Catalysis ChE 481 581 Spring 2015 Complete

8/10/2019 Catalysis ChE 481 581 Spring 2015 Complete

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Gene and Linda Voiland School of Chemical Engineering and Bioengineering

CatalysisFromFundamentals to

ApplicationsProfessor Norbert Kruse

Prerequisites:Undergraduate courses of chemical reaction kinetics and

engineering


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CHE 481/581: Catalysis (Spring 2015)

Prof. Norbert Kruse

Office: Wegner 155A

Email:[email protected]

Office Phone: (509) 335 6601

Office Hours: Fridays 11:00 am1 pm, with an appointment

Tanya Stewart, Secretary Senior

Office: Wegner 155C

Email:[email protected] Phone: (509) 335 1256

Course:

a) Aim: get acquainted with the fundamentals of (heterogeneous) catalysis. Think in terms of

kinetics and mechanisms and use the surface science approach for doing so. Get an overview

on the major large-scale applications of catalysis.

b) Textbooks: there is no unique textbook treating all the topics covered by the course. A

copy of the slides will be provided.

for Surface Chemistry: Gary Attard and Colin Barnes, Surfaces, Oxford Chemistry Primers

for Spectroscopy: J. W. Niemantsverdriet, Spectroscopy in Catalysisan Introduction,Wiley-VC

for Theory in Surface Chemistry: Roald Hoffman, Solids and Surfaces: A Chemists View of

Bonding in Extended Structures, VCHc) Schedule: Terrell Lib. 24. Tu and Th 12pm to 1:15pm until May 1st. No classes on January23rd, March 25thand 27th.

d) Methodology: Avoid a monologue, but rather try to engage a discussion when appropriate.

Slides provide a scaffold. They are to be completed by the students according to theindividual needs. Instructor jumps back to the basics when necessary. More detailed
mailto:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]


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considerations will be either developed at the blackboard or defined as homework. Try toavoid lengthy mathematical derivation of formulas homework

Grading:

Oral (30 to 40 min each student): 70%. Project presentation: 30%

Projects will be assigned according to a list of subjects provided until end of March. A

subject may range from a hot topic in catalysis to the presentation of a large-scale catalyticprocess not subject to the course. Every student has to present his/her project in PPT format

during 12 min., followed by questions during 6 min. Presentations take place on 22nd, 24th,30thApril and 1stMay. Room TBD.

Homework will be defined occasionally and is intended to digest and deepen certainaspects of a subject. No written homework is required, however, there is expectation of

background knowledge to support the new knowledge provided. The oral exam may includequestions on such homework issues.

Oral exams will take in my office at Wegner 155. Oral exam and PPT presentation will beweighted as defined as above.

Grading Scale:

90-100% A 77-79% B-

87-89% A- 73-76% C+

83-86% B+ 70-72% C

80-82% B 0-69% F

Course Website

The contents of this course will be available online on Angel:http://lms.wsu.eduor via email.
http://lms.wsu.edu/http://lms.wsu.edu/http://lms.wsu.edu/


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Students with Disabilities

Reasonable accommodations are available for students with a documented disability. If youhave a disability and need accommodations to fully participate in this class, please either visitor call the Access Center (Washington Bldg 217; 509-335-3417) to schedule an appointment

with an Access Advisor. All accommodations MUST be approved through the AccessCenter.

Academic Integrity

I encourage you to work with classmates on assignments. However, each student must turn in

original work. No copying will be accepted. Students who violate WSUs Standards of

Conduct for Students will receive an F as a final grade in this course, will not have the optionto withdraw from the course and will be reported to the Office of Student Standards and

Accountability. Cheating is defined in the Standards for Student Conduct WAC 50-26-010(3). It is strongly suggested that you read and understand these definitions:http://apps.leg.wa.gov/wac/default.aspx?cite=504-26-010

Safety

Washington State University is committed to maintaining a safe environment for its faculty,

staff, and students. Safety is the responsibility of every member of the campus communityand individuals should know the appropriate actions to take when an emergency arises. Insupport of our commitment to the safety of the campus community the University has

developed a Campus Safety Plan,http://safetyplan.wsu.edu/.It is highly recommended that

you visit this web site as well as the University Emergency Management web site athttp://oem.wsu.edu/ to become familiar with information provided.

Caveat

The schedule and procedures outlined in this syllabus are subject to change in the event ofcircumstances beyond the instructors control or in response to ongoing assessment oflearning.
http://apps.leg.wa.gov/wac/default.aspx?cite=504-26-010http://safetyplan.wsu.edu/http://oem.wsu.edu/http://oem.wsu.edu/http://safetyplan.wsu.edu/http://apps.leg.wa.gov/wac/default.aspx?cite=504-26-010


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Catalysis Kinetic Phenomenon

Some early definitions:

A catalyst increases the rate of a reaction without being consumed by a reactionWilhelm Ostwald (1853-1932)

A chemical reaction has to be thermodynamically feasible in order to beaccelerated Alfred Mittasch (1869-1953) *

A catalyst doesnt appear in the stoichometric equation.

However: Catalysts undergo structural or chemical changes during the catalyticreaction (mainly in heterogeneous reactions).

The catalyst is formed by the catalytic reaction.

Agatha Christie (1891-1976)

(The Mysterious Mr. Quinn, 1930): Do you happen to know anything about

catalysis? The young man stared at him. Never heard of it. What is it? Mr.Satterthwaite quoted gravely, A chemical reaction depending for its success onthe presence of a certain substance which itself remains unchanged.

(Curtain, Poirots last case, 1940): So we get the curious result that we have herea case of catalysisa reaction between two substances that takes place only inthe presence of a third substance, apparently taking no part in the reaction andremaining unchanged. That is the position. It means that where X was present,

crimes took placebut X did not actively take part in these crimes.

_____________________________________________________________________________________________

* non-catalytic

catalytic


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Some general comments on the importance of catalysis:

About 80% of the industrial production is based on the application of the catalytic

process at least in one intermediate stage.

High economic importance

About 90% of all catalytic processes are heterogeneous in nature.

Incidental remark: homogeneous -heterogeneous catalysis

Reactants and catalyst in the same phase not in the same phase

However: there is no general theory of catalysisBlack Magic

Two examples for the importance of empirical research:

Ammonia synthesis:

N2+ 3H22NH3 About 20,000 different catalysts were tested

Fischer-Tropsch Reaction:

CO +H2 2O About 15,000 different catalysts were testedEq. not equilibrated!

In both cases, metals are used to catalyze the reaction (mainly nano-sized

particles on an oxidic support.)


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A historical example

Formation of water:

2H2+ O22H2O Dbereiner (1822)

First construction of a lighter:

The lever eallows contacting a piece of zinc with

sulfuric acid in a glass vessel: Zn + 2H+ Zn2+ + H2

H2is produced and mounts in the jack so as to mix up with O 2. Sponge-like Pt

is placed inside the nozzle fand allows ignition of the reaction.


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For a catalyst with a pore structure (texture) diffusion processes into the pores

and out of them have to be added.

b)Homogeneous Catalysis:

Catalytic Cycle :

A physical separation (unit operations like distillation, extraction, etc.) is

necessary in order to obtain the pure product.

Two examples involving multiple intermediate steps:

Ammonia Synthesis (heterogeneous catalysis):

All intermediate steps were clarified in a Nobel Prize winning work by G. Ertl

(Nobel Prize 2007)

Reaction Advancement

PotentialEnergy


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Hydroformylation (Homogeneous catalysis)

Note that the catalytic cycle above includes changes in the number of metal

valence electrons between 18 and 16, back and forth.

Catalysts are coordination compounds of Rh and Co


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Isokinetic temperature

Kinetic compensationreality or fiction?

We can always find a temperature for which k0has the samenumerical value, no matter which catalyst for a givenreaction is considered.


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Example: methanation

CO + 3 H2 CH4+ H2O

Compensation effect for the methanation of CO for a number of metals, both base

and noble.

kJ/molEA

10

20

60 80 100 120

.

.

.

.

.

. .

.

Ru

CoNi

Rh

Fe

Pd

Pt

Ir

ln k0


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Conversion of crotonicaldehyde: Butyraldehyde selectivity:

Note the distinction between:

TON is frequently used in homogeneous catalysis:

Processes start to become economic when TON > 20,000


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Some statistical thermodynamics

Comparing reaction rates of heterogeneous catalysis with those of non-catalytic

reactions according to the Transition State Theory (TST)

Consider a bimolecular reaction (non-catalytic)

AB is the activated complex in homogeneous phase.

The same bimolecular reaction under catalytic conditions reads:

AB*2

is the activated complex bound to the surface of the catalyst.

The reaction rate according to TST then is:

n

nn


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in general:

k: Boltzmann constant

h: Planck constant

:Partition function of component iEo

: Difference in energy of initial energy T=0 K

(

)


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Heterogeneous Catalysis occurs at the surface of a solid

Transmission Electron Microscopy (TME) of metal

particles on a support (Ag/Al2O3).

Metal particles are well separated from each other

and the pore structure of the catalyst becomes visible(texture).

Image blurring indicates carbon deposition

High Resolution Transmission

Electron Microscopy (HRTEM)revealing the morphology of Rh

nanosized particles on a TiO2

support.

10nm


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Ball model of a single metal grain to visualize the different sites exposed.

In white are: low coordination atoms have empty valence orbitals which are

supposed to be preferred binding sides for adsorbing atoms and molecules

(gasses or liquid molecules).

Field Ion Microscopy (FIM)

of a single Rh particle, in top

view, atom by atom. Not all atoms areseen; only those in low coordination.

Miller indexes of individual surface

facets are also given.


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Metal particles on a support should be nanosized so as to create as large asurface area as possible.

n n n

r ~ N

Number of atoms located along the particle radius.

Ns

= 2 N2

Number of atoms at the surfaceNt = 2/3 N3

Total number of atoms in the particle

Surface fraction is a function ofradius for a hemispherical particleon a flat support.


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Some comments on the nature of catalysts and their supports

Supports should provide high surface area so as to optimize the dispersion of the

catalytically active phase. For metal-based catalysts, frequently the followingsupports are used.

Al2O3, SiO2, TiO2, ZnO, MgO, C

Some of these materials can be prepared with specific surface areas as large as1,000 m2/g (equivalent to the size of a football field).

The catalytically active metal phase is frequently supplied by impregnation using

an aqueous solution of a suitable precursor compound such as Me-nitrate.

Impregnation can be performed wet using immersion techniques or by incipient

wetness. In the latter case, the volume of the aqueous precursor solution matchesthe volume of the support pores so the solid doesnt appear wet. The preparationin water enables solvated metal cations to bind to surface hydroxyl of the support.

The details of the binding mechanism depend on the pH conditions and the point

of zero charge (PZC) of the support.


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pH > PZC cationic adsorption

pH < PZC anionic adsorption

A different type of catalyst are zeolites whose primary construction units are

tetrahedric [SiO4]4-and [AlO4]5- .These materials are highly crystalline withspecific surface areas sometimes exceeding 1,000 m2/g. They have acidic

properties and may or may not contain metallic cations.

[SiO4]4-and [AlO4]

5- units share oxygen atoms so as to form Si-O-Al bridges.

Starting from a 3D SiO2network the replacement of Si by Al creates localized


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charges that have to be compensated in order to maintain electroneutrality (per

Al-either one H+or one Me+is needed).

General formula:

x [ (Me+, Me2+) AlO2]. y SiO2

. z H2O

The occurrence of protons causes Brnsted acidity which allows hydrocarbon

cracking in petrochemical industry.

Heat treatment allows Brnsted acidity to turn into Lewis acidity.

Similar to the construction of secondary building blocks in silica highly

symmetric polyhedral units can be formed to build zeolites.

Heating

Lewis Center


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a) Sodalite

b)Zeolite A

c) Faujasite (zeolites X and Y)

Windows vary between 2.6 in sodalite and 7.4 in faujasite.

The number of basic zeolites can be constructed from sodalite units in which

for reasons of simplicity, the bent Si-O-Al bridges are considered as straight

lines. Corners contain either Si or Al.

Sodalite unit

Sodalite unit Sodalite unit

Large cavity


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More recent developments aim at synthesizing ordered silicas so as to create

mesopores molecular sieves.

1992 - Scientists of Mobil Oil Corporation (USA) applied self-assembledtemplate (micelles of CTAB surfactant) and synthesized a family of ordered

mesoporous silicas named MCM (Mobil Composition of Matters) 41 and 48*

The CTAB molecule (cetyltrimethylammoniumbromide) consists of a long

(cetyl) hydrocarbon skeleton causing hydrophobic properties (tail) and a terminalionic group causing hydrophilic properties (head).

CTAB micelle Silicate self-assembling around the micelle

*Kresge C.T., Leonowicz M.E., Roth W.J., Vartuli J.C., Beck J.S., Nature, 1992, 359, 710

712

Template-

directed

Inorganic

precursor

Template

Template

removalCondensation

MesoporousTemplate-oxide

N(CH3)3

+

Br-


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MCM-41 is characterized by a unique pore size distribution. Pores usually have

diameters between 2-5 nm with hexagonal structure. The total specific surface

area may range from 900-1,500 m2/g.

TEM image and model of MCM-41

V. Meynen et al. / Microporous and Mesoporous Materials 125 (2009) 170223

Micelles can be shaped by use of proper surfactants.

g = V/al

gpacking parameterVvolume of the hydrophobic part ofsurfactant

(including solubilized compounds)aeffective area of the hydrophilic head

(depends also on counter-ions)


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Micelle shape and type versus g

More recent developments have lead to new silicates like SBA-15 or KIT-6

KIT-6 pore size vsHTT temperature Model of KIT-6 doublegyroid Mesostructure

Y. Doi et al, Chem. Commun., 2010, 46, 63656367


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Physisorption and Chemisorption

Physisorption Chemisorption

Forces dispersion valence or electrostatic forcescreate covalent and/or ionic

bonds

van der Waals

a)E Keesom

b) E Debye

c) E London

HphysHcond ~5-20 kJ/mol Hchem 54kJ/mol


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multilayers monolayer limit

Reactants all gasses below reactive gasses

the critical temperature

Reversibility yes yes, in many cases,

but also irreversible.

Dependence on decreases with increasing maybe complicated in

temperature temperature case of an activated

process

Coverage


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Potential diagrams

The above diagram applies to the case of physisorption on top of a chemisorbed

layer. The turnover from the physisorbed state to the chemisorbed state is non-

activated for the observer but can only occur as long as empty chemisorption sites

are available. A diffusion process in the physisorbed state is possible and occurs

with activation energy of Edif.

Chem

Edif


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H chem

The above potential diagram applies to the case of H2adsorption on a Cu surface.

H chem = 34 kJ/mol

EA = 21 kJ/mol

E (*Cu - H) = 233 kJ/mol

Generally: H chem = 2E (M - H) - E(H - H) = 34 kJ/molwhich is low for metals with a closed d-shell.

H chem

*Chemisorption site


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The turnover from the physisorbed state into the chemisorbed state is activated

for H2/Cu. In other cases, like CO on open d-shell metals the dissociation is non-

activated and leads to the deposition of carbon and oxygen. H chem can be

considerably larger in this case (between 100-160 kJ/mol). On the other hand, CO

chemisorption on Cu occurs without dissociation and is weak.


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Dynamics of adsorption

Calculation of the surface residence time of molecules before thermoadsorption

For kinetic for order process:

surface residence time (lifetime)

6 . 1012s-1at 300K

Homework:

Calculate the surface lifetimes at 300K and 600K for the activation energies Ed=4, 20, 40, 80, 160 kJ/mol.

Calculation of the concentration (surface coverage) of adsorbed species:

For physisorption:

phys = J ..wphys wphys probability ofphysisorption

For chemisorption:

chem = J ..s J impingement rate[ L-2.T-1]

s sticking probability


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calculation of the impingement rate:

< =

n mean velocity (m s-1)n number of molecules per m3m molecular mass in kg per molecule

with:

and n J = 2.64 .1024p / (M . T) ( m 2 .s 1) m molecular mass in

atomic mass units

Homework:

Calculate the impingement rate per surface site for nitrogen molecules at 1 barand 273K. Assume the surface site to have a size of 10 2. Calculate the numberof multiple layers assuming Ed= 40 kJ/mol and a probability of physisorptionwphys = 1.

Calculation of the rate of chemisorption:

For a non-activated process: Ra

= J .s

This allows the sticking probability to be defined as:

S = Ra

/ J > tM


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tM= characteristic time of the measurement (molecules must be on the surface tobe measured)

S0 Sticking probability at zero coverage. It can be anticipated that values of

S0are influenced by the surface structure and the temperature.

Example : N2/Fe

S0 7 . 10-8 for the (110) surface (densely packed)

S0 4 . 10-6 for the(111) surface (open structure)

It is interesting to see that the sticking probabilities for N2chemisorption on Fevary in the same manner as the reaction rate of the ammonia synthesis

N2+ 3H22NH3

For comparison, the sticking probability S0 of the CO molecule on transitionmetal surfaces variesbetween 1 and 0.1.

The Langmuir Isotherm:

with (NA

) . Ed

= Hchem

n (Hchem/ RT)


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at constant temperature:

n .pModel assumptions made by Langmuir

1. all surface sites are treated in the same manner: no difference ismade between terrace sites, steps or kinks

2. adsorbed species do not interact laterally3. incoming molecules hitting an occupied site are being reflected

without energy loss

The last argument leads to the following functionaldependence of s vs.coverage :

Assuming every incoming molecule hitting an empty site will get adsorbed,

s0= 1, we will then receive:

with

For application purposes: V / Vs

Vs gas volume giving rise tomonolayer formation


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The experiment consists of introducing a known volume of gas from a calibratedreservoir into a reactor containing the catalyst sample and measuring the volumeconsumed due to adsorption.

An equivalent derivation of the Langmuir equation would be to consider a

dynamic equilibrium between adsorption and thermal desorption.

)[ ] = kd. [ ] [ ] concentration of sites perunit surfaca area

1/p

1/V

1/Vs

tg = 1/ Vs.kL

linearization leads to:


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= ka /kd adsorption coefficient (equilibrium constant of adsorption-desorption), will be denoted later as KA

1

0

p

T3 < T2 < T1T1

T2

T3


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Evaluation of the isosteric heat of adsorption:

dln p /dT| =const

= -Hchem/RT2

analog:ClausiusClapeyron

ln p = f (T) Hchem isosteric heat of adsorption

isosters (only equilibrium states are

considered)

qstHchem

p

V / Vs

p1 p2 p3

T1

T2

T3


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Homework:How does the relative sticking probability s/s0depend on thecoverage in case physisorption on top of a chemisorbed layer is taken intoaccount?

Langmuir Model for the co-adsorption of two species (competitiveadsorption for the same surface sites):

Adsorption and desorption rates for A species:

Ra= ka . pA(1 - -).[ ]Rd= kd

.

Dynamic equilibrium:

Ra = Rd= >

Analog for B:

= =

Homework: Calculate the coverage ratio A/Bassuming both species have thesame pressure pA= pBfor adsorption at room temperature. Species A isconsidered to adsorb by 40kJ/mol stronger than B:HAHB= 40 kJ/mol. Recall that the adsorption enthalpy is given by thedifference of the activation energies for adsorption and desorption.


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Chemisorption of atoms and molecules on metal surfaces: simple

theoretical concepts

Formation of electron bands in solids:


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Adsorption of an atom on the surface of a metal

Free atom


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Moving the free atom to the surface would cause a broadening of the originallysharp electron levels due to a resonance effect. Filled states above the Fermi levelmay lose charge towards the metal.

Donation effect Acceptor effect

Example: Alkalines adsorption Halogens adsorption

on transition metal


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back donation


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d-bands are less broad than s-bands. Therefore, the interaction with d bands givesrise to localized bonding. For the chemisorption of a hydrogen molecule, both theoccupied bonding molecular orbital of H2and the unoccupied molecular orbitalhave to be correlated with the metal d-band. The partial occupation of theantibonding MO by charge transfer from the metal to the H2gives rise toweakening of the H-H bond.

back donationjellium

Consideration of the d-band effect:

1s


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To dissociate the CO molecule the relative position of the Fermi level isimportant. Charge transfer into antibonding MO of the CO molecule causes bondweakening which is the first step to dissociation. As a consequence the COmolecule will tilt to allow its oxygen atom to contact the metal surface. This

process will finally lead to bond breaking with oxygen and carbon atoms beingdeposited into next nearest neighbor sites of the catalyst surface.

Appropriate orbitals: dz, dyz, dxz


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Metallic OrbitalsCO molecular orbitals

5-dz2 provides bonding

dxzdyz-2 * provides bonding

dxzdyz

dz2


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Symmetry of metallic surface planes: Miller indices


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Basic planes of the cubic face-centered system:

Plane (323)


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The direction in a crystal is defined by brackets: [uvw], expressed by the smallestset of integers of a collinear vector of the indicated direction, such that hu+kv+lw= 0

Ordered overlayer structures

a) E.A. WoodJ. Appl. Phys. 35 (1964) 1306

Elementary vectors of the surface a1, a2Elementary vectors of the adsorbate b1, b2

(|b1| / |a1| x |b2| / |a2|) + angle

b) R.L. Park, H.H. MaddenSurface Sci. 11 (1968) 188

b1= m11 a1+ m12 a2

b2= m21a1+ m22a2 M = fcc (100), (110), (111) M =


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Some examples:


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Area of the elementary unit cell

General classification of overlayer structures:1. mij are integers simple structure (M integer)

Adsorbed species are in well-defined local positions.

2. relation between (a1 a2) and (b1b2) given by rational numbers (M fractionalnumber)

two periodic lattices with 3b1 = 4a1or b1= 4/3a1(incommensurate)

3. incoherent structure: irrational numbers between a and b (M irrational number)

|b1|


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Experimental evidence for ordered overlayer structures

Low Energy Electron Diffraction (LEED)

What means low?

Electron wave length : acceleration of electrons by

application of a potential difference

e: elementary charge

54

for U = 100 V

electron microscope: U = 105 V 4


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k = (2/) s

s = unit vector in the samedirection as k


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Electron diffraction at a one-dimensional grating of atoms

Constructive interference is obtained for wavelets propagating along the surfaceof the cones. Diffraction only occurs for certain angles which define the orderof diffraction. For a surface, a second series of cones has to be constructed. For arectangular lattice, constructive interference is obtained where the two sets ofcones intersect. The screen therefore contains a periodic arrangement of spots.

a1 a2 Bragg condition

electron gun

Spherical screen


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ai aj

* = ij(i, j = 1,2)

r: radius of screen curvature in the center of which the sample is placed.

Since distances between spots on the screen are proportional to the reciprocalof distances in real space, the reciprocal lattice can be designed as follows:


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Construction of the reciprocal lattice and the respective lattice vector:

a1*= 1/ a1 sin anglebetween vectors

a2*= 1/ a2 sin

a1a2* = 0 a1a2* a1*a2a2a1* = 0

a1a1* = 1

a2a2* = 1

Nodes of the reciprocal lattice


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Periodic surface structures and ordered overlayers in real and reciprocal space:

It can be shown using matrix calculates that:

(M*is the matrix of M inversely transposed, )

a2*a2


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An example:reciprocal lattice

real space lattice


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Kinetic parameters for elementary reaction steps: desorption energy

for thermal desorption

Evaluation of the data is based on the Polanyi-Wigner equation:

n - is order of the desorptionprocess


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, Ed and n can all be evaluated from dataThe experiment consists in heating the sample according to a temperature

program, which in most cases, is linear.

T = T0+t

Determination of the temperature for which the pressure in the reaction chamberreaches a maximum (the chamber is continuously evacuated)

{

}

Hypothesis:

{n } for the maximum


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Substitution :

n

1storder kinetics:

2ndorder kinetics:

Result: Tm f() for the 1storder process anda constant activation energy

Tmif for a 2ndorder process withconstant activation energy: orfor a 1st order process withcoverage dependence of theactivation energy{Ed= g ()}

n n


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lineshape analysis: for a 1storder process, the desorption trace isasymmetric around the maximumfor a 2ndorder process, we receive a symmetric curvearound Tm

determination of Ed:

1storder: a) hypothesis b) variation of

n n n

n

Differentiation > 2nd order: (symmetrical peak)

n( ) n

verification of the kinetic order:

n n nn

problem:Edand frequently depend on extrapolation 0for increasing temperature T

Best data treatment: simulation of the thermal desorption spectrum by fitting withthe Polanyi-Wigner equation


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Some examples for Temperature Programmed Desorption experiments:

area

H2/ Ni (100)

Coverage (atoms/cm )(a) 4.6 x 1013

(b) 8.8 x 1013

(c) 1.0 x 1014

(d) 1.7 x 1014


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multiple states appear notalways resolved separately butoverlapping, so a deconvolutionhas to be made. The appearanceof different states may dependon the coverage.

CO/ Ni (100)

Desorption spectra of carbon monoxide from clean Ni(100) followingadsorption at 137 K. Coverages were 7.0 x 1013molecules/cm2(I)and 2.6x 1014molecules /cm2(II)

(I)

(II)


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Back to real catalysis: determining the specific surface area of

materials

Adsorption in multiple layers

i is the fraction of the surface covered with i layers. is then the fraction of thesurface remaining empty. Similar to the Langmuir model, the surface isconsidered homogeneous.

i = i0

0 = =

1iii

0 = 1 -

1ii

Under dynamic equilibrium conditionsi values remain constant

i= i0 is i = Ji wi

i0 =Ji wi

surface

3

1

2

4

0

couche 3

couche 2

couche 1

layer 3

layer 2

layer 1


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i =i1 heat of physisorption remains constant in all layers following thefirst one (molecules in direct contact with the surface are assumed to have adifferent heat of physisorption and therefore, a different mean lifetime).

With:Wi= i-1 Ji1i-1= i 0

i i-2=

i =

0

i =

0 i = i-1 =i-2 =1

1 0

we can now express the overall coverage of the surface as:

c i xi

with: x = J1 / 0 andc= 0 / 1

1 - = 1c


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The above formula makes use of the following series expansion:

= (1 + x + x2 i) -1=

xi = x(1+2x +3x2i-1) = using:

and with:

=

we obtain:

V/Vs if pq (condensation on the surface)

then we can identify qp0 as the saturation vapor pressure

literature: S. Brunauer, P.H. Emmett, E. Teller, J. Amer.Chem.Soc. 60 (1938) 309


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one of the advantages of the BET equation is that it can be linearized:

a b

determination of the constant c:

Hphysis usually larger

than Hcondsince the molecules interact more strongly

with the surface than with themselves in the multilayer. The larger the difference,the steeper the slope in the above figure.

range of application:

0.05 p/p00.35

p/p0

p/V(p0-p)


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if c >> 1 we receive:

Vs= V (1 - p/ p0)

For pressures far below the saturation pressure, that is p/p0


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Some examples of BET measurements:

V

p/p0

Similar to Langmuir:indicates a microporousmaterial

p/p0

V

H2O/graphite

Br2/SiO2

The catalyst surface is hydrophobicand the formation of the first layer isnot visible.

H2O/graphiteBr2/SiO2


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Some criticism as to the BET equation: Surfaces are energetically uniform. Lateral interactions between physisorbed molecules do not exist. Condensed molecules are treated identically to the liquid phase of these

molecules.


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Catalytic reaction kinetics

For reasons of simplicity, we shall consider a sequence of steps involvingadsorption, reaction, and desorption. Each of these steps will follow first order

kinetics. The scheme therefore reads:

k1 k2 k3A Aad Bad B

k -1 k -2 k -3

It is assumed that the concentration of active sites is much smaller than theconcentration of reactants and products. The steady state kinetics of the reactionwill then be determined by the surface reaction step (the concentration of activesites being the bottle-neck of the overall speed).

R = RiRi = R2R-2 = R3 R-3= (k2Ak-2 .B) [ ] using:

quasi-equilibrium

rate determining for theoverall reaction

quasi-equilibrium


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At the beginning of the reaction, with little product formation:

(1)

pA+ pB= p

()


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Five different case studies:

1) weak adsorption (A + B) facilitates equation (1)

R = k2KA PA[ ] 1st order kinetics with regard to A

Attention: HAis negative!

2) strong adsorption (A)

KB pB> 1

R = k2 .[ ] zero kinetic order with regard to A and B

Bad


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3) strong adsorption (B)

KB pB >> KA pA

positive 1storder kinetics with regard to A, negative 1storder kinetics for B

the apparent activation energy is now larger than in case 1 (the desorption energyis positive and equal to the adsorpton enthalpy).

4) adsorption determines the reaction rate

R = k1.pA.[ ] surface coverage is low

5) desorption determines the reaction rate

R = k3.B.[ ] KB . pB >> 1

1/T


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What changes for bimolecular reactions?

DAssuming that the surface reaction is the slowest step (C and D are weaklyadsorbed) we receive:

both HA and HBare negative!z is the number of next-nearest neighbor sites

for strong adsorption of A we receive:

; reaction follows 1storder kinetics in A and negative 1storder kinetics in B ; theoverall reaction order is zero.


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Mechanistic alternative for reactions on the surface of oxidic

catalysts:

example: SO2+ O2 SO3 catalyst: V2O5

redox mechanism:

CatO + A Cat- + AO

Cat+ O2 CatO_____________________________

A + O2 AO sum reaction

Rred = R oxyd for steady state conditions

kr.pA(1-)

.[ ] = ko.pO2

..[ ] fraction of the reducedsurface

a) if kr pA>>k0pO2 is close to 1 oxidation islimiting

zero reaction order for A

reduction\

of catalyst

oxidation

/


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b) if isvery small compared to1 zero order reaction kineticsfor oxygen

R = kr . pA

The difference with respect to the Langmuir-Hinshelwood type mechanism onmetals is that according to the above Mars-Krevelen mechanism is a fullyreversible chemical alteration of the catalyst surface takes place while for L-Hthis does not occur at all.

Examples with industrial application:

a. oxidation of hydrocarbons on mixed oxides of V, Mo

b.oxidation of ortho-xylene to phtalic anhydride on V2O5/SiC

Documents

Catalysis ChE 481 581 Spring 2015 Complete