METHANATION - A STUDY WITH ISOTOPIC TRANSIENT METHODS (CATALYSIS, CARBON MONOXIDE, NICKEL)

The objectives of this study were to investigate the reaction mechanism, fractional coverage of reaction intermediates, reactivity, aging processes, H/D isotope effect and catalyst coverage of H(,2) and CO under methanation conditions. Three types of nickel catalysts (Raney nickel, 60 wt % Ni/SiO(,2) and nickel powder) were studied, using isotopic transient methods. A differential tubular reactor was used in this work. The temperature range varied from 160(DEGREES)C to 250(DEGREES)C. The H(,2)/CO ratio varied from 3 to 40. The total pressure was maintained at 1 bar. For some experiments on 60 wt % Ni/SiO(,2), the total pressure was maintained at 4 bar.;The catalyst aging study on Raney nickel showed that the formation of methane occurred via two parallel pathways. The origin of catalyst aging in this particular case is in essence not a process of site blocking, but rather a slow "deterioration" of the Raney nickel itself, causing pronounced kinetic heterogeneity.;The coverage of reaction intermediates and their reactivities are significantly dependent on the nature of nickel catalysts.;The origin of an H/D isotope effect was found to be a thermodynamic isotope effect (the coverage of reaction intermediates effect). The overall H/D isotope effect is sensitive to the nature of the nickel catalysts.;The proposed mechanism for methanation involves both adsorbed hydrogen atoms and CO to be in equilibrium with gas phase hydrogen and CO. The dissociation of CO is a fast and irreversible process. The reaction of surface carbon proceeds through a slow step (carbon-carbon depolymerization or changes in the carbon-metal coordination) and then reacts very rapidly via a reversible sequence of steps to form a CH(,x) (x = 1-3) group. The CH(,3ad) species further reacts with H(,ad) to form CH(,4(g)).;The amount of adsorbed CO is not sensitive to the reaction conditions. A large amount of hydrogen coexisting with approximately a monolayer of CO found under reaction conditions. Under reaction conditions, the adsorbed hydrogen is associated with carbon and with nickel atoms.