Catalytic Activity of Highly Dispersed Palladium. I. The Mechanism of Cyclohexene Hydrogenation and the Role of ZrO2 Support

Abstract

Abstract The hydrogenation of cyclohexene vapor on dispersed palladium was studied in the pressure range of 10–40 Torr at temperatures between 283 and 323 K. Palladium was impregnated on ZrO2 and α-Al2O3 over the wide range of 0.03 to 2.1 wt%, and the percentage exposed was varied from 13 to 96% for Pd/ZrO2 and from 2 to 15% for Pd/α-Al2O3. The hydrogenation activity, Vg, per unit of weight of the Pd dispersed on ZrO2, exhibited a sharp maximum at 0.05 wt%, but the turn-over frequency, Vs, per exposed atom around this maximum remained almost constant, which shows the reaction to be structure-insensitive. The value of Vs for Pd/ZrO2 at 301 K was larger than those for Pd/α-Al2O3 and Pd bulk metal by one order of magnitude. The ZrO2 carrier affected the catalytic behavior of palladium; the reaction orders with respect to the hydrogen and cyclohexene pressures changed to 0.73±0.04 and 0.64±0.04 respectively from the corresponding values, 0.97±0.03 and 0.07±0.04 for Pd/α-Al2O3 and Pd metal. The deuterium distributions in the reactant and product molecules in the reaction with D2 suggested that the hydrogenation on Pd/ZrO2 and Pd/α-Al2O3 proceeds via the associative mechanism ; the slow step is assumed to be the reaction of the surface cyclohexyl radical with the adsorbed hydrogen atom. The poisoning due to preadsorbed CO decreased the catalytic activities of Pd/ZrO2 and Pd/α-Al2O3 almost linearly with the amounts of surface CO and caused a complete deactivation at about a half coverage of the surface Pd atoms, irrespective of the percentage exposed. On the basis of these results, the structure-insensitivity of the cyclohexene hydrogenation on Pd and the carrier effect of ZrO2 are discussed.