Enhanced electron-transfer for peroxymonosulfate activation by Ni single sites adjacent to Ni nanoparticles

Oriented generation of specific reactive oxygen species (ROS) has been challenging in peroxymonosulfate (PMS)-based advanced oxidation processes (AOPs). In this work, we constructed a multifunctional catalyst composed of Ni NPs embedded in N-doped carbon nanotubes (NCNTs) with exposed Ni single-atom sites (Ni-NCNTs). The Ni-N₄ single sites adjacent to the Ni NPs are more efficient for PMS adsorption and activation, resulting in enhanced production of singlet oxygen (¹O₂). More interesting, we demonstrated that the superoxide anion radical (O₂^•-) was generated from ¹O₂ reduction via the electron transfer from the graphitic-N sites of Ni-NCNTs rather than from O₂ reduction or PMS decomposition as reported in previous studies. Thus, Ni-NCNTs can act as both electron acceptor and donor to trigger the cascade production of ¹O₂ and O₂^•-, respectively, leading to fast and selective degradation of aqueous organic pollutants. The graphitic-N adjacent to the aromatic π-conjugation of NCNTs facilitated chemisorption of ¹O₂ onto NCNTs via the strong π*-π interactions, and more importantly, donated the lone pair electrons to trigger the reduction of ¹O₂ to O₂^•-. This study unravels the mechanisms for enhanced production of ROS in the nanoconfined Fenton-like systems and shed new light on the application of multifunctional nanocatalyst for rapid wastewater decontamination.