Morphology-Controllable Hexagonal-Phase Indium Oxide In Situ Structured onto a Thin-Felt Al ₂ O ₃ /Al-Fiber for the Hydrogenation of CO ₂ to Methanol

Indium oxide is as a promising catalyst for the hydrogenation of CO ₂ to methanol. Previous studies mainly focused on the cubic-phase In ₂ O ₃ (C-In ₂ O ₃ ), but the hexagonal-phase In ₂ O ₃ (H-In ₂ O ₃ ) with a corundum structure has been rarely studied because of the difficulty in its synthesis. Herein, the morphology-controllable H-In ₂ O ₃ in situ structured onto a thin-felt Al ₂ O ₃ /Al-fiber is prepared by a facile mixed solvothermal method. The amount of H-In ₂ O ₃ is strongly dependent on the urea/In molar ratio during the mixed solvothermal synthesis. The optimal catalyst obtained at a urea/In ratio of 4.5 possesses the highest amounts of oxygen vacancy and strong basic site. Such catalyst achieves a turnover frequency of 47.8 h ⁻¹ (the number of CO ₂ converted into methanol per oxygen vacancy site per hour) and is stable for at least 200 h, being capable of converting 4.4% CO ₂ into methanol at a selectivity of 67.6% at 325 °C (or higher selectivity at the expense of conversion below 325 °C) with a methanol space time yield of 0.20 g _MeOH g _cat ⁻¹ h ⁻¹ . In contrast, the microfibrous-structured C-In ₂ O ₃ (prepared by the incipient wetness impregnation method) delivers a very low methanol selectivity of only 36.8% under identical reaction conditions. In situ Fourier transform infrared spectroscopy experiments reveal that the H-In ₂ O ₃ /Al ₂ O ₃ /Al-fiber catalyzes CO ₂ -to-methanol conversion through formate intermediates.