Surface Characterization and Heterogeneous Asymmetric Catalysis

1. Surface Characterization andHeterogeneous Asymmetric Catalysis Eugene Kwan April 2, 2002.

2. What is Pt-Black? • Also called “platinized platinum”, “Adam’s Catalyst” • Electrochemically deposited platinum on platinum • Very high surface area defect SEM (1450x) of Pt-black 1x1 um AFM of smooth Pt images from Ilic, Maclay, et al. J. Mat. Sci. (2000) 35 4337-3457

3. Why use Pt-Black? • Many reactions are “mass transport limiting” • Catalytic reactions only occur on active surface sites • For example… Reactants and products are formed faster than they can diffuse out Whitesides et al. (MIT) J. Phys. Chem. (1989) 93 768-775 • Found reaction was mass transport limited • Use of H2O2 to try to go around problem oxidized Pt surface:

4. Some Definitions ROUGHNESS FACTOR takes into account “hills and valleys” h r e.g. 2rh PRODUCTIVITY • typical roughness: 200-500 • productivity varies “roughness” in alumina (15x15 um AFM) image from Ilic, Maclay, et al. J. Mat. Sci. (2000) 35 4337-3457

5. Synthesis Of Pt-Black • Platinum is electrochemically deposited from chloroplatinic acid (H2PtCl6) onto pre-treated platinum • Involves three couples: • in acidic solution PtCl62- is the principal species • PRETREATMENT: • Start with Pt gauze/metal • Slight etching with aqua regia/nitric acid • Removes impurities and improves adherence of deposit

6. Synthesis Of Pt-Black PRETREATMENT DEPOSITION • - +50 mV (vs. SHE) potentiostatic deposition • 2% chloroplatinic acid, 1 M HCl • 20 mA / cm2 for 5 minutes against blackened Pt wire counterelectrode DRYING/STORAGE Pt is oxidized in air and poisoned by CO • Rinsed in distilled water • Dried under N2 or argon • Stored in nitric acid !

7. Hydrogen Overvoltage • - theoretically expect to see hydrogen evolution at cathode at 0 V vs SHE • - never seen due to “kinetic effect” – always see it at higher voltage • - called “overvoltage” • - high overvoltage: mercury, tin, lead, cadmium (first step is slow) • - medium: smooth platinum, nickel, palladium, rhodium, nickel, copper • low: Pt-black (second step is slow)

8. Hydrogen Monolayers Hydrogen Evolution Reaction Cyclic Voltammogram of Pt-Black in 0.5 M H2SO4 correction for double layer charging zero Current (mA) integral is amt. of charge for one H2 monolayer Potential (vs. SHE, V) H2 evolution • - In acid, H2 forms on surface of Pt at –(0.0 + ) V (overvoltage) • The hydrogen becomes reversibly adsorbed to the surface • Two peaks correspond to “weak” and “strong” adsorption: complicated analysis CV from Bergens et al. J. Phys. Chem. B (1998), 102 1 195

9. Determining The Surface Area Integrate Charge Obtain the integral from the CV: Account for Fractional Coverage - surface is not completely covered at endpoint - divide by ~0.84 to get charge for readily accessible sites - divide by ~0.77 to get charge for total sites This is the surface for hydrogen, a small molecule. The “hydrogen surface” is not accessible to all molecules. !

10. Conversion of Charge to Real Area Convention is to define: 1 real cm2 = 1.30 x 1015 surface Pt atoms 210 uC / real cm2 number of surface atoms in 1 cm2 of 100 plane Different Crystal Planes of a fcc lattice: 6 11 9 7 11 note different coordination numbers images from Woods, R. Electroanal. Chem. Interfacial Electrochem. (1974) 49 217.

11. Miller Indices • Miller indices specify particular crystal faces (110, 200, etc.) • Decide on a basis. • 2. Look at the cuts. • - Pick a cut next to the origin • - How many times does it cut • the h unit vector? The k? • 3. Label the face. “11” k red = unit vector h, k lattice vectors origin h 1 “-1” 2-D lattice. Method applies to 3D. 3rd axis is called “l” origin

12. Fuel Cells - Chemical batteries: pour fuel in, electricity comes out work anode e¯ cathode e¯ CO2, MeOH, H2O H2O, air MeOH air: O2 polymer: proton exchange membrane

13. Fuel Cells - high efficiency: not Carnot cycle; real life: 40-70% - efficient catalysts like Pt needed with high surface area. - byproduct: carbon monoxide. CO sticks to Pt! SOLUTION: Reaction deposits a Ru submonolayer on the Pt which cuts off the CO but lets the Pt do the fuel cell oxidations. See Bergens, et al. J. Phys. Chem. B. (1998) 102 193-199

14. Science Article, Tom Malouk • Reddington, Mallouk, et al. Science, 280, 1735-1737 (1998) • Carried out a combinatorial search for best fuel cell catalysts • Took salts of Pt, Ru, Os, Ir, and Rh and placed them into an inkjet printer! • Added fluorescent acid/base indicator that changes color with [H+] • “Printed” onto carbon paper with subsequent treatment with NaBH4 • Active catalysts became bright • Previously, a good catalyst was Pt/Ru 50:50 • Found much better: Pt:Ru:Os:Ir 44:41:10:5 • Don’t know why that is better

15. Urea Adsorption on Platinum • Climent, Aldaz, et al. Universitat d’Alcant (Spain) • Looked at urea adsorption on Pt(100) and Pt(111) • Characterization via FTIRS, CV, etc. • Pt(100) • Saturation coverage = 0.25 • Two electrons transferred per urea molecule • Pt(111) • - Saturation coverage = 0.45 • One electron transferred • per urea molecule

16. Ligand Accelerated Catalysis * = chiral center present - Define ratio: rate with ligand : rate without ligand - If ratio > 1, “ligand acceleration”. If ratio < 1 “ligand deceleration”. - Lots of asymmetric processes are ligand decelerated (chiral ligands tend to sterically crowd the binding site on the catalyst) - Asymmetric epoxidation of allylic alcohols is accelerated: (DET=diethyl tartrate)

17. Heterogeneous Asymmetric H2 Only two examples known: 1. Hydrogenation of beta-ketoesters with Nickel/tartaric acid 2. Hydrogenation of alpha-ketoesters with Pt/cinchona alkaloids - Called “Ciba-Geigy” Process or “Orito Reaction”. - Discovered by Orito in 1970s. ethyl pyruvate

18. Various Modifier Structures

19. Effect of Modifier Structure • Large aromatic systems give better ees than smaller ones of the same type. • Do not need a nitrogen in the aromatic ring. • Modifiers containing simple benzene/pyridine ring show no chiral induction. • Aromatic system must be flat. • 1. Acetic acid gives best ees. • 2. Fastest rates in EtOH and toluene. Effect of Solvent

20. Inductive Effects • Electron withdrawing groups increase rate and ee. • Electron donating groups decreaase rate and ee. • Steric effects in m and p positions also important. ee up to 92%

21. Inductive Effects image from Arx, Baiker, et al. Tet. Asym. 12 3089-3094 (2001)

22. Inductive Effects image from Arx, Baiker, et al. Tet. Asym. 12 3089-3094 (2001)

23. Kinetics • Modifier must be adsorbed on metal surface to be effective. • Modifiers greatly increase reaction rate and ee. • Linear relationship between ee and 1/rate.

24. Chiral Metal Surfaces Surprise! Metal surfaces can be chiral! Attard, G. J. Phys. Chem. B.105, 3158-3167, (2001) If the surface isn’t smooth, you get “kink” sites. Edges must be of unequal length: no chirality 111 100 100 100 100 111 111 111 110 110 110 110 “R” “S”

25. Observations 1. CV of Glucose Oxidation a, b: D-glucose oxidation on Pt{643}S, Pt{643}R 50 mV/sec c, d: L-glucose “ 0.1 M H2SO4, 0.005 M glucose image from Attard, G. J. Phys. Chem. B.105, 3158-3167, (2001)

26. Visualization: Pt{643}S D-glucose L-glucose

27. Observations 2. Adsorption differs depending on chirality. Theory predicts energy differences in adsorption—confirmed by experiment. 3. Should consider Pt surface as a racemate of R, S kink sites. Preferential adsorption of modifiers, such as the cinchona alkaloid may lead to enantioselective hydrogenation. 4. Experiments by Zhao on Cu{001} with Lysine parallel these results.