Evaluation of cobalt oxide, copper oxide and their solid solutions as heterogeneous catalysts for Fenton-degradation of dye pollutants

Abstract

A series of CoO, Co_0.75Cu_0.25O, Co_0.5Cu_0.5O and CuO nanoparticles were synthesized via the calcination of corresponding oxalates and further examined as catalysts for the heterogeneous Fenton reaction. The structures of the as-prepared oxides were characterized by field emission electron microscopy, transmission electron microscopy, energy dispersive X-ray spectroscopy and X-ray diffraction. The catalytic activity of the oxides was evaluated by the degradation of Congo red. It was found that, among the four catalysts, Co_0.5Cu_0.5O showed the best catalytic performance. Subsequently, the effects of operating parameters including the substrate concentration, pH, H₂O₂ concentration and reaction temperature in the catalytic performance of the Co_0.5Cu_0.5O were systematically studied. Under optimized conditions of catalyst loading = 200 mg L⁻¹, pollutant concentration = 100 mg L⁻¹, H₂O₂ concentration = 3 wt%, temperature = 30 °C and pH = 9, the Co_0.5Cu_0.5O catalyst could completely degrade the Congo red within 60 min. The degradation products were analyzed by a liquid chromatography-mass spectrometer and the degradation pathway was revealed. To investigate the catalytic mechanism, the pH and concentrations of H₂O₂ and metal ions were monitored during the Fenton process. Mechanistic studies revealed that hydroxyl radicals and superoxide radicals derived from the activation of H₂O₂ molecules by metal centers were mainly responsible for the degradation of Congo red, and that copper ions played a critical role in the superior catalytic performance of the Co_0.5Cu_0.5O catalyst. The Co_0.5Cu_0.5O catalyst showed negligible metal leaching and outstanding recyclability, which are highly favorable for the practical application in the Fenton process.