Photocatalytic Splitting Of Water Into Hydrogen Under Visible Light Irradiation

Energy becomes one of the most important problems facing human beings with the rapid development of population and economy. Solar energy is enormous and belongs to renewable energy. Hydrogen is an ideal energy carrier for its high energy density, and the combustion of hydrogen is pollution-free for the environment. Photocatalysis is an advanced potential technique to convert solar energy into chemical energy and has many advantages in energy conversion. Photocatalytic water splitting to generate hydrogen has attracted much attention, and is developing rapidly.In the present work, CaCr2O4, Cr-Fe/TiO2, calcium ferrite compounds were synthesized by a sol-gel method and Cu2O was prepared by the hydrolysis of CuCl. The catalysts obtained were characterized by TGA, DTA, UV-Vis DRS,XRD, FT-IR, XPS etc., and studied in the splitting of water into hydrogen under visible light irradiation. Calcium ferrite compounds which showed higher photocatalytic activity were studied particularly. The influences of photocatalysts and reaction conditions were investigated. The photocatalytic reaction mechanism was also discussed.The results showed that H2 could be generated over Pt/CaCr2O4 from starch solution; Cr-Fe co-doped TiO2 had good response to visible light and had good photocatalytic activity in H2 evolution from methanol solution; Cu2O could split pure water into hydrogen and the activity could be increased by promotion of N-doping; 1.5 wt% NiO/Ca2Fe2O5 showed the highest photocatalytic activity among the photocatalysts studied.Calcium Ferrite compounds showed the highest photocatalytic activity among the ferrite salts. It was found that brownmillerite Ca2Fe2O5 was the main component, independent of the molar ratios of Ca/Fe. The brownmillerite has a perovskite structure with oxygen vacancies. The oxygen vacancies in Ca2Fe2O5 were the absorbing sites of the active oxygen. According to the results of XPS analysis, the content of adsorbed oxygen in NiO/Ca2Fe2O5 powder increased after reactions. The oxygen produced in the photocatalytic reactions adsorbed by Ca2Fe2O5 in the reaction. Addition of NaHCO3 in the water markedly increased the activity. The optimal activity was obtained from 0.272 mol/L ~1.09 mol/L NaHCO3 solution. HCO3? consumed the photogenerated hole and enhanced the photocatalytic activity. NiO/Ca2Fe2O5 had higher photocatalytic activity in lower pH. After reactions, NiO/Ca2Fe2O5 changed into CaCO3 which resulted in the decrease of photocatalytic activity.Fe and Fe3O4 formed from the H2 reduction of Fe2O3 favored the transfer of electron to surface and played an important role in the H2 generation. The rate of H2 generation increased drastically when the reaction system was bubbled by CO2. CO32? formed from HCO3? was photocatalytically reduced to formic acid over reduced Fe2O3 under visible light. CoO doped Fe2O3 had long-term photocatalytic stability. In the continuous flow reaction system, the generated hydrogen is removed from the system, favoring the evolution of H2. IR results showed that carbonate salts formed after reactions.