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Solid State Aspects of
Oxidation Catalysis
Paul J. Gellings and
Henny J.M. Bouwmeester
University of Twente, Enschede,
the Netherlands
Contents of lecture
Some
concepts of solid state chemistry
 Methods of computational modelling
 Examples of applications of 1 and 2 to
specific catalytic reactions
 Challenges for extension of use of solid
state considerations to catalysis
 Possibilities for new applications: use of
membranes
Solid State Aspects of Oxidation Catalysis
2
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
3
Defect notation
Atom type:
cation, anion, foreign
ion, vacancy (V)

VO
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
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Important types of defects
metal vacancy

VM
dopant, higher
charge
oxygen vacancy
VO
dopant, lower
charge
NM
metal interstitial
M 
i
free electron
e
free (electron)
hole
h
oxygen interstitial
Oi
Solid State Aspects of Oxidation Catalysis
DM
5
Example of defect
equilibrium
A typical example is ZrO2:
 , or
2 Zr + O = O 2 + V + 2 ZrZr
x
Zr
x
O
1
2
••
O
2 Zr + O = O 2 + V + 2 e,
x
Zr
x
O
1
2
••
O
or at high p O 2 : Oi + 2 h = 12 O 2
•
electroneutralit ycondit ions
 
 
 
••
•




2 V = ZrZr  or 2 VO = e  or 2 O i  = h
••
O
Solid State Aspects of Oxidation Catalysis
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Kröger-Vink or Brouwer
diagram for ZrO2
Solid State Aspects of Oxidation Catalysis
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Doping and defect
equilibrium
Doping of ZrO2:
with lowervalent metal such as Y:
 
h  2 V  Y 
  or even
2 VO   e YZr


O
Zr
With higher valent metal such as Nb:


NbZr
 2  e

VO
Solid State Aspects of Oxidation Catalysis
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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
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Computational modelling:
surfaces
Crystal with surface divided in two regions:
1: 2-dimensional surface region calculated with individual
coordinates of all particles
2: region below 1. which is considered as ideal crystal
treated as continuum
Calculation: Minimize potential energy of system
with respect to displacement and moment of
surrounding ions
Solid State Aspects of Oxidation Catalysis
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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
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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
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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
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Vanadia: quantum chemical
cluster calculations
As mentioned earlier:
more recently quantum chemical calculations based
on 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) 245
Haber, Witko, Tokarz: Appl.Catal. A:General 157 (1997) 3 & 23
Solid State Aspects of Oxidation Catalysis
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Oxygen exchange
Catalyst
BET area
(m2)
La2O3
%O2 ex- D0  1015
changed (cm2/s)
5.03
68.9
9.83
6.86
79.0
18.86
7.64
72.0
11.18
1 at% SrO /
La2O3
2 at% SrO /
La2O3
Kalenik and Wolf, Catal.Lett. 9 (1991) 441
Solid State Aspects of Oxidation Catalysis
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Oxidation reactions
Methane oxidation
Other compounds
 oxidative
 saturated
dimerization
(oxidative coupling)
 oxidation to synthesis
gas
 total combustion
hydrocarbons
 olefins
 aromatic
hydrocarbons
 nitrogen oxides
Solid State Aspects of Oxidation Catalysis
16
Oxidative coupling of
methane 1
La2O3 catalysts:
doping with Sr2+ and Zn2+:
increased activity and C2-selectivity
doping with Ti4+ and Nb5+:
decreased activity and C2-selectivity
clear correlation with increased oxygen vacancy
concentration and oxygen conductivity
Borchert and Baerns, J. Catal. 190 (1997) 315
Solid State Aspects of Oxidation Catalysis
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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
CH4 (g) + O  CH3 (g) + (OH)
•
O
•
Solid State Aspects of Oxidation Catalysis
•
O
18
Defect structure of Lidoped MgO 1
x
Mg
" Li2O"+ 2 Mg
+ O  2 LiMg + V + 2" MgO"
x
O
••
O
Catlow et al. J. Phys. Chem 94 (1990) 7889
Solid State Aspects of Oxidation Catalysis
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Defect structure of Lidoped MgO 2
V + O2 O + 2 h
••
O
1
2
x
O
•
Solid State Aspects of Oxidation Catalysis
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Solution and oxidation
energies
alkali-metal
oxide
Li2O
Na2O
K2O
K2O
alkaline earth
oxide
MgO
CaO
SrO
BaO
solution energy
(kJ/mole M2O)
525.8
294.3
300.8
142.8
oxidation energy
(kJ/mole vacancy)
-220.0
-166.0
-284.6
-167.9
Solution reaction:
+ O  2 LiMg + V + 2" MgO"
••
•
1
Oxidation reaction: VO + 2 O2  O + 2 h
x
Mg
" Li2O"+ 2 Mg
x
O
Solid State Aspects of Oxidation Catalysis
••
O
x
O
21
Segregation energies
defect
LiMg
O O
Li

O
Mg O

segregation energy
(kJ/mole)
-17.4
-91.7
-164.0
Note formation of defect associates
Solid State Aspects of Oxidation Catalysis
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Ammoxidation of propane
and toluene
Cn H2n +2 + NH3 + O2  Cn-1H2n -1CN + 3 H2O
3
2
using vanadium antimonate catalysts with Sb:V
ratios 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
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Active site for selective
ammoxidation
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Partial oxidation of
iso-butane
Hydrogen abstraction:
•

Nb + O  Nb Nb + OO
x
Nb
x
O
C 4 H10 + O•O  •C 4 H 9 + (OH)•O
Formation of iso-butoxide ion:
•

C4H9 + VO• + OOx  OC4H9-

••
+
V
O
ads
Re-formation of active site:
NbNb + 2 (OH)•O  NbxNb + O•O + H2O(gas)
I. Matsuura, H. Oda, and K. Oshida, Catal. Today, 16 (1993) 547
Solid State Aspects of Oxidation Catalysis
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Membranes
 (micro)porous
membranes:
any oxidic material either intrinsically
catalytically active or covered with active
(mono)layer
 dense membranes:
ionic or mixed ionic-electronic conducting
material
Solid State Aspects of Oxidation Catalysis
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Membrane reactors
Modes of operation:
Reductant 
Reductant 
Reductant 
R
Oxygen 
Oxygen 
Oxygen 
Chemical
potential
driven
Electric
potential
driven
Solid oxide
fuel cell
mode
Solid State Aspects of Oxidation Catalysis
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ABO3 perovskite structure
high values of ionic and
electronic conductivity
e.g, in:
La1-xAxCo1-yFeyO3-
A=Sr, Ba
SrFeCo0.5O3.25-
Solid State Aspects of Oxidation Catalysis
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Mixed oxygen/ionic
conducting dense membrane
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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
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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 rate
molecular oxygen formation, see next transparency
Solid State Aspects of Oxidation Catalysis
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Oxygen conducting
membranes - III
In general two competing reactions
x
2 CH4 (g)  OO
x
2 OO


 2 h  2 CH3  H 2O(g)
 4h


 VO

 O 2 (g)  2 VO
Oxygen formed in second reaction can cause oxidation
in gas phase  high oxygen flux not necessarily
favourable for high C2-selectivity!
Solid State Aspects of Oxidation Catalysis
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Proton conduction
Equilibria for compound : SrCe0.95Yb0.05O3-Hx
1
2
O 2 (g) + VO•• = O Ox + 2 h • : K1
O Ox + VO•• + H 2 O(g) = 2 OH•O : K 3
O Ox = 12 O 2 (g) + VO•• + e : K O
H 2 (g) + 12 O 2 (g) = H 2 O(g) : K W
0 = e + h • : K e
electroneutralitycondition:
  
  
2 VO•• + OH•O + h • =  e +  YbCe 
Schober et al. Solid State Ionics 86/88 (1996) 653
Solid State Aspects of Oxidation Catalysis
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Predominance diagram for
SrCe0.95Yb0.05O3-Hx
Solid State Aspects of Oxidation Catalysis
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Methane coupling with proton
conducting membrane
Hamakawa et al. J.Electrochem.Soc. 140 (1993) 459, 141 (1994) 1720
Solid State Aspects of Oxidation Catalysis
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Defect equilibria
1 O  V  O x  2 h 
O
O
2 2


x
H 2 O(g)  VO  2 H  O O
x


H 2 O(g)  O O  VO  2 OH O
or
Increasing pO2 leads to increased hole concentration
(reaction 1) and decreased proton concentration (reaction
2 & 3) and thus simultaneously to increased hole conduction and decreased proton conduction
Solid State Aspects of Oxidation Catalysis
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Further experiments
• 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
Langguth et al. Appl. Catal. A, 158 (1997) 287
Solid State Aspects of Oxidation Catalysis
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Concluding remarks
Solid state aspects:
1. Interpretation of catalytic properties using
solid state concepts: but in many cases this
must still begin
2. Making use of special properties of solids
in membrane reactors: much knowledge must
still be obtained
Solid State Aspects of Oxidation Catalysis
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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
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