Atomically dispersed Fe-N4 sites in activation of peroxymonosulfate for wastewater purification: Mechanism of non-radical pathways and their quantification

Single-atom catalysts are promising alternatives to homogeneous and heterogeneous catalysts in the advanced oxidation processes for contaminant degradation; however, quantifying the exact contribution of nonradical reactive oxygen species has remained challenging. Herein, Fe single atoms anchored on nitrogen-doped carbonaceous substrates (Fe SAs-N-C) were synthesized for peroxymonosulfate (PMS) activation toward pollutant degradation. Fe SAs-N-C achieved high activity within 30 min, which is 17.20 and 5.08 times higher than those of N-C catalysts without or with Fe nanoparticles, respectively. A synergistic mechanism of singlet oxygen (O-1(2)) and electron transfer for dominating pollutant degradation was revealed. The former contributed 78.50 % while the latter contributed 21.50 % in pollutant elimination. Density functional theory calculations uncovered that the O atom in -SO4 moiety of PMS was adsorbed on the Fe atoms, leading to the OH bond elongation and generating O-1(2). While the absorbed O atom in OO bond near to -SO4 moiety of PMS tended to enlarge the OO bond and accelerate the electron transfer. The adsorbed PMS was prone to being decomposed into O-1(2) due to a lower energy barrier (0.43 eV) of O-1(2) rate-determining step than that of electron transfer (0.95 eV). Fe-N-4 sites are both electron transfer channels and O-1(2) formation sites. This work provides insights into the quantitative contributions of non-radical active species to pollutant degradation.