining empty Pt atoms. 2-propylidene prefers to adsorb on the bridge site between the two
rows of clean Pt atoms. The methyl groups are orientated above the nearby fcc and hcp site.
The Pt atoms where these species are adsorbed on, are protruded above the top layer of the
catalyst.
To obtain useful insight on the stability of the intermediates with respect of each other, an
energy profile is constructed for the considered reactions. The reaction energies of the first
elementary steps are strongly endothermic on Gr/Pt(111) as it is unfavorable to adsorb propane
dissociatively on the surface. 1-propyl is the least unstable with respect to 2-propyl, as the
perpendicular orientation of its ethyl moiety reduces the repulsion by the cokes, while both
methyl groups of 2-propyl are on the same height as the graphene ribbon. Here are the steric
effects with the graphene the largest.
Figure 6-6: Energy profile of adsorbed C3Hx (x= 6–8) species on the Gr/Pt(111) surface (4×2 unit cell). Electronic energies are determined relative to gaseous propane. Dehydrogenated hydrogen is optimized in a separate unit cell.
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)
C3H8 (g) C3H8* C3H7
* + H* C3H6* + 2H* C3H6 (g) + 2H*
Propane (g)
Propane, phys
1-propyl
2-propyl1
Propylene,phys
Propylene
2-propylidene
1-propylidene Propylene (g)
Chapter 6: Propane dehydrogenation kinetics on graphene ribbon covered Pt(111) as a model for cokes deactivated Pt catalysts 146
Furthermore, the reaction energies for the second elementary steps are also endothermic, so
further dehydrogenation leads to even more unstable species. However, the reaction energies
are the most endothermic for 1-propyl as the dehydrogenated products are less stabilized, while
the reaction energies for 2-propyl are at least one magnitude lower as the reactant is less
stabilized. The most stable C3H6-species is 2-propylidene as it adsorbs in between the rows of
clean Pt atoms. Second most stable species is propylene, which is 2 kJ/mol less stable than 2-
propylidene, while 1-propylidene is 20 kJ/mol less stable than propylene (see Figure 6-5).
Thermodynamically, it is expected that propylene is still the most favorable species on the
surface (if deeply dehydrogenated species are neglected, but these are discussed in the next
section). The difference in relative energy between propylene and 2-propylidene is small, so
both species are expected on the surface. However, propylene can be formed by
dehydrogenation of 1-propyl and 2-propyl, while 2-propylidene solely via 2-propyl. As 1-
propyl is more stable than 2-propyl, the reaction paths via 1-propyl are favored. Additionally,
propylene can desorb to a more stable physisorbed state, inducing extra driving force for this
reaction.
The considered isomerization reactions are excluded in this reaction network as it was shown
in 4.4.1, that they generally have a large reaction barrier on Pt(111). This makes those reactions
improbable to occur.
It is important to conclude that steric effects between the cokes and the adsorbates lead to
unstable species on the surface. Thermodynamically speaking less or even no reaction will
occur on this model of cokes deactivated Pt(111) surface compared to the clean Pt(111) surface.
6.3.2 Deep dehydrogenation of propylene
The considered deeply dehydrogenated species on Gr/Pt(111) are 1-propenyl, 2-propenyl and
1-propylidyne. 1-propylidyne is formed via α-dehydrogenation of 1-propylidene and adsorbs in
a hollow fcc site. Steric interactions cause the 1-propylidyne to bend slightly away from the
graphene ribbon so that its ethyl moiety is orientated over the neighboring hcp site. 1-propenyl
is formed either via β-dehydrogenation of 1-propylidene or dehydrogenation of the outer
bonded C atom of propylene. 2-propenyl is formed either via β-dehydrogenation of 2-
propylidene or dehydrogenation of the middle C atom of propylene. These species orientate
themselves on a bridge and a top site, respectively with the double bonded C atom and single
bonded C atom.
Chapter 6: Propane dehydrogenation kinetics on graphene ribbon covered Pt(111) as a model 147 for cokes deactivated Pt catalysts
Table 6-6. Comparison between reaction energies for deep dehydrogenation on clean Pt(111) and cokes deactivated Pt(111) (4×2 unit cell). A coverage of 0.13 ML is achieved on these catalyst models. This work uses an optPBE functional.
ΔEr (kJ/mol)
# Surface reaction Pt(111) Gr/Pt(111)
14 Propylene → 1-propenyl + H Dehydrogenation 20 12
15 Propylene → 2-propenyl + H Dehydrogenation 7 7
17 1-propylidene → 1-propylidyne + H Dehydrogenation -67 -74
20 1-propylidene → 1-propenyl + H Dehydrogenation -10 -9
21 2-propylidene → 2-propenyl + H Dehydrogenation -24 5
Based on the reaction energies (see Table 6-6) and energy profile (see Figure 6-7), 1-
propylidyne is the most thermodynamically favored species on Gr/Pt(111). The same is
observed on Pt(111). Furthermore, 1-propenyl is less stable than 2-propenyl, which is also
observed on Pt(111). The reaction energies are similar on clean Pt(111) and Gr/Pt(111). This
indicates that no additional repulsion occurs from the graphene ribbon in the reactions towards
C3H5-species.
Figure 6-7: Energy profile of adsorbed C3Hx (x= 5–8) species on the Gr/Pt(111) surface (4×2 unit cell). Electronic energies are determined relative to gaseous propane. Dehydrogenated hydrogen is optimized in a separate unit cell.
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)
C3H8 (g) C3H8* C3H7
* + H* C3H6* + 2H* C3H5
* + 3H*
Propane (g)
Propane, phys
1-propyl
2-propyl
Propylene,phys
Propylene
2-propylidene
1-propylidene
1-propylidyne
2-propenyl
1-propenyl
Chapter 6: Propane dehydrogenation kinetics on graphene ribbon covered Pt(111) as a model for cokes deactivated Pt catalysts 148
To quantify the selectivity towards propylene with respect to deactivation by deep
dehydrogenation reactions, the same reaction paths (desorption and dehydrogenation paths) as
on Pt(111) are considered. The propylene desorption pathway consists of 1-propyl
dehydrogenation to propylene and propylene desorption via physisorbed propylene, while the
dehydrogenation pathway consists of consecutive dehydrogenations of 1-propyl to 1-
propylidyne via 1-propylidene. Based on thermodynamical aspects and stability of the
intermediates, it is shown that propylene desorption is less endothermic near the graphene
ribbon, while the stabilization of 1-propylidyne remains constant with respect to clean Pt(111).
On the Gr/Pt(111) surface, propylene desorption will occur easier and side reactions to 1-
propylidyne will occur less, thereby decreasing the deactivation of the catalyst.
6.3.3 Hydrogenolysis of C3-intermediates
Cracking of C3-species leads to C1 and C2 species on the surface. They either hydrogenate and
desorb from the surface as alkanes such as methane and ethane or dehydrogenate further,
leading to coke precursors and adsorbed carbon. On Pt(111), it was shown that cracking is
solely favorable for deeply dehydrogenated species, as higher reaction barriers are obtained for
C-C cleavage than C-H bond breaking. On Gr/Pt(111), the dissociation of propane is primarily
evaluated to determine if this and other cracking reactions are probable to occur near the
graphene ribbon. The optimized geometry of the dissociated ethyl and methyl shows clear the
effect of repulsion by the graphene ribbon, see Appendix E. Both the methyl and ethyl adsorb
on top of a Pt atom. The Pt-C bond of ethyl is elongated with respect to Pt(111); also, the Pt
atom on which the ethyl is adsorbed is protruded, as the distance between the two top layers is
3.2 Å while the bulk distance 2.82 Å. This distortion is due the repulsion of the two hydrocarbon
species and the graphene ribbon. It is expected that the steric effects result in a strong
endothermic reaction energy for the dissociation reaction. The resulted reaction energy is 265
kJ/mol. This is strongly endothermic, indicating that the reaction is highly improbable to occur
on the Gr/Pt(111) surface. Hence, the electronic reaction energy for this reaction is not
calculated, as it has to be higher than 265 kJ/mol.
Chapter 6: Propane dehydrogenation kinetics on graphene ribbon covered Pt(111) as a model 149 for cokes deactivated Pt catalysts
6.4 Kinetics
The determination of the kinetic descriptors is based on the transition states, in this case the
electronic activation energies. The electronic energy of the optimized transition states with an
imaginary frequency along the reaction coordinate is used to determine the electronic activation
energy of a reaction, a measure for the reaction barrier. The electronic activation energy Δ‡E is
determined with respect to the electronic energy of the reactant, conform Chapter 4.
The description of the kinetics is divided into two sections: propane dehydrogenation to
propylene (6.4.1) and deep dehydrogenation of propylene (6.4.2). The kinetics of the
dissociation reactions are not considered as these products are thermodynamically unfavorable.
The first section quantifies the activity of the cokes deactivated Pt(111), while the second
describes the selectivity towards propylene with respect to deactivation reactions that lead to
cokes precursor formation.
As the transition state geometries are calculated with the NEB/dimer method, an initial and final
state has to be provided. In contrast to the assumption of low hydrogen coverage for the
adsorbate optimization, the final state of dehydrogenation reactions has to be optimized with
the hydrogen nearby the intermediate in the same unit cell. For all optimized transition state
geometries, one imaginary frequency corresponding to the reaction coordinate is calculated.
6.4.1 Propane dehydrogenation to propylene
Electronic activation energies of all considered reactions in the thermodynamics section are
determined. It is assumed that the adsorption and desorption of the gaseous C3-species (propane
and propylene) are not activated. However, the reaction barrier for desorption is considered to
be equal to the desorption energy. On Gr/Pt(111), the reaction energy for desorption of
propylene is negative, so it is assumed that the reaction barrier is equal to zero. The other
elementary steps are indeed activated and the reaction barriers of these reactions are much
higher with respect to the same reactions on Pt (111). The steric effects of the graphene ribbon
destabilize the transition states in a similar way as the intermediates, leading to higher electronic
activation energies.
Chapter 6: Propane dehydrogenation kinetics on graphene ribbon covered Pt(111) as a model for cokes deactivated Pt catalysts 150
Table 6-7. Comparison between electronic activation energies for propane dehydrogenation towards propylene on Pt(111) and Gr/Pt(111) (4×2 unit cell). A coverage of 0.13 ML is achieved. This work uses an optPBE vdW-DF functional.
Δ‡E (kJ/mol)
Pt(111) Gr/Pt(111) # Surface reaction
0 Propane(g) → Propane,phys Physisorption 0 0
1 Propane,phys → 1-propyl + H Adsorption 80 173
2 Propane,phys → 2-propyl + H Adsorption 68 193
4 1-propyl → 1-propylidene + H Dehydrogenation 83 156
5 1-propyl → propylene + H Dehydrogenation 79 150
7 2-propyl → propylene + H Dehydrogenation 84 143
8 2-propyl → 2-propylidene + H Dehydrogenation 93 126
13a Propylene→ propylene,phys Desorption 92 0
13b Propylene,phys →Propylene (g) Desorption 43 47
The repulsion between the graphene ribbon and the intermediate prevents chemisorption on the
surface. Additionally, all reactions on the surface are so highly activated that they probably do
not occur. In Figure 6-8, the reaction towards propylene via 1-propyl is highlighted, as this is
least activated pathway on Gr/Pt(111). However, with respect to the same path on Pt(111), all
intermediates and transition states are more unstable due to repulsive interaction with the
graphene ribbon, except for physisorbed propane. The transition state of the first elementary
step is 81 kJ/mol more unstable on Gr/Pt(111) than on Pt(111), for the second the difference is
even 133 kJ/mol. On both the clean as the cokes deactivated Pt catalyst model, the first
elementary step has the highest electronic activation energy. The relative energy of propylene
desorption is 34 kJ/mol lower on Gr/Pt(111) than on Pt(111). This discrepancy is equal to the
difference in adsorption energies of two atomic hydrogen on the respective models. Cokes
deactivation leads not only to blockage of adsorption sites, but also lowers the activity of the Pt
catalyst. However, it should be noted that the observed magnitude of graphene ribbon repulsion
is due to the selection of the 4×2 unit cell. If this unit cell is periodically extended, the adsorbed
intermediates are surrounded on two sides by the graphene ribbons.
Chapter 6: Propane dehydrogenation kinetics on graphene ribbon covered Pt(111) as a model 151 for cokes deactivated Pt catalysts
Figure 6-8: Comparison of reaction path towards propylene via 1-propyl on clean Pt(111) (black) and Gr/Pt(111) (blue) (4×2 unit cell). The energy is calculated relative to gaseous propane.
6.4.2 Deep dehydrogenation of propylene
The kinetics of the deep dehydrogenation reactions are necessary to describe the selectivity
towards gaseous propylene with competes with formation of coke precursors via deep
dehydrogenation. Thermodynamically, 1-propylidyne is the most stable species on the surface
on the cokes deactivated Pt catalyst, so this species is selected a model compound for the coke
precursors. To define the selectivity descriptor between propylene desorption and deep
dehydrogenation towards 1-propylidyne, their respective reaction paths are compared, see
Figure 6-8. For the deep dehydrogenation reaction path, the transition state of dehydrogenation
of 1-propylidene to 1-propylidyne is calculated. The electronic activation energy of this reaction
is 18 kJ/mol, which is 27 kJ/mol lower than on Pt(111).
Both reaction paths start at 1-propyl, which can either dehydrogenate to propylene (desorption
reaction path) or 1-propylidene (coke precursor reaction path). However, for the first step, the
reaction barrier towards propylene is lower than for 1-propylidene and subsequently the latter
leads to a more unstable intermediate. The second step is the competition of 1-propylidene
dehydrogenation and propylene desorption via a physisorbed species. As last step in the
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)
Propane (g)
Propane, phys
1-propyl + H*
Propylene,phys + 2H
Propylene + 2H*
TS1
TS5
Reactant Product
Propylene,phys + H2(g)
Propylene (g) + H2(g)
Chapter 6: Propane dehydrogenation kinetics on graphene ribbon covered Pt(111) as a model for cokes deactivated Pt catalysts 152
formation 1-propylidyne has the lowest reaction barrier, this step is kinetically favored.
However, it should be noted that solely enthalpic factors are included in the energy profile.
Figure 6-9: Relative energy profile for propylene desorption (···) and dehydrogenation ( ̶ ̶ ) reaction paths as descriptor for the propylene selectivity on Gr/Pt(111) (4×2 unit cell). The energy is calculated relative to gaseous propane.
Referring to the Gibbs free energy diagram in 4.4.2, the inclusion of entropic contributions,
which are neglected in the relative energy profile, results into that gaseous propylene (+ H2
(g)) has a lower Gibbs free energy than 1-propylidyne (+ 3/2 H2 (g)) at 900K. As gaseous
species are independent of the catalyst model, the Gibbs free energy remains constant.
Furthermore, it is assumed that the entropic contributions of adsorbates are of the same
magnitude on different catalyst models. However, it should be noted that this assumption is less
applicable because the intermediates are weaker bonded to the catalyst surface. This leads to
different vibration modes and thus entropic contributions. So the relative stability of gaseous
propylene with respect to 1-propylidyne is mostly determined by the electronic energy
difference. On clean Pt(111), the energetic difference between gaseous propylene and 1-
propylidene, (accounting for the conservations laws, by adding extra gaseous hydrogen), is 132
kJ/mol. However, a general upward shift in electronic energies for adsorbate species is observed
on Gr/Pt(111). Hence, this leads to a smaller energetic difference on Gr/Pt(111) of 47 kJ/mol.
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Propane (g)
Propane, phys
1-propyl +H*
Propylene,phys + 2H*
Propylene + 2H*
Propylene (g) + 2H*
TS1
TS5
1-propylidene+ 2H*
TS17
TS4
1-propylidyne + 3H*
Reactant Product
Propylene (g) + H2 (g)
1-propylidyne + 3/2 H2 (g)
Chapter 6: Propane dehydrogenation kinetics on graphene ribbon covered Pt(111) as a model 153 for cokes deactivated Pt catalysts
The decrease points out that the formation of 1-propylidyne is less favored on Gr/Pt(111) with
respect to Pt(111).
6.5 The range and strength of cokes influence on propane dehydrogenation
In the previous sections, a 4×2 unit cell was used to describe the cokes deactivated Pt catalyst.
It can be concluded that the inclusion between graphene ribbons induces destabilization of the
adsorbates. The steric effects are so great that catalytic activity near these ribbons is small and
dehydrogenation is improbable to occur. However, due to the weakening of the adsorbate bonds
with the surface, the selectivity towards propylene is improved as the physisorbed species are
more stabilized than the chemisorbed species. It is expected that adsorbates less enclosed by
the graphene ribbons are more stable and the catalytic activity is retained. Hence, the 5×2 unit
cell is constructed (see 6.1) to determine the range and strength of the graphene ribbon on the
adsorbate stability and catalytic activity. Thermodynamically, the intermediates are expected to
form further away from the graphene ribbon. The reaction path for propane dehydrogenation
towards propylene via 2-propyl is considered to evaluate this effect. This reaction path is
compared with the 4×2 cokes deactivated Pt(111) unit cell and the clean Pt(111) 4×2 unit cell.
The optimized geometries can be found in Appendix E, together with a graphical representation
of the considered reaction path based on those geometries. Further, this section is divided into
two parts: thermodynamics and kinetics.
6.5.1 Thermodynamics
For the considered dehydrogenation reactions, the most stable adsorption site of hydrogen is
determined in a separate unit cell, as it is assumed that the hydrogen coverage is low. The most
stable adsorption site of hydrogen is on top on the 5×2 Gr/Pt(111) unit cell. The energy for
dissociative hydrogen adsorption on the 5×2 unit cell is -57 kJ/mol. As expected, the value is
situated between the ΔEads (H2) of -76 kJ/mol on clean Pt(111) and of -42 kJ/mol on cokes
deactivated Pt(111) (4×2 unit cell).
The optimized geometry of all intermediates can be found in Appendix E. Under strict
convergence criteria, the physisorbed propane and propylene have a residual RMS forces of
respectively 0.017 and 0.021 eV/Å, which is acceptable for these metastable species.
Chapter 6: Propane dehydrogenation kinetics on graphene ribbon covered Pt(111) as a model for cokes deactivated Pt catalysts 154
Furthermore, vibrational analyses are conducted to evaluate their stability. No imaginary
frequencies are found, except for physisorbed propane and propylene. These species have
respectively three and four imaginary frequencies, which can be assigned to translation and
external rotation above the surface.
Table 6-8. Comparison between reaction energies for propane dehydrogenation towards propylene via 2-propyl on clean Pt(111) and on two catalyst models for cokes deactivated Pt(111). This work uses an optPBE vdW-DF functional.
ΔEr (kJ/mol) 0.13 ML 0.1 ML Surface reaction Pt(111)
Gr/Pt(111)
(4×2) Gr/Pt(111)
(5×2) Propane(g) → Propane,phys Physisorption -43 -55 -49
Propane,phys → 2-propyl + H Adsorption -14 120 22
2-propyl → propylene + H Dehydrogenation -15 20 9
Propylene→ propylene,phys Desorption 92 -33 60
Propylene,phys → Propylene (g) Desorption 43 47 40
The calculated reaction energies on the larger unit cell show that the steric effects caused by
the graphene inclusion of the intermediates are reduced because the intermediates can adsorb
further from the graphene ribbon. It is logical that the largest decrease in reaction energy (-98
kJ/mol) is observed for the dehydrogenation of propane to form 2-propyl, as during this step
steric effects are too large when enclosed by cokes. The reverse is observed for the desorption
step (adsorbed propylene to physisorbed propylene), as now the chemisorbed species are
favorable with respect to the physisorbed.
Chapter 6: Propane dehydrogenation kinetics on graphene ribbon covered Pt(111) as a model 155 for cokes deactivated Pt catalysts
Figure 6-10: Relative energy profile for propane dehydrogenation towards propylene via 2-propyl on clean Pt(111) (black), 4×2 Gr/ Pt(111) (blue) and 5×2 Gr/Pt(111) (purple). Energies are determined relative to gaseous propane. Dehydrogenated hydrogen is optimized in a separate unit cell.
However, the graphene still induces repulsive interaction with the adsorbates, as can be seen in
the endothermic nature of the reaction energies. In the constructed energy profile, relative
energies of intermediates on the 5×2 Gr/Pt(111) model situate themselves between
intermediates of the two other models. The relative energies of the clean Pt(111) intermediates
are the most stabilized, while those of the 4×2 Gr/Pt(111) model are strongly endothermic with
respect to gaseous propane. It can be concluded that the 4×2 Gr/Pt(111) catalyst model was too
small to accurately describe the effect of cokes on the reaction mechanism since this catalyst
would not be active anymore.
6.5.2 Kinetics
The determination of the electronic activation energy is identical as in 6.4. It is assumed that
adsorption and desorption are not activated. In the case of desorption, the electronic activation
energy is equal to the desorption energy. For the considered activated reactions, it is observed
that on the larger unit cell of Gr/Pt(111), the reaction barriers are lower with respect to the
smaller unit cell, see Table 9 and Figure 6-10. The decrease of steric effects helps to stabilize
the transition states in a similar way as the intermediates.
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)
C3H8 (g) C3H8, phys C3H7* + H* C3H6
* + 2H* C3H6 (g) + 2H*
Propane (g)
Propane, phys 2-propyl
Propylene,phys
Propylene
Propylene (g)
C3H6, phys + 2H*
Chapter 6: Propane dehydrogenation kinetics on graphene ribbon covered Pt(111) as a model for cokes deactivated Pt catalysts 156
Table 6-9. Comparison between electronic activation energies for propane dehydrogenation towards propylene via 2-propyl on clean Pt(111) and on two catalyst models for cokes deactivated Pt(111). This work uses an optPBE vdW-DF functional.
Δ‡E (kJ/mol) 0.13 ML 0.1 ML Surface reaction Pt(111)
Gr/Pt(111)
(4×2) Gr/Pt(111)
(5×2) Propane(g) → Propane,phys Physisorption 0 0 0
Propane,phys → 2-propyl + H Adsorption 68 193 108†
2-propyl → propylene + H Dehydrogenation 84 143 115
Propylene→ propylene,phys Desorption 92 0 60
Propylene,phys →Propylene (g) Desorption 43 47 40 † The frequency analysis of the transition state for reaction 2 reported two imaginary frequencies.
The relative energy profile (Figure 6-11) shows that the activity is retained on 5×2 Gr/Pt(111)
in comparison with 4×2 Gr/Pt(111), both the energies of the intermediates and transition states
are shifted downwards. Kinetically, it is expected that the reaction rates are the highest on clean
Pt(111) and the proximity of graphene will lower the reaction rate until all reaction are
improbable to occur. In the final steps, all adsorbate are desorbs and the total reaction energy is
identical for all catalyst models, as electronic energies of the gaseous species do not depend on
the catalyst model.
Figure 6-11: Comparison of reaction path towards propylene via 2-propyl on clean Pt(111) (black), 4×2 Gr/Pt(111) (blue) and 5×2 Gr/Pt(111) (purple)
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ProductReactant
Propane (g)
Propane, phys 2-propyl + H*
Propylene,phys + 2H
Propylene + 2H*
TS2
TS7
Propylene,phys + H2(g)
Propylene (g) + H2(g)
Chapter 6: Propane dehydrogenation kinetics on graphene ribbon covered Pt(111) as a model 157 for cokes deactivated Pt catalysts
6.6 Conclusions
In this chapter, a model is constructed to quantify the effect of cokes deactivated Pt on the
activity towards propylene and deactivation of the catalyst during propane dehydrogenation.
Characterization of coke deposits on pure Pt catalyst have led to the selection of graphene as
model compound to represent the cokes on the Pt surface. Initially, a 4×2 unit cell is employed
for the calculations on Gr/Pt(111), directly extended from the Pt(111) catalyst model of Chapter
4. However, the catalytic activity on this catalyst model is poor as steric effects of the graphene
ribbon strongly destabilize the adsorbates. It is postulated that dehydrogenation reactions are
improbable to occur on this catalyst surface. Aside the poor activity, an improved selectivity
towards propylene with respect to deactivation reactions is observed. Thermodynamically, 1-
propylidyne is favored on the catalyst surface and an exothermic reaction energy is observed
for its formation, but all preceding intermediates are more unstable than on clean Pt(111). This
leads to a selectivity descriptor that favors propylene desorption with respect to the same
descriptor on Pt(111). So a distinct trade-off is made between activity and propylene selectivity.
It is clear that the steric effects in the 4×2 unit cell are too large, so a 5×2 unit cell is constructed
on which intermediates can adsorb further away from the graphene ribbon. This model restores
most of its activity compared with the smaller unit cell. The strength of the steric interactions
is reduced as they have a small range, however repulsion by the graphene ribbon still affects
the intermediates, but on a smaller scale. For the considered reaction path, the catalytic activity
is lower than on the clean Pt(111) catalyst model.
6.7 References
1. Siddiqi, G., et al., Catalyst performance of novel Pt/Mg(Ga)(Al)O catalysts for alkane dehydrogenation. Journal of Catalysis, 2010. 274(2): p. 200-206.
2. Preto, I., Graphene formation and oxidation on Pt/Mg(Ga)AlOx dehydrogenation catalysts, in Department of Chemical Engineering and Technical Chemistry. 2015, University of Ghent: Ghent.
3. Larsson, M., et al., The effect of reaction conditions and time on stream on the coke formed during propane dehydrogenation. Journal of Catalysis, 1996. 164(1): p. 44-53.
4. Vu, B., et al., Electronic density enrichment of Pt catalysts by coke in the propane dehydrogenation. Korean Journal of Chemical Engineering, 2011. 28(2): p. 383-387.
5. Peng, Z., et al., High-resolution in situ and ex situ TEM studies on graphene formation and growth on Pt nanoparticles. Journal of Catalysis, 2012. 286: p. 22-29.
6. Zaera, F. and D. Chrysostomou, Propylene on Pt(111) I. Characterization of surface species by infra-red spectroscopy. Surface Science, 2000. 457(1-2): p. 71-88.
Chapter 6: Propane dehydrogenation kinetics on graphene ribbon covered Pt(111) as a model for cokes deactivated Pt catalysts 158
7. Valcárcel, A., et al., Theoretical study of the structure of propene adsorbed on Pt(111). Surface Science, 2002. 519(3): p. 250-258.
Chapter 7: Propane dehydrogenation kinetics on Pt(211) catalyst model 159
Chapter 7 Propane dehydrogenation kinetics on Pt(211) catalyst model The solid state Pt catalyst consists of several surface planes such as the (111) and (110) planes
of which the Pt(111) is the most abundant. However, the transition of one plane to another is
coupled to the formation of undercoordinated edges. Furthermore, irregularities on the surface
leads to undercoordinated sites such as steps and kinks. Literature has shown that those
undercoordinated sites have a high catalytic activity and are preferable to the terrace Pt(111)
sites at the initial stage of propane dehydrogenation. Apart from high activity, a low
propylene selectivity and large coke accumulation on these sites are observed at this stage,
indicating that coke deactivates these sites rather fast. [1-3]
It is opted to construct the catalyst model based on the undercoordinated step site. Pt(211) is
the smallest fcc surface that consists of step sites. Furthermore, Yang et al. conducted a DFT
study on the effect of step sites and also employed a Pt(211) unit cell. However, a coverage of
0.25 ML is employed, which is higher than the propylene monolayer coverage. Furthermore,
Yang et al. use the PBE functional that does not account for van der Waals interactions. To
obtain qualitative insight, the reaction path via 2-propyl to form propylene on Pt(211) is
evaluated based on its comparison with Yang et al. [2]
7.1 Catalyst model
The Pt(211) catalyst model is represented by a 3×2 unit cell, see Figure 7-1. The a-direction
consists of two Pt atoms and the b-direction of three atoms. In the latter direction, the step is
160 Chapter 7: Propane dehydrogenation kinetics on Pt(211) catalyst model
located. In the c-direction, four layers of Pt atoms are used. The upper two layers can relax as
they represent the surface atoms, while the bottom two layers are kept fixed as they represent
the bulk structure of the model. In this direction, a vacuum layer of 12 Å and an artificial
dipole layer are constructed to avoid periodic interactions. A Monkhorst-Pack grid of 5×5×1
is utilized for the Brillouin zone integration. The optimization is completed under strict
convergence criteria with optPBE vdW-DF as functional. The unit cell dimensions, including
vacuum layer, are 5.64×6.91×17.46 ų
Figure 7-1: Isometric representation of the 4×2 unit cell of the Pt(211) catalyst model
7.1.1 Adsorption site nomenclature
Four types of adsorption sites conform the nomenclature of Chapter 4 are available in this
model. While most of adsorption sites are identical with respect of those in Chapter 4, the
focus will be on those nearby the step. The considered adsorption sites are illustrated in
Figure 7-2. Hence, to avoid confusion with the Pt(111) adsorption sites, the suffix “near-
edge” is added during comparison with the Pt(111) model.
Step
b
c
a
Chapter 7: Propane dehydrogenation kinetics on Pt(211) catalyst model 161
Figure 7-2: Top view of Pt(211) unit cell. Adsorption sites near the edge on the Pt(211) catalyst model are shown: top ●, bridge █ , three-folded hcp ▲ and fcc ▼.
7.1.2 Determination of the degree of coverage
The degree of coverage as defined in 4.1.2, is used in this chapter as well. Therefore, for one
adsorbate per six surface atoms, the coverage equals 1/6 ML (0.17 ML). This is lower than the
monolayer coverage of propylene (0.20 ML). [4]
7.2 Adsorption
The main compounds of propane dehydrogenation that occur in the gasphase are propane,
propylene and H2. Cracking reactions can lead to eventually to formation of smaller gaseous
hydrocarbons such as methane, ethane and ethylene. However, their adsorption will not be
considered on Pt(211). The optimized geometries can be found in Appendix F.
7.2.1 Propane
Propane is a saturated C3 hydrocarbon that weakly adsorbs on the Pt surface. During
physisorption, no binding is formed with the Pt surface and propane is solely stabilized by van
Step
Step
b
a
162 Chapter 7: Propane dehydrogenation kinetics on Pt(211) catalyst model
der Waals interactions. Wang et al. studied the molecular propane adsorption on a Pt(655)
surface. This surface is formed by separating Pt(111) terraces by the Pt(100) surface of single
atom height and it can be considered as a step surface. The favorable adsorption zone for
propane was near the step edge on the Pt surface. [5]
The initial position of physisorbed propane on Pt(211) is located in the favorable adsorption
zone proposed by Wang et al. The resulted geometry is located above the step, parallel with
surface. The distance between the Pt and closest C atom of propane is 3.3 Å, which is similar
to the distance calculated on Pt(111). The C-C bonds of physisorbed propane are identical to
C-C bonds of gaseous propane, so no strong interaction with the Pt surface is observed.
Additionally, frequency analyses have shown only one imaginary frequency. This imaginary
frequency corresponds to external rotation of the physisorbed propane species.
Table 7-1. Comparison of physisorption energies of propane on Pt(111) and Pt(211) surface. Yang et al. used a PBE functional, while in this an optPBE vdW-DF functional is employed.
Pt(111) Pt(211)
ΔEads (kJ/mol) Yang et al. (0.11 ML) [6]
This work (0.13 ML)
Yang et al. (0.25 ML) [2]
This work
(0.17 ML) Propane physisorption -6 -43 -4 -53
The large discrepancy for adsorption energies of propane between the DFT studies can be
explained due the difference of the utilized DFT functionals. In this work, van der Waals
interaction are included, which lead to (over-)stabilization of the physisorbed species. In the
work of Yang et al., no distinct effect of the step sites on the propane physisorption can be
observed as energetic difference between Pt(111) and Pt(211) lays within the error margin of
the employed PBE functional. However, in this work, an additional stabilization is found
nearby Pt steps of ~10 kJ/mol.
7.2.2 Propylene
As propylene has an unsaturated bond, it can directly decompose and adsorb on the Pt surface.
Zaera et al. propose four different adsorption modes in function of the employed propylene
coverage of which is the most important, followed by π propylene. Literature indicates that
the di-σ adsorption mode is more stable than the π adsorption mode. Conform 4.2.2.1; it is
opted to solely study di-σ adsorption mode of propylene. [7, 8]
On the step sites, di-σ propylene prefers to adsorb on the bridge site near the step edge, as
indicated in Figure 7-2. This is the same site as on the flat Pt(111) surface. The C=C bond of
Chapter 7: Propane dehydrogenation kinetics on Pt(211) catalyst model 163
adsorbed propylene is weakened as its bond length is 1.49 Å, which is more similar to the C-
C bond length of gaseous propane (1.54 Å) than gaseous propylene (C=C length of 1.36 Å).
Furthermore, the same geometry is observed as on the flat Pt(111) surface, with the methyl
moiety perpendicular to the surface. Aside from the di-σ adsorption mode of propylene, a
physisorption mode is investigated. In this adsorption mode, covalent bonds are formed with
the surface and physisorbed propylene retains its gasphase geometry. The most stable
adsorption site is over the edge of the step site. However, this mode did not converge under
strict criteria as an RMS force of 0.027 eV/Å remains. Still, this is adequate for a metastable
adsorption mode. Additionally, three imaginary frequencies are observed and can be assigned
to modes of translation and external rotation. The physisorbed propylene is slightly tilted with
respect to the surface, to stabilize the methyl moiety of propylene. The C=C is not stretched
and bond length of 1.34 is similar to that of gaseous propylene (1.36 Å).
Table 7-2. Comparison of adsorption energies of di-σ propylene on Pt(111) and Pt(211) surface. Yang et al. used a PBE functional, while in this a vdW-DF functional is employed.
Pt(111) Pt(211)
ΔEads (kJ/mol) Yang et al. (0.11 ML) [6]
This work (0.13 ML)
Yang et al. (0.25 ML) [2]
This work
(0.17 ML)
Propylene adsorption -90 -135 -138 -158
In both studies, the adsorption energies of Pt(211) are much lower than on Pt(111), indicating
that di-σ propylene prefers to adsorb on undercoordinated step sites. This additional
adsorption strength on Pt(211) will make desorption more difficult on these sites as for
desorption the reaction barrier will be more endothermic. Yang et al. did not report the
adsorption energy of physisorbed propylene. However, this work indicates that this adsorption
mode is more stabilized on the step sites as an adsorption energy of -105 kJ/mol is observed,
with respect to -43 kJ/mol on the Pt(111) terrace surface.
7.3 Thermodynamics
In this section, the reaction path towards propylene via 2-propyl is investigated as part of the
reaction network proposed in Chapter 4 to obtain qualitative insight on the effects of step sites
on the stability of the adsorbed intermediates. The following intermediates are considered for
this reaction path: physisorbed propane, 2-propyl, di-σ propylene and physisorbed propylene.
For each component, the most stable geometry on the Pt(211) catalyst model is calculated and
164 Chapter 7: Propane dehydrogenation kinetics on Pt(211) catalyst model
a frequency analysis is conducted to evaluate its stability. A condensed version of this
reaction network is shown in Figure 7-3. The optimal geometries can be found in Appendix F,
together with a visualized reaction path based on those optimal geometries. It should be noted
that this network path is a part of the proposed reaction network proposed in Chapter 4 as the
same numbering for the reactions is used.
Figure 7-3: Reaction path towards propylene via 2-propyl on Pt(211).
This reaction path was selected as it is important to quantify the catalytic activity towards
propylene. The considered reaction path consists solely of adsorption/desorption (reaction 0
and 13) and dehydrogenation steps (reaction 2 and 7). Complementary to previous chapters,
the hydrogen formed during dehydrogenation reaction diffuses away from the hydrocarbon
adsorbate. This is implemented in our calculations by optimizing the adsorbate and hydrogen
in separate unit cells. To determine the reaction energies, equation (3), (4) and (5) of section
4.3 are used. It is essential to locate the most stable adsorption site for atomic hydrogen.
Hydrogen prefers to adsorb either in a hollow fcc site or on top. However, previous pure Pt
catalyst model (see Chapter 4 and 6) have shown that the top site is the most stable adsorption
for hydrogen with the selected optPBE vdW-DF functional. So hydrogen was optimized on
various top positions: on the flat surface and near the edge. The energetic difference is 4
kJ/mol in favor for the hydrogen adsorbed nearby the edge. This most stable geometry of
adsorbed hydrogen will be used in reaction energy calculations of the dehydrogenation
reactions. With respect to Pt(111), atomic hydrogen adsorbs dissociatively 8 kJ/mol stronger
on the Pt(211) surface.
2-propyl
CH2=CH-CH3,phys
CH2=CH-CH3(g)
7CH3-CH-CH3 + H
Pt Pt
_ _2CH3-CH2-CH3,phys
CH3-CH2-CH3(g)Gaseous propane
0
Physisorbed propane
CH2-CH-CH3 + H
Pt
__
Pt
_
PtPropylene
Physisorbed propylene
Gaseous propylene
13
Chapter 7: Propane dehydrogenation kinetics on Pt(211) catalyst model 165
7.3.1 Propane dehydrogenation to propylene
In the considered reaction path, two elementary steps are considered: dehydrogenation of the
methylene group of propane (reaction 2) and β-dehydrogenation of 2-propyl towards
propylene. Prior to these reaction steps, the physisorption of propane is considered. However,
this reaction is discussed in the adsorption section. Physisorbed propane is a metastable
species and acts as a precursor to C3H7-species.
In the first elementary step, 2-propyl adsorbs on top of a Pt atom, conform the adsorption on
Pt(111), but now near the edge. However, the most stable geometries differ between this work
and Yang et al., especially the orientation of the outer two methyl groups. In the optimized
geometry of Yang et al, the two methyl groups remain parallel with the step and neither of
them hangs over the step more than the other. In this work, various geometries, obtained from
rotating the propyl group around the Pt-C bond, are proposed as initial guess and the minimal
electronic energy is found where one methyl group is orientated over the edge and the other
above the terrace. For the second elementary step, propylene adsorbs on the bridge site, near
the edge. However, the orientation of methyl moiety of propylene differs between the DFT
studies. In this work, the methyl moiety is perpendicular to the surface, while Yang et al.
reported a geometry with the methyl group parallel to the surface orientated over the edge.
The different optimized geometry can be explained based on the long-range stabilization of
van der Waals interaction by the selected functional in this work.
The reported reaction energies by Yang et al. are considerably lower than in this work, except
for the physisorption of propane as the employed PBE functional cannot describe this state of
propane very well. Furthermore, reaction energies of the dehydrogenation step are 35-40
kJ/mol lower than in this work, indicating that either the used PBE functional or the higher
coverage strongly stabilize the intermediates. The general trend is that higher coverage
destabilizes the intermediates on the surface, so most probably the use of a different
functional is the reason for the discrepancy.
166 Chapter 7: Propane dehydrogenation kinetics on Pt(211) catalyst model
Table 7-3. Comparison between reaction energies for the propane dehydrogenation reaction path via 2-propyl on Pt(111) and Pt(211). Yang et al. used a PBE functional, while in this an optPBE vdW-DF functional is employed.
ΔEr (kJ/mol) Pt(111) Pt(211)
Reaction # Yang et al. (0.11 ML) [6]
This work (0.13 ML)
Yang et al. (0.25 ML) [2]
This work
(0.17 ML)
0 -2 -43 -4 -53
2 -6 -14 -51 -22
7 -23 -15 -59 -24
13a 90 92 138 53
13b - 43 - 105
The step sites are more active, leading to more stable intermediates and lower reaction
energies in both DFT studies. However, no general trend is observed in the literature. No
distinct energetic difference is found for the physisorption of propane, while the reaction
energies of the first and second elementary step are considerably lower, respectively 45
kJ/mol and 36 kJ/mol, see Figure 7-4. In this work, a decrease of ~9 kJ/mol is observed for
the reaction energies with respect to Pt(111) as can be seen in Figure 7-4.
Thermodynamically, the adsorption sites near the step are preferred with respect to the
Pt(111) adsorption sites.
Figure 7-4: Energy profile of adsorbed C3Hx (x= 3–6) species on the (black) Pt(111) and (red) Pt(211) surface (respectively coverage of 0.13 ML and 0.17 ML). Energies are determined relative to gaseous propane, while the detached hydrogen is optimized in separate unit cell for dehydrogenation reactions.
-120
-100
-80
-60
-40
-20
0
20
40
60
80
Rel
ativ
e en
ergy
(kJ/
mol
)
C3H8 (g) C3H8, phys C3H7* + H* C3H6
* + 2H* C3H6,phys + 2H*
Propane (g)
Propane, phys2-propyl
Propylene,phys
Propylene
Propylene (g)
C3H6 (g)+ 2H*
Chapter 7: Propane dehydrogenation kinetics on Pt(211) catalyst model 167
The reaction pathway towards propylene via 2-propyl can be treated as a catalytic cycle and,
if correct, the total reaction energy should correspond to the gasphase reaction of propane to
propylene and H2. If the reaction energies of the considered elementary steps are summated, a
total reaction energy of 138 kJ/mol is found, which corresponds with the gas phase reaction
energy.
7.4 Kinetics
To determine the reaction barriers of the proposed reactions, the corresponding transition
states of these reactions have to be found. Hence, by combining the electronic energy of the
transition state and the reactant, the electronic activation energy Δ‡E is calculated conform the
equation in Chapter 4. This energy is an estimation of the reaction barrier of that reaction.
In the considered reaction pathways, it is assumed that the physisorption of propane is not
activated and hence, has a reaction barrier equal to zero. The two considered elementary steps
are activated, but the transition states on the steps are more stable and lower barriers are found
with respect to Pt(111) for both DFT studies, see Table 4. It is concluded that propane
dehydrogenation on step sites is more active. The optimized geometries are similar to those of
Yang et al., especially the bond length between the detached hydrogen and carbon atoms are
identical. These geometries can be found in Appendix F.
Table 7-4. Comparison of the electronic activation energies on Pt(111) and Pt(211). Yang et al. used a PBE functional, while in this an optPBE vdW-DF functional is employed.
Δ‡E (kJ/mol) Pt(111) Pt(211)
Reaction # Yang et al. (0.11 ML) [6]
This work (0.13 ML)
Yang et al. (0.25 ML) [2]
This work
(0.17 ML)
0 0 0 0 0
2 68 68 27 49†
7 61 84 32 22
13a 90 92 138 53
13b - 43 - 105 † This transition state has an additional imaginary frequency; however, it can be assigned to lattice vibration.
However, some discrepancy is observed between the energies of the DFT studies. The
electronic energies of Yang et al. are similar for the two elementary steps on Pt(211), while in
this work, the first elementary step is higher activated (49 kJ/mol) and the second is lower
activated (22 kJ/mol), indicating that the activation of propane is the rate determining step.
168 Chapter 7: Propane dehydrogenation kinetics on Pt(211) catalyst model
Figure 7-5: Relatieve energy profile for propylene reaction path via 2-propyl on (black) Pt(111) and (red) Pt(211). (respectively coverage of 0.13 ML and 0.17 ML).The energy is calculated relative to gaseous propane while dehydrogenated hydrogen is optimized in a separate unit cell.
Based on the relative energy profile (see Figure 7-5), it is observed that adsorption sites near
the step of Pt(111) have a higher activity as lower reaction barriers are observed for the
propane dehydrogenations reactions towards propylene on Pt(211). However, the reaction
barrier for propylene desorption from adsorbed species to gasphase is 158 kJ/mol on Pt(211),
which 25 kJ/mol more than on Pt(111), indicating that propylene desorption is less likely on
Pt(211) than Pt(111).
It is also expected that the selectivity towards propylene is lower on the Pt(211) surface than
on the Pt(111) because further dehydrogenation of the adsorbed propylene can be favored.
Yang et al. reports results concerning deeply dehydrogenated species, confirming the low
selectivity towards propylene on step sites, as deep dehydrogenation steps are both
thermodynamically and kinetically favored until formation of propyne. For these types of
species, cracking reactions have low reaction barriers, leading to the formation of cokes. This
confirms the results of Peng et al. that undercoordinated sites such as steps are the initiating
site for coke formation on Pt catalysts [2, 3]
Furthermore, Yang et al. observed the hydrogenolysis of propane, which is identified as an
important step towards gaseous side product as its dissociation forms ethyl and methyl on the
-100
-50
0
50
100
150
Rel
ativ
e en
ergy
(kJ/
mol
)
Reactant Product
Propane (g)
Propane, phys
2-propyl + H*
Propylene,phys + 2H*
Propylene +2H*
Propylene (g) + 2H*
TS2
Propylene (g) + H2(g)
TS7
Chapter 7: Propane dehydrogenation kinetics on Pt(211) catalyst model 169
surface. The reported reaction barriers are 235 kJ/mol on Pt(111) and 157 kJ/mol on Pt(211).
These values are both higher than the respective reaction barriers for dehydrogenation.
However, relative speaking, the reaction barrier for propane hydrogenolysis decrease more
than the barriers for propane dehydrogenation, indicating that steps sites are more prone to
propane dehydrogenation, and subsequently the formation of gaseous side products.
Preliminary calculations on the propane hydrogenolysis on Pt(211) have shown that the
transition state does not convergence under strict criteria.
It should be noted that apart of the effect of the steps, the coverage is also different for both
catalyst models. At lower coverages and under the saturation coverage, it is expected that
intermediates are weaker adsorbed on the Pt surface. [9] However, this effect is not observed,
as the coverage effect is subjected to the effect of the step site.
7.5 Conclusions
To obtain qualitative insight in the effect of steps on the Pt surface on propane
dehydrogenation characteristics, the reaction path towards propylene via 2-propyl is
investigated and compared with literature. As catalyst model for Pt(211), a 3×2 unit cell is
employed to obtain a coverage of 0.17 ML. The hydrocarbon intermediates are optimized in
adsorption sites located near the step.
The resulted thermodynamics and kinetics on Pt(211) are shown as a reaction profile relative
to gaseous propane and are compared with the reaction profile on Pt(111) in Figure 7-5. Both
thermodynamically and kinetically, the steps are preferred as adsorption sites due their higher
reactivity. However, it should be noted that by strengthening the propylene bonds with the
surface, a higher reaction barrier is observed for propylene desorption, indicating that
propylene is less likely to desorb on Pt(211) than on Pt(111).
Further, Yang et al. proposed that steps sites are more sensitive for deep dehydrogenation and
hydrogenolysis. Deep dehydrogenation steps are both thermodynamically and kinetically
favored until formation of propyne. Consecutively, these species have low barriers for
hydrogenolysis, leading to the formation of cokes.
170 Chapter 7: Propane dehydrogenation kinetics on Pt(211) catalyst model
7.6 References
1. Siddiqi, G., et al., Catalyst performance of novel Pt/Mg(Ga)(Al)O catalysts for alkane dehydrogenation. Journal of Catalysis, 2010. 274(2): p. 200-206.
2. Yang, M.L., et al., DFT study of propane dehydrogenation on Pt catalyst: effects of step sites. Physical Chemistry Chemical Physics, 2011. 13(8): p. 3257-3267.
3. Peng, Z., et al., High-resolution in situ and ex situ TEM studies on graphene formation and growth on Pt nanoparticles. Journal of Catalysis, 2012. 286: p. 22-29.
4. Tsai, Y.L., C. Xu, and B.E. Koel, Chemisorption of ethylene, propylene and isobutylene on ordered Sn/Pt(111) surface alloys. Surface Science, 1997. 385(1): p. 37-59.
5. Wang, J.-C., Effects of surface step on molecular propane adsorption. Surface Science, 2003. 540(2–3): p. 326-336.
6. Yang, M.L., et al., Density functional study of the chemisorption of C-1, C-2 and C-3 intermediates in propane dissociation on Pt(111). Journal of Molecular Catalysis a-Chemical, 2010. 321(1-2): p. 42-49.
7. Zaera, F. and D. Chrysostomou, Propylene on Pt(111) I. Characterization of surface species by infra-red spectroscopy. Surface Science, 2000. 457(1-2): p. 71-88.
8. Valcárcel, A., et al., Theoretical study of the structure of propene adsorbed on Pt(111). Surface Science, 2002. 519(3): p. 250-258.
9. Nykänen, L. and K. Honkala, Density Functional Theory Study on Propane and Propene Adsorption on Pt(111) and PtSn Alloy Surfaces. The Journal of Physical Chemistry C, 2011. 115(19): p. 9578-9586.
Chapter 8: Conclusions and prospects 171
Chapter 8 Conclusions and prospects Recently, the catalytic dehydrogenation of light paraffins (C2-C3) has emerged as promising
technology for on purpose production of light olefins, independent of the olefin production via
steam cracking. The focus of this thesis is on the catalytic dehydrogenation of propane towards
propylene.
Ab initio calculations have been employed to obtain fundamental insight in propane
dehydrogenation kinetics. In the defined computational framework, a non-local optPBE vdW-
DF functional is used to describe the geometries of the intermediates and transition states in the
proposed reaction network on the considered catalyst models.
Platinum metal catalysts are an adequate choice as catalyst for light alkane dehydrogenation as
they exhibit a high activity for the dehydrogenation steps. However, those catalysts lack a high
selectivity towards olefins, as many side reactions occur on the surface. Eventually, these side
reactions initiate coke formation, leading to unwanted deactivation of the catalyst. [1] To
determine the propane dehydrogenation characteristics on pure Pt catalysts, a catalyst model of
the most abundant Pt(111) phase is constructed. The DFT calculations are conducted on a 4×2
Pt(111) unit cell, corresponding to a molecular coverage of 0.13 ML. The proposed reaction
network is divided in three main sections: propane dehydrogenation to propylene, deep
dehydrogenation of C3H6-species and hydrogenolysis of C3 intermediates. This first section is
employed to quantify the catalyst activity of pure platinum as reaction barriers and energies of
the elementary dehydrogenation steps towards propylene are important. In the second section,
the deep dehydrogenation towards coke precursors is investigated. The selectivity towards these
species is an indication for the catalyst deactivation. In the third section, the hydrogenolysis
reactions are taken into account. These reactions form C1 and C2 adsorbates on the surface.
172 Chapter 8: Conclusions and prospects
These species eventually lead to gaseous side products such as methane, ethane and ethylene
and affect the selectivity towards propylene.
Both the thermodynamics and kinetics of propane dehydrogenation are evaluated for the
considered reactions. The propane dehydrogenation reaction towards propylene can occur via
1-propyl and via 2-propyl but, the reaction path via 2-propyl is preferred as it has a lower
reaction barrier with respect to 1-propyl. In contrast to literature [2], adsorbed 2-propyl is more
stable than 1-propyl in this work. This catalyst model serves as reference for more advanced
models. However, as the Gibbs free energies at 873 K of these species and the reaction barriers
of the elementary step are similar, it is expected that both reaction paths contribute to the
formation of propylene. Both 1-propyl and 2-propyl preferentially dehydrogenate further to
propylene, as it is the thermodynamically most stable C3H6-species. The resulting electronic
activation energy is similar for both reactions.
Deep dehydrogenation leads to the thermodynamically most stable species on the surface, i.e.
1-propylidyne on the Pt(111) surface. Multiple reaction paths towards 1-propylidyne are
possible if isomerization reactions are included. However, it is shown that the isomerization
reactions are highly activated and unlikely to occur. Furthermore, the α-dehydrogenation of 1-
propylidene towards 1-propylidyne is kinetically favored. The most probable path is via
propane →1-propyl →1-propylidene →1-propylidyne. However, this is solely based on the
electronic energy (enthalpic contributions). The entropic contributions are included from
harmonic oscillator calculations and the Gibbs free energy of the intermediates and transition
states is determined. The entropic contributions are greater for gaseous species than for
adsorbates, so based on the Gibbs free diagram at 900 K, gaseous propylene is 11 kJ/mol more
stable than 1-propylidyne.
Hydrogenolysis reactions compete with dehydrogenation reactions and C-C bond breaking
leads to the formation of less stable products with respect to the dehydrogenated species, except
for the cracking of 2-propylidene into ethylidyne and methyl. However, C-C cleavage of 2-
propylidene and other hydrogenolysis reactions are averagely 95 kJ/mol more activated than
the competing dehydrogenation reactions. Therefore, it can be concluded that these reactions
are less favored for the considered species on the Pt(111) surface.
To validate the calculated thermodynamics and kinetics, a microkinetic simulation of the
propane dehydrogenation on Pt(111) is conducted, initially at 873 K and total reactant pressure
of 0.4 bar (C3H8/H2: 3/1). Three time regimes are investigated: short (0.1 s), intermediate (10
s) and long (109 s). The highest TOF for propylene and hydrogen gas (1191 s-1) is observed at
short simulated time. Initially, the surface is for 80% covered with 1-propylidyne, consistent
Chapter 8: Conclusions and prospects 173
with the ab initio calculations. At intermediate simulated time (10 s), the coverage of 1-
propylidyne starts to decline and ethylidyne (CH3C≡Pt) and methylidyne (HC≡Pt) are formed.
This indicates that 1-propylidyne is kinetically preferred as adsorbate (fast coke precursors),
while the other species are thermodynamically favored (slow coke precursors). At a long
simulated time (109 s), no catalyst activity for propane dehydrogenation is reported and the
surface is completely covered with slow coke precursors. The intermediate simulated time is
further employed as it still shows high catalytic activity, while 1-propylidyne is converted
substantially to more stable coke precursors. While varying the simulation temperature, a
maximum TOF for propylene dehydrogenation is observed at 948 K. However, at this
temperature, formation of gaseous side products, especially methane and ethylene is also
increased, lowering the selectivity towards gaseous propylene. Increasing the pressure of the
products, i.e. propylene, methane, ethylene and ethane, does not have a significant influence on
the catalytic activity and selectivity.
The recent experimental studies on Ga-promoted Pt catalysts during light alkane
dehydrogenation have already shown an increase in catalytic activity and selectivity towards
propylene with respect to an unmodified Pt catalyst on the same support [3]. Saerens [4] already
identified the most likely PtxGay alloy at the catalytic surface by comparing frequencies of CO
adsorption experiments with the results of theoretical CO vibrational frequencies on Pt-Ga
catalyst models with various Pt/Ga ratios. The most likely candidate among the different studied
catalyst models is the Pt3Ga bulk alloy, the studied model catalyst with the lowest Ga content.
To assess the role of Ga on propane dehydrogenation kinetics on Pt-Ga catalysts, a 4×2
Pt3Ga(111) unit cell is constructed. The same molecular coverage is achieved as on Pt(111).
Propane dehydrogenation towards propylene consists of two elementary steps and occurs via
two reaction pathways (via 1-propyl and via 2-propyl). The reaction barriers of the two
elementary steps towards propylene define the catalytic activity. On Pt3Ga(111), both 1-propyl
and 2-propyl are more stabilized than on Pt(111). In contrast to Pt(111), is 2-propyl
thermodynamically more stable than 1-propyl and hence the reaction pathway via 2-propyl is
selected to describe the catalytic activity on Pt3Ga(111). Furthermore, adsorbed propylene
bonds stronger on the Pt3Ga(111) surface and the calculated reaction barriers are lower than on
Pt(111), while the propylene desorption barrier remains constant. It can be concluded that the
catalytic activity is potentially larger on Pt3Ga(111) than on Pt(111).
In addition, the deep dehydrogenation of C3H6-species is investigated on Pt3Ga(111). The
formation of these species are an indication for the deactivation of the catalyst. Similar to
174 Chapter 8: Conclusions and prospects
Pt(111), 1-propylidyne is thermodynamically the most favored species on the Pt3Ga(111)
surface. To quantify the selectivity to the coke precursors, the reaction energies and barrier for
the reaction path via 1-propyl and 1-propylidene towards 1-propylidyne are determined. This
reaction path has higher reaction barriers on Pt3Ga(111) than on Pt(111) and consequently 1-
propylidyne is not as kinetically favored as on Pt(111). Furthermore, the electronic energy
difference between propylene and 1-propylidyne on Pt3Ga(111) is much smaller than on
Pt(111). As it can be assumed that the entropic contributions for adsorbed species such as 1-
propylidyne are similar on Pt3Ga(111) as on Pt(111), the relative Gibbs free energy of 1-
propylidyne on Pt3Ga(111) with respect to Pt(111) is determined by the difference in electronic
energy on Pt(111) and Pt3Ga(111). Based on this reasoning, it is expected that 1-propylidyne is
less likely to be formed on Pt3Ga(111) than on Pt(111), both from a thermodynamic and a
kinetic point of view. This indicates that fewer coke precursors will be formed and that Ga
reduces the formation of cokes on the surface, which is confirmed based on the reduced carbon
formation on Pt-Ga/Mg(Ga)(Al)Ox.
The hydrogenolysis of propane is studied on Pt3Ga(111) as this reaction forms C1 and C2
adsorbates, which are critical for the formation of gaseous side products such as ethane,
ethylene and methane. However, this reaction is highly activated (191 kJ/mol), 4 kJ/mol more
than on Pt(111) while the reaction barriers for the competing dehydrogenation reactions are
lower on Pt3Ga(111). Hence, the formation of gaseous side products via this way is unlikely.
This result is supported by the experimental results that show that the propylene selectivity is
above 98% on Pt-Ga/Mg(Ga)(Al)Ox for the total time on stream. [3]
The coke formation on the surface of the Pt catalysts leads inherently to the deactivation of the
catalyst surface. Furthermore, the effect of cokes deactivated Pt on nearby Pt atoms is
investigated in terms of catalytic activity and further deactivation of the catalyst during propane
dehydrogenation. [5] Graphene is selected as representation of the coke on the cokes
deactivated Pt catalyst based on TEM observations. [6] The catalyst model Gr/Pt(111) is
primarily constructed in a 4×2 unit cell, in which half of the supercell is covered with a
continuous 1-D graphene ribbon. The catalytic activity on this catalyst model is poor as steric
effects of the graphene ribbon strongly destabilize the adsorbates. It is postulated that
dehydrogenation reactions are improbable to occur on this catalyst surface based on the resulted
high reaction barriers and energies. In contrast the poor activity, an improved selectivity
towards propylene with respect to deactivation reactions is observed. Thermodynamically, 1-
propylidyne is favored on the catalyst surface and an exothermic reaction energy is observed
for its formation, but all preceding intermediates are more unstable than on clean Pt(111). This
Chapter 8: Conclusions and prospects 175
leads to a selectivity descriptor that favors propylene desorption with respect to the same
descriptor on Pt(111). So a distinct trade-off is made between activity and propylene selectivity.
It is clear that the steric effects in the 4×2 unit cell are too large due to the graphene ribbon
inclusion, so a 5×2 unit cell is constructed on which intermediates can adsorb further away from
the graphene ribbon. This model restores most of its activity compared with the smaller unit
cell. The strength of the steric interactions is reduced as they have a small range, however
repulsion by the graphene ribbon still affects the intermediates, but on a smaller scale. For the
considered reaction path, the catalytic activity remains lower than on the clean Pt(111) catalyst
model.
Literature reports that the origin of gaseous side products and initial coke formation can be
assigned to the reactivity of undercoordinated sites such as steps and edges of the Pt catalyst.
[6] To obtain qualitative insight on the effects of undercoordinated sites on propane
dehydrogenation characteristics, a Pt(211) catalyst model is constructed as a representation of
an undercoordinated step on the Pt surface. For this model, a 3×2 unit cell is employed to obtain
a coverage of 0.17 ML, which is higher than the reference coverage of 0.13 ML. Furthermore,
the hydrocarbon intermediates are optimized in adsorption sites located near the step and the
reaction path towards propylene via 2-propyl is investigated.
Both thermodynamically and kinetically, the steps are preferred as adsorption sites due to their
higher reactivity than the terrace plane (Pt(111)). However, it should be noted that by
strengthening the propylene bonds with the surface, propylene adsorbs stronger, indicating that
propylene is less likely to desorb on Pt(211) than on Pt(111). The higher coverage of this model
(0.17 ML) compared to the previously considered catalyst models (0.13 ML) induces a minor
destabilization of the surface species mutually. However, as the reverse energetic effect
(stabilization) is reported, it is concluded that coverage affects the adsorbates weaker than the
step sites. Furthermore, Yang et al. proposed that steps sites are more sensitive for deep
dehydrogenation and hydrogenolysis reactions. [7] Deep dehydrogenation steps considered by
Yang et al., are both thermodynamically and kinetically favored until formation of propyne.
Consecutively, these species have low barriers for hydrogenolysis, leading to C1 and C2-species
on the catalyst surface.
Summarizing, Ga alloying of Pt catalysts show superior catalytic properties than alloying with
Sn. With respect to the reference Pt(111), the Pt3Ga catalyst model shows an increased catalytic
activity for propane dehydrogenation. Furthermore, it is shown that the formation of coke
precursors are reduced, indicating a higher deactivation resistance. The hydrogenolysis of
176 Chapter 8: Conclusions and prospects
propane is improbable and higher selectivity towards to propylene are obtained on this Pt-Ga
catalyst. On the cokes deactivated Pt catalyst, the propane dehydrogenation does not occur near
the graphene ribbon due steric effects with the adsorbates. Beyond the short-ranged steric
effects, the catalytic activity is retained, however it is lower compared to clean Pt(111) due to
the repulsion by the graphene ribbon. Undercoordinated sites on the Pt catalysts are
thermodynamically and kinetically preferred as sites for propane dehydrogenation to propylene
with respect to the terrace Pt surface. In addition, deep dehydrogenation and hydrolysis
reactions are also less activated, indicating that these sites initiate unwanted side reactions.
Future work
For the continuation of this work, the following suggestions are made. Apart from the Pt(111)
reference catalyst model, the main focus of this work was on the quantification of the catalytic
activity and the catalyst deactivation by the formation of deeply dehydrogenated coke
precursors. However, the proposed reaction network on the more advanced catalyst models
should include the hydrogenolysis reactions and consecutive formation of gaseous side
products. The description of these products are essential to correlate the selectivity towards
propylene with experimental data. Obtaining a better understanding of the Pt-Ga particle by
investigating the Ga distribution over the catalyst particle and the interaction with the calcined
hydrotalcite support can improve the catalyst model used in the ab initio calculations.
Furthermore, the condense reaction network of the advanced catalyst models, such as the
Pt3Ga(111) catalyst model, should be extended conform the mechanism on Pt(111) and the
resulted data should be implemented in the microkinetic model for propane dehydrogenation.
Next, the proposed reaction network on the Pt(111) catalyst model can be refined by including
additional deeply dehydrogenated species and the interconversion of C1- and C2-species.
References
1. Vora, B.V., Development of Dehydrogenation Catalysts and Processes. Topics in Catalysis, 2012. 55(19-20): p. 1297-1308.
2. Yang, M.L., et al., Density functional study of the chemisorption of C-1, C-2 and C-3 intermediates in propane dissociation on Pt(111). Journal of Molecular Catalysis a-Chemical, 2010. 321(1-2): p. 42-49.
3. Siddiqi, G., et al., Catalyst performance of novel Pt/Mg(Ga)(Al)O catalysts for alkane dehydrogenation. Journal of Catalysis, 2010. 274(2): p. 200-206.
Chapter 8: Conclusions and prospects 177
4. Saerens, S., Determination of the active phase of a Pt/Mg(Ga)AlOx catalyst of the catalytic dehydrogenation of propane, in Department of Chemical Engineering and Technical Chemistry. 2014, University of Ghent: Ghent.
5. Vu, B., et al., Electronic density enrichment of Pt catalysts by coke in the propane dehydrogenation. Korean Journal of Chemical Engineering, 2011. 28(2): p. 383-387.
6. Peng, Z., et al., High-resolution in situ and ex situ TEM studies on graphene formation and growth on Pt nanoparticles. Journal of Catalysis, 2012. 286: p. 22-29.
7. Yang, M.L., et al., DFT study of propane dehydrogenation on Pt catalyst: effects of step sites. Physical Chemistry Chemical Physics, 2011. 13(8): p. 3257-3267.
Appendix A
178
Appendix A Examples of INCAR files
A.1 INCAR file for strict geometry optimization
Electronic minimization PREC = NORMAL GGA = OR #For optPBE vdw-DF functional. Change to “PE” for PBE functional LUSE_VDW = .TRUE. #Only necessary in case of vdw-DF functional AGGAC = 0.0000 #Only necessary in case of vdw-DF functional VOSKOWN = 1 LREAL = auto #Use FALSE for small systems (bulk unit cell) ALGO = FAST ENCUT = 400 #Plane wave cutoff in eV EDIFF = 1e-8 #Strict electronic energy convergence criterion ISYM = 0 ISPIN = 1 NELM = 400 LWAVE = .FALSE. LCHARG = .FALSE. LVTOT = .FALSE. Ionic relaxation IDIPOL = 3 LDIPOL = .TRUE. ISIF = 0 ISTART = 0 ; ICHARG=2 LORBIT = 11 EDIFFG = -0.015 #Strict ionic convergence criterion NSW = 200 IBRION = 1 #1=Quasi Newton RMM-DIIS, 2=Conjugate Gradient POTIM = 0.25 DOS related values ISMEAR = 1
179 Appendix A
SIGMA = 0.2 Parallelization NSIM = 4 NPAR = 1 LPLANE = .TRUE. LSCALU = .FALSE.
A.2 INCAR files for transition state optimization
A.2.1 NEB optimization
Electronic minimization PREC = NORMAL GGA = OR #NEB calculations are only conducted with the optPBE vdw-DF functional. LUSE_VDW = .TRUE. AGGAC = 0.0000 VOSKOWN = 1 LREAL = auto ALGO = FAST ENCUT = 400 EDIFF = 1e-4 #Loose electronic energy convergence criterion ISYM = 0 ISPIN = 2 NELM = 400 LWAVE = .FALSE. LCHARG = .FALSE. LVTOT = .FALSE. Ionic relaxation IDIPOL = 3 LDIPOL = .TRUE. ISIF = 0 ISTART = 0 ; ICHARG=2 LORBIT = 11 EDIFFG = -0.20 #Loose ionic convergence criterion NSW = 200 IBRION = 1 #1=Quasi Newton POTIM = 0.30 DOS related values ISMEAR = 1 SIGMA = 0.2 Parallelization NSIM = 2 NPAR = 2 LPLANE = .TRUE. LSCALU = .FALSE. TRANSITION STATE IMAGES = 8 #Number of images between initial and final state ICHAIN = 0
Appendix A
180
SPRING = -5 SPRING2 = -5 ISPRING = 1 SPOWER = 1 EFIRST = -219.09992 #Energy of initial state ELAST = -218.94877 #Energy of final state LCLIMB = .FALSE. # If TRUE, the climbing NEB method is used LTANGENT = .TRUE.
A.2.2 Dimer optimization
Electronic minimization PREC = NORMAL GGA = OR #Dimer calculations are only conducted with the optPBE vdw-DF functional. LUSE_VDW = .TRUE. VOSKOWN = 1 AGGAC = 0.0000 LREAL = auto ALGO = FAST ENCUT = 400 EDIFF = 1e-8 #Strict electronic energy convergence criterion ISYM = 0 ISPIN = 1 NELM = 400 LWAVE = .FALSE. LCHARG = .FALSE. LVTOT = .FALSE. Ionic relaxation IDIPOL = 3 LDIPOL = .TRUE. ISIF = 0 ISTART = 0 ; ICHARG=2 LORBIT = 11 EDIFFG = -0.015 #Strict ionic energy convergence criterion NSW = 550 IBRION = 3 POTIM = 0 DOS related values ISMEAR = 1 SIGMA = 0.2 Parallelization NSIM = 4 NPAR = 2 LPLANE = .TRUE. LSCALU = .FALSE. TRANSITION STATE IOPT = 2 #Type of optimizer employed ICHAIN = 2 #Enables the dimer method
181 Appendix A
DdR = 5E-3 DRotMax = 2 DFNMin = 0.01 DFNMax = 1.0
A.3 INCAR file for frequency calculations
Electronic minimization PREC = NORMAL GGA = OR #Frequency calculations are conducted with the optPBE vdw-DF functional. AGGAC = 0.0000 VOSKOWN = 1 LREAL = auto IALGO = 48 ENCUT = 400 EDIFF = 1e-8 ISYM = 0 ISPIN = 1 NELM = 400 LWAVE = .FALSE. LCHARG = .FALSE. Ionic relaxation IDIPOL = 3 LDIPOL = .TRUE. ISIF = 0 ISTART = 0 ; ICHARG=2 LORBIT = 11 NSW = 400 IBRION = 5 #5=Vibration POTIM = 0.015 #Step size for the numerical Hessian matrix calculation DOS related values ISMEAR = 1 SIGMA = 0.2 Parallelization NSIM = 4 NPAR = 2 LPLANE = .TRUE. LSCALU = .FALSE.
Appendix B 182
Appendix B Optimized geometries on the Pt(111) catalyst model In this appendix, first the proposed reaction models are illustrated with the optimized
geometries of the intermediates. Next, the optimized geometries are enlarged and given in the
following order: gasphase, hydrogen, propane and propylene dehydrogenation, dissociation
products, C1 and C2 hydrocarbons and transition states. The following color code is employed
for the elements: carbon (gray), hydrogen (white) and platinum (blue). As platinum atoms are
rather large, solely the top layer of the catalyst model is shown. Additionally, the radius of
platinum is reduced from 1.40 Å to 1.00 Å. For the optimized geometries, interatomic lengths
are given between specific atoms. For intermediates, the lengths of the C-C bonds, one C-H
bonds and Pt-C bonds (and eventual Pt-H bond) are determined. For transition states, the bond
length between atoms that participate in the reaction are reported. All distances are determined
in Å.
183 Appendix B
B.1 Reaction network
Figure B-1: Reaction network of propane dehydrogenation on Pt(111) catalyst model. In the dehydrogenation reactions, hydrogen is formed. However, the formed hydrogen is not shown in this figure.
Appendix B 184
Figure B-2: Reaction network for formation of gaseous side products
B.2 Optimized geometries
B.2.1 Gasphase species
Figure B-3: Gaseous propane
Figure B-4: Gaseous propylene
Gaseous methanePhysisorbed methaneMethyl
A1 A2
Ethyl
Physisorbed ethane
Physisorbed ethylene
Gaseous ethane
Gaseous ethylene
B1
B2
C1C3C2
Di-sigma ethylene
+ H-Pt
+ H-Pt
- H-Pt
185 Appendix B
Figure B-5: Gaseous ethane
Figure B-6: Gaseous ethylene
Figure B-7: Gaseous methane
Figure B-8: Gaseous H2
Appendix B 186
B.2.2 Hydrogen
Figure B-9: Hydrogen on top site
Figure B-10: Hydrogen in fcc site
Figure B-11: Hydrogen in hcp site
187 Appendix B
B.2.3 (Deeply) dehydrogenated intermediates
Figure B-12: Physisorbed propane
Figure B-13: On top 1-propyl
Figure B-14: On top 2-propyl
Figure B-15: Bridged 1-propylidene
Figure B-16: Bridged 2-propylidene
Figure B-17: Di-sigma propylene
Appendix B 188
Figure B-18: Physisorbed propylene
Figure B-19: 1-propylidyne in fcc site
Figure B-20: 1-propenyl on top and in bridge site
Figure B-21: 2-propenyl on top and in bridge site
189 Appendix B
B.2.4 Dissociation products
Figure B-22: On top methyl and ethyl
Figure B-23: Bridged methylene and on top ethyl
Figure B-24: On top methyl and bridged ethylidene
Figure B-25: Bridged ethylidene and bridged methylene
Figure B-26: On top ethyl and methylidene in fcc site
Figure B-27: On top methyl and ethylidyne in fcc site
Appendix B 190
B.2.5 C1 and C2 hydrocarbon intermediates
Figure B-28: Physisorbed methane
Figure B-29: On top methyl
Figure B-30: Physisorbed ethane
Figure B-31: On top ethyl
Figure B-32: Physisorbed ethylene
Figure B-33: Di-sigma ethylene
191 Appendix B
B.2.6 Transition states
Figure B-34: TS1 Propane – 1-propyl
Figure B-35: TS2 Propane – 2-propyl
Figure B-36: TS3 Propane dissociation
Figure B-37: TS4 1-propyl – 1-propylidene
Figure B-38: TS5 1-propyl – propylene
Figure B-39: TS6 1-propyl dissociation
Appendix B 192
Figure B-40: TS7 2-propyl – propylene
Figure B-41: TS8 2-propyl – 2-propylidene
Figure B-42: TS9 2-propyl dissociation
Figure B-43: TS10 1-propyl – 2-propyl
Figure B-44: TS11 1-propylidene – propylene
Figure B-45: TS12 2-propylidene – propylene
193 Appendix B
Figure B-46: TS14 Propylene – 1-propenyl
Figure B-47: TS15 Propylene – 2-propenyl
Figure B-48: TS16 Propylene dissociation
Figure B-49: TS17 1-propylidene – 1-propylidyne
Figure B-50: TS18 1-propylidene dissociation
Figure B-51: TS19 2-propylidene dissociation
Appendix B 194
Figure B-52: TS20 1-propylidene – 1-propenyl
Figure B-53: TS21 2-propylidene – 2-propenyl
Figure B-54: TSA1 Methane – methyl
Figure B-55: TSB1 Ethane – ethyl
Figure B-56: TSC1 Ethyl – ethylene
Appendix C 195
Appendix C Thermodynamic and kinetic parameters of the elementary steps for the Pt(111) catalyst model In this appendix, the thermodynamic and kinetic parameters of the reaction network on Pt(111)
will be provided. The reaction network is the same as illustrated in Appendix B. All kinetic and
thermodynamic parameters have been determined at 900 K. All adsorbates were considered to
be immobile on the surface (including hydrogen) and only for the gaseous species free rotation
and translation have been taken into account. Spurious imaginary frequencies have been dealt
with in an adequate manner, as discussed in Chapter 4.
C.1 Thermodynamic parameters
For the thermodynamic parameters, only the reaction enthalpy, entropy, Gibbs free energy and
thermodynamic equilibrium coefficient are shown (see Table C-1).
196 Appendix C
Table C-1. Thermodynamic parameters for the elementary propane dehydrogenation reaction steps on Pt(111). All values are determined at 900 K.
∆rH ∆rS ∆rG K kJ/mol J/molK kJ/mol Total reaction 129.0 208.6 -58.7 2.56E+03 Propane physisorption -56.1 -184.8 110.3 3.98E-07 Reaction 1 8.0 55.7 -42.1 2.78E+02 Reaction 2 0.4 45.7 -40.8 2.32E+02 Reaction 3 -1.3 14.8 -14.6 7.00E+00 Reaction 4 -1.1 -11.9 9.6 2.77E-01 Reaction 5 -30.1 -28.9 -4.1 1.72E+00 Reaction 6 10.7 -36.7 43.8 2.87E-03 Reaction 7 -22.5 -18.9 -5.4 2.06E+00 Reaction 8 7.2 -0.8 7.9 3.47E-01 Reaction 9 10.2 -9.7 19.0 7.88E-02 Reaction 10 -7.7 -10.0 1.3 8.36E-01 Reaction 11 -29.0 -17.0 -13.7 6.23E+00 Reaction 12 -29.6 -18.1 -13.3 5.93E+00 Reaction 14 11.2 9.0 3.2 6.54E-01 Reaction 15 -1.5 13.5 -13.6 6.18E+00 Reaction 16 52.7 13.4 40.7 4.36E-03 Reaction 17 -72.7 5.0 -77.2 3.03E+04 Reaction 18 -31.3 -22.1 -11.4 4.56E+00 Reaction 19 -70.0 -5.4 -65.1 6.01E+03 Reaction 20 -17.7 -8.0 -10.5 4.07E+00 Reaction 21 -31.1 -4.6 -26.9 3.66E+01 Reaction A1 -4.6 4.2 -8.4 3.09E+00 Reaction B1 5.9 -9.9 14.8 1.38E-01 Reaction C1 -13.8 15.0 -27.2 3.81E+01 Propylene adsorption -137.2 -253.1 90.6 5.53E-06 Ethane physisorption -42.2 -166.9 108.0 5.38E-07 Ethylene adsorption -119.5 -159.3 23.9 4.11E-02 Methane physisorption -83.7 -141.4 43.5 2.97E-03
C.2 Kinetic parameters
For the kinetic parameters, only the forward reaction coefficient, pre-exponential factor and
activation energy are shown (see Table C-2). For adsorption, molecular flux expressions have
been used (see Chapter 4). Desorption and reverse reaction parameters have been determined
using thermodynamic consistency (see Chapter 4).
Appendix C 197
Table C-2. Kinetic parameters for the elementary propane dehydrogenation reaction steps on Pt(111). All values are determined at 900 K and units are given for first order reactions.
kforward
s-1 Aforward
s-1 Ea,forward
kJ/mol Reaction 1 1.42E+11 1.22E+16 85.0 Reaction 2 1.34E+11 1.89E+15 71.5 Reaction 3 5.07E+04 8.08E+15 193.0 Reaction 4 1.66E+08 1.13E+12 66.0 Reaction 5 1.04E+09 4.28E+12 62.3 Reaction 6 1.00E+01 1.90E+09 142.6 Reaction 7 1.07E+09 8.00E+12 66.8 Reaction 8 8.06E+07 2.05E+12 75.9 Reaction 9 3.29E+02 3.49E+12 172.7 Reaction 10 6.09E+02 3.55E+13 185.5 Reaction 11 1.62E+04 2.69E+11 124.4 Reaction 12 1.00E+01 1.90E+09 142.6 Reaction 14 3.09E+08 5.44E+12 73.2 Reaction 15 9.80E+08 5.38E+12 64.4 Reaction 16 4.26E+01 1.81E+13 200.4 Reaction 17 3.68E+08 2.69E+09 14.9 Reaction 18 4.32E+04 3.53E+10 101.9 Reaction 19 1.61E+05 5.19E+12 129.4 Reaction 20 8.80E+08 1.43E+12 55.3 Reaction 21 1.97E+09 1.76E+12 50.8 Reaction A1 1.54E+08 3.18E+12 74.4 Reaction B1 1.01E+06 7.24E+09 66.4 Reaction C1 2.46E+08 6.76E+12 76.5
Appendix D 198
Appendix D Optimized geometries on the Pt3Ga(111) catalyst model In this appendix, first the proposed reaction models are illustrated with the optimized
geometries of the intermediates. Next, the optimized geometries are enlarged and given in the
following order: gasphase, hydrogen, (deeply) dehydrogenated species, dissociation products
and transition states. The following color code is employed for the elements: carbon (gray),
hydrogen (white), platinum (blue) and gallium (green). As platinum and gallium atoms are
rather large, solely the top layer of the catalyst model is shown. Additionally, the radius of the
surface atoms is reduced to 1.00 Å. For the optimized geometries, interatomic lengths are given
between specific atoms. For intermediates, the lengths of the C-C bonds, one C-H bonds and
Pt-C bonds (and eventual Ga-X bonds) are determined. For transition states, the bond length
between atoms that participate in the reaction are reported. All distances are determined in Å.
199 Appendix D
D.1 Reaction network
Figure D-1: Reaction network on the Pt3Ga(111) catalyst model. In the dehydrogenation reactions, hydrogen is formed. However, the formed hydrogen is not shown in this figure.
2-propyl
1-propylidene
10
13
2-propenyl1-propenyl
14 1517 20 21
Methyl and ethyl
4 5
2-propylidene
7 8
1-propyl
1 2 3
Gaseous propane0
Physisorbed propane
Propylene
11
1-propylidyne Physisorbed propylene
Gaseous propylene
12
Appendix D 200
D.2 Optimized geometries
D.2.1 Gasphase species
The optimized geometries of gaseous species can be found in B.2.1 as they are identical.
D.2.2 Hydrogen
Figure D-2: Hydrogen adsorbed on Pt3-fcc-Pt2Ga site
Figure D-3: Hydrogen adsorbed on Pt3-hcp-Ga site
Hydrogen will preferentially adsorb in hcp or fcc sites on Pt3Ga. Other sites, such as bridge and
top, are less favored and therefore not shown.
D.2.3 (Deeply) dehydrogenated intermediates
Figure D-4: Physisorbed propane
Figure D-5: On top 1-propyl
201 Appendix D
Figure D-6: On top 2-propyl
Figure D-7: 1-propylidene on Pt2-bridge-Pt2Ga site
Figure D-8: 2-propylidene on Pt2-bridge-Pt2Ga site
Figure D-9: Di-sigma propylene on Pt2-bridge-Pt2Ga site
Figure D-10: Physisorbed propylene
Figure D-11: 1-propylidyne in Pt3- hcp site
Appendix D 202
Figure D-12: 1-propenyl in Pt3- hcp site
Figure D-13: 2-propenyl in Pt3- hcp site
D.2.4 Dissociation products
Figure D-14: On top methyl and ethyl
D.2.5 Transition states
Figure D-15: TS1 Propane – 1-propyl
Figure D-16: TS2 Propane – 2-propyl
203 Appendix D
Figure D-17: TS3 Dissociation propane
Figure D-18: TS4 1-propyl – 1-propylidene
Figure D-19: TS7 2-propyl – propylene
Figure D-20: TS 17 1-propylidene – 1-propylidyne
Appendix E 204
Appendix E Optimized geometries on the graphene ribbon covered Pt(111) catalyst model In this appendix, first the proposed reaction models are illustrated with the optimized
geometries of the intermediates. Next, the optimized geometries are enlarged and given in the
following order: gasphase, hydrogen, (deeply) dehydrogenated species, dissociation products
and transition states. First, all geometries on the 4×2 unit cell are reported, followed by those
on the 5×2 unit cell. The following color code is employed for the elements: carbon (gray),
hydrogen (white) and platinum (blue). As platinum atoms are rather large, solely the top layer
of the catalyst model is shown. Additionally, the radius of platinum is reduced from 1.40 Å to
1.00 Å. For the optimized geometries, interatomic lengths are given between specific atoms.
For intermediates, the lengths of the C-C bonds, one C-H bond and Pt-C bonds (and eventual
Pt-H bond) are determined. For transition states, the bond length between atoms that participate
in the reaction are reported. All distances are determined in Å. In some of the shown figures,
isolated hydrogen atoms are found. This is due to the periodic nature of the unit cell and the
fact that the graphene ribbon is quite large in the unit cell: the hydrogens that terminate the
ribbon are found in the opposite position of the unit cell due to periodicity reasons.
205 Appendix E
E.1 Reaction network
Figure E-1: Reaction network on the cokes deactivated Pt(111) catalyst model (4×2 unit cell). In the dehydrogenation reactions, hydrogen is formed. However, the formed hydrogen is not shown in this figure.
2-propyl
1-propylidene
10
13
2-propenyl1-propenyl
14 1517 20 21
Methyl and ethyl
4 5
2-propylidene
7 8
1-propyl
1 2 3
Gaseous propane0
Physisorbed propane
Propylene
11
1-propylidyne Physisorbed propylene
Gaseous propylene
12
Appendix E 206
Figure E-2: Reaction path via 2-propyl towards propylene on 5×2 Gr/Pt(111) catalyst model. In the dehydrogenation reactions, hydrogen is formed. However, the formed hydrogen is not shown in this figure.
E.2 Optimized geometries in the 4×2 unit cell
E.2.1 Gasphase species
The optimized geometries of gaseous species can be found in B.2.1 as they are identical.
2-propyl
72
Gaseous propane
0
Physisorbed propane Propylene
Physisorbed propylene
Gaseous propylene
13
207 Appendix E
E.2.2 Hydrogen
Figure E-3: Hydrogen on top site
Figure E-4: Hydrogen in fcc site
Figure E-5: Hydrogen on bridge site
E.2.3 (Deeply) dehydrogenated intermediates
Figure E-6: Physisorbed propane
Figure E-7: On top 1-propyl
Appendix E 208
Figure E-8: On top 2-propyl
Figure E-9: Bridged 1-propylidene
Figure E-10: Bridged 2-propylidene
Figure E-11: Di-sigma propylene
Figure E-12: Physisorbed propylene
Figure E-13: 1-propylidyne in fcc site
209 Appendix E
Figure E-14: 1-propenyl on top and in bridge site
Figure E-15: 2-propenyl on top and
in bridge site
E.2.4 Dissociation products
Figure E-16: On top methyl and ethyl
E.2.5 Transition states
Figure E-17: TS1 Propane – 1-propyl
Figure E-18: TS2 Propane – 2-propyl
Appendix E 210
Figure E-19: TS4 1-propyl – 1-propylidene
Figure E-20: TS5 1-propyl – propylene
Figure E-21: TS7 2-propyl – propylene
Figure E-22: TS8 2-propyl – 2-propylidene
Figure E-23: 1-propylidene – 1-propylidyne
E.3 Optimized geometries in the 5×2 unit cell
E.3.1 Gasphase species
The optimized geometries of gaseous species can be found in B.2.1 as they are identical.
211 Appendix E
E.3.2 Hydrogen
Figure E-24: Hydrogen on top site
Figure E-25: Bridged hydrogen
Figure E-26: Hydrogen in fcc site
E.3.3 Dehydrogenated species
Figure E-27: Physisorbed propane
Figure E-28: On top 2-propyl
Figure E-29: Di-sigma propylene
Figure E-30: Physisorbed propylene
Appendix E 212
E.3.4 Transition states
Figure E-31: TS2 Propane – 2-propyl
Figure E-32: TS7 2-propyl – propylene
Appendix F 213
Appendix F Optimized geometries on the Pt(211) catalyst model In this appendix, first the proposed reaction models are illustrated with the optimized
geometries of the intermediates. Next, the optimized geometries are enlarged and given in the
following order: gasphase, hydrogen, propane dehydrogenation products and transition states.
The following color code is employed for the elements: carbon (gray), hydrogen (white) and
platinum (blue). As platinum atoms are rather large, solely the two top layers of the catalyst
model is shown. Additionally, the radius of platinum is reduced from 1.40 Å to 1.00 Å. For the
optimized geometries, interatomic lengths are given between specific atoms. For intermediates,
the lengths of the C-C bonds, one C-H bonds and Pt-C bonds (and eventual Pt-H bond) are
determined. For transition states, the bond length between atoms that participate in the reaction
are reported. All distances are determined in Å.
214 Appendix F
F.1 Reaction network
Figure F-1: Reaction path via 2-propyl towards propylene on Pt(211) catalyst model. In the dehydrogenation reactions, hydrogen is formed. However, the formed hydrogen is not shown in this figure.
F.2 Optimized geometries
F.2.1 Gasphase species
The optimized geometries of gaseous species can be found in B.2.1 as they are identical.
2-propyl
7
Pt
_2
Gaseous propane
0
Physisorbed propane Propylene
Physisorbed propylene
Gaseous propylene
13
Appendix F 215
F.2.2 Hydrogen
Figure F-2: Hydrogen on top site near the edge
Figure F-3: Hydrogen on top site not near the edge
F.2.3 Dehydrogenated intermediates
Figure F-4: Physisorbed propane
Figure F-5: On top 2-propyl
216 Appendix F
Figure F-6: Di-sigma propylene
Figure F-7: Physisorbed propylene
F.2.4 Transition states
Figure F-8: TS2 Propane – 2-propyl
Figure F-9: TS7 2-propyl – propylene
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