Criteria of active sites in nonradical persulfate activation process from integrated experimental and theoretical investigations: boron-nitrogen-co-doped nanocarbon-mediated peroxydisulfate activation as an example

Carbon-catalyzed persulfate activation is a metal-free advanced oxidation process for abating aqueous organic micropollutants. Recently, the electron-transfer mechanism in the activation of peroxydisulfate (PDS) has attracted tremendous interest due to its unknown nonradical reaction pathways. The conventionally used atomic-scale descriptors of adsorption energy (E-ads), O-O bond length (l(O-O)) and S-O bond length (l(S-O)) cannot accurately reflect the ability of the functionalities of PDS in its activation. In this work, a new descriptor, local electrophilicity index (omega), which represents the oxidative capacity of adsorbed S2O82-, was included to identify the intrinsic active sites in carbocatalysts via density functional theory calculations. To verify the reliability of the proposed criteria, the catalytic performances of a series of highly boronated and nitrogenated carbon nanotube/nanosheet composites (BCN-NT/NS) with tailored physicochemical properties were comparatively studied for activating PDS to degrade phenol. By integrating the computational and experimental results, the catalytic activity of BCN-NT/NS was determined to not only be related to the contents of heteroatom dopants (B and N), but also the positions of B and N in the co-doping configurations. This study offers reliable criteria for determining the intrinsic catalytic sites in carbocatalysts for the activation of PDS based on an electron-transfer mechanism, which assists the rational design of nanocarbons as advanced catalysts for metal-free oxidation and water remediation. Environmental significance In recent years, the application of carbon-catalyzed persulfate-based advanced oxidation processes (PS-AOPs) in abating aqueous organic micropollutants has been widely studied due to the rich source, biocompatibility and tunable activity of carbocatalysts. Recently, nonradical carbon/PS oxidative systems, especially electron-transfer mediated nonradical activation processes, have aroused great interest due to their unknown reaction pathways. Thus, understanding the electron-transfer mechanism and identification of active sites in carbocatalysts is important. Adsorption energy, O-O bond length and S-O bond length are previously considered as important descriptors in density functional theory (DFT) for determining the active sites in radical-based PSAOPs; however, they cannot accurately reflect the ability of the functionalities in carbocatalysts for activating persulfate via an electron-transfer mechanism. Therefore, a new descriptor indexing the oxidative capacity of the persulfate adsorbed on the carbocatalyst was proposed by DFT calculations, and a series of boron, nitrogen-co-doped nanocarbons with different structural and chemical properties was used as model peroxydisulfate activators to explore the criteria of active sites in nonradical PS-AOPs in this work. By integrating the experimental and theoretical results, we found that the above four descriptors should be considered together to identify the active sites in the electron-transfer mechanism. The outcomes of this study provide reliable criteria for the identification of the active sites to mediate an electron-transfer mechanism in persulfate activation and also insightful understanding of the nonradical regime in nanocarbon-based AOPs, assisting the rational design of advanced carbocatalysts for water remediation.