High-Density Single-Atomic Mn–N–C Site in Hierarchical Porous Biochar for Superoxide Radical-Dominated Ozonation

In this work, biomass-derived single-atomic (Mn–N–C) materials with hierarchical porous structures are synthesized through a double-anchoring strategy. Expandable biomass of semen sterculia lychnopherae is used with strong adsorption ability for Mn precursors. High contents of atomic Mn (7.8 wt %) is bonded with astonishing doped N (20.74 at %) to form pyridinic-type Mn–N4 coordination. For ozonation, 100% of 50 ppm 4-nitrophenol could be rapidly degraded within 30 min with negligible Mn leaching (∼0.04 mg/L), better than lots of commercial MnOx catalysts. Compared with nitrogen-doped biomass carbon (BNC), introduction of atomic Mn will convert the dominated reactive oxidation species (ROSs) from ·OH into O2·– with longer lifespans. According to DFT calculations, O3 will be first adsorbed on the catalyst to form Mn–N4–O*ad intermediate, which will react with H2O to generate ·OH on BNC and react with dissolved O3 to produce O2·– on the Mn–N4 sites, thus revealing a different O3 activation mechanism on Mn–N–C catalysts.