Kinetics, isotope effects, and mechanism of the hydrogenation of carbon monoxide on a supported palladium catalyst

Abstract

Kinetics and isotope effects for the C-O bond dissociation in the hydrogenation of CO on palladium/alumina (Pd/Al/sub 2/O/sub 3/) catalyst were investigated by using pulse surface reaction rate analysis (PSRA). The rate-determining step was found to be the C-O bond dissociation of the adsorbed CO species. The rate constant for the C-O bond dissociation process per adsorbed CO molecule was determined at various temperatures and Arrhenius parameters of the rate constant were obtained. The rate constant in flowing deuterium (D/sub 2/)(k/sub D/) was considerably larger than that in flowing H/sub 2/(k/sub H/), indicating an inverse isotope effect. The average value of k/sub H//k/sub D/ was 0.61. The extent of the inverse isotope effect for Pd was more significant than that for nickel (Ni)(k/sub H//k/sub D/ = 0.75). The preexponential factor for k/sub H/ on Pd (1.6 x 10/sup 4/s/sup -1/) was much smaller than that on Ni (5.4 x 10/sup 6/s/sup -1/). It was concluded that adsorbed CO is not directly dissociated to surface carbon and oxygen atoms but hydrogen atoms play an important role in the C-O bond dissociation. Further details of the mechanism for the C-O bond dissociation were discussed from the observed kinetics and isotope effects. 5more » figures.« less