N-Doping-Induced Nonradical Reaction on Single-Walled Carbon Nanotubes for Catalytic Phenol Oxidation

Metal-free materials have been demonstrated to be promising alternatives to conventional metal-based catalysts. Catalysis on nanocarbons comparable to that of cobalt- or manganese-based catalysts in peroxymonosulfate (PMS) activation has been achieved, yet the catalyst stability has to be addressed and the mechanism also needs to be elucidated. In this study, N-doped carbon nanotubes (NoCNTs) were employed as metal-free catalysts for phenol catalytic oxidation with sulfate radicals and, more importantly, a detailed mechanism of PMS activation and the roles of nitrogen heteroatoms were comprehensively investigated. For the first time, a nonradical pathway accompanied by radical generation (•OH and SO4•–) in phenol oxidation with PMS was discovered upon nitrogen heteroatom doping. The NoCNTs presented excellent stability due to the emerging nonradical processes. The findings can be used for the design of efficient and robust metal-free catalysts with both superior catalytic performance and high stability for various heterogeneous catalytic processes.