Defect structure and transport properties of ceria-zirconia-based oxides
March 10-14, 2019
CeO2-ZrO2 based oxides (CZ) showing excellent oxygen storage capacity (OSC) are widely used in three-way catalysts. To further improve their storage capacity and kinetics at low temperatures, defect structural analysis appears to play an important role. In this study, 89Y NMR spectroscopy is performed to probe the preference sites of oxygen vacancies in CZ under not only oxidizing but also reducing atmospheres. Figure 1 shows the 89Y NMR spectra taken for (Ce1-xZrx)0.8Y0.2O2-d annealed under (a) air and (b) Ar-5%H2 atmospheres. 7- and 8-coordinated environments are clearly observed for both cases. Based on their integrated intensities as a function of Zr content (Fig. 1 (c)), the oxygen vacancies appear to favor Zr rather than Y and Ce; the same trend has been reported for various fluorite-type oxides [1, 2]. This suggests that doping of smaller cations is effective to enhance their OSC. Furthermore, OSC and kinetics of CoFe2O4-added CZ at around 400°C are evaluated. The spinel-type oxides such as CoFe2O4 has been well known to enhance oxygen transport properties of ceria-based oxides [3, 4]. In this study, CoFe2O4-added CZ were prepared by the Pechini and solid-state reaction methods. 5 vol% CoFe2O4-added Ce0.5Zr0.5O2-d shows higher OSC and faster kinetics at 400°C than Ce0.5Zr0.5O2-d itself. The microstructure including the distribution of CoFe2O4 was analyzed by TEM; their morphology was found to strongly depend on the volume fraction of CoFe2O4 and fabrication techniques. Their surface exchange kinetics is also discussed based on pulse isotope exchange results for powder samples.
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Hitoshi Takamura, Akihiro Fujimaki, Itaru Oikawa, and Yoko Sugawara, "Defect structure and transport properties of ceria-zirconia-based oxides" in "Nonstoichiometric Compounds VII", ECI Symposium Series, (2019). https://dc.engconfintl.org/nonstoichiometric_vii/30