Optimizing methanol synthesis from CO2: Are bulk hexagonal indium oxide structures superior to cubic ones?

Recently, catalysts based on bulk indium oxide (In₂O₃) have been used in CO₂ valorization; however, several studies correlate crystal phase with performance without considering possible changes during the reaction. In this context, we investigated different crystal phases of bulk In₂O₃ (pure cubic, hexagonal, or mixed-phased) in CO₂ hydrogenation, where we observed variations in catalytic activity associated with phase transitions occurring under reaction conditions. We systematically compared the crystal phase and surface area before and after the reaction, showing that, at 350°C, independent from the initial In₂O₃ structure, there is a tendency to form the cubic phase accompanied by the loss of surface area. To reach these results, we employed various synthetic methods that tailored structural and textural characteristics to achieve desired properties; for the first time, we obtained a cubic major mixed-phase In₂O₃ structure at a 3-hour synthesis time by using a microwave-assisted method. Such material presented the best methanol productivity. Thus, as not previously reported, our results revealed that utilizing bulk hexagonal In₂O₃ may not be interesting under this temperature; also, a higher surface area does not necessarily provide improved conversion rates. XPS, XRD, EPR, MEV, N₂ physisorption, CO₂-TPD, and H₂-TPR were performed and corroborated our investigations.