Fe 3d Electron Delocalization in Asymmetric Fe Single Atoms Enables Nonradical Peroxymonosulfate Activation for Micropollutant Degradation

Nonradical peroxymonosulfate (PMS) activation provides a selective route for micropollutant degradation, yet the orbital-level origin of oxidant speciation at single-atom sites remains unclear. Herein, an asymmetric Fe single-atom catalyst (Fe–N 3 –V N –C) is synthesized via molten-salt-assisted pyrolysis of a zeolitic imidazolate framework precursor, where NaCl etching enriches pyrrolic-N anchors and stabilizes low-coordination Fe centers. Fe–N 3 –V N –C exhibits efficient degradation of electron-rich micropollutants, exemplified by sulfamethoxazole, over a wide pH range of 3.5–9.5 and in wastewater matrices. The study through quenching experiments and electron paramagnetic resonance spectroscopy indicates singlet oxygen ( 1 O 2 ) as the dominant reactive species, with only minor radical contributions. The density functional theory calculations reveal that asymmetric coordination induces Fe 3 d electron delocalization, which reinforces Fe 3 d –O 2 p coupling, strengthens Fe–O covalency, accelerates interfacial charge transfer, and stabilizes Fe–O intermediates. In addition, adjacent pyrrolic N cooperates with Fe to polarize adsorbed pollutants, forming a dual-site center that promotes selective 1 O 2 generation. This work establishes Fe 3 d electron delocalization as a key electronic origin of nonradical PMS activation and provides a mechanistic basis for designing single-atom catalysts for practical water purification.