Kinetic studies of the methanation of CO over a Ni/γ-Al2O3 catalyst using a batch reactor

Abstract

The methanation of CO was investigated in a gradientless, spinning-basket reactor at temperatures 443–473K and pressures up to 16bar. The reactor was operated in batch and the composition of its contents was determined periodically. Temperature programmed studies and DRIFTS analysis were performed to gain an understanding of the nature of the surface of the catalyst. In all experiments, the reaction initially proceeded with a constant rate period. This was followed by a marked increase in the rate of production of CH4 after the depletion of CO, attributed to the hydrogenation of remaining carbonyl groups on the surface as well as the hydrogenolysis of long-chained paraffins in the reactor. The selectivity for CH4 was found to be significantly lower than that observed in CO2 methanation, consistent with the low H2 to CO ratio on the surface of the catalyst. Temperature-programmed studies and DRIFTS studies of the spent catalyst identified two main types of carbonaceous species on the surface of the catalyst, with the results being consistent with the presence of (i) carbonyl species on nickel clusters and (ii) formate groups on nickel sites which have a stronger interaction with the alumina support. The former were found to be reactive at the temperatures studied. Finally, the rate of methanation was found to be insensitive to H2O. This was attributed to the strong affinity of the nickel catalyst for CO, which saturates the surface of the catalyst leaving little opportunity for the adsorption of H2O. Two models were derived assuming that the rate-limiting steps were either (i) the adsorption of H2 on the catalyst, or (ii) the reaction of gaseous H2 with adsorbed CO. The strong adsorption of CO on the surface of the catalyst, evident from various experimental observations, is consistent with both mechanisms.