1 / 39

Solid State Aspects of Oxidation Catalysis

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

reynard
Download Presentation

Solid State Aspects of Oxidation Catalysis

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

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

  2. 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

  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 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 Catalysis

  5. Important types of defects Solid State Aspects of Oxidation Catalysis

  6. Example of defect equilibrium A typical example is ZrO2: 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: With higher valent metal such as Nb: 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 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

  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 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

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

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

  17. 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

  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 Solid State Aspects of Oxidation Catalysis

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

  20. Defect structure of Li-doped MgO 2 Solid State Aspects of Oxidation Catalysis

  21. Solution and oxidation energies Solution reaction: 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 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

  24. Active site for selective ammoxidation Solid State Aspects of Oxidation Catalysis

  25. 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 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: Reductant  Reductant  Reductant  R Oxygen  Oxygen  Oxygen  Chemical potential driven Electric potential driven Solid oxide fuel cell mode Solid State Aspects of Oxidation Catalysis

  28. ABO3 perovskite structure high values of ionic and electronic conductivity e.g, in: La1-xAxCo1-yFeyO3- A=Sr, Ba SrFeCo0.5O3.25-d 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 rate molecular oxygen formation, see next transparency Solid State Aspects of Oxidation Catalysis

  32. 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 Catalysis

  33. Proton conduction Equilibria for compound : SrCe0.95Yb0.05O3-Hx 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 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 • 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 Langguth et al. Appl. Catal. A, 158 (1997) 287 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 begin 2. 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 Solid State Aspects of Oxidation Catalysis

More Related