Solid State Aspects of Oxidation Catalysis Paul J. Gellings and Henny J.M. Bouwmeester University of Twente, Enschede, the Netherlands

Solid State Aspects of Oxidation Catalysis2 Contents of lecture Ê Some concepts of solid state chemistry Ë Methods of computational modelling Ì Examples of applications of 1 and 2 to specific catalytic reactions Í Challenges for extension of use of solid state considerations to catalysis Î Possibilities for new applications: use of membranes

Solid State Aspects of Oxidation Catalysis3 Solid state concepts The solid state concepts considered are: u atomic, ionic and electronic defects u defect structure u defect concentrations u type of conduction u conductivity u crystal structure

Solid State Aspects of Oxidation Catalysis4 Defect notation Atom type: cation, anion, foreign ion, 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 Catalysis5 Important types of defects

Solid State Aspects of Oxidation Catalysis6 Example of defect equilibrium A typical example is ZrO 2 :

Solid State Aspects of Oxidation Catalysis7 Kröger-Vink or Brouwer diagram for ZrO 2

Solid State Aspects of Oxidation Catalysis8 Doping and defect equilibrium Doping of ZrO 2 : with lowervalent metal such as Y: With higher valent metal such as Nb:

Solid State Aspects of Oxidation Catalysis9 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 Catalysis10 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 Catalysis11 Defect energies near surface Two examples : Y 3+ -dopant in ThO 2 Oxygen vacancy in ThO 2 Catlow et al. J. Phys. Chem. 94 (1990) 7889

Solid State Aspects of Oxidation Catalysis12 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 Catalysis13 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 Catalysis14 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 (1999) 245 Haber, Witko, Tokarz: Appl.Catal. A:General 157 (1997) 3 & 23

Solid State Aspects of Oxidation Catalysis15 Oxygen exchange Kalenik and Wolf, Catal.Lett. 9 (1991) 441

Solid State Aspects of Oxidation Catalysis16 Oxidation reactions Methane oxidation u oxidative dimerization (oxidative coupling) u oxidation to synthesis gas u total combustion Other compounds u saturated hydrocarbons u olefins u aromatic hydrocarbons u nitrogen oxides

Solid State Aspects of Oxidation Catalysis17 Oxidative coupling of methane 1 La 2 O 3 catalysts: doping with Sr 2+ and Zn 2+ : increased activity and C2-selectivity doping with Ti 4+ and Nb 5+ : 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 Catalysis18 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

Solid State Aspects of Oxidation Catalysis19 Defect structure of Li- doped MgO 1 Catlow et al. J. Phys. Chem 94 (1990) 7889

Solid State Aspects of Oxidation Catalysis20 Defect structure of Li- doped MgO 2

Solid State Aspects of Oxidation Catalysis21 Solution and oxidation energies Solution reaction: Oxidation reaction:

Solid State Aspects of Oxidation Catalysis22 Segregation energies Note formation of defect associates

Solid State Aspects of Oxidation Catalysis23 Ammoxidation of propane and toluene using vanadium antimonate catalysts with Sb:V ratios of 1 to 5 A. Andersson, et al. Appl. Catal. A, 113 (1994) J. Nilsson, et al. J. Catal. 160 (1996) J. Nilsson, et al. Catal. Today, 33 (1997)

Solid State Aspects of Oxidation Catalysis24 Active site for selective ammoxidation

Solid State Aspects of Oxidation Catalysis25 Partial oxidation of iso-butane 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 Catalysis26 Membranes u (micro)porous membranes: any oxidic material either intrinsically catalytically active or covered with active (mono)layer u dense membranes: ionic or mixed ionic-electronic conducting material

Solid State Aspects of Oxidation Catalysis27 Membrane reactors Modes of operation: Oxygen  Reductant  R Chemical potential driven Electric potential driven Solid oxide fuel cell mode

Solid State Aspects of Oxidation Catalysis28 ABO 3 perovskite structure high values of ionic and electronic conductivity e.g, in: La 1-x A x Co 1-y Fe y O 3-  A=Sr, Ba SrFeCo 0.5 O 

Solid State Aspects of Oxidation Catalysis29 Mixed oxygen/ionic conducting dense membrane

Solid State Aspects of Oxidation Catalysis30 Oxygen conducting membranes - I Examples of perovskites used: La 1-x Sr x Co y Fe 1-y O 3-  : 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 ) BaCe 1-x Gd x O 3-  : Hibino et al. ( J. Chem. Soc. Faraday Trans. 91 (1995) 4419 ) La 1-x Ba x Co y Fe 1-y O 3-  : Xu and Thomson (see above) SrFeCo 0.5 O 3-  : Ma and Balachandran ( Solid State Ionics 100 (1997) 53 )

Solid State Aspects of Oxidation Catalysis31 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 Catalysis32 Oxygen conducting membranes - III In general two competing reactions 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 Catalysis33 Proton conduction Equilibria for compound : SrCe 0.95 Yb 0.05 O 3-  H x Schober et al. Solid State Ionics 86/88 (1996) 653

Solid State Aspects of Oxidation Catalysis34 Predominance diagram for SrCe 0.95 Yb 0.05 O 3-  H x

Solid State Aspects of Oxidation Catalysis35 Methane coupling with proton conducting membrane Hamakawa et al. J.Electrochem.Soc. 140 (1993) 459, 141 (1994) 1720

Solid State Aspects of Oxidation Catalysis36 Defect equilibria Increasing pO 2 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 Catalysis37 Further experiments Langguth et al. Appl. Catal. A, 158 (1997) 287 Without oxygen: some CO and C 2 (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 CO 2 formation as a consequence of decreased oxygen conduction and increased proton con- duction

Solid State Aspects of Oxidation Catalysis38 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 Catalysis39 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