Synergetic Manipulation Mechanism of Single-Atom M–N4 and M–OH (M = Mn, Fe, Co, Ni) Sites for Ozone Activation: Theoretical Prediction and Experimental Verification

Carbon-based single-atom catalysts (SACs) have been gradually introduced in heterogeneous catalytic ozonation (HCO), but the interface mechanism of O₃ activation on the catalyst surface is still ambiguous, especially the effect of a surface hydroxyl group (M-OH) at metal sites. Herein, we combined theoretical calculations with experimental verifications to comprehensively investigate the O₃ activation mechanisms on a series of conventional SAC structures with N-doped nanocarbon substrates (MN₄-NCs, where M = Mn, Fe, Co, Ni). The synergetic manipulation effect of the metal atom and M-OH on O₃ activation pathways was paid particular attention. O₃ tends to directly interact with the metal atom on MnN₄-NC, FeN₄-NC, and NiN₄-NC catalysts, among which MnN₄-NC has the best catalytic activity for its relatively lower activation energy barrier of O₃ (0.62 eV) and more active surface-adsorbed oxygen species (O_ads). On the CoN₄-NC catalyst, direct interaction of O₃ with the metal site is energetically infeasible, but O₃ can be activated to generate O_ads or HO₂ species from direct or indirect participation of M-OH sites. The experimental results showed that 90.7 and 82.3% of total organic carbon (TOC) was removed within 40 min during catalytic ozonation of p-hydroxybenzoic acid with MnN₄-NC and CoN₄-NC catalysts, respectively. Phosphate quenching, catalyst characterization, and EPR measurement further supported the theoretical prediction. This contribution provides fundamental insights into the O₃ activation mechanism on SACs, and the methods and ideals could be helpful for future studies of environmental catalysis.