Robust Fe-N₄-C₆O₂ single atom sites for efficient PMS activation and enhanced Fe^IV = O reactivity

The microenvironment regulation of Fe-N₄ single atom catalysts (SACs) critically governs peroxymonosulfate (PMS) activation. Although conventional heteroatom substitution in primary coordination enhances activity, it disrupts Fe-N₄ symmetry and compromises stability. Herein, we propose oxygen doping in the secondary coordination shell to construct Fe-N₄-C₆O₂ SAC, which amplifies the localized electric field while preserving the pristine coordination symmetry, thus trading off its activity and stability. This approach suppresses Fe-N bond structural deformation (bond amplitude reduced from 0.875-3.175 Å to 0.925-2.975 Å) during PMS activation by lowering Fe center electron density to strengthen Fe-N bond, achieving extended catalytic durability (>240 h). Simultaneously, the weakened coordination field lowers the Fe=O σ* orbital energy, promoting electrophilic σ-attack of high-valent iron-oxo towards bisphenol A, and increasing its degradation rate by 41.6-fold. This work demonstrates secondary coordination engineering as a viable strategy to resolve the activity-stability trade-off in SAC design, offering promising perspectives for developing environmental catalysts.