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

A series of CoO, Co(0.7)5Cu(0.25)O, Co0.5Cu0.5O 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, Co0.5Cu0.5O 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 Co0.5Cu0.5O 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 degrees C and pH = 9, the Co0.5Cu0.5O 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 Co0.5Cu0.O-5 catalyst. The Co0.5Cu0.5O catalyst showed negligible metal leaching and outstanding recyclability, which are highly favorable for the practical application in the Fenton process.