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 NiN4 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 center dot-) 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 center dot-, respectively, leading to fast and selective degradation of aqueous organic pollutants. The graphitic-N adjacent to the aromatic pi-conjugation of NCNTs facilitated chemisorption of 1O2 onto NCNTs via the strong pi*-pi interactions, and more importantly, donated the lone pair electrons to trigger the reduction of 1O2 to O2 center dot-. 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.