Ultrastable Fe-N4-C6O2 Single Atom Sites for Highly Efficient PMS Activation and Enhanced FeIV=O Reactivity

The local environment modulation of iron sites in Fe-N₄ single atom catalysts (SACs) plays a crucial role in the efficient peroxymonosulfate (PMS) activation. Many reported modulation strategies involve the partial replacement of N in the first coordination shell of Fe-N₄ sites with foreign elements to facilitate the PMS activation via disrupting the structural symmetry, suffering from undesired catalytic stability. Herein, we demonstrate that Fe-N₄-C₆O₂ sites, which are prepared by substituting C in the second coordination shell of Fe-N₄ sites with O, can activate PMS more efficiently and stably by providing an enhanced localized electric field without destroying their symmetric coordination structure in the first coordination shell, and thus achieve an unprecedented catalytic durability of at least 240 h. The O doping in the second coordination shell strengthened the Fe-N bond by reducing the electron density of Fe center, and weakened the amplitude of Fe-N bond from 0.875 ~ 3.175 Å to 0.925 ~ 2.975 Å during the PMS activation, therefore effectively prevented the demetallation of Fe-N₄ sites. Meanwhile, this O doping also lowered the energy of Fe = O σ* orbitals by weakening the coordination field to promote the electrophilic σ-attack of high-valent iron-oxo (Fe^IV=O) towards electron-rich contaminants, thus enhancing the bisphenol A degradation rate from 1.08 × 10³ M^{− 1} s^{− 1} to 4.6 × 10⁴ M^{− 1} s^{− 1} by a factor of 41.6. This work sheds light on the importance of second coordination shell doping on the ultrastability of Fe-N₄ SACs, and provides a novel strategy to design metal SACs by balancing a trade-off between exceptional activity and long-term stability.