Revealing the fundamental role of MoO2 in promoting efficient and stable activation of persulfate by iron carbon based catalysts: Efficient Fe2+/Fe3+ cycling to generate reactive species

Abstract

Electron-rich iron sites are the main sites for iron-based catalysts to activate persulfate (PS) to generate reactive species, while blocked Fe²⁺/Fe³⁺ cycling usually reduces the catalytic performance of iron-based materials and hinders the generation of reactive species in the reaction. To solve the bottleneck, we synthesized an iron-carbon nanocomposite catalyst loaded with MoO₂ (Fe/Mo-CNs). The promotion of MoO₂ on the Fe²⁺/Fe³⁺ cycle in the system allowed Fe/Mo-CNs to exhibit excellent catalytic performance and environmental adaptability. The degradation rate of bisphenol S (BPS) by the Fe/Mo-CNs/PS system was significantly increased to 0.080 min^-1 compared with the iron-carbon based catalyst/persulfate system, and the degradation efficiency of BPS was maintained at around 85% after four cycles. Density functional theory (DFT) calculations showed that the introduction of MoO₂ reduced the reaction energy barrier of persulfate activated by catalysts to produce reactive species, especially promoted the production of more high valent iron (Fe(IV)). Fe(IV) and reactive oxygen species (SO₄·^-, ·OH, ·O₂^- and ¹O₂) worked together on the efficient degradation of BPS. In addition, the test of an automatic circulating degradation plant had proved that Fe/Mo-CNs had a good practical application prospect. BPS was mainly degraded by ring cleavage and O=S=O bond cleavage, and the toxicity of BPS and its intermediates were also evaluated. This work clarifies the mechanism of improving the catalytic performance of heterogeneous iron-based catalysts by MoO₂ in sulfate radical-based advanced oxidation processes (SR-AOPs), providing a new idea for solving the blockage of Fe²⁺/Fe³⁺ cycle in SR-AOPs.

Keywords: BPS degradation; Fe(IV); Fe/Mo-CNs; MoO(2); Persulfate.