Role of Zirconia in Indium Oxide-Catalyzed CO₂Hydrogenation to Methanol

Monoclinic zirconia has been uncovered as a carrier able to substantially boost the activity of indium oxide for CO2 hydrogenation to methanol. Here, electronic, geometric, and interfacial phenomena associated with this peculiar effect are investigated. Generating mixed In–Zr oxides by coprecipitation does not improve performance, excluding a primary role of electronic parameters. Because even only 1 mol % of indium stabilizes the metastable tetragonal phase of zirconia, the relevance of its crystalline structure is explored in impregnated solids. Both tetragonal and monoclinic ZrO2 permit epitaxial growth of In2O3, but a more pronounced lattice mismatching leads to a lower dispersion of the oxide on the second, which is observed in the form of subnanometric islands on the carrier, and to more pronounced tensile forces. The latter triggers the formation of a surplus of oxygen vacancies only in this system, which is in line with its greatly enhanced indium-specific activity. Hence, a deposition synthesis method is essential to unlock the role of monoclinic zirconia. According to kinetic analyses, the monoclinic ZrO2-based catalyst can also better activate both reactants, likely because of a superior character of oxygen vacancies on supported In2O3 and a direct contribution of zirconia to CO2 activation on its own oxygen vacancies, which was investigated in comparison with In2O3 supported on alumina and ceria. Elucidating the nature of the active sites at the phase boundary and the impact of the defect chemistry of zirconia are identified as aspects to be prioritized in upcoming studies to shed further light on interfacial effects in this relevant catalytic system.