Highly Stabilized and Finely Dispersed Cu₂O/TiO₂: A Promising Visible Sensitive Photocatalyst for Continuous Production of Hydrogen from Glycerol:Water Mixtures

Cu-doped TiO2 with varying amounts of Cu (0.2, 0.3, 0.5, 1, 2, and 5) are prepared by impregnation method and calcined at 350 and 450 °C for 5 h. These catalysts are characterized by X-ray diffraction, diffuse reflectance spectroscopy (DRS), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy energy-dispersive X-ray spectroscopy (EDAX), and transmission electron microscopy (TEM). The DRS studies are clearly showing the expanded photo response of TiO2 into the visible region on impregnation of copper ions. TEM images are depicting the fine dispersion of Cu particles on TiO2 surface. XPS studies are showing change in the binding energy values of Ti 2p, O 1s, and Cu 2p, indicating that copper ions are in interaction with TiO2. XPS results are also confirming that the oxidation state of copper is +2 in samples calcined at 350 °C and +1 in samples calcined at 450 °C. EDAX analysis supports the presence of copper species on the surface layers of TiO2. Photocatalytic hydrogen production activity studies are conducted over CuO/TiO2 and Cu2O/TiO2 catalysts in pure water and glycerol:water mixtures under solar irradiation. Maximum hydrogen production of 265 and 290 μmol h−1 is observed over 2 wt % CuO/TiO2 and Cu2O/TiO2 catalysts in pure water. A significant improvement in hydrogen production is observed in glycerol:water mixtures and maximum hydrogen production of 16,500 and 20,060 μmol h−1 is obtained over 0.5 wt % CuO/TiO2 and Cu2O/TiO2 catalysts in 5% glycerol aqueous solutions. No hydrogen production activity is observed on reduced catalysts under solar irradiation. Furthermore, when these catalysts are studied under UV irradiation, 2−3 fold increase in activity is observed on calcined catalysts, and the same level of activity is observed on reduced catalysts, but under these conditions the activity is limited by the dissolution of Cu ions into the solution. However, under solar irradiation a continuous and stable activity is observed over Cu2O/TiO2 catalyst. On the basis of the characterization and hydrogen production activity results, finely dispersed Cu in +1 oxidation state that is in interaction with TiO2 is proposed as a promising visible sensitive photocatalyst for the continuous production of hydrogen from glycerol:water mixtures.