Oxygen vacancy-mediated d-band center regulation in NiCu_0.4Fe_1.6O₄ via Cu-doping achieves efficient peroxymonosulfate activation for durable water purification

Element-doped catalysts with oxygen vacancies (OVs) are highly effective in water purification by activating oxidants; however, the interfacial mechanisms and specific contributions of reactive oxygen species (ROS) in metal-doped catalysts featuring OVs for peroxymonosulfate (PMS) activation remain insufficiently elucidated. In this study, Cu-doped NiFe₂O₄ catalysts with abundant OVs were synthesized via a hydrothermal-calcination process, endowing efficient PMS activation for ROS generation. Remarkably, it mediated more sulfate radicals (SO₄·⁻) and hydroxyl radicals (·OH) generation, accounting for 50.4 % and 36.5 % of 2,4-dichlorophenoxyacetic acid (2,4-D) degradation, respectively. Benefiting from the formation of OVs, it significantly enhanced PMS adsorption (E_ads = −7.17 eV), markedly lower than that of the pristine NiFe₂O₄ (E_ads = −4.11 eV). Meanwhile, inducing the regulation of the d-band center of Fe and Ni sites towards the Fermi level, thereby enhancing more electron transfer (0.84 e of NiCu_xFe_2-xO₄ compared to 0.60 e of NiFe₂O₄) from OVs to surface-complexed PMS, and thus voluntarily forming SO₄·⁻ with lower free energy (△G = −11.7 eV). This work elucidates the OVs-mediated structural regulation of the d-band center and interfacial electron transfer pathway, highlighting a synergistic multi-metal activation mechanism in response to PMS activation, which offers a novel strategy for designing efficient catalysts to trigger interfacial ROS formation.