CO HYDROGENATION ON A NICKEL-CATALYST - 2 - A MECHANISTIC STUDY BY TRANSIENT KINETICS AND INFRARED-SPECTROSCOPY

The CO hydrogenation into methane was studied over a Ni/SiO2 catalyst by means of in situ transient techniques (DRIFT, SSITKA) in order to h ave access to a detailed description of the reacting surface and to un derstand the ageing process related to the kinetic behaviour at steady state. After an initial period of carbon deposition, fast sintering, and particle smoothing via nickel carbonyl transfer, the reacting surf ace can be described as a monolayer of nickel carbide, largely covered by CO adspecies with an average bonding stoichiometry of CO/2Ni(s). T he rate of hydrogenation is controlled by the probability for a hydrog en molecule to collide with an active site formed of one to two adjace nt Ni atoms free from adsorbed CO. The concentration of active sites i s statistically determined by the CO coverage. Carbon atoms belonging to the carbidic layer associated to the active site are hydrogenated b y hydrogen activated on the free Ni atoms. The regeneration of the car bidic layer is in turn ensured by CO dissociation after methane desorp tion. A rate equation is presented with physically meaningful kinetic parameters. (C) 1998 Academic Press.