Hydrogenation of cyclohexanone on Pt-Sn surface alloys

Hydrogenation of cyclohexanone was studied over Pt(111) and two ordered, Pt-Sn alloy surfaces formed by vapor deposition of Sn on the Pt(111) single crystal surface - the (2 x 2) Sn/Pt(111) and (3 x 3)R30° Sn/Pt(111) surface alloys. The influence of alloying Sn, alloy structure, partial pressures of hydrogen and cyclohexanone, and temperature on the activity and selectivity of these model catalysts toward formation of the corresponding alcohol was investigated. All surfaces were characterized before and after catalytic reaction by XPS and LEED to ensure proper alloy formation and stability. The hydrogenation activity at low temperature (325 K) was substantially higher for both Pt-Sn alloy catalysts than that of the Pt(111) surface, with the activity proportional to the amount of Sn in the surface layer. The apparent activation energy for the hydrogenation of cyclohexanone at 325 K was 16.2, 13.4, and 12.4 kcal/mol for Pt(111), (2 x 2) Sn/Pt(111) alloy, and (3 x 3)R30° Sn/Pt(111) alloy, respectively, and the reaction order in H₂ changed from 0.5 on Pt(111) to 1.5 on both alloys. The selectivity over the two Pt-Sn alloys was the same as for Pt(111), but large changes occurred in the products formed as a function of temperature on all three surfaces. Cyclohexanol was the only product detected below 400 K, but cyclohexene and cyclohexane were the sole products at 425 and 500 K, and the C₁₂-coupling product cyclohexylcyclohexene was formed exclusively at 600 K over all catalysts.