Evidence of entropy effects and changes in re-oxidation behavior of Ceria-Zirconia solutions as function of thermal pre-treatment

Kurzfassung

The utilization of clean and sustainable energy becomes important for solving global environmental problems and securing future energy supply. A possible method for producing fuels is a two-step water splitting reaction in a solar thermo-chemical cycle. Especially doped and undoped CeO2 are considered as promising redox materials for thermochemical purposes [1]. So redox kinetics of pure CeO2-d and Ce0.85Zr0.15O2-d, synthesized via Pechini method [2] and thermally pre-treated at 1473K and 1923K, respectively, have been investigated in view of their applicability as redox material for thermochemical hydrogen production. Reduction values d of thermal pre-treated samples were determined by thermal analyses as function of temperature and pO2. With these data redox-enthalpies and entropies were identified. The investigation indicates that the redox behavior of Ce0.85Zr0.15O2-d mixed oxide is influenced by high-temperature pre-treatment at ambient pressure conditions. Both, enthalpy and entropy of redox-reaction are found to be affected by thermal pre-treatment apart from the fact that Zr addition anyway affects the redox thermodynamics of CeO2. In particular, the redox entropy of higher pre-treated (1923K) samples is strikingly reduced. This finding is explained by a decrease of configuration entropy, which in turn is attributed to a pyrochlore-based superstructure. Furthermore, the oxygen vacancy-stabilizing pyrochlore-type structure counteracts the re-oxidation and water splitting behavior. So Ce0.85Zr0.15O2 samples pre-treated at 1923K need temperatures above 700K to re-oxidize in air and are not able to split water in contrast to samples pre-treated at 1473K/1573K which re-oxidize in air already at 400K and could be re-oxidized by water vapour.