Atom-level tuning of Co single-atom catalysts to regulate the peroxymonosulfate activation pathway for wastewater...

The precise manipulation of reactive species generation in advanced oxidation processes remains a fundamental challenge in environmental catalysis, primarily due to the elusive structure-activity relationships at the atomic scale. In this study, we systematically synthesized a variety of cobalt-based SACs with precisely tuned nitrogen coordination numbers (Co-N₄, Co-N₃, and Co-N₂), creating a gradient of nitrogen vacancies. This special structure directly modulated the electronic density of cobalt active sites and enhance catalytic performance in peroxymonosulfate (PMS) activation. The Co-N₂ catalysts displayed the best catalytic performance with only 0.2 mM PMS, and 95.0 % of sulfathiazole (STZ) was degraded within 40 min followed by Co-N₃ (91.0 %) and Co-N₄ (75.5 %) catalysts. The k_obs values in cobalt-based SACs/PMS systems correlated well (R² = 0.9952) with N vacancy intensity as well as C chemical state (C-C/C-N/C-O, R² = 0.9994-0.6829) as well as N chemical state (pyrrolic N/pyridinic N, R² = 0.7991-0.9990) between the STZ and cobalt-based SACs/PMS systems. Particularly, while Co-N₄ primarily facilitated non-radical pathways (¹O₂), the reduced coordination in Co-N₃ and Co-N₂ promoted a dual radical/non-radical mechanism, generating sulfate radicals (SO₄^•-), hydroxyl radicals (•OH), and ¹O₂ simultaneously. Density functional theory calculations confirm that reduced N-coordination (Co-N₂) optimizes charge redistribution (increased the Bader charge transfer from 0.766 to 0.859 e), lengthening the O-O bond (from 1.468 Å to 1.522 Å) in PMS and boosting the contribution of radical active species 15.8-fold versus Co-N₄. The Co-N₂/PMS system exhibits high activity and strong anti-interference capability against various pollutants, coexisting ions, and natural water matrices. This work provides atomic-level insights into how coordination engineering governs reactive species evolution, offering a new insight for designing high-performance SACs in environmental remediation applications.