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+.