Insights into the mechanism of carbocatalysis for peracetic acid activation: Kinetic discernment and active site identification

Peracetic-acid-based advanced oxidation processes (PAA-AOPs) on metal-free catalysts have emerged as charming strategies for water contaminant removal. However, the involved reactive species and their corresponding active sites are ambiguous. Herein, using carbon nanotube (CNT) as a model carbocatalyst, we demonstrated that, under neutral conditions, the CNT−PAA* complex was the dominant reactive species to oxidize phenolic compounds via electron-transfer process (ETP), whereas the surface-bound hydroxyl radicals (·OHsurface) played a minor role on the basis of quenching and electrochemical tests as well as Raman spectroscopy. More importantly, the experimental and density functional theory (DFT) calculation results collaboratively proved that the active site for ETP was the sp2-hybridized carbon on the CNT bulk, while that for radical generation was the edge-located hydroxyl group (C−OH), which lowered the energy barrier for cleaving the O−O bond in CNT−PAA* complex. We further discerned the oxidation kinetic constants (koxid) of different pollutants from the apparent kinetic constants in CNT/PAA system. The significant negative linear correlation between lnkoxid and half-wave potential of phenolic compounds suggests that the pollutants with a lower one-electron oxidation potential (i.e., stronger electron-donating ability) are more easily oxidized. Overall, this study scrutinizes the hybrid radical and non-radical mechanism and the corresponding active sites of the CNT/PAA system, providing insights into the application of PAA-AOPs and the development of ETP in the remediation of emerging organic pollutants.