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ChE 553 Lecture 16

ChE 553 Lecture 16. Mechanisms Of Surface Reactions. Objective. Start to talk about mechanisms of reactions on solid surfaces. Mechanisms Of Reactions On Surfaces. Generally surface reactions follow catalytic cycle with adsorption, reaction, desorption Form adsorbed radicals Radicals react

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ChE 553 Lecture 16

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  1. ChE 553 Lecture 16 Mechanisms Of Surface Reactions

  2. Objective • Start to talk about mechanisms of reactions on solid surfaces

  3. Mechanisms Of Reactions On Surfaces • Generally surface reactions follow catalytic cycle with adsorption, reaction, desorption • Form adsorbed radicals • Radicals react • Molecules desorb

  4. Generic Types Of Surface Reactions Most catalytic reactions Most film growth reactions

  5. 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)

  6. Example Catalytic Mechanisms: Olefin Hydrogenation Transfer one atom at a time

  7. Example CH3OHCO+2H2 Transfer one atom at a time Figure 5.14 The Mechanism of Methanol Decomposition on Pt(111).

  8. The Mechanism Of Ethanol Decomposition On Pt(111) Transfer one atom or one ligand at a time

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

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

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

  12. Bond Fragmentation Reactions

  13. Association Reactions Typical on metal surfaces Typical on metal clusters

  14. Reactions Continued

  15. Recombinative Desorption (Reductive Elimination)

  16. Ridel-Eley Displacement Reaction

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

  18. Can usually predict mechanism on metal surfaces by just considering the 5 steps Dissociative adsorption Associative Desorption Molecular adsorption Associative hydrogen migration

  19. What Happens If We Run The Reaction The Opposite Way? H H H C H H H H C H Associative recombination H H Dissociative Adsorption H H C H H H C H H C H H H H C H Molecular desorption H H H H H H H C C C H H H H C H H H H H β-elimination C C H H

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

  21. Example: Ethanol Decomposition On Pt(111) Transfer one atom or one ligand at a time

  22. Summary • Reactions on surfaces go by Rideal-Eley or Langmuir Hinschelwood Mechanisms • RE  film growth • LH  catalysis • Reactions on metals • Only 5 characteristic reactions • Usually transfer a ligand or atom to or from surface • No direct isomerizations

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