Photocatalysis of aqueous PFOA by common catalysts of In2O3, Ga2O3, TiO2, CeO2 and CdS: influence factors and mechanistic insights

Gallium oxide (Ga₂O₃), titanium dioxide (TiO₂), cerium dioxide (CeO₂), indium oxide (In₂O₃) and cadmium sulfide (CdS) were commonly used under UV light as photocatalysis system for the pollutants' degradation. In this study, these five catalysts were applied for the photodegradation of perfluorooctanoic acid (PFOA), a well-known perfluoroalkyl substance (PFAS). As a result, the PFOA photodegradation performance was sequenced as: Ga₂O₃ > TiO₂ > CeO₂ > In₂O₃ > CdS. To further explain the photocatalysis mechanism, the effects of initial pH, photon energy and band gap were evaluated. The initial pH of 3 ± 0.2 hinders the catalytic reaction of CdS, resulting in low degradation of PFOA, while it has no significant effect on Ga₂O₃, TiO₂, CeO₂ and In₂O₃. In addition, quantum yield was sequenced as TiO₂ > CeO₂ > Ga₂O₃ > In₂O₃, which may not be the main factor determining the degradation effect. Notably, the band gap energy from large to narrow was as: Ga₂O₃ > TiO₂ > CeO₂ > In₂O₃ > CdS, which exactly matched their degradation performance.