Effects of In₂O₃-Ga₂O₃ metal oxides on different ZrO₂ phases for CO₂ hydrogenation activity to methanol

The surface properties of In₂O₃-Ga₂O₃ promoted ZrO₂ were significantly altered by the different ZrO₂ phases, such as tetragonal (t), amorphous (a), and monoclinic (m), which modified surface oxygen vacant sites through newly formed phase-dependent strong metal-support interaction, resulting in the variations of CO₂ conversion and methanol selectivity. The In₂O₃-Ga₂O₃-promoted tetragonal ZrO₂(t) phase, compared to other ZrO₂ phases, demonstrated enhanced CO₂ activation, which was attributed to the presence of less reducible Ga₂O₃ phases by forming strong interaction with Zr³⁺ sites in the tetragonal ZrO₂ structures, creating larger Ga-Ov-Zr oxygen vacant sites due to the closer interactions of Ga₂O₃-In₂O₃ metal oxides themselves. Additionally, the partially reducible In₂O₃ surfaces (such as In⁰, In⁺ or In²⁺ species) facilitated easier H₂ dissociation to further selective hydrogenation of surface intermediates to methanol. Those synergy effects on the In₂O₃-Ga₂O₃ promoted tetragonal ZrO₂ (InGa/ZrO₂(t)) were responsible for an enhanced CO₂ conversion of 15.8 % with higher methanol selectivity of 73.4 % owing to a suppressed reverse water–gas shift (RWGS) reaction activity, measured at T = 280 °C and P = 5.0 MPa. However, the In₂O₃-promoted tetragonal ZrO₂ (In/ZrO₂(t)) showed a higher CO selectivity with 34.5 % formed by reverse water–gas shift (RWGS) reaction at a higher CO₂ conversion of 17.3 % (lower methanol selectivity of 62.7 %), which were mainly attributed to the over-reduction of In₂O₃ by enhancing the dissociation of H₂ due to the larger surface coverage of In₂O₃ metal oxide on the tetragonal ZrO₂ surfaces.