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ModernMethods inHeterogeneous Catalysis Research
SolidStateAspects inOxidationCatalysis
20thJanuary 2012
MaikEichelbaum/FHI
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How chemists like to see the world:Everything is molecular
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1.Introduction
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Solid State Aspects: Goingunderneath the surface and beyond
the molecule
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1.Introduction
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1.Introduction
Dehydrogenation of formic acid with different bronzealloys: activation energy andresistivity
HCOOH CO2 + H2
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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 + O2 CO2
Explanation: donor reaction
defect electron + Ni2+
= active site
fast
CO chemisorption rate-determining
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1.Introduction
Excess electrons in ZnO (with Znexcess): n-type semiconductor
Increase of quasi-free electrons byGa2O3, decrease by Li2O addition:CO + O2 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)
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1.Introduction
Krger-Vink notation for point defects in crystalsF. A. Krger, 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 bulk
vacancies, interstitials, surface vacancies, adsorbed ions etc.
Examples: Ni3+ in Ni2+O2- on normal lattice position NiNi
Interstitial Zn+ in Zn2+O2- iZn
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2. Electrical Conductivity in SolidOxides
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2.Electrical Conductivity inSolidOxides
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
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2.Electrical Conductivity inSolidOxides
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
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2.Electrical Conductivity inSolidOxides
Example: Oxidation of propene to acrylic acid on MoVTeNbOx(M2)
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2.Electrical Conductivity inSolidOxides
humid propene-oxygen-helium (2-8-90) mixture
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1) Proton conduction in oxides
2) Oxygen conduction in oxides
1) Proton conduction in oxides
2) Oxygen conduction in oxides
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2.Electrical Conductivity inSolidOxides
Oxygen conduction orHow is oxygen incorporated into oxides?
Existing point defects (oxygen vacancies: VO, oxygen interstitials: Oi, electronicpoint defects: h, e) allow a finite compositional flexibility in a binary oxide MOwith phase width MO1-
R. Merkle, J. Maier,
Example: Fe-doped SrTiO3- (quasi-binary due to fixed Sr/Ti ratio below 1200C)
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2.Electrical Conductivity inSolidOxides
Phase diagram for MO1-
R.Merkle,J.Maier,
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2.Electrical Conductivity inSolidOxides
Reaction and transport of oxygen: kinetic limitations
R.Merkle,J.Maier,
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2.Electrical Conductivity inSolidOxides
Fedoped
SrTiO3single
crystal,
surface
reaction
is
limiting,
T=923
K,
pO2:
0.01
>1
bar,
ttotal=24
h
falsecolorimage concentrationprofile
http://www.fkf.mpg.de/maier/downloads.html
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2.Electrical Conductivity inSolidOxides
Fedoped
SrTiO3single
crystal,
bulk
diffusion
is
limiting,
T=823
K,
pO2:
0.1
>1
bar,
ttotal=6300
s
falsecolorimage concentrationprofile
http://www.fkf.mpg.de/maier/downloads.html
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2.Electrical Conductivity inSolidOxides
Blocking grain boundary formation of space charge layers
R.Merkle,J.Maier,
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2.Electrical Conductivity inSolidOxides
array of circular gold microelectrodes(20 m diameter) on polycrystalline Fe-
doped SrTiO3 platinumcontacttips
R.Merkle,J.Maier,
Localized Impedance Spectroscopy
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Insertion: Impedance spectroscopy
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3.Impedance Spectroscopy
Impedance measurements (measurement at alternating voltages):
)(
)(
)(
)(
I
U
ti
ti
et
et
+
+
=
=
II
UU
22XRZ +==
I
U
Impedance []:
)( IUii eZeZiXR ==+=ZComplex impedance:
Resistance Reactance
ieZIIZU ==Ohm`s law:
Phase difference
between voltage andcurrent
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3.Impedance Spectroscopy
Impedance of an ideal resistor:
Impedance of an ideal inductor:
Impedance of an ideal capacitor: )2(
2
11
==
==
=
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
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3.Impedance Spectroscopy
Impedance spectroscopy (IE): Sinosoidal voltage, frequency variation (typicallybetween 106 and 10-3 Hz)
According to Ohms 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 (Helmholtzlayer) can be described by a capacitor
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3.Impedance Spectroscopy
Bode plot (phase angle vs. frequency)e.g. oxide layer on Al
one time constant ( = R C)e.g. chromate layer on oxide layer on Al
two time constants
Equivalent circuit:
D. Ende, K.-M. Mangold, ChiuZ1993, 3, 134
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3.Impedance Spectroscopy
Simplified equivalent circuit for aredox reaction:
Cdl
Rct
-ImZ
ReZ
RctCdl = 1
Rct
= 0 =
Corresponding Nyquist diagram forRC parallel circuit:
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The Physicists View of a Solid
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4.ThePhysicist`s Viewof aSolid
A.W.Bott,CurrentSeparations1998,17,87
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4.ThePhysicist`s Viewof aSolid
Occupancy of electrons in a band is determined byFermi-Dirac statistics
Fermi-Dirac distribution (for an electron gas):
potential
Chemical(Electro-)...
eTemperatur...
constantBoltzmann...
Energy...
1exp
1)(
T
k
E
Tk
EEf
B
B
+
=
2/1)( ==Ef
= 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 Eregion
(T = 0) =EFermi
P.A.Tipler,Physik,SpektrumVerlag
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Space charge region
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4.ThePhysicist`s Viewof aSolid
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 (largerwork function) until the Fermi levels of both metals are equalized
Valence band
Conductionband
Contact voltage = (WA1 -WA2)/e
P.A.Tipler,Physik,SpektrumVerlag
h h i i ` i f S lid
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4.ThePhysicist`s Viewof aSolid
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 few
lattice constants (high concentration of free charge carriers)
In SCs: Debye shielding, LD, can amount from 10 nm to 1 mi
BD
nq
TkL
22
=
torsemiconducintrinsicincarrierschargeofionConcentrat...charge;Electron...crystal;oftypermittiviDielectric... inq
LD LD
A.W.Bott,CurrentSeparations1998,17,87
4 Th Ph i i ` Vi f S lid
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4.ThePhysicist`s Viewof aSolid
External PotentialControl
Positivepotential
Negativepotential
Flatband potential
A.W.Bott,CurrentSeparations1998,17,87
4 Th Ph i i t` Vi f S lid
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4.ThePhysicist`s Viewof aSolid
Conductionband
Conductionband
Valenceband
Valenceband
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
Potentialdifference
(highresistance)
Energy
P.A.Tipler,Physik,
SpektrumVerlag
++
forwardbias
reversebias
breakdownvoltage
4 The Ph si ist`s Vie of a Solid
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4.ThePhysicist`s Viewof aSolid
Metal-semiconductor junction (Schottky barrier)
Ohmic contact
Schottky barrier
K.W.Kolasinski,Surface Science:Foundations of Catalysis and Nanoscience,JohnWiley &Sons2008
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Back to Chemistry
5 Oxidation Catalysis
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5.OxidationCatalysis
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)
5 Oxidation Catalysis
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5.OxidationCatalysis
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 reactor
concept)
2) Different oxidation states of the transition metal are monitored independence 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. inToF-SIMS experiments)
5) Conductivity changes upon reaction conditions, correlation betweenconductivity and activity/selectivity for differently doped semiconductors
5 Oxidation Catalysis
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5.OxidationCatalysis
Example:
5 Oxidation Catalysis
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5.OxidationCatalysis
Participation of lattice oxygen is often associated with the Mars-vanKrevelen mechanism
5 Oxidation Catalysis
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5.OxidationCatalysis
Original derivation of the Mars-van Krevelen rate expressionAnanalysis by
5. Oxidation Catalysis
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5.OxidationCatalysis
kinetic gas theory allows only n= 1 only O2 single-site adsorption represented (two-site adsorption: (1-)2), however involvement of Olattice ions necessitates O2 dissociation into atoms severe inconsistency
no elementary step, Eley-Rideal reaction veryunlikely, multiple bond-breaking, multipleintermediates, sequence of elementary steps needed
no intermediate species of final productsincluded, only O ions considered
Mars-van Krevelen rate equation:
5. Oxidation Catalysis
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5.OxidationCatalysis
Simple elementary steps for adsorption ofO2 on a lattice vacancy (active site)Alternative derivations by Vannice:
Mars-van Krevelen rate equation:
(n= 1)
5.OxidationCatalysis
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5. O dat o Cata ys s
Simple elementary steps forlattice O ions involved in the reaction
Mars-van Krevelen rate equation:
++= 1
1642
O
2
1
2
R
2
2
O1
R2R2R
22Pk
Pk
Pk
PkPkr
?
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5.OxidationCatalysis ElectronicTheory
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y y
Selective oxidation of hydrocarbons, consideration of bands and frontier orbitals:
J.Haber,M.Witko,J.Catal.2003,216,416424
Rigid band assumption (no local surface states)
No site isolation totalconduction!
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Surface States
5.OxidationCatalysis ElectronicTheory
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Surface states of a 3D crystal
Intrinsic surface states: Onideal surfaces (perfecttermination with 2D
translational symmetry)
Extrinsic surface states: Onsurfaces with imperfections(e.g. missing atom)
H. Lth, Solid Surfaces, Interfaces and Thin Films, Springer 2001
5.OxidationCatalysis ElectronicTheory
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J.Haber,M.Witko,J.Catal.2003,216,416424
Electrocatalytic oxidation of catechol on vanadium-doped TiO2electrodes
Flat band potential(no external potential)
With external potential
Surface states
5.OxidationCatalysis ElectronicTheory
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Surface statecharge
Space charge (to compensatesurface charge)
Neutrality condition determines the Fermi level
ECEFED
EV
acceptors
e
Build-up ofuncompensatednegative charge insurface acceptor state
unstable
Hypothetical flatband situation: Formation of (depletion) space charge layer:
H. Lth, Solid Surfaces, Interfaces and Thin Films, Springer 2001
5.OxidationCatalysis ElectronicTheory
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H. Lth, Solid Surfaces, Interfaces and Thin Films, Springer 2001
Band schemes:
Free chargecarrier
densities:
Local
conductivity:
n-SC: p-SC:
DEPLETION
pb
nb
5.OxidationCatalysis ElectronicTheory
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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
5.OxidationCatalysis ElectronicTheory
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H. Lth, 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 sameamount of e due to band bending independence on chemical potential of
adsorbate (no surface dipoles)
5.OxidationCatalysis ElectronicTheory
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Valence band spectra (h = 35 eV) of In2O
3surface in dependence on P
O2
A. Klein, Appl. Phys. Lett.2000, 77, 2009-2011
Literature
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Reviews, e.g.:Solid state aspects of oxidation catalysis: P. J. Gellings, H. J. M. Bouwmeester, CatalysisToday2000, 58, 1-53; ibid.1992, 12, 1-105The role of redox, acid-base and collective properties and of cristalline sate ofheterogeneous catalysts in the selective oxidation of hydrocarbons, J. C. Vdrine, Top. Catal.
2002, 21, 97-106In 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 VCH1988
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)
Top Related