Mechanistic study of low temperature CO2 methanation over Rh/TiO2 catalysts

CO2 methanation at low temperature and atmospheric pressure was studied over Rh/TiO2 catalysts focusing on the effect of Rh particle size on the activity and reaction mechanism. Catalysts with different Rh contents (0.5–5 wt.%) were prepared in order to obtain different mean cluster sizes. The activity was measured between 85 and 165 °C, with a H2/CO2 ratio equal to 4. The rate of methane production per surface Rh atoms increases as metal particle size increases up to ca. 7 nm. Beyond this size, the rate does not change appreciably. Higher activation energies (up to 28.7 kcal/mol) are obtained for catalysts with small cluster size (ca. 2 nm), whereas for larger particles (>7 nm), the activation energy is lower and does not change with size (ca. 17 kcal/mol). Reaction order with respect to CO2 is near zero for large clusters, whereas it decreases to −0.36 for lower size clusters. From the analysis of adsorbed species using operando-DRIFTS, it is proposed that smaller Rh particles tend to bind CO(ads) intermediate stronger than larger ones. The activation energy for the dissociation of adsorbed CO species does not vary with Rh particle size, which suggests that smaller particles are not intrinsically less active, but they present less active sites than larger ones. The study of the kinetic parameters permits to propose that CO(ads) dissociation is aided by the presence of H species and that a likely surface intermediate is Rh carbonyl hydrides.