Coking and decoking during methanation and methane decomposition on Ni-Cu supported catalysts

Abstract

The effect of the composition of silica supported Ni-Cu alloy catalysts on the process of coking and decoking during methane decomposition and during methanation was considered. The kinetics of methanation was studied and compared to those of carbon deposition and of strong adsorption of hydrogen. Initiation of the formation of filamentous carbon formation on mono-metallic surfaces may take place if the ratio of the partial pressures, pCO/pH2, is larger than 2 (T < 673 K) or 1 (T > 673 K). Once the process starts, the chemical potential of the gas phase may be reduced to lower values without interruption of filament growth. Besides, it was concluded that the methanation reaction includes two steps: the dissociative adsorption of CO and the hydrogenation of the adsorbed species. It was possible to establish the mechanism through which Cu affects the activity of Ni. The effect of the composition of the alloy catalysts on the methane formation and on the simultaneous carbon deposition indicates that those reactions belong to group I and to group II, respectively, following Ponec's classification. It was possible to find the optimal Cu concentration that maximises methanation and minimises carbon deposition. The kinetics of methane decomposition was also considered and is well described by adapting a model developed by other authors for Fe catalysts.