Insight into the degradation of tetracycline hydrochloride by non-radical-dominated peroxymonosulfate activation with hollow shell-core Co@NC: Role of cobalt species

Cobalt species often play a crucial role in the sulfate radical ((SO4-)-S-center dot) generation during peroxymonosulfate (PMS) activation, but their roles in the formation of non-radical-dominated PMS activation are largely unclear. Here, shell-core hollow Co@NC with exposed two different cobalt species (i.e., Co nanoparticles and Co-N-x site) was prepared and used to activate PMS for tetracycline hydrochloride (TCH) degradation. The experimental results and theoretical calculations showed cobalt species play a crucial role in the O-1(2)-dominated TCH degradation in which Co-N-x directly served as active sites to adsorb PMS (*PMS) to facilitate PMS decomposition to form (SO5-)-S-center dot, and thereafter evolved into O-1(2) by the rapid self-reaction of (SO5-)-S-center dot, while Co nanoparticles indirectly promote O-1(2) generation via electron transfer due to their excellent conductivity. With exposed cobalt species and unique structure, Co@NC showed a remarkable catalytic activity for TCH degradation, outperforming the synthesized NC, Co-NC, and commercial Co3O4, Fe3O4 and MnO2. Meanwhile, the good stability and reusability, high environmental robustness and universal adaptability of Co@NC were demonstrated. The TCH degradation pathways including aniline ring oxidation, the cleavage of functional groups and ring-opening reactions were also proposed. The improved understanding on the roles of cobalt species in the non-radical-dominated PMS activation may inspire the development of efficient, selective, and robust cobalt-based catalysts.