Understanding the active sites and associated reaction pathways of metal-free carbocatalysts in persulfate activation and pollutant degradation

Environmental remediation based on persulfate (i.e., peroxomonosulfate (PMS) and peroxydisulfate (PDS))-activation-enabled radical and non-radical oxidation has captured growing interest due to the strong oxidation power, long life of reactive oxygen species, and wide pH adaptability. While metal-based catalysts suffer from environmental problems (e.g., metal ion leaching), metal-free carbocatalysts become the focus of current activities for establishing environmentally benign and efficient carbocatalyst-driven PMS/PDS-based advanced oxidation processes (AOPs). However, the complex structure and nonstoichiometry of carbocatalysts cause the decoding of their active sites and associated pathways in activating persulfates and degrading pollutants to be challenging. Moreover, the ambiguous active sites and structure-activity-performance relationships, as well as the inability to differentiate the oxidation capabilities based on different reaction pathways, present an enormous obstacle to designing and fabricating highly active and durable carbocatalysts for PMS/PDS-based AOPs. Hereby, this paper makes an effort to unravel the catalytic sites of carbocatalysts and their associated pathways in PMS/PDS-based AOPs. Apart from various non-doped active sites, we summarize the diverse active sites induced by single-doping, dual-doping, and tri-doping carbon substrates with different kinds of heteroatoms. Meanwhile, the relationships between these active sites and associated pathways in PMS/PDS-based AOPs are simultaneously analyzed before presenting the challenges and future perspectives.