Renewable energy technologies are crucial in view of global warming and limited fossil fuel resources. One promising solution for the environmental-friendly production of essential components (H2, CO) for synthetic fuels is the application of thermochemical cycles with concentrated solar energy as heat source. The core of such thermochemical process is a redox material which allows water or carbon dioxide splitting at feasible temperature conditions. Cerium oxide is a very attractive redox material for such thermochemical cycles due to its redox thermodynamics and fast redox kinetics. In the first endothermic step of a ceria-based thermochemical cycle, CeO2 is partially reduced to CeO2−δ. In the second step Ce3+ re-oxidizes exothermically to Ce4+ in the presence of H2O/CO2 to produce H2/CO.
The oxidation kinetics comprising the “low” temperature oxidation step of such a thermochemical process with carbon dioxide atmosphere were studied on reduced nominally undoped dense ceria pellets as well as on trivalent and tetravalent doped ceria samples in the temperature range between 300 and 900 °C, where the oxidation step occurs in the mixed control or in the diffusion control regime, respectively. From oxygen isotope exchange in combination with SIMS depth profiling and line scan technique oxygen exchange coefficients, K, and oxygen diffusivities, D, as well as the apparent activation enthalpies were determined for so-called non-equilibrium measurements (µO2 (solid) ≠ µO2 (gas)) and compared to data obtained from equilibrium experiments (µO2 (solid) = µO2 (gas)). From the obtained values for K and D the (oxidation) equilibrium surface exchange rates for the ceria samples were determined and differences regarding the gas atmosphere (carbon dioxide vs. pure oxygen) and the different dopants were discussed.