Al Promotion of In₂O₃ for CO₂ Hydrogenation to Methanol

In₂O₃ is a promising catalyst for the hydrogenation of CO₂ to methanol, relevant to renewable energy storage in chemicals. Herein, we investigated the promoting role of Al on In₂O₃ using flame spray pyrolysis to prepare a series of In₂O₃-Al₂O₃ samples in a single step (0-20 mol % Al). Al promoted the methanol yield, with an optimum being observed at an Al content of 5 mol %. Extensive characterization showed that Al can dope into the In₂O₃ lattice (maximum ∼ 1.2 mol %), leading to the formation of more oxygen vacancies involved in CO₂ adsorption and methanol formation. The rest of Al is present as small Al₂O₃ domains at the In₂O₃ surface, blocking the active sites for CO₂ hydrogenation and contributing to higher CO selectivity. At higher Al content (≥10 mol % Al), the particle size of In₂O₃ decreases due to the stabilizing effect of Al₂O₃. Nevertheless, these smaller particles are prone to sintering during CO₂ hydrogenation since they appear to be more easily reduced. These findings show subtle effects of a structural promoter such as Al on the reducibility and texture of In₂O₃ as a CO₂ hydrogenation catalyst.