Deciphering the Origin of Higher Shell Coordination on Single Iron Catalysts for Resilient Modulating Persulfate Oxidation Into Singlet Oxygen Pathway

Precise manipulation of coordination structure of single-atom sites and establishment of schematic microenvironment-oxidation pathway relations remain significant challenges in Fenton-like chemistry. Herein, incorporating sulfur heteroatoms into the higher coordination shell of FeN₄ structure (Fe-NSC) exhibited a volcano trend of p-hydroxybenzoic acid oxidation, aligning with the number and positions of sulfur dopant. Specifically, higher shell S coordination with moderate electronegativity and larger atomic radii triggers long-range electronic interactions, which provoke Fe 3d orbital splitting and spin electron rearrangement, resulting in a spin crossover with orbital states d_xy² d_yz¹ d_xz² d_z²¹. As a result, the partial filling of e_g and t_2 g orbitals and moderate σ/π antibonding states between 3d and 2p atomic states optimized the adsorption–desorption behaviors of the key oxygenated intermediates from peroxymonosulfate activation. Thus, the optimal binding configuration weakens the Fe─O bonding and accelerates PMS dissociation to yield C-S-N₄Fe-O*, which subsequently couples to form ¹O₂ with nearly 100% selectivity. The Fe-NSC-functionalized membrane exhibited outstanding long-term reusability in a continuous flow reactor which further validated practical application perspective. This study provides insight at both atomic and electronic levels for rational design of spin-polarized catalysts and its functions in fine-tuning oxidation pathways in environmental catalysis.