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 Aspects of Oxidation Catalysis 3
Solid state concepts
The solid state concepts considered are:
atomic, ionic and electronic defects defect structure defect concentrations type of conduction conductivity crystal structure
Solid State Aspects of Oxidation Catalysis 4
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)
Solid State Aspects of Oxidation Catalysis 5
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
Solid State Aspects of Oxidation Catalysis 6
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
Solid State Aspects of Oxidation Catalysis 7
Kröger-Vink or Brouwer diagram for ZrO2
Solid State Aspects of Oxidation Catalysis 8
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
Solid State Aspects of Oxidation Catalysis 9
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
Solid State Aspects of Oxidation Catalysis 10
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
Solid State Aspects of Oxidation Catalysis 11
Defect energies near surface
Two examples:Y3+-dopant in ThO2 Oxygen vacancy in ThO2
Catlow et al. J. Phys. Chem. 94 (1990) 7889
Solid State Aspects of Oxidation Catalysis 12
Vanadia: morphology
Equilibrium shape: planes (001) (major)(110), (101), (200), (301)
Sayle et al. J. Mater. Chem. 6 (1996) 653
Solid State Aspects of Oxidation Catalysis 13
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
Solid State Aspects of Oxidation Catalysis 14
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
Solid State Aspects of Oxidation Catalysis 15
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
Solid State Aspects of Oxidation Catalysis 16
Oxidation reactions
Methane oxidation oxidative
dimerization(oxidative coupling)
oxidation to synthesis gas
total combustion
Other compounds saturated
hydrocarbons olefins aromatic
hydrocarbons nitrogen oxides
Solid State Aspects of Oxidation Catalysis 17
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
Solid State Aspects of Oxidation Catalysis 18
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
Solid State Aspects of Oxidation Catalysis 19
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
Solid State Aspects of Oxidation Catalysis 20
Defect structure of Li-doped MgO 2
•xO22
1••O h2+OO+V
Solid State Aspects of Oxidation Catalysis 21
Solution and oxidation energies
MgO""2+V+iL2O+Mg2+O"Li" ••OMg
xO
xMg2
Solution reaction:
•xO22
1••O h2+OO+V Oxidation reaction:
Solid State Aspects of Oxidation Catalysis 22
Segregation energies
Note formation of defect associates
Solid State Aspects of Oxidation Catalysis 23
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
Solid State Aspects of Oxidation Catalysis 24
Active site for selective ammoxidation
Solid State Aspects of Oxidation Catalysis 25
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
Solid State Aspects of Oxidation Catalysis 26
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
Solid State Aspects of Oxidation Catalysis 27
Membrane reactors
Modes of operation:
Oxygen Oxygen Oxygen
Reductant Reductant Reductant
R
Chemicalpotential
driven
Electricpotential
driven
Solid oxidefuel cellmode
Solid State Aspects of Oxidation Catalysis 28
ABO3 perovskite structure
high values of ionic and electronic conductivitye.g, in:
La1-xAxCo1-yFeyO3-
A=Sr, Ba
SrFeCo0.5O3.25-
Solid State Aspects of Oxidation Catalysis 29
Mixed oxygen/ionic conducting dense membrane
Solid State Aspects of Oxidation Catalysis 30
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 )
Solid State Aspects of Oxidation Catalysis 31
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
Solid State Aspects of Oxidation Catalysis 32
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!
Solid State Aspects of Oxidation Catalysis 33
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
Solid State Aspects of Oxidation Catalysis 34
Predominance diagram for SrCe0.95Yb0.05O3-Hx
Solid State Aspects of Oxidation Catalysis 35
Methane coupling with proton conducting membrane
Hamakawa et al. J.Electrochem.Soc. 140 (1993) 459, 141 (1994) 1720
Solid State Aspects of Oxidation Catalysis 36
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
Solid State Aspects of Oxidation Catalysis 37
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 con- duction
Solid State Aspects of Oxidation Catalysis 38
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 State Aspects of Oxidation Catalysis 39
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