Mechanistic study of vacuum UV catalytic oxidation for toluene degradation over CeO₂ nanorods

The present study specifically investigates vacuum ultraviolet (VUV) catalytic oxidation for toluene degradation over CeO₂ nanorods. Synergetic effects of ultraviolet photocatalytic oxidation (UV-PCO) and ozone catalytic oxidation (OZCO) were manifested in the results of toluene removal and COx generation, while the combination of UV-PCO and OZCO (UV-OZCO) did not lead to improvement of mineralization. All the processes contribute to ozone decomposition, but no obvious synergetic effects of the different processes can be observed. Intermediate analysis results indicated that more toluene was oxidized into by-products, such as benzyl alcohol and benzaldehyde, by UV-OZCO rather than forming COx. Both hydroxyl radical (·OH) and superoxide radical (·O₂⁻) were found in all the processes of the VUV-PCO-OZCO system (combination of VUV photolysis, UV-PCO, OZCO and UV-OZCO processes). In the UV-OZCO process, the formation of hydroxyl radical was promoted, while that of superoxide radical was impeded, resulting in lower mineralization level of toluene. The mechanistic study of toluene degradation over CeO₂ nanorods in the VUV-PCO-OZCO system revealed that with the formation of ·O₂⁻ and ·OH, toluene is first oxidized to intermediates, followed by further ring-opening reaction and, finally, degradation into CO₂ and H₂O. CeO₂ nanorods function as both ozonation catalyst and photocatalyst, and the redox pair of Ce³⁺ and Ce⁴⁺ are interconvertible and can keep a balance.