Redox studies on dense zirconia-doped ceria pellets were carried out by thermogravimetric investigations and dilatometry. Up to 1600 K reduction parameters determined by both methods correspond to each other. At higher temperatures, however, thermogravimetry overestimates the degree of reduction since mass loss is not only due to oxygen exsolution but also to selective evaporation of CeO
2 whose vapour pressure is considerably higher than that of ZrO
2. As a consequence surface segregation of zirconia occurs in (Ce,Zr)O
2−δ pellets leading to a porous surface zone of Ce
2Zr
2O
7 pyrochlore which gradually grows in thickness. Surface enrichment of zirconia is detrimental for splitting CO
2 or H
2O since re-oxidation temperatures of (Ce,Zr)O
2−δ are known to be shifted towards lower temperatures with increasing ZrO
2 content. Thus, very harsh reduction conditions should be avoided for the (Ce,Zr)O
2−δ redox system. The kinetics investigations comprised the high temperature reduction step (
T ≅ 1600 K) and the “low” temperature oxidation reaction with a carbon dioxide atmosphere (
T ≅ 1000 K). The reduction kinetics (at around 1600 K and an oxygen activity of 7 × 10
−4 in the gas phase) directly yield the (reduction) equilibrium exchange rate of oxygen in the order of 10
−7 mol·O/(cm
3·s) as the kinetics are surface controlled. The oxidation step at around 1000 K, however, occurs in the mixed control or in the diffusion control regime, respectively. From oxygen isotope exchange in combination with SIMS depth profiling oxygen exchange coefficients,
K, and oxygen diffusivities,
D, were determined for so-called equilibrium experiments as well as for non-equilibrium measurements. From the obtained values for
K and
D the (oxidation) equilibrium exchange rates for differently doped ceria samples were determined. Their dependency on the oxygen activity and the nature and the concentrations of a tetravalent dopant (Zr) and trivalent dopants (La, Y, Sm) could be semi-quantitatively rationalised on the basis of a master equation for the equilibrium surface exchange rate.
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