Two-step pyrolysis to anchor ultrahigh-density single-atom FeN5 sites on carbon nitride for efficient Fenton-like catalysis near 0 °C

Low temperatures approaching 0 °C will slash the efficiency of Fenton-like catalysis for organic pollution control. Herein, a two-step pyrolysis strategy is developed to fabricate the carbon nitride nanosheet-supported single-atom Fe catalyst with ultrahigh Fe loading amount of 16.64 wt%. The secondary pyrolysis generates surface vacancies to convert Fe coordination structure from FeN3 to FeN5, which exhibits ultralow activation energy of 6.54 kJ mol−1 in peroxymonosulfate activation for sulfamethoxazole degradation via a new ‘surface contact oxidation’ path. The catalyst-dose-normalized kinetic rate constant on FeN5 site reaches 21.38 L min−1 g−1 at 2 °C, even exceeding that on FeN3 site and reported values by 0.61–70.27 time(s) at 25–30 °C. Density-functional-theory calculations reveal that additional N ligands (L) make less charges transfer from Fe toward -SO4 in critical [FeL-SO4] intermediates, enabling their thermodynamically favorable electron seizure from pollutants, and cause easier -SO4H desorption for rapid site regeneration, contributing to excellent low-temperature resistance.