Identiﬁcation of the Structural Phases of Ce x Zr 1-x O 2 by Eu(III) Luminescence Studies Tiziano Montini, † Adolfo Speghini,* ,‡ Loredana De Rogatis, † Barbara Lorenzut, † Marco Bettinelli, § Mauro Graziani, † and Paolo Fornasiero* ,† Chemistry Department, ICCOM-CNR, INSTM, Center of Excellence for Nanostructured Materials (CENMAT), UniVersity of Trieste, Via L. Giorgieri 1, 34127 Trieste, Italy, DiSTeMeV and INSTM, UniVersity of Verona, Via della PieVe 70, 37029 San Floriano, Verona, Italy, and Laboratory of Solid State Chemistry, Department of Biotechnology, INSTM, UniVersity of Verona, Strada Le Grazie 15, 37128 Verona, Italy Received June 23, 2009; E-mail: pfornasiero@units.it; adolfo.speghini@univr.it Abstract: Despite the wide application of ceria-zirconia based materials in Three Way Catalysts (TWCs), Solid Oxides Fuel Cells (SOFCs), and H 2 production and puriﬁcation reactions, an active debate is still open on the correlation between their structure and redox/catalytic performances. Existing reports support the need of either (i) a homogeneous solid solution or (ii) materials with nanoscale heterogeneity to obtain high activity and stability. Here we report on a simple and inexpensive approach to solve this problem taking advantage of the luminescence properties of Eu(III), used as a structural probe introduced either in the bulk or on the surface of the samples. In this way, the real structure of ceria-zirconia materials can be revealed even for amorphous high surface area samples. Formation of small domains is observed in catalytically important metastable samples which appear homogeneous by conventional XRD. Introduction Ceria (CeO 2 ) based systems are extensively investigated due to their wide applications in materials science. In fact ceria is able to easily form oxygen vacancies releasing surface and lattice oxygen. 1 Due to the high mobility of their lattice oxygen ions, CeO 2 -based materials are widely used as promoters in Three Way Catalysts (TWCs) 2 and appear to be promising as active supports/cocatalysts for reforming 3,4 and hydrogen puriﬁcation reactions, 5,6 as well as electrolyte or electrode promoters in SOFC fabrication. 7,8 Doped with Zr(IV), CeO 2 acquires a particular thermal stability and excellent redox properties, forcing enormous and still growing attention to these materials. Ce x Zr 1-x O 2 solid solutions can be prepared with particular attention to their morphology, to maximize their performance as catalyst supports. 9,10 Ce x Zr 1-x O 2 can exist in three stable phases (monoclinic (m), tetragonal (t), cubic (c)) and in various metastable phases (t′, t′′, κ, and t*) depending on the composition and preparation conditions. 11-14 Oxygen deﬁcient structures, such as pyrochlore, may also be formed (either in the bulk 13,15 or as nuclei on the surface 16,17 ), and they have been related with the low reduction temperature of Ce x Zr 1-x O 2 . 18 Due to the complexity of the phase diagram, a great effort has been devoted to verify the phase homogeneity of these materials. Conventional techniques, such as XRD and Raman spectroscopy, fail in the investigation of the samples with low crystallinity which, however, are of major interest in catalysis. 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