Spin States of Fe‐N ₄ Sites Regulates PMS Activation Pathways for Ultrafast Sulfamethoxazole Degradation

ABSTRACT 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 FeN ₄ catalyst (Fe ₁ NC) 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 FeN ₄ preferentially binds PMS at the hydroxyl oxygen and strengthens Fe 3d‐O 2p orbital hybridization, thermodynamically promoting ¹ O ₂ formation; high‐spin FeN ₄ stabilizes the N ₄ Fe‐O*‐SO ₄ 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 ( ¹ O ₂ : 57.0%, O ₂ ⁻ : 23.7%, Fe(IV) = O: 15.0%) that confers high selectivity toward electron‐rich contaminants and minimal matrix interference. A Fe ₁ NC‐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.