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Modern Methods in Heterogeneous Catalysis Research
Solid State Aspects in Oxidation Catalysis
20th January 2012
Maik Eichelbaum / FHI
Lecture Series Catalysis: Solid State Aspects in Oxidation Catalysis
2Maik Eichelbaum
How chemists like to see the world: Everything is molecular
Lecture Series Catalysis: Solid State Aspects in Oxidation Catalysis
3Maik Eichelbaum
1. Introduction
Lecture Series Catalysis: Solid State Aspects in Oxidation Catalysis
4Maik Eichelbaum
Solid State Aspects: Goingunderneath the surface and beyond
the molecule
Lecture Series Catalysis: Solid State Aspects in Oxidation Catalysis
5Maik Eichelbaum
1. Introduction
Lecture Series Catalysis: Solid State Aspects in Oxidation Catalysis
6Maik Eichelbaum
1. Introduction
Dehydrogenation of formic acid with different bronzealloys: activation energy and resistivity
HCOOH CO2 + H2
Lecture Series Catalysis: Solid State Aspects in Oxidation Catalysis
7Maik Eichelbaum
1. Introduction
Defect electrons (holes) in NiO (withO excess): p-type semiconductor
Ni3+ Ni2+
Ni3+
Ni2+ Ni2+ Ni2+
Ni2+
Ni2+Ni2+Ni2+Ni2+
Ni2+ Ni2+
Increase of defect electrons byLi2O, decrease by Cr2O3 addition: CO + ½O2 CO2
Explanation: donor reactiondefect electron + Ni2+ = active site
fast
CO chemisorption rate-determining
Lecture Series Catalysis: Solid State Aspects in Oxidation Catalysis
8Maik Eichelbaum
1. Introduction
Excess electrons in ZnO (with Znexcess): n-type semiconductor
Increase of quasi-free electrons byGa2O3, decrease by Li2O addition: CO + ½O2 CO2
Explanation: acceptor reactionZn2+ + (1 or 2) quasi-free electrons = active site
fast
Oxidation of active site rate-determing(strong influence of oxygen on rate)
Lecture Series Catalysis: Solid State Aspects in Oxidation Catalysis
9Maik Eichelbaum
1. Introduction
Kröger-Vink notation for point defects in crystalsF. A. Kröger, H. J. Vink, Solid State Phys. 1956, 3, 307
P. J. Gellings, H. J. M. Bouwmeester, Catal. Today 2000, 58, 1
Precise description of point defects possible; differentiation between bulkvacancies, interstitials, surface vacancies, adsorbed ions etc.
Examples: Ni3+ in Ni2+O2- on normal lattice position •NiNi
Interstitial Zn+ in Zn2+O2- •iZn
Lecture Series Catalysis: Solid State Aspects in Oxidation Catalysis
10Maik Eichelbaum
2. Electrical Conductivity in Solid Oxides
Lecture Series Catalysis: Solid State Aspects in Oxidation Catalysis
11Maik Eichelbaum
2. Electrical Conductivity in Solid Oxides
1) Proton conduction in oxides
2) Oxygen conduction in oxides
1) Proton conduction in oxides
2) Oxygen conduction in oxides
Lecture Series Catalysis: Solid State Aspects in Oxidation Catalysis
12Maik Eichelbaum
2. Electrical Conductivity in Solid Oxides
Formation of protonic defects:
„normal“ lattice oxygen
Doubly ionized oxygen vacancy
hydroxyl group on regular oxygen position
1) Reaction with water
2) Reaction with hydrogen and electron holes
3) Reaction with hydrogen and formation of free electrons
Oxygen vacancy
Lecture Series Catalysis: Solid State Aspects in Oxidation Catalysis
13Maik Eichelbaum
2. Electrical Conductivity in Solid Oxides
Proton conduction mechanisms:
1) Proton hopping (Grotthus mechanism): protonhops between adjacent oxygen ions (large H+/D+
isotope effect)
2) Hydroxyl-ion migration (vehicle mechanism): negligible H+/D+ isotope effect
Lecture Series Catalysis: Solid State Aspects in Oxidation Catalysis
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2. Electrical Conductivity in Solid Oxides
Example: Oxidation of propene to acrylic acid on MoVTeNbOx (M2)
Lecture Series Catalysis: Solid State Aspects in Oxidation Catalysis
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2. Electrical Conductivity in Solid Oxides
“humid“ propene-oxygen-helium (2-8-90) mixture
Lecture Series Catalysis: Solid State Aspects in Oxidation Catalysis
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1) Proton conduction in oxides
2) Oxygen conduction in oxides
1) Proton conduction in oxides
2) Oxygen conduction in oxides
Lecture Series Catalysis: Solid State Aspects in Oxidation Catalysis
17Maik Eichelbaum
2. Electrical Conductivity in Solid Oxides
Oxygen conduction or How is oxygen incorporated into oxides?Existing point defects (oxygen vacancies: VO
••, oxygen interstitials: Oi‘‘, electronic
point defects: h•, e‘) allow a finite compositional flexibility in a binary oxide MO with phase width MO1-δ
R. Merkle, J. Maier,
Example: Fe-doped SrTiO3-δ (quasi-binary due to fixed Sr/Ti ratio below 1200°C)
Lecture Series Catalysis: Solid State Aspects in Oxidation Catalysis
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2. Electrical Conductivity in Solid Oxides
Phase diagram for MO1-δ
R. Merkle, J. Maier,
Lecture Series Catalysis: Solid State Aspects in Oxidation Catalysis
19Maik Eichelbaum
2. Electrical Conductivity in Solid Oxides
Reaction and transport of oxygen: kinetic limitations
R. Merkle, J. Maier,
Lecture Series Catalysis: Solid State Aspects in Oxidation Catalysis
20Maik Eichelbaum
2. Electrical Conductivity in Solid Oxides
Fe‐doped SrTiO3 single crystal, surface reaction is limiting, T=923 K, pO2: 0.01‐>1 bar, ttotal=24 h
false‐color image concentration profile
http://www.fkf.mpg.de/maier/downloads.html
Lecture Series Catalysis: Solid State Aspects in Oxidation Catalysis
21Maik Eichelbaum
2. Electrical Conductivity in Solid Oxides
Fe‐doped SrTiO3 single crystal, bulk diffusion is limiting, T=823 K, pO2: 0.1‐>1 bar, ttotal=6300 s
false‐color image concentration profile
http://www.fkf.mpg.de/maier/downloads.html
Lecture Series Catalysis: Solid State Aspects in Oxidation Catalysis
22Maik Eichelbaum
2. Electrical Conductivity in Solid Oxides
Fe‐doped SrTiO3 bicrystal, blocking grain boundary, T=873 K, pO2: 0.1‐>1 bar, ttotal=24 h7.5° tilt grain boundary
false‐color image concentration profile HRTEM image, structural model
http://www.fkf.mpg.de/maier/downloads.html
Lecture Series Catalysis: Solid State Aspects in Oxidation Catalysis
23Maik Eichelbaum
2. Electrical Conductivity in Solid Oxides
Blocking grain boundary formation of space charge layers
R. Merkle, J. Maier,
Lecture Series Catalysis: Solid State Aspects in Oxidation Catalysis
24Maik Eichelbaum
2. Electrical Conductivity in Solid Oxides
array of circular gold microelectrodes (20 µm diameter) on polycrystalline Fe-doped SrTiO3 platinum
contact tips
R. Merkle, J. Maier,
Localized Impedance Spectroscopy
Lecture Series Catalysis: Solid State Aspects in Oxidation Catalysis
25Maik Eichelbaum
Insertion: Impedance spectroscopy
Lecture Series Catalysis: Solid State Aspects in Oxidation Catalysis
26Maik Eichelbaum
3. Impedance Spectroscopy
Impedance measurements (measurement at alternating voltages):
)(
)(
ˆ)(
ˆ)(I
U
ti
ti
et
etφω
φω
+
+
=
=
II
UU
22 XRZ +==IUˆˆ
Impedance [Ω]:
)( IUii eZeZiXR φφθ −==+=ZComplex impedance:
Resistance Reactance
θieZIIZU ==Ohm`s law:
Phase differencebetween voltage andcurrent
Lecture Series Catalysis: Solid State Aspects in Oxidation Catalysis
27Maik Eichelbaum
3. Impedance Spectroscopy
Impedance of an ideal resistor:
Impedance of an ideal inductor:
Impedance of an ideal capacitor: )2
(
2
11 π
π
ωω
ωω−
==
==
=
i
C
i
L
R
eCCi
Z
LeLiZ
RZ
Total impedance calculated by using rules for combining impedances in seriesand parallel:
Series combination: Ztot = Z1 + Z2 + … + Zn
Parallel combination: 1/Ztot = 1/Z1 + 1/Z2+ … + 1/Zn
Lecture Series Catalysis: Solid State Aspects in Oxidation Catalysis
28Maik Eichelbaum
3. Impedance Spectroscopy
Impedance spectroscopy (IE): Sinosoidal voltage, frequency variation (typicallybetween 106 and 10-3 Hz)
According to Ohm’s law the impedance of a sample can be calculated by complex division of the voltage and current
Looking for an “equivalent circuit“ with certain impedance elements thatdescribes best the frequency-dependent behavior of the sample
The impedance elements are related to certain physico-chemical properties, e.g. electrochemical double layer between electrode and electrolyte ions (Helmholtz layer) can be described by a capacitor
Lecture Series Catalysis: Solid State Aspects in Oxidation Catalysis
29Maik Eichelbaum
3. Impedance Spectroscopy
Bode plot (phase angle θ vs. frequency)e.g. oxide layer on Al
one time constant (τ = R C)e.g. chromate layer on oxide layer on Al
two time constants
Equivalent circuit:
D. Ende, K.-M. Mangold, ChiuZ 1993, 3, 134
Lecture Series Catalysis: Solid State Aspects in Oxidation Catalysis
30Maik Eichelbaum
3. Impedance Spectroscopy
Simplified equivalent circuit for a redox reaction:
Cdl
Rct
-ImZ
ReZ
ωRctCdl = 1
Rct
ω = 0ω = ∞
Corresponding Nyquist diagram forRC parallel circuit:
Lecture Series Catalysis: Solid State Aspects in Oxidation Catalysis
31Maik Eichelbaum
The Physicist‘s View of a Solid
Lecture Series Catalysis: Solid State Aspects in Oxidation Catalysis
32Maik Eichelbaum
4. The Physicist`s View of a Solid
A. W. Bott, Current Separations 1998, 17, 87
Lecture Series Catalysis: Solid State Aspects in Oxidation Catalysis
33Maik Eichelbaum
4. The Physicist`s View of a Solid
Occupancy of electrons in a band is determined by Fermi-Dirac statistics
Fermi-Dirac distribution (for an electron gas):
potential Chemical(Electro-)...
eTemperatur...constantBoltzmann ...
Energy...
1exp
1)(
μ
μ
TkE
TkE
Ef
B
B
+⎟⎟⎠
⎞⎜⎜⎝
⎛ −=
2/1)( == μEf= Fermi level
Population densityE
W
EFermi
Vacuum level
Conduction band
WA(work function)
The Fermi curve determines thepopulation of occupied states, independent of the existence ofstates in the regarded E region
μ (T = 0) = EFermi
P. A. Tipler, Physik, Spektrum Verlag
Lecture Series Catalysis: Solid State Aspects in Oxidation Catalysis
34Maik Eichelbaum
4. The Physicist`s View of a Solid
Intrinsicsemiconductor
(Extrinsic) n-type semiconductor
(Extrinsic) p-type semiconductor
Donor level Acceptorlevel
e.g. Si (band gap at300 K = 1.12 eV)
e.g. As-doped Si e.g. Ga-doped Si
additional electrons
hole
hole
P. A. Tipler, Physik, Spektrum Verlag
Lecture Series Catalysis: Solid State Aspects in Oxidation Catalysis
35Maik Eichelbaum
Space charge region
Lecture Series Catalysis: Solid State Aspects in Oxidation Catalysis
36Maik Eichelbaum
4. The Physicist`s View of a Solid
Metals
Metal 1 Metal 1Metal 2 Metal 2
free electrons
a) Energy states of two different metals with different Fermi energies andwork functions. b) With contact, electrons will flow from the metal with higherFermi energy (lower work function) to the one with lower Fermi energy (larger work function) until the Fermi levels of both metals are equalized
Valence band
Conductionband
Contact voltage = (WA1 -WA2)/e
P. A. Tipler, Physik, Spektrum Verlag
Lecture Series Catalysis: Solid State Aspects in Oxidation Catalysis
37Maik Eichelbaum
4. The Physicist`s View of a Solid
Semiconductorsn-type SC in contact with electrolyte p-type SC in contact with electrolyte
Space Charge Region Space Charge Region
Electrochemical double layer
In metals: penetration depth (space charge region to compensate surface charge) of only a fewlattice constants (high concentration of free charge carriers)
In SCs: Debye shielding, LD, can amount from 10 nm to 1 µmi
BD nq
TkL 22πε
=
torsemiconduc intrinsicin carriers charge ofion Concentrat... charge;Electron ... crystal; ofty permittivi Dielectric... inqε
LD LD
A. W. Bott, Current Separations 1998, 17, 87
Lecture Series Catalysis: Solid State Aspects in Oxidation Catalysis
38Maik Eichelbaum
4. The Physicist`s View of a Solid
External Potential Control
Positive potential
Negative potential
Flatband potential
A. W. Bott, Current Separations 1998, 17, 87
Lecture Series Catalysis: Solid State Aspects in Oxidation Catalysis
39Maik Eichelbaum
4. The Physicist`s View of a Solid
Conductionband Conduction
band
Valenceband Valence
band
Junction(charge depleted zone)
n sidep side
p-n junction
Electrons migrate to p side, holes migrate to n side untilelectrochemical potentials (Fermi energies) are equilibrated
Potential difference
(highresistance)
Energy
P. A. Tipler, Physik, Spektrum Verlag
‐++‐
forwardbias
reversebias
breakdown voltage
Lecture Series Catalysis: Solid State Aspects in Oxidation Catalysis
40Maik Eichelbaum
4. The Physicist`s View of a Solid
Metal-semiconductor junction (Schottky barrier)
Ohmic contact
Schottky barrier
K. W. Kolasinski, Surface Science: Foundations of Catalysis and Nanoscience, John Wiley & Sons 2008
Lecture Series Catalysis: Solid State Aspects in Oxidation Catalysis
41Maik Eichelbaum
Back to Chemistry…
Lecture Series Catalysis: Solid State Aspects in Oxidation Catalysis
42Maik Eichelbaum
5. Oxidation Catalysis
Oxidation catalysis with transition metal ions:
1) Oxidation of the substrate by the catalyst (being reduced)
2) Reoxidation of the catalyst (usually by gas phase oxygen)
Lecture Series Catalysis: Solid State Aspects in Oxidation Catalysis
43Maik Eichelbaum
5. Oxidation Catalysis
Experimental hints for the participation of “lattice“ oxygen (oxygencovalently bound to the active transition metal ion) in the reaction:
1) Reaction runs (awhile) without gas phase oxygen present (riser reactorconcept)
2) Different oxidation states of the transition metal are monitored in dependence on the partial pressures of reaction gases or on contact time
3) By using 18O2 in isotope exchange experiments, first 16O (from thecatalyst) is found in the reaction product, and only after some time 18O
4) After long 18O/16O exchange, 18O is found in the catalyst lattice (e.g. in ToF-SIMS experiments)
5) Conductivity changes upon reaction conditions, correlation betweenconductivity and activity/selectivity for differently doped semiconductors
Lecture Series Catalysis: Solid State Aspects in Oxidation Catalysis
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5. Oxidation Catalysis
Example:
Lecture Series Catalysis: Solid State Aspects in Oxidation Catalysis
45Maik Eichelbaum
5. Oxidation Catalysis
Participation of “lattice oxygen“ is often associated with the Mars-van Krevelen mechanism
Lecture Series Catalysis: Solid State Aspects in Oxidation Catalysis
46Maik Eichelbaum
5. Oxidation Catalysis
Original derivation of the Mars-van Krevelen rate expressionAn analysis by
Lecture Series Catalysis: Solid State Aspects in Oxidation Catalysis
47Maik Eichelbaum
5. Oxidation Catalysis
kinetic gas theory allows only n = 1only O2 single-site adsorption represented (two-
site adsorption: (1-θ)2), however involvement of O lattice ions necessitates O2 dissociation into atoms
severe inconsistency
no elementary step, Eley-Rideal reaction veryunlikely, multiple bond-breaking, multiple intermediates, sequence of elementary steps needed
no intermediate species of final productsincluded, only O ions considered
Mars-van Krevelen rate equation:
Lecture Series Catalysis: Solid State Aspects in Oxidation Catalysis
48Maik Eichelbaum
5. Oxidation Catalysis
Simple elementary steps for adsorption of O2 on a lattice vacancy (active site)Alternative derivations by Vannice:
Mars-van Krevelen rate equation:
(n = 1)
Lecture Series Catalysis: Solid State Aspects in Oxidation Catalysis
49Maik Eichelbaum
5. Oxidation Catalysis
Simple elementary steps for lattice O ions involved in the reaction
Mars-van Krevelen rate equation:
⎟⎟
⎠
⎞
⎜⎜
⎝
⎛++= 1
164 2O
21
2R
22
O1
R2R2R
22Pk
PkPkPkPkr
?
Lecture Series Catalysis: Solid State Aspects in Oxidation Catalysis
50Maik Eichelbaum
Unifying Physics and Chemistry andCatalysis; an Attempt
Lecture Series Catalysis: Solid State Aspects in Oxidation Catalysis
51Maik Eichelbaum
5. Oxidation Catalysis – Electronic Theory
Selective oxidation of hydrocarbons, consideration of bands and frontier orbitals:
J. Haber, M. Witko, J. Catal. 2003, 216, 416‐424
Rigid band assumption (no local surface states)
No site isolation total conduction!
Lecture Series Catalysis: Solid State Aspects in Oxidation Catalysis
52Maik Eichelbaum
Surface States
Lecture Series Catalysis: Solid State Aspects in Oxidation Catalysis
53Maik Eichelbaum
5. Oxidation Catalysis – Electronic Theory
Surface states of a 3D crystal
Intrinsic surface states: On ideal surfaces (perfecttermination with 2D translational symmetry)
Extrinsic surface states: On surfaces with imperfections(e.g. missing atom)
H. Lüth, Solid Surfaces, Interfaces and Thin Films, Springer 2001
Lecture Series Catalysis: Solid State Aspects in Oxidation Catalysis
54Maik Eichelbaum
5. Oxidation Catalysis – Electronic Theory
J. Haber, M. Witko, J. Catal. 2003, 216, 416‐424
Electrocatalytic oxidation of catechol on vanadium-doped TiO2 electrodes
Flat band potential(no external potential)
With external potential
Surface states
Lecture Series Catalysis: Solid State Aspects in Oxidation Catalysis
55Maik Eichelbaum
5. Oxidation Catalysis – Electronic Theory
Surface statecharge
Space charge (to compensatesurface charge)
Neutrality condition determines the Fermi level
ECEFED
EV
acceptors
e‐
Build-up ofuncompensatednegative charge in surface acceptor state
unstable
Hypothetical flatband situation: Formation of (depletion) space charge layer:
H. Lüth, Solid Surfaces, Interfaces and Thin Films, Springer 2001
Lecture Series Catalysis: Solid State Aspects in Oxidation Catalysis
56Maik Eichelbaum
5. Oxidation Catalysis – Electronic Theory
H. Lüth, Solid Surfaces, Interfaces and Thin Films, Springer 2001
Band schemes:
Free chargecarrierdensities:
Localconductivity:
n-SC: p-SC:
DEPLETION
pb
nb
Lecture Series Catalysis: Solid State Aspects in Oxidation Catalysis
57Maik Eichelbaum
5. Oxidation Catalysis – Electronic Theory
p-type semiconductor (e.g. (VO)2P2O7 for butane oxidation to maleic anhydride)
Adsorption (catalysis) at surface states:
Local surface state = active (isolated single) site selective oxidation
Oxidation Reduction
Lecture Series Catalysis: Solid State Aspects in Oxidation Catalysis
58Maik Eichelbaum
5. Oxidation Catalysis – Electronic Theory
H. Lüth, Solid Surfaces, Interfaces and Thin Films, Springer 2001
Spectroscopic evidence? Photoemission spectroscopy
adsorbate-induced extrinsicsurface state
Core levels, valence states, workfunction at the surface shift by same amount of eΔΦ due to band bending in dependence on chemical potential ofadsorbate (no surface dipoles)
Lecture Series Catalysis: Solid State Aspects in Oxidation Catalysis
59Maik Eichelbaum
5. Oxidation Catalysis – Electronic Theory
Valence band spectra (hν = 35 eV) of In2O3 surface in dependence on PO2
A. Klein, Appl. Phys. Lett. 2000, 77, 2009-2011
Lecture Series Catalysis: Solid State Aspects in Oxidation Catalysis
60Maik Eichelbaum
Literature
Reviews, e.g.:“Solid state aspects of oxidation catalysis“: P. J. Gellings, H. J. M. Bouwmeester, CatalysisToday 2000, 58, 1-53; ibid. 1992, 12, 1-105“The role of redox, acid-base and collective properties and of cristalline sate ofheterogeneous catalysts in the selective oxidation of hydrocarbons“, J. C. Védrine, Top. Catal. 2002, 21, 97-106“In situ and operando spectroscopy for assessing mechanisms of gas sensing“, A. Gurlo, R. Riedel, Angew. Chem. Int. Ed. 2007, 46, 3826-3848
Surface physics and surface chemistry, e.g.:S. R. Morrison, “The chemical physics of surfaces“, Plenum Press New York and London 1977(surface physics and surface chemistry, includes chapter on heterogeneous catalysis)H. Lueth, “Solid surfaces, interfaces and thin films“, Springer 2010P. A. Cox, “The electronic structure and chemistry of solids”, Oxford University Press 1989R. Hoffmann, “Solids and surfaces : a chemist's view of bonding in extended structures” VCH 1988
Fundamental textbooks, e.g.:N. W. Ashcroft, N. D. Mermin, “Solid state physics“, Brooks/Cole Cengage Learning 2009H. Ibach, H. Lueth, “Solid-state physics : an introduction to principles of materials science”, Springer 2009
(Examples)