| Environment-friendly and high efficient hydrogen is the most potential alternative to fossil fuels. Using solar energy to produce hydrogen is an ideal way. Compared with direct water-splitting at a high temperature, the thermochemical cycle that comprises multi-step reactions allows for a much lower temperature and minimizes the problem of H2 and O2 separation. The two-step thermochemical cycle based on metal oxide redox pairs is one of the most widely studied cycles. It consists of two steps:The first step is the metal oxide reduction at high temperatures, this step may use concentrated solar energy as the heat source. The second step is H2O dissociation through the reduced metal oxide or metal generated from the first step at relatively low temperatures, metal oxide is then recycled back to the first step. Thus, the net reaction is H2O splitting into H2 and O2, and separation of H2 and O2 is skillfully avoided. The biggest challenge facing this cycle is the extremely high temperature needed for the reduction of metal oxide, typically above 1500 ℃ for single metal oxide. Doping other metal ions to form mixed metal oxide is the most widely used method to overcome this problem currently. However, but temperatures above 1200 ℃ are still required to acquire a reasonable amount of hydrogen.Our research group introduced a photochemical reaction into the thermochemical cycle and established a novel photo-thermochemical cycle of water-splitting for hydrogen production. In this new cycle, the process by which metal oxides are reduced through concentrated solar energy is replaced with a photochemical reaction while water is still dissociated via a thermochemical reaction. Thus, photo-thermochemical cycles that combine these two reactions can be initiated at relatively low temperatures.TiO2 was first used as the circulating material of photo-thermochemical cycle. Through a series of comparative experiments, the feasibility of the cycle was preliminary proved. The influence of irradiation time, heating time and temperature on hydrogen production was studied. Five successive cycles were repeated, the exposure time for the photochemical reaction was 40 min, and the heating time for the thermochemical reaction was 1 h at 600 ℃. The TiO2 sample exhibited good cyclability. An average amount of 0.421 mL/g hydrogen was generated and the amount of hydrogen generated during cycles was stable. A preliminary mechanism for the cycle is established according to the XPS and EPR results. Crystal structure and specific surface area were compared before and after cycles, no obvious change was observed, this may the reason for its good cyclability.0.5% Fe was doped into TiO2, and it was compared with pure TiO2 through TEM, SEM, UV-VIS and PL. The results showed that:0.5% Fe doped TiO2 particles was more disperse through TEM and SEM; the absorption spectra of 0.5% Fe doped TiO2 were broadened and the absorptivity was promoted in the range of 300-500 nm through UV-VIS; the recombination rate of electron hole pair was lowered through PL. The influence of irradiation time, heating time and temperature on hydrogen production was studied. Five successive cycles was repeated with the exposure time 30 min and heating time 1 h at 600 ℃, it showed that the 0.5% Fe doped TiO2 also showed a good cyclability, an average amount of 0.747 mL/g hydrogen was generated, which is 1.77 times that of TiO2. |
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