Kinetics, isotope effects, and mechanism for the hydrogenation of carbon monoxide on supported nickel catalysts

Abstract

Kinetics and hydrogen-deuterium isotope effects in the methanation of adsorbed CO molecules on a Ni/SiO/sub 2/ catalyst were precisely measured by using pulse surface reaction rate analysis (PSRA). When a CO pulse was injected into flowing hydrogen, it was immediately adsorbed on the catalyst and gradually hydrogenated to CH/sub 4/ and H/sub 2/O. The amounts of CH/sub 4/ and H/sub 2/O produced by the hydrogenation of the adsorbed CO were determined up to various times, and it was found that CH/sub 4/ and H/sub 2/O were produced at the same rate. When O/sub 2/ instead of CO was injected, H/sub 2/O was immediately produced. From these results, the rate-determining step of the reaction was found to be C-O bond dissociation of an adsorbed CO molecule or a partially hydrogenated CO species. By PSRA, the rate constant for the C-O bond dissocition process per adsorbed CO molecule (k/sub H/) was determined at various temperatures, and the Arrhenius parameters of the rate constant were obtained. The rate constant in flowing deuterium (k/sub D/) was also determined. it was found that k/sub D/ is considerably larger than k/sub H/, indicating an inverse isotope effect. The average value of k/sub H//k/sub D/ was 0.75.more » From these results, it was concluded that adsorbed CO is not directly dissociated to surface carbon and oxygen atoms but it is partially hydrogenated before C-O bond dissociation under the conditions of the PSRA experiment. 8 figures.« less