Kinetic studies of CO₂ methanation over a Ni/γ-Al₂O₃ catalyst

The production of methane by reacting CO₂ with H₂ (CO₂ methanation) has the potential for producing synthetic natural gas, which could be exported using the existing infrastructure for the distribution of natural gas. The methanation of CO₂ was investigated over a wide range of partial pressures of products and reactants using (i) a gradientless, spinning-basket reactor operated in batch mode and (ii) a laboratory-scale packed bed reactor operated continuously. The rate and selectivity of CO₂ methanation, using a 12 wt% Ni/γ-Al₂O₃ catalyst, were explored at temperatures 445-497 K and pressures up to 20 bar. Research with the batch reactor showed that the rate increased with increasing partial pressures of H₂ and CO₂ when the partial pressures of these reactants were low; however, the rate of reaction was found to be insensitive to changes in the partial pressures of H₂ and CO₂ when their partial pressures were high. A convenient method of determining the effect of H₂O on the rate of reaction was also developed using the batch reactor and the inhibitory effect of H₂O on CO₂ methanation was quantified. The kinetic measurements were compared with a mathematical model of the reactor, in which different kinetic expressions were explored. The kinetics of the reaction were found to be consistent with a mechanism in which adsorbed CO₂ dissociated to adsorbed CO and O on the surface of the catalyst with the rate-limiting step being the subsequent dissociation of adsorbed CO. The ability of the kinetic expressions to predict the results from the continuous, packed-bed reactor was explored, with some discrepancies discussed.