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

Kinetics and isotope effects for the C-O bond dissociation in the hydrogenation of CO on Pd/Al₂O₃ 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 D₂ (k_D) was considerably larger than that in flowing H₂ (k_H), indicating an inverse isotope effect. The average value of k_H/k_D was 0.61. The extent of the inverse isotope effect for Pd was more significant than that for Ni (k_H/k_D = 0.75). The preexponential factor for k_H on Pd (1.6 × 10⁴ s^-1) was much smaller than that on Ni (5.4 × 10⁶ s^-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.