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

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^[rad]−) 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^[rad]−. 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^[rad]−. 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^[rad]− 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^[rad]− 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.