O2-releasing reactivity of ceria-based reactive ceramics on irradiation of artificial concentrated solar beam for solar hydrogen production

• Enhancement of O2 release on CeO2–MOx (M = Sc, Y, Dy, Zr or Hf) is observed at 1500 °C.
• Promotion effect is due to the crystal-chemical effect and ionicity of M–O bond.
• XRD and TG techniques were employed to reveal the O2 release reactivity.
• Ce0.9Zr0.1O2 or Ce0.9Hf0.1O2 was promising material for solar hydrogen production.

In the present paper, different types of metal doped ceria CeO2–MOx (M = Sc, Y, Dy, Zr and Hf) with fluorite structure have been prepared with a complex polymerization method. The O2-releasing potential of prepared samples was evaluated at 1500 °C for a two-step water splitting process. The partially oxygen defected CeO2–MO1.5 (M = Sc3+, Y3+ or Dy3+) mixed oxide exhibited an enhancement of the O2 evolution on CeO2–ScO1.5 owing to the crystal-chemical effect that cation with smaller ionic radius (Sc3+) compensates the volume expansion of crystal lattice resulted in the reduction of Ce4+ with smaller ionic radius into Ce3+ with larger ionic radius. The CeO2–MO2 (M = Zr4+ and Hf4+) produced larger amount of O2 than that for CeO2–MO1.5 mixed oxides, since CeO2–MO2 has no oxygen vacancy initially. While the ionic radii of Zr4+ and Hf4+ are almost equal, the higher O2-releasing reactivity of Ce0.9Hf0.1O2 than that of Ce0.9Zr0.1O2 is explained in terms of ionicity of M–O bond. Also, the weight losses of CeO2, Ce0.9Zr0.1O2 and Ce0.9Hf0.1O2 in the mitigation process from high PO2PO2 (O2 gas stream) to low PO2PO2 (Ar gas stream) atmosphere were determined. The quasi activation energies in the initial period of the O2-release for CeO2, Ce0.9Zr0.1O2 and Ce0.9Hf0.1O2 were evaluated 105, 84.2 and 75.0 kJ/mol, respectively. The O2-releasing reactivity is considered from the standpoint of an activation energy.