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

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 (O_v) 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 O_v 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 O_v 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 (ΔE_a) 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.