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

The addition of MO x (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-MO x (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-MO x samples were solid solutions between CeO2 and MO x with the fluorite structure through the X-ray diffractometry measurement. Two-step water-splitting reactions with CeO2-MO x 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-MO x increased with an increase in the reaction temperature. The amounts of H2 evolved in the H2-generation reaction with CeO2-MO x 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-MO x 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