Halogen-Driven Electronic Modulation of Single-Atom Fe Sites for Universal Non-Radical Catalysis

Selective and matrix-tolerant advanced oxidation processes (AOPs) are essential for treating real waters. Iodine-polarized Fe-N₄ single-atom sites in N-doped carbon (Fe-NI-C) are engineered to molecularly adsorb peroxymonosulfate (PMS) and lower its activation barrier, enabling ultrafast, non-radical phenol oxidation. Fe-NI-C/PMS achieves complete removal within 2 min (k_obs  =   4.098 min^-1, 195-fold over N-C) with 71.2% total organic carbon abatement, maintaining near-quantitative activity from pH 3-11 and in tap, river, and seawater. Electrochemical, quenching, and EPR assays decipher an electron-transfer-to-singlet-oxygen cascade (ETP → ¹O₂): a surface Fe-PMS* adduct triggers interfacial electron flux, producing ¹O₂ as the selective oxidant while suppressing free-radical and matrix scavenging. Spin-polarized density functional theory (DFT) shows iodine upshifts the Fe 3d band toward E_F, strengthens Fe 3d-PMS σ* coupling, stabilizes side-on O-O adsorption (E_ads  =   -1.89 eV), and lowers the transition-state barrier to 0.35 eV. Activity follows I  >  Br > Cl > F, consistent with dopant polarizability and Bader charge transfer (up to 0.88 e). Intrinsic metrics (turnover frequency, TOF; K_ac) confirm site-level acceleration beyond textural effects. These results establish soft-ligand electronic polarization of Fe-N₄ as a programmable route to radical-free, selective, and matrix-robust PMS activation for advanced water purification.