CO2 hydrogenation at low temperature over Rh/gamma-Al2O3 catalysts: effect of the metal particle size on catalytic performances and reaction mechanism

Low temperature CO2 methanation was studied on Rh/γ-Al2O3 catalysts. Catalysts were prepared with varying amounts of Rh to obtain different mean particle sizes, ranging between 3.6 and 15.4 nm. In the conditions studied, selectivity to methane was 100%. The intrinsic activity of Rh/γ-Al2O3 catalysts in CO2 hydrogenation to methane was found to do not depend on particle size at temperatures between 185 and 200 °C, whereas at lower temperatures larger particles favored higher activity. Apparent activation energies were found to be in the range 14.5–22.7 kcal/mol. Lower activation energies were obtained for catalysts with larger particles. Operando-DRIFTS (Diffuse Reflectance Infrared Fourier Transform Spectroscopy) experiments showed that CO2 is readily dissociated on these catalysts at 50 °C giving rise to different Rh carbonyl and formates species. Rh–CO species are proposed to be associated with H forming Rh carbonyl hydrides. According to transient experiments performed in different gas atmospheres, it is proposed that adsorbed CO is an important intermediate and that formates are spectators in the reaction path. Activation energy for CO dissociation is found to be similar to that of the overall reaction which suggests the importance of CO bond dissociation in the reaction path.