Photodegradation of methylene blue and methyl orange with CuO supported on ZnO photocatalysts: The effect of copper loading and reaction temperature

Copper oxide supported on zinc oxide catalysts was tested for the photocatalytic degradation of methylene blue (MB) and methyl orange (MO) dyes under UV irradiation at 25, 35 and 45 °C. These Cu/ZnO catalysts with variable copper loadings (2.5, 5 and 7.5 wt%) were prepared by impregnation and characterized by atomic absorption spectroscopy (AAS), X-ray diffraction (XRD), N₂ adsorption–desorption, transmission electron microscopy (TEM), diffuse reflectance UV–visible absorption spectroscopy, and X-ray photoelectron spectroscopy (XPS). XRD and XPS data revealed that copper was in the form of copper oxide (CuO), and the crystal size was 240.8 and 49.3 nm for ZnO and CuO, respectively. Experiments showed that xCu/ZnO catalysts were highly active for MB and MO degradation compared with the zinc oxide supports due to the presence of copper oxide acting as an electron trap inhibiting electron-hole recombination. Furthermore, photodegradation activity was improved with increasing temperature. The 5Cu/ZnO catalyst exhibited greater degradation activity in both dyes, reaching 99% degradation at 45 °C after 15 and 75 min for MB and MO, respectively, due to its greater dispersion of CuO in ZnO and higher concentration of hydroxyl groups on the surface compared with other catalysts. Thermodynamic calculations indicated that the interaction of MB with the surface was stronger than that of MO and possibly related to the presence of hydroxyl groups on the surface, as indicated in the XPS results. The effects of different scavengers of the main species involved, such as hydroxyl radicals, superoxide radical anions and positive holes, were studied to propose photodegradation mechanisms of the dyes on these systems. The superoxide radical was the main reactive species during the photocatalytic degradation of MB and MO.