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ChE 553 Lecture 20

ChE 553 Lecture 20. Mechanisms On Metal Surfaces. Objective. Use Findings To Compute Mechanisms On Metal Surfaces. Typical Reactions On Metal Surfaces. Mechanism On Metal Surface Similar To Radical Reactions In Gas Phase – But Radicals Bound To Surface.

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ChE 553 Lecture 20

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  1. ChE 553 Lecture 20 Mechanisms On Metal Surfaces

  2. Objective • Use Findings To Compute Mechanisms On Metal Surfaces

  3. Typical Reactions On Metal Surfaces

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

  5. In Lect 3 we Noted That Metals Have Many Free Electrons

  6. 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

  7. Catalytic Cycles For The Production Of Water a) Via Disproportion Of OH groups, b) Via The Reaction OH(ad)+H(ad)H2O

  8. An Illustration Of The Adsorbed Phase When Ethylene Adsorbs On Platinum

  9. Molecular Adsorption vs Dissociative Adsorption

  10. Mechanisms Of Surface Reactions • Mechanisms on metal surfaces similar to gas phase reactions except initiation is formation of a bare surface site • All surface reactions occur in cycles where bare surface sites are formed and destroyed

  11. Example CH3OHCO+2H2 Figure 5.14 The Mechanism of Methanol Decomposition on Pt(111).

  12. The Mechanism Of Ethanol Decomposition On Pt(111)

  13. Notation S=surface site (5.156)

  14. Rules To Predict Mechanisms On Surfaces Polayni relationship + proximity effect Figure 5.16 Bond Energies in Methanol Figure 5.17 The transition state for C-H scission in adsorbed ethanol.

  15. Pictures of molecule • Notice that carbons can never get close to the surface • We will show later: • Ea= Ea0 + (energy of bonds that break when getting to transition state)- (energy of new bonds that form in getting to the transition state). = Ea0 + ΔHr

  16. Implications • For OH bond scission Ea=Ea0 +(energy loss of broken OH bond)- (energy gain of new hydrogen-surface bond)-(energy gain of new oxygen-surface bond) • For CH bond scission Ea=Ea0 +(energy loss of broken CH bond)- (energy gain of new hydrogen-surface bond)-(energy gain of new carbon-surface bond) • OH Bond scission wins

  17. Example Methanol Decomposition OH breaks first proximity effect CH next Yields CO Figure 5.18 Bond energy in ethanol.

  18. Example Ethanol OH CH Eventually get to CH3CO(ad)

  19. How Does CH3C=O Decompose? • C-C Bond in CH3CO(ad) is only 55 kcal/mole compared to 100 kcal/mole for C-H bond. • Enthalpy wins!!

  20. The Mechanism Of Ethanol Decomposition On Pt(111)

  21. New Topic: Generic Types Of Surface Reactions Figure 5.20 Schematic of a) Langmuir-Hinshelwood, b) Rideal-Eley, c) precursor mechanism for the reaction A+BAB and ABA+B.

  22. A Langmuir-Hinshelwood Mechanism For The Reaction C2H4+H2C2H6

  23. A Rideal-Eley Mechanism For Diamond Deposition

  24. A Precursor Mechanism For The Reaction 2CO+O2CO2

  25. Rules Of Thumb • Reactions on solid catalysts (-600C) usually go by Langmuir-Hinshellwood • Reactions in semiconductor processing usually Rideal- Eley • Reactions on enzymes often precursor (can also be Langmuir)

  26. Summary Mechanisms on metals similar to gas phase- Key difference - species bound to surface proximity effect di-radicals, tri-radicals possible

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