Methanation of CO2: Further insight into the mechanism over Rh/γ-Al2O3 catalyst

The methanation of CO2 was performed on Rh/γ-Al2O3 catalyst at temperatures between 50 and 150°C at 2bar of pressure in a pulse reactor. Experiments confirm the formation of methane at low temperature and pressure. The formation of formiate species during the adsorption of CO2 can be excluded. After reaction with CO2, the catalyst is oxidized. Oxidation is not observed in the presence of CO. The CO2 is adsorbed dissociatively, forming CO (ads) and O (ads). Gem-dicarbonyl Rh(CO)2 species are more reactive than the Rh–CO linear species. The type of adsorbed species depends on the Rh oxidation state. The formation of methane by hydrogenation of CO2 and CO is carried out with 100% selectivity. The activation energy for the hydrogenation of CO2 and CO is lower than values presented in the literature which have been obtained at higher temperature. In the presence of CO2 and CO, the reaction of methanation of CO2 seems to be inhibited by CO. When oxygen is added in low amount in the reactant gas feed, a positive effect on methanation is observed. When the amount of oxygen is too high, oxygen has a negative effect. These results are in agreement with thermodynamics equilibrium calculations, except when O2 is present, confirming the importance of kinetic effects in the reaction. These results open new perspectives of application of catalysis, in order to recycle CO2 in the presence of H2.