Maik Eichelbaum Solid State Aspects in Oxidation Catalysis 120120

8/12/2019 Maik Eichelbaum Solid State Aspects in Oxidation Catalysis 120120

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ModernMethods inHeterogeneous Catalysis Research

SolidStateAspects inOxidationCatalysis

20thJanuary 2012

MaikEichelbaum/FHI


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Lecture SeriesCatalysis:SolidStateAspects in

OxidationCatalysis 2MaikEichelbaum

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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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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Example: Oxidation of propene to acrylic acid on MoVTeNbOx(M2)


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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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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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Phase diagram for MO1-

R.Merkle,J.Maier,


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Reaction and transport of oxygen: kinetic limitations

R.Merkle,J.Maier,


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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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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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Blocking grain boundary formation of space charge layers

R.Merkle,J.Maier,


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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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Lecture SeriesCatalysis:SolidStateAspects inOxidationCatalysis

26MaikEichelbaum

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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Lecture SeriesCatalysis:SolidStateAspects inOxidationCatalysis

27MaikEichelbaum


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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Lecture SeriesCatalysis:

Solid

State

Aspects in



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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Solid

State

Aspects in



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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Solid

State

Aspects in



Simplified equivalent circuit for aredox reaction:

Cdl

Rct

-ImZ

ReZ

RctCdl = 1

Rct

= 0 =

Corresponding Nyquist diagram forRC parallel circuit:


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Solid

State

Aspects in


The Physicists View of a Solid


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Solid

State

Aspects in


4.ThePhysicist`s Viewof aSolid

A.W.Bott,CurrentSeparations1998,17,87


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Solid

State

Aspects in



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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Solid

State

Aspects in


Space charge region


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Solid

State

Aspects in



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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Solid

State

Aspects in



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


4 Th Ph i i ` Vi f S lid


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Solid

State

Aspects in



External PotentialControl

Positivepotential

Negativepotential

Flatband potential


4 Th Ph i i t` Vi f S lid


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Solid

State

Aspects in



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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Solid

State

Aspects in



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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Solid

State

Aspects in


Back to Chemistry

5 Oxidation Catalysis


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Solid

State

Aspects in


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)



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Solid

State

Aspects in



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



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Solid

State

Aspects in



Example:



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Solid

State

Aspects in



Participation of lattice oxygen is often associated with the Mars-vanKrevelen mechanism



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Solid

State

Aspects in



Original derivation of the Mars-van Krevelen rate expressionAnanalysis by

5. Oxidation Catalysis


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Solid

State

Aspects in



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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Solid

State

Aspects in



Simple elementary steps for adsorption ofO2 on a lattice vacancy (active site)Alternative derivations by Vannice:


(n= 1)



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Solid

State

Aspects in


5. O dat o Cata ys s

Simple elementary steps forlattice O ions involved in the reaction


++= 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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Solid

State

Aspects in


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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Solid

State

Aspects in


Surface States



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Solid

State

Aspects in


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



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Solid

State

Aspects in


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



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Solid

State

Aspects in

OxidationCatalysis 55Maik

Eichelbaum

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:




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Solid

State

Aspects in


Eichelbaum


Band schemes:

Free chargecarrier

densities:

Local

conductivity:

n-SC: p-SC:

DEPLETION

pb

nb



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Solid

State

Aspects in


Eichelbaum

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



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Solid

State

Aspects in


Eichelbaum


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)



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Solid

State

Aspects in


Eichelbaum

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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Solid

State

Aspects in


Eichelbaum

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)

Maik Eichelbaum Solid State Aspects in Oxidation Catalysis 120120

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