Electro-Fenton reaction (EF) has been considered as an efficient advanced oxidation technology (AOP) for sewage treatments, while the design of Fenton catalysts remains a great challenge in simultaneous hydrogen peroxide (H2O2) production and activation to achieve oxygen to hydroxyl radical conversion. Here, the atomically dispersed Fe anchored in porous defective carbon structure (Fe-SA@PC) was loaded on carbon felt as the cathode for EF to degrade amoxicillin (AMX). Compared with iron nanoparticles (Fe-NP@PC), Fe-SA@PC had favorable mass transfer rate and charge transfer characteristics, which was more prone to the 3-electron transfer path in the oxygen reduction reaction (ORR). The pseudo-first-order reaction rate constant (kobs) of Fe-SA@PC (0.249min-1) was almost six orders of magnitude higher as compared to that of Fe-NP@PC for degrading AMX. Electron spin resonance revealed that hydroxyl radical was the key reactive specie, the accumulative concentration of hydroxyl radical by Fe-SA@PC surpassed that of Fe-NP@PC and blank carbon felt by almost 2 times. Experimental and density function theory calculations demonstrated that the atomically dispersed Fe was used as an active site, its different valence state of Fe induced different reaction paths. That is, the low-valence Fe configuration (C2-Fe-Cl2) was thermodynamically favorable for the adsorption of oxygen, and it was liable to activate H2O2 to form hydroxyl radical. However, the high-valence Fe configuration (C3-Fe-Cl) was more conducive to the production of H2O2, although it was verified a rate-limiting step in the adsorption of oxygen. Meanwhile, Fe-SA@PC was more practical in the neutral conditions, and achieved favorable removal efficiency for a variety of antibiotics. Based on the ultraperformance liquid chromatography/quadrupole time-of-flight mass spectrometry analysis, the possible thirty-four intermediates and four degradation pathways were proposed according to the different vulnerable groups of AMX structure. These above results provide a new insight into the synergistic mechanisms induced by feasible valence configurations of atomically dispersed Fe in the EF system.

Atomically dispersed Fe sites on porous defective carbon for simultaneous hydrogen peroxide generation and activation: application in electro-Fenton catalysis

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