Cerium ion redox system in CeO₂–xFe₂O₃ solid solution at high temperatures (1,273–1,673 K) in the two-step water-splitting reaction for solar H₂ generation

CeO₂–xFe₂O₃ (x = 0.026–0.214) solid solutions with different Ce:Fe mole ratios (Ce:Fe = 9.5:0.5–7.0:3.0) were prepared as reactive ceramics with the combustion method for solar hydrogen production. The prepared CeO₂–xFe₂O₃ solid solutions were characterized by X-ray diffractometry, ICP atomic emission spectrometry, and Mössbauer spectroscopy. Two-step water-splitting reaction with the CeO₂–xFe₂O₃ solid solution proceeded at 1,673 K for the O₂-releasing reaction and at 1,273 K for the H₂-generation reaction by irradiation of an infrared imaging lamp as a solar simulator. The amounts of H₂ gas evolved in the H₂-generation reaction with CeO₂–xFe₂O₃ solid solutions were 0.97–1.8 cm³/g, the evolved H₂/O₂ ratio was approximately equal to 2 of the stoichiometric value. The amounts of H₂ and O₂ gases were independent of the Ce:Fe mole ratio in the CeO₂–xFe₂O₃ solid solution. It was suggested that the O₂-releasing and H₂-generation reactions with the CeO₂–xFe₂O₃ solid solution were repeated with the reduction and oxidation of Ce⁴⁺–Ce³⁺ enhanced by the presence of Fe³⁺–Fe²⁺.