ZIF-8-derived hollow carbon polyhedra with highly accessible single Mn-N6 sites as peroxymonosulfate activators for efficient sulfamethoxazole degradation

The adsorption and protective calcination methods were combined to synthesize single Mn atoms catalyst with more Mn-Nx exposure for peroxymonosulfate (PMS) activation. During the carbonization process, the surface adsorbed Mn species in zeolitic imidazolate framework-8 (ZIF-8) and the surrounding graphitic carbon nitride (g-C3N4) jointly maintained the morphology of precursor and enriched Mn-N6 sites near the catalyst surface. The hollow and hierarchically porous structures induced by the erosive function of g-C3N4 further facilitated the exposure of Mn-N6 sites and mass transfer processes. Herein, the derived hollow carbon polyhedron catalyst (C-Z8-Mn@g-C3N4) possessed highly accessible edge-host Mn-N6 sites and ultra-high specific surface area (1846.91 m2 g−1). C-Z8-Mn@g-C3N4 exhibited remarkable catalysis for sulfamethoxazole (SMX) degradation at an extremely low dosage (0.01 g L−1), particularly, up to 3.8 times of promotion was obtained compared to traditional single Mn atoms catalyst (66.9 vs 17.8 L g−1 min−1). Besides, the catalytic performance of C-Z8-Mn@g-C3N4 was not obviously affected by initial pH (3.14–11.00), inorganic anions and natural organic matters (NOMs). Experiments and Density functional theory (DFT) calculations revealed that Mn-N6 sites not only had stronger PMS adsorption capacity than graphitic N, but also enhanced the electron transfer between catalyst and PMS, thus promoting the redox and self-decomposition of PMS to produce SO4−, OH and singlet oxygen (1O2) (as major active oxygen species). This work provides new insights into the regulation of active sites exposure in single-atom catalysts for enhanced catalytic performance.