1. ChE 551 Lecture 29
Catalysis By Metals

2. Metals Work By Same Mechanisms As Other Catalysts
Metal catalysts can help initiate reactions
Metal catalysts can stabilize the intermediates of a reaction
Metal catalysts can hold the reactants in close proximity and in the right configuration to react
Metal catalysts can be designed to block side reactions
Metal catalysts can stretch bonds and otherwise make bonds easier to break
Metal catalysts can donate and accept electrons
Metal catalysts can act as efficient means for energy transfer

3. Mechanisms Of Reactions On Metals
Generally metal catalyzed reactions follow catalytic cycle with adsorbtion, reaction, desorption
Form adsorbed radicals
Radicals react
Molecules desorb

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

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

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

7. Bond Fragmentation Reactions

8. Association Reactions

9. Reactions Continued

10. Recombinative Desorption (Reductive Elimination)

11. Displacement Reaction

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

13. Example Catalytic Mechanisms: Olefin Hydrogenation

14. Principles Of Catalytic Reaction
Metals can help initiate reactions
Metals can stabilize the intermediates of a reaction
Metals can hold the reactants in close proximity and in the right configuration to react
Metals can stretch bonds and otherwise make bonds easier to break
Metals can donate and accept electrons

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

16. Electrons In Metals Solvate Radicals
Metals are solvents for radicals. They lower the energy of radical species which allows initiation-propagation reactions to occur.
Adsorbed radicals have lower energies than gas phase radicals which leads to higher concentrations.
Intrinsic barriers of species adsorbed on metals similar to gas phase radicals.

17. Electrons In Metals Solvate Radicals
Figure The electron density extending out from a metal surface. (Note 1 bohr =0.52Å)

18. Metals Stabilize Intermediates

19. 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

20. Metals Initiate Reactions
Figure The metals which dissociate CO, NO, H2, O2 and CO at various temperatures.

21. Activation Energy For H2 Dissociation
Metal
Heat of dissociative adsorption of H2
Intrinsic barrier for dissociative adsorption of H2 Pt
-13 kcal/mole
<6 kcal/mole Si
-54 kcal/mole
~80 kcal/mole Al
~-70 kcal/mole
>90 kcal/mole K

22. D-bands Help Tear Bonds Apart
Figure A diagram of the key interactions during the dissociation of hydrogen on platinum.

23. Redox Chemistry
(14.53)
(14.54)
(14.55)
(14.56)
(14.57)

24. Metals Hold Reactants In Correct Configuration To React
Figure Balandin's suggested multiplet for the decomposition of ethanol a) to form ethylene b)to form acetyladehyde. The asterisks in the figures represent places on the surface where reaction can occur.

25. Metals Hold The Reactants In The Correct Configuration To React
(12.91)
Figure The active site for reaction (12.91) on a palladium catalyst.

26. Structure Sensitive Reactants
Figure The rate of nitric oxide dissociation on several of the faces of platinum along the principle zone axes of the stereographic triangle. Adapted from Masel[1983].

27. Structure Sensitive Reactions
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]

28. Different Reactions Show Different Structure Sensitivity

29. A picture of a supported metal
Practical Catalysts Are Supported Structures With Multiple Exposed Faces
Figure 12.4
A picture of a supported metal
catalyst.

30. Summary
Metals can help initiate reactions - in particular they facilitate bond scission processes
Metals can stabilize the intermediates of a reaction particularly radical intermediates
Metals can lower the intrinsic barriers to bond scission. The d-electrons promote bond scission and bond formation. The s-electrons promote redox chemistry.
D’s convert forbidden reactions to allowed reactions. Produces tremendous rate enhancements.

31. Query
What did you learn new in this lecture?