MOF-templated hollow cobalt sulfide as an enhanced Oxone activator for degradation of UV Absorber: Key role of sulfur Vacancy-Induced highly active Co^II sites

This study aimed to design hollow rhombic cobalt sulfide (HRCS) via a single-step sulfidation of cobaltic metal organic framework (CoMOF) as a template. The obtained HRCS with abundance of defects and sulfur vacancy (SV) was then employed for degradation of Novantisol (NVT), a sunscreen agent, through Oxone activation. The superior catalytic performance of HRCS was attributed to its more electroactive sites and low charge transfer resistance that were enhanced by highly active Co^II due to the existence of SV for increased generation of SO₄^•− as a predominant species. Although ^•OH and ¹O₂ were proved to be generated obviously from activation of Oxone over HRCS, their contribution to NVT degradation was marginal. While ^•OH and SO₄^•− were generated mainly by Co^II-activated Oxone, the formation of SO₄^•− was accelerated by sulfur species and the disproportionation of SO₅^•−. The limited conversion of SO₄^•− by reacting with ⁻OH and undirect self-hydrolysis of Oxone, on the other hand, contributed to enhanced ^•OH generation. Further experiments on furfuryl alcohol (FFA) consumption showed that ¹O₂ generated from O₂^•− as an intermediate species did not account for the NVT degradation but rather from self-decomposition of Oxone, dissociation and self-combination of SO₅^•−, and disproportionation of ^•OH. The degradation pathway was also investigated and unveiled in details via DFT calculation, which further validated that HRCS appeared to be a superior catalyst for NVT degradation through Oxone activation.