Activation of peroxymonosulfate on visible light irradiated TiO2 via a charge transfer complex path

Abstract

Photo-induced activation of peroxymonosulfate (PMS) has been enabled by either the direct photolysis of the peroxide bond or the semiconductor bandgap-excited photocatalysis. Whereas the existing approaches utilize UV light, this study first studied the utilization of visible light for the PMS activation in which the dual roles of PMS as a complexing ligand on TiO2 and a precursor of sulfate radical (SO4 center dot-) are enabled via ligand-to-metal charge transfer (LMCT) mechanism. In this LMCT-mediated photocatalysis, PMS coordinated to TiO2 as a surface complex is photoexcited by visible light to inject electrons to the CB of TiO2, which subsequently activate PMS to yield SO4 center dot-. Despite the lack of visible light activity of both TiO2 and PMS, the addition of PMS induced a significant degradation of 4-chlorophenol and dichloroacetate on TiO2 under visible light irradiation. Together with several spectroscopic analyses, the result revealed the formation of an interfacial charge transfer (CT) complex of PMS on TiO2 and the LMCT-mediated PMS conversion into SO4 center dot-. Multi-activity assessment showed that the oxidizing capacity of TiO2/PMS varied depending on the substrate type; benzoic acid and acetaminophen were rapidly decomposed whereas nitrophenol oxidation was insignificant. The role of SO4 center dot- as the main oxidant was identified based on (1) quenching effect of methanol as a radical quencher, (2) coumarin hydroxylation as an indication of SO4 center dot- formation, and (3) EPR spin-trapping technique. The comparison of TiO2/PMS versus Co3O4/PMS suggested that the repeated acetaminophen decay was achievable with TiO2/PMS without the loss of activating capacity whereas a gradual reduction in degradation efficiency was observed with Co3O4/PMS.