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 CeO₂ whose vapour pressure is considerably higher than that of ZrO₂. As a consequence surface segregation of zirconia occurs in (Ce,Zr)O_2−δ pellets leading to a porous surface zone of Ce₂Zr₂O₇ pyrochlore which gradually grows in thickness. Surface enrichment of zirconia is detrimental for splitting CO₂ or H₂O since re-oxidation temperatures of (Ce,Zr)O_2−δ are known to be shifted towards lower temperatures with increasing ZrO₂ 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/(cm³·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.

The development of the oxide scale on model Fe20Cr5Al-type alloys unmodified and containing implanted yttrium was studied in oxygen-dominated atmosphere at 1,100 °C for up to 1 h. A two-stage-oxidation exposure was applied with the use of ¹⁸O₂ as a tracer. The choice of the exposure durations ensured the possibility to follow the consecutive stages of scale development. The oxidized samples were characterized using SEM (morphology); PLS (phase composition), and SIMS (elemental distributions). The obtained results are discussed in terms of the mechanism of the development of protective α-Al₂O₃ scale and the effect of the additions on this process taking into account the necessity of distinguishing the mechanism and kinetics of the scale evolution. Similar scale evolution stages were found on both studied materials and in both cases the protective α-Al₂O₃ scale developed rapidly, already after the exposure for 3 min. Implanted yttrium appeared to have a negligible effect on the evolution of the scale. It only slightly retarded the evolution which can be attributed rather to a kinetic effect than to mechanistic one. However, the mechanical failure of the scales via formation of cracks at the asperities of convolutions occurred on the yttrium-implanted alloy but not on the non-implanted one.

The surface exchange and bulk transport of oxygen are highly relevant to ceria-based redox materials, which are envisaged for the solar thermochemical splitting of carbon dioxide in the future. Experimental investigations of oxygen isotope exchange on CeO_2-δ, Ce_0.9M³⁺_0.1O_1.95-δ (with M³⁺ = Y, Sm) and Ce_0.9M⁴⁺_0.1O_2-δ (with M⁴⁺ = Zr) samples were carried out for the first time utilizing oxygen-isotope-enriched C¹⁸O₂ gas atmospheres as the tracer source, followed by Secondary Ion Mass Spectrometry (SIMS), at the temperature range 300 ≤ T ≤ 800 °C. The experimental ${\tilde{K}}_{\mathrm{O}}$ and ${\tilde{D}}_{\mathrm{O}}$ data reveal promising results in terms of CO₂ splitting when trivalent (especially Sm)-doped ceria is employed. The reaction temperatures are lower than previously proposed/reported due to the weak temperature dependency of the parameters ${\tilde{K}}_{\mathrm{O}}$ and ${\tilde{D}}_{\mathrm{O}}$. The majority of isotope exchange experiments show higher values of ${\tilde{K}}_{\mathrm{O}}$ and ${\tilde{D}}_{\mathrm{O}}$ for Sm-doped cerium dioxide in comparison to Y-doped and Zr-doped ceria, as well as nominally undoped ceria. The apparent activation energies for both ${\tilde{K}}_{\mathrm{O}}$ and ${\tilde{D}}_{\mathrm{O}}$ are lowest for Sm-doped ceria. Using Zr-doped cerium oxide exhibits various negative aspects. The Zr-doping of ceria enhances the reducibility, but the possible Zr-based surface alteration effects and dopant-induced migration barrier enhancement in Zr-doped ceria are detrimental to surface exchange and oxygen diffusion at lower temperatures of T ≤ 800 °C.

The authors would like to add Stefan Beschnitt, who performed SIMS line scans, to the authors’ list in 5th position of their paper published in Inorganics