In this work, we report the facile hydrothermal synthesis of manganese cobaltite nanoparticles (MnCo₂O_4.5 NPs) which can efficiently activate peroxymonosulfate (PMS) for the generation of sulfate free radicals (SO₄˙^-) and degradation of organic dyes. The synthesized MnCo₂O_4.5 NPs have a polyhedral morphology with cubic spinel structure, homogeneously distributed Mn, Co, and O elements, and an average size less than 50 nm. As demonstrated, MnCo₂O_4.5 NPs showed the highest catalytic activity among all tested catalysts (MnO₂, CoO) and outperformed other spinel-based catalysts for Methylene Blue (MB) degradation. The MB degradation efficiency reached 100% after 25 min of reaction under initial conditions of 500 mg L^-1 Oxone, 20 mg L^-1 MnCo₂O_4.5, 20 mg L^-1 MB, unadjusted pH, and T = 25 °C. MnCo₂O_4.5 NPs showed a great catalytic activity in a wide pH range (3.5-11), catalyst dose (10-60 mg L^-1), Oxone concentration (300-1500 mg L^-1), MB concentration (5-40 mg L^-1), and temperature (25-55 °C). HCO₃ ^-, CO₃ ^2- and particularly Cl^- coexisting anions were found to inhibit the catalytic activity of MnCo₂O_4.5 NPs. Radical quenching experiments revealed that sulfate radicals are primarily responsible for MB degradation. A reaction sequence for the catalytic activation of PMS was proposed. The as-prepared MnCo₂O_4.5 NPs could be reused for at least three consecutive cycles with small deterioration in their performance due to low metal leaching. This study suggests a facile route for synthesizing MnCo₂O_4.5 NPs with high catalytic activity for PMS activation and efficient degradation of organic dyes.

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