Second-shell engineering of single MnN4 sites steering catalytic ozonation from singlet oxygenation toward high-valent Mn(IV)=O-mediated selective oxidation of water pollutants

Regulating the microenvironment of single-atom catalysts (SACs) is critical for steering ozone (O 3 ) activation toward selective on-demand reactive species generation. While first-shell coordination engineering modulates the center metal activity, it often disrupts the robust Mn-N 4 symmetry and thus compromises structural stability. Here, we developed atomically dispersed Mn-N 4 -S sites (S-MnN 4 ) that feature sulfur doping in the second coordination shell for catalytic ozonation and water pollutant degradation. Second-shell sulfur incorporation subtly elevated the d -band center and increased the spin state of Mn from low to intermediate, enhancing O 3 adsorption, dissociation and the preferred generation of high-valent Mn(IV)=O species. Consequently, S-MnN 4 achieved a 2.7-fold higher ozone utilization efficiency and double the turnover frequency versus pristine MnN 4 which catalyzed O 3 to primarily yield singlet oxygen. A continuous-flow system using a S-MnN 4 -packed column reactor demonstrated efficient chemical oxygen demand removal and exceptional durability over 170 h for the treatment of real-world wastewater. This work highlights second-shell coordination engineering as a feasible approach to overcome SAC activity-stability trade-off, advancing rational SAC design toward a sustainable environmental remediation.