Breaking Local Symmetry with Axial Cl to Steer Nonradical Polymerization of Phenol over a Co Single-Atom Catalyst

Single-atom catalysts (SACs) with asymmetrically engineered coordination environments are promising for advancing peroxymonosulfate (PMS)-based advanced oxidation. Yet, achieving precise control over the electronic and geometric structure of the metal center to direct reaction pathways beyond mineralization remains a substantial challenge. To address this challenge, we reported a planar-axial dual-regulation strategy to fabricate a cobalt SAC featuring a tailored Co–N3 plane and an axial chlorine ligand (denoted as Cl–Co–N3). At a rate constant of 0.208 min–1, which exceeded most reported heterogeneous catalysts, this asymmetric site enabled ultrarapid (>98% within 30 min) phenol removal by optimizing the Co electronic state. The system performed robustly across a wide pH range and resisted common water-matrix interfering substances. Integrated experimental probes and theoretical simulations revealed that the reaction proceeds predominantly by means of a nonradical electron-transfer mechanism. The axial Cl coordination modulated the d-band center of Co, enhanced charge transfer capacity, and strengthened PMS adsorption/activation, leading to the generation of a high-potential surface-activated PMS* complex. This complex drives efficient one-electron oxidation of phenol to generate phenoxyl radicals, which subsequently undergo oxidative coupling and chain-growth polymerization, transforming pollutants into separable macromolecular products on the catalyst surface. Consequently, this unique nonmineralization pathway achieved an exceptionally high electron utilization efficiency of 181.6%, drastically reducing oxidant consumption. Our work offers a molecular-level design principle for engineering efficient SACs toward oxidant-economical, polymerization-based water decontamination.