In-situ Methods to Determine High Oxygen Deficits in Ceria-Zirconia Mixed Oxides for Exhaust Gas Aftertreatment

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Besides its good catalytic properties, ceria provides a high oxygen storage capacity and is, therefore, highly suited for the application in storage catalysts and steam reformers. To in-crease the stability, ceria-zirconia mixed oxides are used. A comparatively high oxygen defi-ciency can be established, which has only been studied with indirect or elaborated laboratory methods. To make the above-mentioned systems more effective, the oxygen deficiency of ceria-zirconia mixed oxides must be monitored in situ. At this point, resonant nano balances and microwave-based approaches represent promising opportunities. The latter are already studied in automotive exhausts without, however, detailed knowledge of the fundamental materials properties. Further, the defect chemical mechanisms of the ceria-zirconia mixed oxides must be better understood, especially for high oxygen deficiencies.Therefore, the aim of this project is to provide from a materials science point of view the re-quirements for in situ analysis of the oxygen deficiency in ceria-zirconia mixed oxides of se-lected compositions. For this purpose, a direct measurement of the oxygen deficiency by means of a resonant high-temperature nano scale (TU Clausthal) and an indirect microwave-based method (Univ. Bayreuth) are envisaged. The influence of the electrical and mechani-cal properties of mixed oxides on frequencies and damping is used. The measurements are performed at high temperatures that allow a fast adjustment of the defect equilibrium.The preliminary work confirms, in principle, the suitability of the proposed methods to investi-gate the oxygen deficiency. Besides the development of the new in situ methods, the stability limits of the highly oxygen deficient mixed oxides should be determined by simultaneous ap-plication of conventional methods. The properties of interest include the defect and transport mechanisms as well as the effect of sample morphology. Further, the measurement of other thermo physical properties of the mixed oxides such as chemical expansion is anticipated in order to evaluate the mechanical stability. Finally, the microwave dielectric properties of the ceria-zirconia mixed oxides should be determined and included in a model that describes the microwave properties of three-way-catalysts under operation.The scientific approach includes the preparation of common samples and their investigation in both laboratories. Furthermore, the data should be correlated and included in a common model. Thereby, the project takes advantage of the complementary competencies of both partners. Partial redundancy of the data improves their reliability. The expected results broaden the understanding of the defect chemistry of highly oxygen deficient ceria-zirconia mixed oxides and enable the in situ monitoring of related high-temperature systems.