N, S, and Cl tri-doped carbon boost the switching of radical to non-radical pathway in Fenton-like reactions: Synergism of N species and defects.

Heteroatom doping represents a promising strategy for enhancing the generation of singlet oxygen (1O2) during the activation of peroxymonosulfate (PMS) using carbon-based catalysts; however, it remains a formidable challenge. In this study, we systematically controlled the structure of metal-free carbon-based materials by introducing different heteroatoms to investigate their efficacy in degrading organic pollutants in water via PMS activation. The results of reactive oxygen species detection showed that the dominant free radical in the four samples was different: CN (•SO4- and •OH), CNS (•O2-), CNCl (1O2), and CNClS (1O2). This led to the transformation of active species from free radicals to non-free radicals. The tri-doped carbons with nitrogen, sulfur, and chlorine (CNClS) exhibited exceptional performance in PMS activation and achieved a remarkable degradation efficiency of 95% within just 6 min for tetracycline. Moreover, a strong linear correlation was observed between the ratio of pyridine-N/graphite-N and ID/IG with the yield of 1O2, indicating that N species and defects play a crucial role in CNClS as they facilitate the transition from radical to non-radical pathways during PMS activation. These findings highlight the possibility that adjustable tri-heteroatom doping will expand the Fenton-like reaction for the treatment of actual wastewater.

Facilitating Redox Cycles of Copper Species by Pollutants in Peroxymonosulfate Activation.

The redox behavior of metal active sites determines the rate of heterogeneous catalysis in peroxymonosulfate activation. Previous reports focused on the construction of catalysts for accelerating interfacial electron transfer. In this work, a new strategy was proposed for facilitating valence cycles of Cu+/Cu2+ by using pollutants. The 2.5Cu/CeO2/PMS system was capable of achieving the efficient removal of pollutants, including tetracycline, oxytetracycline, and rhodamine B, in a wide pH working range. In the presence of tetracycline, a Cu-N bond was formed between the -NH2 group of tetracycline and the Cu site of the catalyst, showing that the coordination of Cu active sites changed to CuO4N1. The charge of CuO4N1 active sites rearranged, making it easier to obtain electrons and promote the PMS oxidation, thereby accelerating the reduction of Cu2+ to Cu+ and PMS activation. The PMS activation system showed excellent sustainability and selectivity for the removal of organic pollutants. This study provides a novel routine to promote peroxymonosulfate activation by utilizing pollutants to accelerate the redox behavior of metal species.

Almost 100 % Peroxymonosulfate Conversion to Singlet Oxygen on Single-Atom CoN2+2 Sites.

Single-atom CoN4 active sites have demonstrated excellent efficiency in peroxymonosulfate activation. However, the identification of CoN4 active sites and the detailed singlet oxygen generation mechanism in peroxymonosulfate activation remains ambiguous. We demonstrate a strategy to regulate the generation of reactive oxygen species by atomically dispersed cobalt anchored on nitrogen-doped carbon. As indicated by experiment and DFT calculations, CoN2+2 was the active site and singlet oxygen was the predominant reactive oxygen species with a proportion of 98.89 %. Spontaneous dissociation of adsorbed peroxymonosulfate on the CoN2+2 active sites was energetically unfavorable because of the weakly positive Co atoms and CoN2+2 coordination, which directed PMS oxidation by a non-radical pathway and with simultaneous singlet oxygen generation. The generated singlet oxygen degraded several organic pollutants with high efficiency across a broad pH range.