Mechanistic understanding of support effect on the activity and selectivity of indium oxide catalysts for CO2 hydrogenation

Abstract

Herein we present a mechanistic study on the support effect (ZrO2 and CeO2) of In2O3 catalysts in CO2 hydrogenation by a combined experimental and computational approach. Kinetic experiments and surface characterization suggested that the activity of In2O3 catalysts cannot be simply correlated with the abundance of surface oxygen vacancies (Ov) formed by either H2-reduction or thermal treatment, which has been frequently invoked in previous studies. The support effect should originate from the electronic interactions between In2O3 and the support oxide, rather than geometric factors or the difference in the particle size of In2O3. Theoretical modelling revealed that surface Ov facilitate the formation and stabilization of the formate (HCOO*) intermediate. While a carbonate-like structure is favored for CO2 adsorption on CeO2-supported or unsupported In2O3 catalysts, CO2 tends to bind strongly in a bent configuration on the Ov site at the In2O3-ZrO2 interface. The distinct CO2 adsorption structures on different supported In2O3 catalysts may account for the different reaction energy profiles in the subsequent hydrogenation reactions, especially the rate-limiting step, i.e., hydrogenation of HCOO* to CH2O* and methoxy (CH3O*). The relatively higher methanol selectivity of In2O3 catalyst supported on ZrO2 with respect to that on CeO2 is suggested to stem from the greater energy difference (ΔEa) between the parallel hydrogenation and C-O bond cleavage of HCOO*, which leads to the formation of methanol and CO, respectively. This study underlines the important role of metal-oxide-interface in determining the catalytic behavior of oxide-supported In2O3 catalysts in CO2 conversion.