The role of Fe-Nx single-atom catalytic sites in peroxymonosulfate activation: Formation of surface-activated complex and non-radical pathways

Transition-metal and nitrogen co-doped carbon-based catalysts receive much attention in peroxymonosulfate (PMS) activation. However, the contribution of doped metal and the kinetic mechanisms of the emerging nonradical pathways especially enabled by reactive singlet oxygen species remain unclear. Herein, we report Fe-N-co-doped carbon-based catalysts (FeCNx) for peroxymonosulfate activation. The catalysts achieve fast removal of a broad spectrum of organic pollutants in a wide pH range without the common Fe leaching. Both catalyst surface-activated PMS complex (catalyst-PMS*), surface-bound radicals, and singlet oxygen are identified. The new mechanism is enabled by the Fe-Nx single-atomic-site that enhances the adsorption of PMS and facilitates the formation of catalyst-PMS*. The formation route of singlet oxygen species is elucidated for the first time for Fe, N-co-doped carbon-based catalysts with density functional theory simulation. This study reveals the intrinsic role of Fe-Nx site in catalysts and provides new insights into the origin and kinetics of non-radical pathways for peroxymonosulfate activation.