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-N single sites adjacent to the Ni NPs are more efficient for PMS adsorption and activation, resulting in enhanced production of singlet oxygen (O). More interesting, we demonstrated that the superoxide anion radical (O) was generated from O reduction via the electron transfer from the graphitic-N sites of Ni-NCNTs rather than from O 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 O and O, 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 O onto NCNTs via the strong π*-π interactions, and more importantly, donated the lone pair electrons to trigger the reduction of O to O. 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.