Iron 3D-Orbital Configuration Dependent Electron Transfer for Efficient Fenton-Like Catalysis

Transition metals are excellent active sites to activate peroxymonosulfate (PMS) for water treatment, but the favorable electronic structures governing  reaction mechanism still remain elusive. Herein, the authors construct typical d-orbital configurations on iron octahedral (Fe_Oh ) and tetrahedral (Fe_Td ) sites in spinel ZnFe₂ O₄ and FeAl₂ O₄ , respectively. ZnFe₂ O₄ (136.58 min^-1 F^-1 cm² ) presented higher specific activity than FeAl₂ O₄ (97.47 min^-1 F^-1 cm² ) for tetracycline removal by PMS activation. Considering orbital features of charge amount, spin state, and orbital arrangement by magnetic spectroscopic analysis, ZnFe₂ O₄ has a larger bond order to decompose PMS. Using this descriptor, high-spin Fe_Oh is assumed to activate PMS mainly to produce nonradical reactive oxygen species (ROS) while high-spin Fe_Td prefers to induce radical species. This hypothesis is confirmed by the selective predominant ROS of ¹ O₂ on ZnFe₂ O₄ and O₂ ^•- on FeAl₂ O₄ via quenching experiments. Electrochemical determinations reveal that Fe_Oh has superior capability than Fe_Td for feasible valence transformation of iron cations and fast interfacial electron transfer. DFT calculations further suggest octahedral d-orbital configuration of ZnFe₂ O₄ is beneficial to enhancing Fe-O covalence for electron exchange. This work attempts to understand the d-orbital configuration-dependent PMS activation to design efficient catalysts.