MECHANISMS OF METHANATION OF CARBON-DIOXIDE AND CARBON-MONOXIDE OVER NICKEL ALUMINA CATALYSTS

By the use of diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS) and the temperature-programmed reaction (TPRx) method, it was shown that two types of adsorbed CO in bridged structures (referred to as weakly adsorbed bridged CO(a) and strongly adsorbed bridged CO(a) species) were predominantly present along with formate species in the methanation of CO2 on Ni/Al2O3. The amount of carbidic carbon species (referred to as C(a) species) and adsorbed CO in linear structures (referred to as linear CO(a) species) were practically negligible. In the methanation of CO2 a considerable amount of linear CO(a), weakly and strongly adsorbed bridged CO(a) and C(a) species was present along with formate, methoxide and surface hydrocarbon species. C(a) species were produced by dissociation of strongly adsorbed bridged CO(a) species, being hydrogenated to methane. On the basis of the reactions under transient states and those of C(a) species with H2, CO2/H2 and CO/H2, it was suggested that one of the steps involved in the hydrogenation of C(a) species to methane was markedly retarded in the presence of linear CO(a) species.