Photocatalytic reduction of CO2 with water using catalysts of γ-Ga2O3 supported by α-Ga2O3: mechanism and roles of each phase

Abstract

Gallium oxide (Ga₂O₃), consisting of the mixed phases of α and β, β and γ, and α and γ, is known as a photocatalyst for the reduction of CO₂ with water, producing CO, H₂ and O₂. In previous studies, we investigated Ga₂O₃ consisting of the mixed phases of α-Ga₂O₃ and γ-Ga₂O₃, systematically varying the contents of γ-Ga₂O₃ as catalysts for the photoreduction of CO₂ with water, and proposed a crude reaction mechanism of the photocatalytic reduction of CO₂. However, the mechanism should be refined to clarify the roles of each phase and the effects of the morphology of the mixture. To do this, we have investigated the photocatalytic activity of γ-Ga₂O₃ supported by α-Ga₂O₃ instead of their mixed phases previously examined. With increasing contents of γ-Ga₂O₃, H₂ production rates monotonically decreased, whereas CO production rates increased, reached a maximum at 60–80% of the γ-Ga₂O₃ content, and decreased significantly. These trends are consistent with those observed in the previous studies using the mixed phases. Based on the previously suggested mechanism, we have proposed the detailed mechanism as follows: (1) the surfaces of α-Ga₂O₃ and γ-Ga₂O₃ particles are hydro-oxidated to GaOOH in water, and GaOOH on the γ-Ga₂O₃ particles absorbs CO₂ as bicarbonate; (2) GaOOH on α-Ga₂O₃ is photo-decomposed to α-Ga₂O₃ producing H; (3) the produced H migrates to the γ-Ga₂O₃ particles and reduces the adsorbed bicarbonate to CO; and (4) without UV photons, the surfaces of α-Ga₂O₃ and γ-Ga₂O₃ return to their initial states of GaOOH and bicarbonate-absorbing state, respectively.