Mechanistic Study of Low-Temperature CO2 Hydrogenation over Modified Rh/Al2O3 Catalysts

The hydrogenation of CO2 on Rh/Al2O3 catalysts modified with Ni and K was studied by in situ and operando DRIFTS spectroscopy comprising transient and isotopic exchange experiments to study the influence of this modification on the catalytic performance in CO and methane formation at 250–350 °C and to gain mechanistic insight. Catalytic testing and spectroscopic studies revealed that the modification with particularly K promotes the formation of CO being the highest over Rh, K, Ni/Al2O3, whereas methane formation is preferred over the unmodified catalyst. It was found that CO2 does not dissociatively adsorb but is adsorbed at the support, forming mainly hydrogen carbonate, and in the presence of K, also carbonate species. The dissociative adsorption of H2 proceeds on Rh. The activated H2 reacts mainly with the hydrogen carbonate species forming CO adsorbed on Rh and formate (F1) species stably adsorbed on the support. On the K-containing catalysts, an additional formate species (F2) was identified as more reactive than F1 formate and can act as a reaction intermediate in the CO formation pathway. Furthermore, adsorbed formyl species were detected, which are assumed to be intermediates in the methanation reaction. The modifying additives change the surroundings of the Rh particles. This influences the strength of CO adsorption and the activation ability of Rh for H2 dissociation. Thus, desorption of the formed CO from the catalyst surface is favored, and the methanation of CO is hindered. The modification with K enhances the ability for CO2 fixation by formation of additional carbonate species which cover adsorption sites for unreactive F1 formate species and favors the formation of reactive F2 formate species.