Synergetic Manipulation Mechanism of Single-Atom M-N 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 O 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 O activation mechanisms on a series of conventional SAC structures with N-doped nanocarbon substrates (MN-NCs, where M = Mn, Fe, Co, Ni). The synergetic manipulation effect of the metal atom and M-OH on O activation pathways was paid particular attention. O tends to directly interact with the metal atom on MnN-NC, FeN-NC, and NiN-NC catalysts, among which MnN-NC has the best catalytic activity for its relatively lower activation energy barrier of O (0.62 eV) and more active surface-adsorbed oxygen species (O). On the CoN-NC catalyst, direct interaction of O with the metal site is energetically infeasible, but O can be activated to generate O or HO 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 -hydroxybenzoic acid with MnN-NC and CoN-NC catalysts, respectively. Phosphate quenching, catalyst characterization, and EPR measurement further supported the theoretical prediction. This contribution provides fundamental insights into the O activation mechanism on SACs, and the methods and ideals could be helpful for future studies of environmental catalysis.