1. Mechanisms Of Surface Reactions
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. Transfer one atom at a time
Example CH3OHCO+2H2
Transfer one atom at a time
Figure 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 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?
Associative recombination H H
Dissociative
Adsorption H C H H H H H C C H H H H H C H
Molecular desorption H H H H H C H H C C H H H H C H H H H H C C
β-elimination H H

20. Example CH3OHCO+2H2
Figure 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