Thermodynamics and kinetics of CO2 splitting with the redox system Ce3+/Ce4+ at high temperatures

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For the reduction of CO2 to CO, (solar) thermal cycles based on the Ce+3/Ce4+ redox system are well suited. The information on the thermodynamics of pure cerium oxide (CeO2) and, especially, on the thermodynamics of solid solutions with homovalent (Zr) oxides or oxides with lower cation valences (Gd, Sm, Y, Ca, Mg, ...), albeit extensive, is partially incomplete and contradictory and comprises primarily only the dependency of the oxygen stoichiometry on temperature, oxygen partial pressure and dopants. With respect to the kinetics of the surface exchange of oxygen in the global CO2 splitting reaction at the CeO2 surface, as well as for the oxygen transport in the (doped/undoped) CeO2 matrix information is even more scarce. Therefore, the aim of the concerted action is to provide reliable thermodynamic information on the solid solutions with potential technological relevance, and to experimentally quantify the dependency of the surface exchange coefficient K, the diffusion coefficient D and the equilibrium exchange rate of oxygen, R°, on the process variables (temperature, oxygen potential or CO2/CO ratio, kind and concentration of dopants) of the CO2 splitting process, and to present a consistent model. To this purpose, a model published recently by the applicants shall be expanded. This model quantitatively describes the relation between K, D and R°.In order to gain the necessary experimental data two complementary experimental approaches are combined in this concerted action, if possible on the same samples. The first method is the exchange of rare stable isotopes (13C, 18O) in combination with SIMS depth profiling. The second method comprises the time dependent gravimetric and dilatometric thermal analysis in order to detect the relaxation kinetics of bulk samples after an oxygen potential change.The data gained from these complementary experiments will be checked for consistency with the above mentioned model. They will yield K, D and the equilibrium exchange rate of oxygen, R°, at the respective surface. The envisaged concerted action will enable the gathering of the information needed to understand and apply CO2 splitting with the redox system Ce+3 /Ce4+ in technologically relevant solid solutions. Thus, it will simultaneously supply the scientific basis for a technological realization. This implies that the results can be principally transferred to other material systems. Further, the present state of the phenomenological kinetic model developed by the applicants supports the expectation that the model will also enable to quantitatively describe other reaction systems where a fluid and a solid phase exchange a common component.