Rational Control of Oxygen Vacancy Density in In₂O₃ to Boost Methanol Synthesis from CO₂ Hydrogenation

Oxygen vacancies (O_v) in reducible metal oxides are the vital active sites for methanol synthesis via a CO₂ hydrogenation technology. However, the relationship between the density of O_v and the methanol synthesis performance is still ambiguous, and it still shows a lack of a versatile strategy to precisely tailor the number of O_v. In this study, with In₂O₃ as a representatively catalytic component, the density functional theory computation confirms that the O_v property, especially O_v density, is pivotal to enhancing methanol selectivity of CO₂ hydrogenation by suppressing the undesirable reverse water-gas shift reaction for CO formation, which is attributed to the unique electronic density of In atoms around O_v. To verify the theoretical results, we report a protocol to optimize the concentration of O_v on In₂O₃ by sequential carbonization and oxidation (SCO) treatments of In-based metal-organic frameworks, during which the consumption of carbon species and the structural reconstruction of the In₂O₃ crystal regulated the particle size and O_v concentration of In₂O₃ by varying the oxidation temperature. The In₂O₃-5 catalyst carbonized and oxidized at 500 °C exhibits good methanol selectivity (72.3%) at a CO₂ conversion of 9.9% under 330 °C, 3 MPa, and high space velocity of 12,000 L^-1 kg_cat^-1 h^-1. Multiple in situ characterizations clarify that the proposed O_v property regulating the SCO strategy is convenient to boost methanol synthesis by altering the CO₂ hydrogenation process to the HCOO* intermediate-dominated pathway. Our work provides the catalyst design strategy and will shed light on the rational design of reducible metal oxide-based catalysts with a controllable O_v density.