solid state aspects of oxidation catalysis paul j. gellings and henny j.m. bouwmeester university of...

Solid State Aspects of Oxidation Catalysis

Paul J. Gellings andHenny J.M. Bouwmeester

University of Twente, Enschede, the Netherlands

Solid State Aspects of Oxidation Catalysis 2

Contents of lecture

Some concepts of solid state chemistryMethods of computational modellingExamples of applications of 1 and 2 to

specific catalytic reactionsChallenges for extension of use of solid

state considerations to catalysisPossibilities for new applications: use of

membranes


Solid state concepts

The solid state concepts considered are:

atomic, ionic and electronic defects defect structure defect concentrations type of conduction conductivity crystal structure


Defect notation

OV

Atom type:cation, anion, foreignion, vacancy (V)

Effective charge with respect to ideal lattice: = positive, ' = negative,x = neutral

Position in lattice:cation site, anion site, interstitial site (i)


Important types of defects

metal vacancy MV dopant, highercharge

MD

oxygen vacancy OV dopant, lower

chargeMN

metal interstitial iM free electron e

oxygen interstitial iO free (electron)hole

h


Example of defect equilibrium

A typical example is ZrO2:

•i

••OZr

••O

221•

iO

••O22

1xO

xZr

Zr••

O221x

OxZr

h=O2ore=V 2orrZ=V 2

conditionstrality electroneu

O=h2+O:phigh at or

,e2+V+O=O+Zr2

or,rZ2+V+O=O+Zr2

2


Kröger-Vink or Brouwer diagram for ZrO2


Doping and defect equilibrium

Doping of ZrO2:with lowervalent metal such as Y:

ZrO

ZrO

YV 2h

evenorYeV 2

With higher valent metal such as Nb:

eV 2Nb OZr


Computational modelling:defects

Crystal with defects divided in two regions:1: inner region containing defect with number of

neighbours and calculation with individual coordinates of all particles

2: outer region which is considered as dielectric continuum

(Mott-Littleton method)Calculation: Minimize potential energy of system

with respect to displacement and moment of surrounding ions


Computational modelling:surfaces

Crystal with surface divided in two regions:1: 2-dimensional surface region calculated with individual

coordinates of all particles2: region below 1. which is considered as ideal crystal

treated as continuum

Calculation: Minimize potential energy of systemwith respect to displacement and moment of

surrounding ions


Defect energies near surface

Two examples:Y3+-dopant in ThO2 Oxygen vacancy in ThO2

Catlow et al. J. Phys. Chem. 94 (1990) 7889


Vanadia: morphology

Equilibrium shape: planes (001) (major)(110), (101), (200), (301)

Sayle et al. J. Mater. Chem. 6 (1996) 653


Vanadia: ethene sorption

The (001) and (301) planes have high V=O concentrations, which are of special importance in catalytic reactions.

Sorption energies of ethene (kJ/mole)(001) -33(200) -23(301) -77

Sayle et al. J. Mater. Chem. 6 (1996) 653


Vanadia: quantum chemical cluster calculations

As mentioned earlier: more recently quantum chemical calculations basedon clusters of vanadium and oxygen atoms.These are not discussed further here, because they probably were the subject of Witko's paper of this morning.

Some references to her work:Hermann, Witko et al.: J. Electron. Spectr. 98-99 (1999) 245Haber, Witko, Tokarz: Appl.Catal. A:General 157 (1997) 3 & 23


Oxygen exchange

Catalyst BET area

(m2)

%O2 ex-

changed

D0 1015

(cm2/s)

La2O3 5.03 68.9 9.83

1 at% SrO /

La2O3 6.86 79.0 18.86

2 at% SrO /

La2O3 7.64 72.0 11.18

Kalenik and Wolf, Catal.Lett. 9 (1991) 441


Oxidation reactions

Methane oxidation oxidative

dimerization(oxidative coupling)

oxidation to synthesis gas

total combustion

Other compounds saturated

hydrocarbons olefins aromatic

hydrocarbons nitrogen oxides


Oxidative coupling of methane 1

La2O3 catalysts: doping with Sr2+ and Zn2+:

increased activity and C2-selectivitydoping with Ti4+ and Nb5+:

decreased activity and C2-selectivityclear correlation with increased oxygen vacancy

concentration and oxygen conductivity

Borchert and Baerns, J. Catal. 190 (1997) 315


Oxidative coupling of methane 2

MgO catalysts: doping with Li+ :

increased activity and C2-selectivity:active species O- -ion, abstracts hydrogen from methane under formation of methyl radical in gas phase

(OH)+)g(CHO+)g(CH •O3

••O4


Defect structure of Li-doped MgO 1

Catlow et al. J. Phys. Chem 94 (1990) 7889

MgO""2+V+iL2O+Mg2+O"Li" ••OMg

xO

xMg2


Defect structure of Li-doped MgO 2

•xO22

1••O h2+OO+V


Solution and oxidation energies

MgO""2+V+iL2O+Mg2+O"Li" ••OMg

xO

xMg2

Solution reaction:

•xO22

1••O h2+OO+V Oxidation reaction:


Segregation energies

Note formation of defect associates


Ammoxidation of propane and toluene

OH3+CNHCO +NH+HC 21-2n1-n223

32+2nn

using vanadium antimonate catalysts with Sb:Vratios of 1 to 5

A. Andersson, et al. Appl. Catal. A, 113 (1994) 43 - 57

J. Nilsson, et al. J. Catal. 160 (1996) 244 - 260

J. Nilsson, et al. Catal. Today, 33 (1997) 97 - 108


Active site for selective ammoxidation


Partial oxidation of iso-butane

•O94

••O104

•ONb

xO

xNb

(OH)+HCO+HC

O+bNO+Nb

••Oads

-94

xO

•O94

• V+HOCO+V+HC

O(gas)H+O+Nb(OH)2+bN 2•O

xNb

•ONb

Hydrogen abstraction:

Formation of iso-butoxide ion:

Re-formation of active site:

I. Matsuura, H. Oda, and K. Oshida, Catal. Today, 16 (1993) 547


Membranes

(micro)porous membranes:any oxidic material either intrinsically catalytically active or covered with active (mono)layer

dense membranes:ionic or mixed ionic-electronic conductingmaterial


Membrane reactors

Modes of operation:

Oxygen Oxygen Oxygen

Reductant Reductant Reductant

R

Chemicalpotential

driven

Electricpotential

driven

Solid oxidefuel cellmode


ABO3 perovskite structure

high values of ionic and electronic conductivitye.g, in:

La1-xAxCo1-yFeyO3-

A=Sr, Ba

SrFeCo0.5O3.25-


Mixed oxygen/ionic conducting dense membrane


Oxygen conducting membranes - I

Examples of perovskites used:La1-xSrxCoyFe1-yO3- : ten Elshof et al.

(Solid State Ionics, 81 (1995) 97, 89 (1996) 81) Xu and Thomson (Am.Inst.Chem.Eng. Journal

Ceram. Processing 43 (1997) 2731)BaCe1-xGdxO3- : Hibino et al.

(J. Chem. Soc. Faraday Trans. 91 (1995) 4419) La1-xBaxCoyFe1-yO3- : Xu and Thomson (see above)

SrFeCo0.5O3- : Ma and Balachandran(Solid State Ionics 100 (1997) 53 )


Oxygen conducting membranes - II

All authors: higher C2 selectivity than conventional

ten Elshof et al. and Xu and Thomson: limiting reaction surface process at methane side. If transport in membrane limiting danger for reduction of membrane lower C2 selectivity. Also if too high oxygen permeation ratemolecular oxygen formation, see next transparency


Oxygen conducting membranes - III

In general two competing reactions

O2xO

O23xO4

V2(g)Oh4O2

VO(g)HCH2h2O(g)CH 2

Oxygen formed in second reaction can cause oxidationin gas phase high oxygen flux not necessarily favourable for high C2-selectivity!


Proton conduction

Equilibria for compound : SrCe0.95Yb0.05O3-Hx

Ce••

O••

O

e•

W2221

2

O••

O221x

O

3•O2

••O

xO

1•x

O••

O221

bY+e=h+OH+V2

:conditiontrality electroneu

K:h+e=0

K:O(g)H= (g)O+(g)H

K:e+V+(g)O=O

K:OH2=O(g)H+V+O

K:h2+O=V+(g)O

Schober et al. Solid State Ionics 86/88 (1996) 653


Predominance diagram for SrCe0.95Yb0.05O3-Hx


Methane coupling with proton conducting membrane

Hamakawa et al. J.Electrochem.Soc. 140 (1993) 459, 141 (1994) 1720


Defect equilibria

OOxO2

xOO2

OH2VOO(g)H

OH2VO(g)H or

h2OVO xOO22

1

Increasing pO2 leads to increased hole concentration (reaction 1) and decreased proton concentration (reaction

2 & 3) and thus simultaneously to increased hole con-duction and decreased proton conduction


Further experiments

Langguth et al. Appl. Catal. A, 158 (1997) 287

• Without oxygen: some CO and C2 (reduction electrolyte and impurity in methane)• With oxygen: C2-products but decreasing selectivity with time (contribution of oxygen conduction)• With oxygen + water: increased C2 selectivity, due to decreased CO and CO2 formation as a consequence of decreased oxygen conduction and increased proton conduction


Concluding remarks

Solid state aspects:

1. Interpretation of catalytic properties using solid state concepts: but in many cases this must still begin2. Making use of special properties of solids in membrane reactors: much knowledge must still be obtained


Solid electrolyte membrane cell

a = solid oxide potentiometry, b = solid oxide fuel cell, c = electrochemical oxygen pump

Eng and Stoukides, Catal.Rev.Sci.Eng. 33 (1991) 375

solid state aspects of oxidation catalysis paul j. gellings and henny j.m. bouwmeester university of...

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