Urea and ammonium fluoride di-nitrogen and Cu & Fe bi-metal co-doped carbon felt as cathode for electro-Fenton degradation of norfloxacin: 1O2-dominated oxidation pathway†

Carbon materials co-doped with both metals and non-metallic heteroatoms have become an important research focus as catalysts for heterogeneous electro-Fenton technology and the removal of refractory organics. However, there is still a lack of in-depth studies on the doping of carbon with multiple nitrogen sources and their possible synergistic mechanism. In this study, two types of nitrogens (urea and ammonium fluoride) and Cu&Fe bimetal co-doped carbon felt electrodes (C–CuFe/N) were designed to explore the effect of co-doping on carbon. C–CuFe/N exhibited a high degradation efficiency towards norfloxacin (97.2%), low ion leaching and high cycling stability (90.3% after ten cycles), better than those shown by C–CuFe, C–CuFe/UN and C–CuFe/FN prepared with none or a single nitrogen dopant. Various characterizations indicated that C–CuFe/N has the largest specific surface area, highest content of Fe(II), most surface oxygen and active sites, and importantly, stable M–N_x bond, indicating that UN and FN exhibited an excellent synergistic effect. According to the quenching experiments, the dominant reactive oxygen species for C–CuFe, C–CuFe/UN and C–CuFe/FN were radical species (·OH and O₂˙⁻), while they changed to non-radical species (¹O₂) for C–CuFe/N under acidic condition. Alternatively, C–CuFe/N showed a good catalytic performance (97.2–92.3%) over a wide initial pH range (1.2–11.3), but during the degradation process, all the pH values changed toward neutral, and the oxidation pathway varied from ¹O₂-dominated under acidic condition to radical-dominated under neutral or alkaline condition. Generally, a good synergistic effect was found to exist between the dual nitrogens, which promoted the catalytic activity as well as stability of the catalyst, thus providing a good strategy to design catalysts.