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ChE 553 Lecture 29

ChE 553 Lecture 29. Catalysis By Metals. Objective. Apply what we have learned to reactions on metal surfaces. Metals Work By Same Mechanisms As Other Catalysts. Metal catalysts can help initiate reactions Metal catalysts can stabilize the intermediates of a reaction

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ChE 553 Lecture 29

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  1. ChE 553 Lecture 29 Catalysis By Metals

  2. Objective • Apply what we have learned to reactions on metal surfaces

  3. Metals Work By Same Mechanisms As Other Catalysts • Metal catalysts can help initiate reactions • Metal catalysts can stabilize the intermediates of a reaction • Metal catalysts can hold the reactants in close proximity and in the right configuration to react • Metal catalysts can be designed to block side reactions • Metal catalysts can stretch bonds and otherwise make bonds easier to break • Metal catalysts can donate and accept electrons • Metal catalysts can act as efficient means for energy transfer

  4. Mechanisms Of Reactions On Metals • Generally metal catalyzed reactions follow catalytic cycle with adsorbtion, reaction, desorption • Form adsorbed radicals • Radicals react • Molecules desorb

  5. Typical Reactions On Metals • Simple molecular adsorption reactions • Dissociative adsorption reactions • Bond scission reactions • Addition reactions • Recombination reactions • Desorption reactions

  6. Adsorption On Metals • Molecular Adsorption CO + S  COad • Dissociative adsorption • oxidative addition H2 + 2S  2Had

  7. Molecular vs Dissociative Adsorption Figure 5.12 The metals which dissociate CO, NO, H2, O2 and CO at various temperatures.

  8. Bond Fragmentation Reactions

  9. Association Reactions

  10. Reactions Continued

  11. Recombinative Desorption (Reductive Elimination)

  12. Displacement Reaction

  13. -scission R2CDCH2(ad) R2C=CH2 + D(ad) (14.15)

  14. Example Catalytic Mechanisms: Olefin Hydrogenation

  15. Principles Of Catalytic Reaction • Metals can help initiate reactions • Metals can stabilize the intermediates of a reaction • Metals can hold the reactants in close proximity and in the right configuration to react • Metals can stretch bonds and otherwise make bonds easier to break • Metals can donate and accept electrons

  16. Mechanism On Surface Similar To Radical Reactions In Gas Phase – But Radicals Bound To Surface

  17. Electrons In Metals Solvate Radicals • Metals are solvents for radicals. They lower the energy of radical species which allows initiation-propagation reactions to occur. • Adsorbed radicals have lower energies than gas phase radicals which leads to higher concentrations. • Intrinsic barriers of species adsorbed on metals similar to gas phase radicals.

  18. Figure 14.18 The electron density extending out from a metal surface. (Note 1 bohr =0.52Å) Electrons In Metals Solvate Radicals

  19. Metals Stabilize Intermediates

  20. Multiple Radicals One key difference between gas phase and surface is that di & tri radicals are stable on metals N Gives possibilities for interesting chemistry

  21. Metals Initiate Reactions Figure 5.12 The metals which dissociate CO, NO, H2, O2 and CO at various temperatures.

  22. D-bands Help Tear Bonds Apart Figure 12.20 A diagram of the key interactions during the dissociation of hydrogen on platinum.

  23. Activation Energy For H2 Dissociation

  24. Redox Chemistry (14.53) (14.54) (14.55) (14.57) (14.56)

  25. Metals Hold Reactants In Correct Configuration To React Figure 14.20 Balandin's suggested multiplet for the decomposition of ethanol a) to form ethylene b)to form acetyladehyde. The asterisks in the figures represent places on the surface where reaction can occur.

  26. Metals Hold The Reactants In The Correct Configuration To React (12.91) Figure 12.15 The active site for reaction (12.91) on a palladium catalyst.

  27. Structure Sensitive Reactants Figure 14.22 The rate of nitric oxide dissociation on several of the faces of platinum along the principle zone axes of the stereographic triangle. Adapted from Masel[1983].

  28. Figure 14.21The rate of the reaction N2 + 3H2 2NH3 over an iron catalyst as a function of size of the iron particles in the catalyst. Data of Boudart et al [1975] Structure Sensitive Reactions

  29. Different Reactions Show Different Structure Sensitivity

  30. Practical Catalysts Are Supported Structures With Multiple Exposed Faces A picture of a supported metal Figure 12.4 catalyst.

  31. Summary • Metals can help initiate reactions - in particular they facilitate bond scission processes • Metals can stabilize the intermediates of a reaction particularly radical intermediates • Metals can lower the intrinsic barriers to bond scission. The d-electrons promote bond scission and bond formation. The s-electrons promote redox chemistry. • D’s convert forbidden reactions to allowed reactions. Produces tremendous rate enhancements.

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