The enhancement of WO3-catalyzed photodegradation of organic substances utilizing the redox cycle of copper ions

WO3 is a visible-light-responsive photocatalyst and an n-type semiconductor photocatalyst, but its photocatalytic activity toward organic substances is low. We utilized several transition metal ions as redox mediators in attempts to improve the photoactivity of WO3, and a drastic improvement was observed only when Cu2+ ions were utilized for WO3-catalyzed photodegradation of organic substances. More specifically, the CO2 generation rates for the photodegradation of formaldehyde and methanol in the presence of Cu2+ ions were about 1000 and 150 times as high, respectively, as the rates in the absence of Cu2+ ions. We confirmed that Cu2+ was reduced to Cu+ through the transfer of a photoexcited electron from WO3 and that Cu+ was easily re-oxidized to Cu2+ by oxygen in air. This redox cycle could assist in the reduction of oxygen in the photocatalytic reaction, and the reaction was particularly efficient when Cu2+ was introduced to the system as a CuCl2 solution. Photodegradation was achieved with a WO3 photoelectrode in a two-compartment cell in which the Cu2+ solution was separated from wastewater containing organic substances by an ion-exchangeable membrane, simulating practical use. The anodic photocurrent generated by the photodegradation of organic substances was observed at the WO3 photoelectrode without applying any external bias.