Reactive ceramics of CeO2–MOx (M=Mn, Fe, Ni, Cu) for H2 generation by two-step water splitting using concentrated solar thermal energy

The addition of MOx (M: di- or tri-valent transition metal ion) into cerium dioxide (CeO2) enhanced the ability of CeO2 for the oxygen (O2)-releasing reaction at lower temperature and swift hydrogen (H2)-generation reaction. CeO2–MOx (M=Mn, Fe, Ni, Cu) reactive ceramics having high melting points were synthesized with the combustion method from their nitrates for solar H2 production. The prepared CeO2–MOx samples were solid solutions between CeO2 and MOx with the fluorite structure through the X-ray diffractometry measurement. Two-step water-splitting reactions with CeO2–MOx reactive ceramics proceeded at 1573–1773 K for the O2-releasing step and at 1273 K for the H2-generation step by irradiation of infrared image furnace as a solar simulator. The amounts of O2 evolved in the O2-releasing reaction with CeO2–MOx increased with an increase in the reaction temperature. The amounts of H2 evolved in the H2-generation reaction with CeO2–MOx systems except for M=Cu were more than that of CeO2 system after the O2-releasing reaction at the temperatures of 1673 and 1773 K. The amounts of H2 evolved in the H2-generation reaction with CeO2–MnO and CeO2–NiO systems were more than those of CeO2–Fe2O3, CeO2–CuO and CeO2 systems after the O2-releasing reaction at the temperature of 1573 K. The amounts of evolved H2 after the O2-releasing reaction at the temperature of 1773 K in cm3 per gram of CeO2–MOx were 0.975–3.77 cm3/g. The O2-releasing reaction at 1673 K and H2-generation reaction at 1273 K with CeO2–Fe2O3 proceeded with repetition of 4 times stoichiometrically.