Kinetics and hydrogen-deuterium isotope effects in the methanation of adsorbed CO molecules on a Ni/SiO2 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 CH4 and H2O. The amounts of CH4 and H2O produced by the hydrogenation of the adsorbed CO were determined up to various times, and it was found that CH4 and H2O were produced at the same rate. When O2 instead of CO was injected, H2O 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 dissociation process per adsorbed CO molecule (kH) was determined at various temperatures, and the Arrhenius parameters of the rate constant were obtained. The rate constant in flowing deuterium (kD) was also determined. It was found that kD is considerably larger than kH, indicating an inverse isotope effect The average value of kH/kD was 0.75. 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. The kinetics and isotope effect were reasonably understood in terms of transition-state theory coupled with the proposed mechanism. Kinetics and the hydrogen-deuterium isotope effect were also determined in steady-state methanation, and an inverse isotope effect (0.77) was also measured. The results obtained by using the continuous flow technique were reasonably explained in terms of the proposed mechanism.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry