Phase-engineered coordinatively unsaturated metal sites in spinel oxides for robust Fenton-like catalysis

Coordinatively unsaturated metal sites (CUS) located at tetrahedral (T_d) in spinel structure are highly effective for activating peroxymonosulfate (PMS) in Fenton-like catalysis. However, the conventional T_d−octahedral (O_h) connectivity in spinel structures restricts internal electron transfer, limiting the regeneration of low-valent metals and creating a trade-off between catalytic activity and long-term stability. Herein, we address this challenge by engineering a novel T_d−T_d connectivity in amorphous CoFeO_x nanosheets (a-CoFeO_x NSs). Soft X-ray absorption spectroscopy (sXAS) measurements reveal that in a-CoFeO_x nanosheets, the ligand field symmetry around Co atoms is dominated by a T_d coordination, in contrast to the O_h coordination in the crystalline state, which introduces T_d−T_d connection. Density functional theory (DFT) calculations confirm that the T_d−T_d connection in a-CoFeO_x structure significantly strengthens electron transfer to activate PMS, which exhibited a first-order kinetic constant (k_obs) of 0.27 min⁻¹ for sulfamethoxazole (SMX) removal with high stability. This study reveals that the phase-engineered CUS can further enhance catalytic activity and provides a simple and scalable strategy for optimizing spinel-type catalysts.