Reactivity and XAFS study on (1−x)CeO2–xNiO (x=0.025–0.3) system in the two-step water-splitting reaction for solar H2 production

The redox reaction of Ce4+–Ce3+ promoted by the catalytic function of nickel ions in a (1−x)CeO2–xNiO solid solution was investigated for solar H2 production by the two-step water-splitting reaction. By irradiation using an infrared imaging lamp as a solar simulator, the O2-releasing reaction with (1−x)CeO2–xNiO solid solution proceeded at 1673–1873 K, and its reduced form was produced. The amounts of H2 gas evolved by the reduced form were 1.2–2.5 cm3/g and the evolved gases amounts ratio of H2/O2 was nearly 2, which is equal to the stoichiometric value of the water-splitting reaction (H2O=H2+1/2O2). The maximum amounts of evolved H2 and O2 gases were obtained at the Ce:Ni mole ratio of 0.95:0.05 (x=0.05) in the (1−x)CeO2–xNiO system. The X-ray absorption fine structure (XAFS) measurement showed that the O2-releasing and H2-generation reactions with (1−x)CeO2–xNiO solid solution were repeatable with the redox system of Ce4+–Ce3+, which was enhanced by the catalytic function of Ni2+–Ni0.