Synergetic Manipulation Mechanism of Single-Atom M-N 4 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 3 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 3 activation mechanisms on a series of conventional SAC structures with N-doped nanocarbon substrates (MN 4 -NCs, where M = Mn, Fe, Co, Ni). The synergetic manipulation effect of the metal atom and M-OH on O 3 activation pathways was paid particular attention. O 3 tends to directly interact with the metal atom on MnN 4 -NC, FeN 4 -NC, and NiN 4 -NC catalysts, among which MnN 4 -NC has the best catalytic activity for its relatively lower activation energy barrier of O 3 (0.62 eV) and more active surface-adsorbed oxygen species (O ads ). On the CoN 4 -NC catalyst, direct interaction of O 3 with the metal site is energetically infeasible, but O 3 can be activated to generate O ads or HO 2 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 MnN 4 -NC and CoN 4 -NC catalysts, respectively. Phosphate quenching, catalyst characterization, and EPR measurement further supported the theoretical prediction. This contribution provides fundamental insights into the O 3 activation mechanism on SACs, and the methods and ideals could be helpful for future studies of environmental catalysis.