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 4 Important types of defects metal vacancy VM dopant, higher charge oxygen vacancy VO dopant, lower charge NM metal interstitial M i free electron e free (electron) hole h oxygen interstitial Oi Solid State Aspects of Oxidation Catalysis DM 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 : Oi + 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 6 Kröger-Vink or Brouwer diagram for ZrO2 Solid State Aspects of Oxidation Catalysis 7 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 8 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 9 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 10 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 11 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 12 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 13 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 14 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 15 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 17 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 LiMg + V + 2" MgO" x O •• O Catlow et al. J. Phys. Chem 94 (1990) 7889 Solid State Aspects of Oxidation Catalysis 19 Defect structure of Lidoped MgO 2 V + O2 O + 2 h •• O 1 2 x O • Solid State Aspects of Oxidation Catalysis 20 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 LiMg + 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 LiMg 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 22 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 23 Active site for selective ammoxidation Solid State Aspects of Oxidation Catalysis 24 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: NbNb + 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 25 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 26 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 27 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 28 Mixed oxygen/ionic conducting dense membrane Solid State Aspects of Oxidation Catalysis 29 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 30 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 31 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 32 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 + YbCe Schober et al. Solid State Ionics 86/88 (1996) 653 Solid State Aspects of Oxidation Catalysis 33 Predominance diagram for SrCe0.95Yb0.05O3-Hx Solid State Aspects of Oxidation Catalysis 34 Methane coupling with proton conducting membrane Hamakawa et al. J.Electrochem.Soc. 140 (1993) 459, 141 (1994) 1720 Solid State Aspects of Oxidation Catalysis 35 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 36 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 37 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 38 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 39