Sulfur-doping gradient modulates spin-orbital engineering in cobalt single-atom catalysts: Optimized d-p orbital...

The rational design of single-atom catalysts (SACs) with tailored electronic configurations is critical for advancing peroxymonosulfate-based heterogeneous advanced oxidation processes (PMS-AOPs). Nevertheless, the intricate interplay between atomic-scale coordination microenvironments, electronic configurations, and resultant catalytic functionalities in SACs remains inadequately understood. Here, an S-gradient doping strategy is developed to engineer a series of Co-S_xN_4-x (x = 0, 1, 2, 3) SACs with progressively tuned coordination geometries, achieving a volcano-type activity trend in PMS-activated sulfadiazine (SDZ) degradation. Multiscale characterization and theoretical analyses demonstrate that S doping induces a structural transition from planar Co-N₄ to distorted tetrahedral oordination. This transition elevates the spin states of Co while optimizing Co 3d-O 2p orbital hybridization. The intermediate-spin Co-S₁N₃ configuration achieves superior catalytic performance by strategically optimizing the σ-bond dominated adsorption strength, thereby stabilizing reactive Co-S₁N₃/PMS* complexes and accelerating interfacial electron transfer kinetics. Mechanistic studies confirm the dominance of a non-radical electron transfer pathway (ETP) in Co-S₁N₃/PMS systems. This work establishes a "spin-orbital" activity framework, wherein sulfur-gradient engineering synergistically modulates spin states and d-p orbital hybridization, providing a universal paradigm for designing high-efficiency SACs in sustainable water remediation.