Probing the activation mechanism of nitrogen-doped carbonaceous materials for persulfates: Based on the differences between peroxymonosulfate and peroxydisulfate

The activation processes of persulfates by metal-free nitrogen-doped carbonaceous material (NCM) remain unclear due to their complex structures and heterogeneous nature. On the other hand, from the perspective of persulfates, it is possible to clarify the reaction between persulfates and NCM by considering the differences in activation behaviors between peroxymonosulfate (PMS) and peroxydisulfate (PDS). Our study aims to compare the differences between NCM-PDS and NCM-PMS using a fully metal-free NCM as a model catalyst. Firstly, NCM-PDS was more efficient than NCM-PMS in degrading phenolic compounds (PCs). Secondly, the stoichiometric ratio between consumed persulfates and DCP removed in the NCM-PDS (0.73) is lower than in the NCM-PMS (1.08). Thirdly, PMS and PDS adsorb on NCM in different ways, suggesting that the peak O-O bond in PDS has blue shifted from 814 cm-1 to 805 cm-1, while that of O-O bond in PMS has shifted from 889 cm-1 to 834 cm-1. Additionally, the hydrogen bond between the phenolic group and oxidants plays a critical role in PCs degradation by NCM-PDS, exhibiting a stronger pH effect and higher kinetic isotope effects (KIEs) than NCM-PMS. A proton-coupled electron transfer process has been proposed for PCs degradation using NCM-PDS, and a scheme of reaction pathways has been provided for the NCM-PMS/PDS-PCs system. The study results provide a deeper understanding of the activation of persulfates by NCM, as well as a strategy for selecting oxidants.