1. Introduction To Surface Reactions
ChE 553 Lecture 13
Introduction To Surface Reactions

2. Objective Start To Discuss Surface Reactions Overview of mechanism
Typical reactions on metals
Highlight well studied examples
Qualitative trends with changing composition and structure

3. Topics General Mechanisms Of Surface Reactions
Mechanisms look like gas phase reactions
Need surface site
Effect of Surface Structure
Effect of composition

4. Some Examples Of Reactions On Metal Surfaces

5. Mechanisms On Surface Similar To Radical Reaction In Gas Phase – But Radicals Bound To Surface

6. Catalytic Cycles Needed
All Surface Reactions Occur In Cycles Where Bare Surface Sites Are Formed And Destroyed (5.154)

7. Catalytic Cycles Where Needed

8. General Rules For Overall Reactions On Surfaces
There must be bare sites on the catalyst to start the reaction.
Then at least one of the reactants must adsorb on the bare sites.
Then there are a series of bond dissociation reactions, fragmentations, association reactions and single atom recombinations which convert the adsorbed reactants into products
Then the products desorb.

9. Example CH3CO+2H2
Figure The Mechanism of Methanol Decomposition on Pt(111).

10. Catalytic Cycles Continued
Figure The Mechanism of Ethanol Decomposition on Pt(111).

11. Notation
S=surface site
(5.156)

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

13. Example: Lanmuir Hinshelwood for C2H4+H2C2H6
Figure A Langmuir-Hinshelwood mechanism for the reaction C2H4+H2C2H6.

14. Rideal-Eley For Film Growth

15. Proposed Precursor Mechanism for 2C0+O22CO2
Figure A precursor mechanism for the reaction 2CO+O2 CO2.

16. Reactions In Forward And Reverse
Figure Schematic of a) Langmuir-Hinshelwood, b) Rideal-Eley, c) precursor mechanism for the reaction A+BAB and ABA+B.
Figure 6.6 Schematic of (a) Langmuir-Hinshelwood; (b) Rideal-Eley; (c) precursor mechanism for the reaction A-B → A + B.

17. Next Mechanisms Of Important Reactions: Olefin Hydrogenation

18. Isomerization
Requires at least 5 carbons in the chain so called 5 center isomerization

19. 3-Centered Isomerization Also Possible But May Require A Metallocarbocation

20. CO Oxidation

21. Partial Oxidation Of Ethylene

22. Hydroformulation CO+RCH=CH2+H2RCH2CH2CHO
Figure The catalytic cycle for hydroformylation over a rhodium hydride cluster.

23. Gas Phase & Solution Show Different Mechanism
Figure 6.7 The geometry of the OH + CH3OH reaction in the gas phase and in solutions.

24. Ethylene Decomposition (For H2 Production)
Figure 6.8 The mechanism of ethylene decomposition on Pt(111). (Proposed by Kesmodel, Dubois and Somorjai [1979] and confirmed by Iback and Lehwald [1978].)

25. Methanol Decomposition For H2 Production
Figure The mechanism of methanol decomposition on the hexagonal faces of group VII and 1b metals. (Proposed by Davis and Barteau [1989].)

26. Summary Of Mechanism Of Reaction On Metals
Mechanisms on metals similar to gas phase- Key difference – species bound to surface proximity effect di-radicals, tri-radicals possible

27. The Effect Of Surface Structure
Reactions often occur faster on special configurations called active sites
Figure The multiplet for the reaction of ethyl alcohol (a) to form either ethylene, (b) to form acetaldehyde and hydrogen, as discussed by Balandin [1929]. The asterisks in the figure represent catalytic centers.

28. The Effects Can Be Huge
Figure The rate on nitric oxide dissociation on several of the faces of platinum along the principle zone axes of the stereographic triangle. (Adapted from Masel [1983].)

29. Rates Also Vary With Particular Size
Figure The 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].)

30. Mechanisms Vary With Surface Structure

31. Another Case Of Variation With Surface Structure
Figure The mechanism of ethylene decomposition on the close packed faces of transition metals. (After Yagasaki and Masel [1994].)
Figure The mechanism of ethylene decomposition on the (100) faces of the transition metals. (After Yagasaki and Masel [1994].)

32. Largest Variation in Rate with Geometry Observed Prior to 1996
Notation
Structure sensitive reactions Structure insensitive reactions Secondary Structure Sensitivity
Reaction
Largest Variation in Rate with Geometry Observed Prior to 1996
2CO + O2 → 2CO2 6
C2H4 + H2 → C2H6 8
CH3OH → + H2O
>100
C2H6 + H2 → 2CH4
104
N2 + 3H2 → 2NH3
105
2NO + 2H2 → N2 + 2H2O
≈ 1021

33. The Effect Of Composition
Figure The relative rate of ethane hydrogenolysis and ethylene hydrogenation over several transition metal catalysts. (Data of Sinfelt [1963].)

34. Summary General Mechanisms Of Surface Reactions
Mechanisms look like gas phase reactions
Need surface site
Multiply bound species
Proximity Effect
Effect of Surface Structure
Structure Sensitive Reactions
Structure insensitive reactions
Models hard - multiple bonding
Effect of composition
Rates change
Key effect - stability of intermediates
Dual Sites