Spin States of Fe-N₄ Sites Regulates PMS Activation Pathways for Ultrafast Sulfamethoxazole Degradation

The precise correlation between the spin configuration of single-atom Fe sites and the selectivity of peroxymonosulfate (PMS) activation pathways remains poorly understood, limiting the rational design of high-performance Fenton-like catalysts. Here, we report an atomically dispersed FeN₄ catalyst (Fe₁NC) featuring engineered low-spin and high-spin states via a sol-gel-derived, gas-phase-assisted carbonization strategy. The coexistence of tunable spin states enables ultrafast PMS activation, achieving over 95% sulfamethoxazole degradation (10 mg L^-1) within 3 min (k_obs = 0.96 min^-1) and robust resistance toward coexisting ions, pH fluctuations, and diverse water matrices. Density functional theory calculations reveal a spin-state-dictated bifurcation of reactive oxygen species (ROS) pathways: low-spin FeN₄ preferentially binds PMS at the hydroxyl oxygen and strengthens Fe 3d-O 2p orbital hybridization, thermodynamically promoting ¹O₂ formation; high-spin FeN₄ stabilizes the N₄Fe-O*-SO₄ intermediate through a downshifted d-band center, kinetically lowering the energy barrier for Fe(IV) = O formation. These spin-regulated channels collectively create a nonradical-dominated oxidation regime (¹O₂: 57.0%, O₂ ^-: 23.7%, Fe(IV) = O: 15.0%) that confers high selectivity toward electron-rich contaminants and minimal matrix interference. A Fe₁NC-integrated membrane reactor further delivers stable > 85% removal over 7 h of continuous operation. This work establishes spin-state engineering as a mechanistic lever for directing ROS selectivity in single-atom PMS catalysis.