1. 0581.5271 Electrochemistry for Engineers LECTURE 6 Lecturer: Dr. Brian Rosen Office: 128 Wolfson Office Hours: Sun 16:00

2. A Birds Eye View of: Methods of Catalyst Action

3. What is a Catalytic Material? A catalyst is a material that increases the rate of a chemical reaction by reducing the free- energy barrier without being consumed Pt, Pd, Ag, Sn, Ni, Cu…. Pt e-e- O2O2 Catalytic Converters Fuel Cells, Electrolyzers

4. Intelligent Design of Catalysts Initiate reactions (break bonds, form radicals) Stabilize intermediates Hold reactants together in proper configuration Block side reactions Stretch bonds (to make them break easier) Donate or accept electrons Efficient energy transfer Pd

5. Intelligent Design of Catalysts Initiate reactions (break bonds, form radicals) Stabilize intermediates Hold reactants together in proper configuration Block side reactions Stretch bonds (to make them break easier) Donate or accept electrons Efficient energy transfer Pd Sabatier’s Principle In order to optimize catalyst performance, one must design a surface that binds the reactants neither too strong nor too weak

6. Types of Heterogeneous Catalysts Supported Powders (e.g. oxide supported) Structured (e.g. fibers and cloths)

7. Importance Of Catalysis 90% of all chemical processes use catalysts Changes in catalysts have a giant influence on rates and selectivity’s of reactions. More than anything else Most real reactor design associated with optimizing performance of catalyst

8. Catalytic Reaction Occurs Via A Catalytic Cycle: reactants + catalyst  complex complex  products + catalyst

9. Catalytic Cycles Rosen et al. J. Phys. Chem. C. 116 (29), 2012. 27

10. Large Changes in Rate ReactionCatalystE A (kcal/mol) Without Catalyst E A (kcal/mol) With Catalyst Enhancement H 2 + I 2  2HIPt4414 10 13 2N 2 O  2N 2 + O 2 Au5829 10 13 (C 2 H 5 ) 2 O  2C 2 H 4 + H 2 OI2I2 5334 10 8

11. The Rate Enhancement In The Presence Of A Catalyst 10 40 10 20 10 42

12. Molecular vs Dissociated Adsorption

13. Leonard-Jones’ Potential Well

14. Leonard-Jones (cont’d)

15. Initiation Often Not Enough. Also Need To Stabilize Intermediates 15

16. Adsorption Sites

17. Adsorption Isotherms (Langmuir) The surface containing the adsorbing sites is perfectly flat plane with no corrugations (assume the surface is homogeneous). The adsorbing gas adsorbs into an immobile state. All sites are equivalent. Each site can hold at most one molecule of A (mono-layer coverage only). There are no interactions between adsorbate molecules on adjacent sites.

18. Adsorption Isotherms (Langmuir) “Coverage” [S]= free surface sites [So] = Total surface sites

19. N 2 adsorption onto W

20. Effects Of Surface Area Consider a platinum catalyzed reaction. You can run the reaction 1)Run the reaction on the wire 2)Take the wire and smash it with a hammer and then run the reaction. The rate will be higher on the wire you smashed with a hammer!

21. Why Does Smashing A Wire Change The Rate? When you squashed the platinum you created more surface area. You also changed the shape of the surface which can affect the rate.

22. Turnover Numbers Rates of catalytic reactions often expressed as turnover number R A = Rate per unit area (molecules/cm 2 -sec) N S = Number of exposed metal atoms / unit area (Atoms/cm 2 )

23. CO 2 conversion on Ag NPs

24. Turnover Numbers For Some Typical Reactions

25. TON for ORR

26. Very Complex Pore Structure 26 Figure 12.4 A diagram of the pore structure in Faugasite. -Macropores -Micropores -Nanopores “Mesoporous”

27. BET Surface Area

28. Electrochemical Surface Area 420 cm 2 /C CO desorbed

29. Comparing ECSA

30. High Current – Low Overpotential

31. Volcano Plots on ΔG H

32. Volcano Plot for ORR

33. Experimental Evidence HCOOH  H 2 +CO 2 (12.75)

34. Biomimicry

35. Y. Tomita, S. Teruya, O. Koga, Y. Hori, J. Elect. Soc. 147, 4164-4167 (2000) Hori, Y., Electrochemical CO 2 reduction on metal electrodes. Modern Aspects of Electrochemistry, (2008). 42: p. 89-189 Aqueous KHCO 3 Product Selectivity

37. Intelligent Design of Catalysts Initiate reactions (break bonds, form radicals) Stabilize intermediates Hold reactants together in proper configuration Block side reactions Stretch bonds (to make them break easier) Donate or accept electrons Efficient energy transfer Pd

38. Holding Reactants In The Right Configuration Figure 12.16 A cartoon of the reaction of ethanol and NAD + on the active site of liver alcohol dehydrogenase. Adapted from Oppenheimer and Handlon (1992) (In the Enzyme, vol 20 (1992) 453. (12.92)

39. Catalysts Make Bonds Easier To Break Figure 12.17 A Picture of Lysozyme 161L. This figure was generated using a program called RASMOL, using data in the protein data base from an x-ray diffraction spectrum generated by Weaver and Matthews[1987]

40. Transition Metals – Weaken Bond By Attaching To Antibonding Orbitals Figure 12.20 A diagram of the key interactions during the dissociation of hydrogen on platinum.

41. Catalyst For PE Production: Block Side Reactions Figure 12.24 A diagram of propylene polymerization in a Ziegler-Natta catalyst.

42. Supported Metal Catalysts Figure.12.3 A picture of a supported metal catalyst. Use support because platinum very expensive and only the surface is active. Spread platinum out on cheap support. Support also provides strength

43. Why are Intermediates Important? CO 2 Conversion Example

44. O 2 -H 2 on Platinum Thesis of E. Hudak

45. O 2 -H 2 on Platinum

46. Pt (hkl) Dependence ORR

47. Effect of crystallographic planes Pt has a face centered cubic crystal structure 3 basal planes: (111), (100), (110) Catalytic activity for oxygen reduction reaction Pt(110) > Pt(100) > Pt(111)

48. Key Substitutions – Noble metals (Re, Ru) Coke resistance due to large E A for carbon formation – Promoters (K, Na) Promotion of CO 2 dissociation by e- donation to active sites. Replenishment of O 2 in lattice – Poisons (S, P) Electronegative atoms to remove electrons from active sites blocking unwanted side reactions

49. Summary 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. 49