Efficient fenton-like catalysis enabled by single cobalt atoms anchored on expanded graphite: Remarkable intrinsic activity of Co-N4 sites and the enhanced mass transfer facilitated by gradient mesopore structure

The single-atom catalysts derived from traditional Zeolitic Imidazolate Framework (ZIF) materials are exclusively characterized by a micropore structure, which severely hampers both the mass transfer efficiency and the accessibility of metal sites during catalysis. Herein, by utilizing mesoporous expanded graphite (EG) and ZIF-67 as precursors, a novel single-atom cobalt catalyst anchored on three-dimensional porous carbon materials (ZIF-67-x@EG) has been developed. Experiments demonstrate that introducing cobalt atoms in the formed bulges of EG increases carrier concentration and significantly enhances carrier mobility. The resulting ZIF-67-1@EG catalyst exhibits remarkable efficiency in degrading levofloxacin (LEVO concentration = 48 mg/L), with a degradation rate constant (0.078 min−1) 3.25 times higher than EG@N (0.024 min−1). The Co amounts in all ZIF-67-x@EG catalysts positively correlated with the reaction rate constant (R2 = 0.995). The ZIF-67-1@EG, with a small amount of cobalt loading (2.29 at%), exhibited an exceptionally high turnover frequency (TOF) value for LEVO (0.49 min−1). The degradation process involves an electron transfer pathway (ETP) and the contribution of singlet oxygen (1O2). The toxicity of the intermediates was characterized by quantitative structure-activity relationship (QSAR) prognostication, and the toxicity was reduced post-reaction. PMS/Co-N4 (-0.37 eV) and PMS/pyridinic N (-0.39 eV) configurations exhibit lower adsorption energies than PMS/pyrrolic N (-0.32 eV) structure, indicating that PMS primarily binds to these sites in the ZIF-67-1@EG/PMS system.