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 have 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, 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 effects 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 the 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, (2) GaOOH on α-Ga2O3 is photo-decomposed to α-Ga2O3 producing H, (3) the produced H migrates to the γ-Ga2O3 particles of which surface is converted to GaOOH and absorbs CO2 as bicarbonate, and reduces the bicarbonate resulting CO. (4) Without UV-photon, the surfaces of α-Ga2O3 and γ-Ga2O3 return to their initial states of GaOOH and bicarbonate absorbing state, respectively.