Enhanced hydrogen production by Rh-Cu supported on TiO2 by the photocatalytic process

Hydrogen production by the photocatalytic process on three distinct photocatalysts containing Rh, Cu, and Rh-Cu supported on TiO₂ synthesized by the sol–gel method is reported. The addition of Rh, Cu, and Rh-Cu onto TiO₂ was performed by the deposition–precipitation with urea method. Methanol, ethanol, isopropanol, and glycerol were used as sacrificial agents in a mixture with water. Hydrogen production over the photocatalysts pretreated under reduced and oxidized conditions was analyzed. The impact of the thermal treatment and the photocatalyst mass is also reported. Photocatalysts were characterized by S_BET, UV–vis, XRD, HAADF-STEM-EDS, H₂-TPR, and XPS spectroscopy.

It was found that the 1Rh-1Cu/TiO₂ photocatalyst produced the largest amount of hydrogen (1,212 µmol/h) with respect to the 1Rh/TiO₂ (363 µmol/h) and 1Cu/TiO₂ (340 µmol/h) photocatalysts. Methanol was the best sacrificial agent for producing hydrogen, followed by ethanol, isopropanol, and glycerol. The photocatalysts pretreated under reducing conditions boosted hydrogen production in comparison with the one pretreated under oxidizing conditions. The presence of Rh⁰ and Cu⁰ species enhanced the hydrogen evolution with respect to the oxidized species (Rh³⁺ and Cu²⁺). The formation of reduced Ti³⁺ species seemed to have no effect on hydrogen production under UV light irradiation. A red shift of the band gap energy was observed in the presence of reduced species. Under UV irradiation, the oxygen vacancies worked as recombination centers for the photo-induced electrons and holes, which hindered hydrogen production. By means of visible light, oxygen vacancies could act as electron donors, facilitating the charge transport and separation, which improved the catalytic activity.