α and γ mixed-phase Ga2O3 as a photocatalyst for CO2 reduction with water: mechanism and the role of each phase

Abstract

Gallium oxides (Ga₂O₃) with mixed phases of α/β, β/γ, and α/γ are known to show high catalytic activity for the photoreduction of CO₂ with water to produce CO, H₂ and O₂. However, the roles of each phase and phase mixing in the photocatalytic CO₂ reduction have not been well understood. In this study, we have synthesized α/γ mixed-phase Ga₂O₃ with controlled mixing ratios and examined their catalytic activity in the photoreduction of CO₂ with water. The catalytic activity or the production rates of H₂ and CO appreciably changed with the γ/α mixing ratio. The H₂ formation dominated on the α-phase, while the CO production increased with increasing the γ-phase content and attained the maximum for a γ-content of 40%. Above 60%, both the H₂ and CO production rates significantly decreased to the similar rates of the pure γ-phase. TEM observations of the mixed-phase Ga₂O₃ revealed the coexistence of spheroid type α-Ga₂O₃ particles with smaller polyhedral type γ-Ga₂O₃ which was not well crystallized to show a large surface area. In the α/γ mixed-phase Ga₂O₃, γ-Ga₂O₃ was embedded in α-Ga₂O₃ with γ-phase contents less than 40%, while γ-Ga₂O₃ covered α-Ga₂O₃ above 60%. Considering that the H₂ and CO production rates change with the γ/α mixing ratio, we have revealed a reaction mechanism of CO production that some of the H produced on the α phase by water splitting is used to reduce CO₂ adsorbed on the γ-phase which has a much larger specific surface area than that of the α-phase but less catalytic activity than the α-phase.