XAFS and XRD analysis of ceria-zirconia oxygen storage promoters for automotive catalysts

Three types of CeO2-ZrO2 (Ce:Zr = 1:1 molar ratio) compounds with different oxygen storage/release capacity (OSC) were characterized by means of the Ce K-edge and Zr K-edge XAFS. In order to investigate the relationship between the OSC and local structure, the quantitative EXAFS curve-fitting analysis was applied. By enhancing the homogeneity of the Ce and Zr atoms in the CeO2-ZrO2 solid solution, the OSC performance increased. Additionally, from the XRD analysis, the homogeneous CeO2-ZrO2 solid solution has an ordered cation arrangement, and exhibits the highest OSC. The crystal structure of this CeO2-ZrO2 solid solution is usually termed as kappa-CeZrO4 phase". However, the OSC performance of kappa-CeZrO4 degrades upon a high-temperature treatment under an oxidative atmosphere. The fresh kappa-CeZrO4 was aged at 973, 1,273 and 1,473 K under an oxidative atmosphere, respectively. The OSC performance deteriorated as: the fresh sample approximate to 973 > 1,273 > 1,473 K-aged samples. We also found that, if the temperature was beyond 1,273 K, the Ce/Zr ordered arrangement would collapse and the local structure around Ce and Zr ions would also changed remarkably. These results indicated that OSC was strongly dependent on its atomic structure.