Local structure investigation in ceria-zirconia "k -phase" by XAFS: unraveling the reason for outstanding oxygen storage capacity

Ceria zirconia solid solutions (CexZryO2, CZ) are well known for buffering oxygen due to fast and easy switch between the oxidation states Ce3+/Ce4+ [1], showing remarkable oxygen storage capacity (OSC), and, thanks to the presence of the zirconium, increased thermal stability with respect to pure ceria. Hence, they are used as catalyst carrier in three way catalysts (TWC) also for their high surface carrier density and as oxygen release material in oxygen deficient atmospheres. In addition to the known catalytic properties of ceria, the mixed oxide system has proven to improve the catalytic activity in HCl oxidation (Deacon process)[2] with respect to pure CeO2. Particularly interesting among the ceria zirconia solutions is the so-called "?-phase". This is known to increase the OSC since its high degree of homogeneity, without local enrichments of Ce or Zr, allows oxygen atoms to rapidly move into and out of the lattice. In addition, the ordered arrangement of the Ce and Zr atoms along the <110> direction limits reduction-induced lattice stress. The exact role of the ZrO2 in the increase of the OSC, on the other hand, is still debated[3]. The OSC is strongly dependent on the atomic structure of the ceria-zirconia solutions, and it is therefore necessary to have a thorough description of the structural features of these systems. In this regard, X-ray absorption fine structure (XAFS) is the optimal choice to investigate the local structure around both the Ce and Zr cations and determine the cation-cation network and the oxygen environment around these. In this framework, we investigate the structural evolution of a series of ceria zirconia solutions characterized by different synthetic approaches and/or thermal history, with the aim of finding the optimal conditions for the easy preparation of the desired "?-phase". This evolution was followed by means of both XRD and XAFS measurements, evidencing how the correct thermal treatment is necessary to obtain a homogenous solid solution. References [1] Di Monte R, Kaspar J Catal. Today 2005, 100, 27 [2] Trovarelli A, Fornasiero P Eds. Catalysis by Ceria and Related Materials, 2nd Edition, Imperial College Press, London, 2013 [3] Nagai Y, Yamamoto T, Tanaka T, Yoshida S, Nonaka T, Okamoto T, Suda A, Sugiura M Top Catal. 2008, 47, 137