Active electronic structure derived by Fe-Cl-C coordination of single-atom cathode applied in antibiotics degradation by electro-Fenton: Enhanced transformation of oxygen to hydroxyl radicals via 3-electron pathway

Designing heterogeneous catalysts with atomically dispersed active sites is vital to promote electro-Fenton (EF) activity, but how to regulate the electronic structure of metal centers to overcome the rate-limiting step over electron transfer triggered by reduction-/oxidation-state cycle in Fenton still remains a great challenge. Herein, we report a systematic investigation into heteroatom-doped engineering for tuning the electronic structure of iron single-atom sites by integrating electron-acceptor chlorine atoms into MOF-derived carbon substrate, in which the conversion of O2 toward •OH in EF were enhanced over the electronic structures of Fe-Cl2C2 and Fe-Cl2C3 formed by iron unsaturated coordination with chlorine and carbon atoms via a 3-electron pathway, and overcame the restriction of the rate-limiting step for reducing oxidized metal ions. The resulting accumulative concentration of •OH by FeCl2Cx/PC surpassed that of iron oxide nanoparticles by almost 2 times. Iron site shielding experiments and density functional theory calculations further demonstrated that the vital effect of Fe-Cl2C3 configuration corresponds to Fe(III) on Fe center contributes to H2O2 production and the dominant role of Fe-Cl2C2 configuration corresponds to Fe(II) in H2O2 activation to form •OH. Meanwhile, FeCl2Cx/PC exhibited less pH dependence, high stability, and efficient applicability for various antibiotics and wastewater remediation. The above results provide a new perspective into the reaction mechanism of multi-electron oxygen reduction pathway on single-atom catalysts by modulating the electronic structure of chlorine coordination.