1. How Do Surfaces Change Activation Barriers
CHBE 553 Lecture 26
How Do Surfaces Change Activation Barriers

2. Objective Describe how surfaces changes rates of reaction
What are key mechanisms
Start to give examples

3. Background: Surfaces Can Change Rates By A Large Amount
1040
1020
1042

4. Today: Mechanisms Of Catalyst Action
Catalysts can be designed to help initiate reactions.
Catalysts can be designed to stabilize the intermediates of a reaction.
Catalysts can be designed to hold the reactants in close proximity.
Catalysts can be designed to hold the reactants in the right configuration to react.

5. Mechanism Of Catalysts, Continued
Catalysts can be designed to block side reactions.
Catalysts can be designed to sequentially stretch bonds and otherwise make bonds easier to break.
Catalysts can be designed to donate and accept electrons.
Catalysts can be designed to act as efficient means for energy transfer.

6. Mechanism Of Catalysis Continued
Is is also important to realize that:
One needs a catalytic cycle to get reactions to happen.
Mass transfer limitations are more important when a catalyst is present.

7. Catalysts stabilize intermediates.
Today: The Role Of Catalysts In Initiating Reactions, Stabilizing Intermediates
Catalysts initiate reactions by help creating active centers (i.e. a radical or ion).
Active center could be catalyst itself
Could be a radical R-O.
Catalysts stabilize intermediates.
Catalyst binds to intermediate, lowering the free energy of the reactive intermediates.
Raises intermediate concentration.
Intermediates less reactive.

8. How Could Catalysts Change H2+Br22HBr
Mechanism Br2→2Br Br+H2→HBr+H H+Br2→HBr+Br 2Br→Br2 Pt
Sulfided pt

9. Key Principles Of Catalytic Mechanisms
Catalysts bind intermediates at distinct sites.
Mechanism often the same in gas phase & on catalyst.
Initiation much faster – in effect do not need initiation reacion.

10. Example Of Catalysts Initiating Reactions
C2H6C2H4+H2
(12.41)
C2H6® 2CH3
(12.42)
CH3+C2H6®C2H5+CH4
(12.43)
C2H5®C2H4+H
(12.44)
H+C2H6®C2H5+H2
(12.45)
2CH3®C2H6
(12.46)
2c2H5®C4H10
(12.47)
CH3®C2H5 ®C3H8
(12.48)
Gas phase mechanism

11. Catalysis By NO2 Consider adding NO2
Consider adding NO2
(12.50)
Catalysts can initiate reactions. The mechanisms are similar to the mechanisms without a catalyst, but the initiation process is much faster with the catalyst.
Effect 109 (small for catalysis)

12. Some Examples Of Reactions Initiated By Catalysts

13. Initiation Often Not Enough. Also Need To Stabilize Intermediates

14. Stabilization Of Ionic Intermediates
Note big barrier to first step

15. Acid Catalyzed Reaction

16. Stabilization Of Intermediates. Can We Have Too Much of a Good Thing?
When we stabilize intermediates we increase the intermediate concentration. We also decrease the reactivity of the intermediates. Which wins? (recall lecture 21)

17. Experimental Evidence HCOOHH2+CO2
(12.75)

18. Sabatier’s Principle
The best catalysts are substances which bind the reactants strongly, but not too strongly.

19. Consider H2+Br2HBr Rideal Eley mechanism
Complicated derivation (see text)
(12.85)
(12.76)
(12.86)

20. For Unlimited Sites
Figure The rate of HBr formation as calculated from Equation (12.85), with [S] = 1e14/cm2 and p= 0.5, T = 500K,

21. For Finite Number Of Sites
(12.87)
Figure The rate of HBr formation calculated from Equation (12.85), with [S] from Equation (12.87) and p= 0.5, T = 500K,

22. Common Plots In Literature
Sachtler-Frahenfort plots: Use heat of oxidation per mole of oxygen as surrogate for heat of formation of product.
Tanaka-Tamaru plots: Use heat of oxidation per mole of metal as surrogate for heat of formation of product.

23. Comparison Of Sachtler-Frahrenfort And Tanaka-Tamaru
Figure A Sachtler-Frahrenfort and Tanaka-Tamaru plot for the hydrogenation of ethylene.

24. Summary So Far
Catalysts work by initiating reactions, stabilizing intermediates.
Leads to 1020 increase in rates – need other effects to get to 1040.
Can stabilize too much.

25. Example: Constructing Sachtler-Frauhenfort And Tanaka-Tamaru Plots
Table 12.E.1 some data for the rate of ethylene hydrogenation on a number of metals 0 C. Next lets construct a Sachtler-Fahrenfort plot of the data.

26. Solution

27. Comparison Of Sachtler-Frahrenfort And Tanaka-Tamaru
Figure A Sachtler-Frahrenfort and Tanaka-Tamaru plot for the hydrogenation of ethylene.

28. Stabilizing Intermediates Not Entire Effect
Leads to 1020 increases in rates –
-need other effects to get to 1040
Does not lead to selectivity

29. Summary
Catalysts work by initiating reactions, stabilizing intermediates.
Leads to 1020 increase in rates – need other effects to get to 1040.
Can stabilize too much.
Other effects change selectivity.