Spectroscopic insight into the interplay between structural disorder and oxidation degree in melt‐grown Ce_0.5Zr_0.5O_2‐y compounds

Abstract

The oxygen storage capacity of CexZr1‐xO2‐y compounds arises from the mixed‐valent character of cerium, which makes the system sensitive to external factors such as temperature and oxygen partial pressure through their influence in the Ce valence, cation ordering and oxygen content. In this work, we analyze the phase relations in Ce0.5Zr0.5O2‐y compounds fabricated by the laser floating zone technique in either oxidising or reducing atmosphere. Chemical, structural, and microstructural properties are studied by X‐ray diffraction, thermogravimetry, scanning electron microscopy (energy dispersive X‐ray spectroscopy and electron backscatter diffraction), and Raman spectroscopy. The latter proves to be determinant to discriminate between phases arising from subtle variations of atomic distribution that are hardly distinguished by X‐ray diffraction or energy dispersive X‐ray analyses. Cation disordered, fluorite‐like phases are obtained when processing is performed in air, whereas cation‐ordered, pyrochlore‐related phases are formed when processing in Ar+5%H2 atmosphere. Processing in a low but non‐negligible pO2 yields, besides the pyrochlore and fluorite‐like phases, an intermediate state with partial disorder that may be relevant for catalysis applications. The vibrational Raman spectrum of the Ce2Zr2O7 pyrochlore has been fully interpreted through polarisation measurements at room and low temperature. Bands arising from crystal field transitions within the ground and first excited multiplets of Ce3+ ions are also identified. The electronic Raman spectrum is found to be a sensitive probe of the degree of cation and oxygen order in these compounds.

Effect of the Structural and Electronic Properties of Rh/CeXZr1-XO2 Catalysts on the Low-temperature Ethanol Steam-reforming

“…2a that this band decreased in intensity and widened as Zr content was increased in the bare CexZr1-xO2-A oxides. Furthermore, the same band shifted for the respective Ce0.75Zr0.25O2-A (459→468 cm -1 ) and Ce0.5Zr0.5O2-A (459→466 cm -1 ) materials due to a change in their cell parameters [32].…”

“…Therefore, that band could be attributed to the O atoms' displacement from its ideal position in the fluorite structure [33]. The band located at 570 cm -1 has been ascribed to O vacancies [32] and can barely be noticed in the supports. Furthermore, a broadening of the main band for the CexZr1-xO2-A mixed samples indicates a less ordered structure than pure CeO2-A.…”

“…2). The main band located among 400-500 cm -1 corresponds to the symmetric F2g vibrational mode of the Ce-O8 unit, where only the O atoms exhibit vibrational modes [30][31][32]. This band is highly sensitive to any displacement at the oxygen sub-lattice caused by stoichiometry changes, composition, or crystallite size.…”

“…CeO2 and CexZr1-xO2 mixed oxides have been recognized as supports for noble metals on ESR and some other oxidation reactions since they possess excellent redox properties and high oxygen mobility [18][19][20]. The oxygen mobility can promote the oxidation of CO2 and CHx adsorbed species while activating water to regenerate the surface of the support and release H2 [21][22][23][24][25][26][27][28][29][30][31][32][33][34]. Studies have exhibited that the addition of ZrO2 to CeO2 could improve its redox properties and increase its oxygen storage capacity and thermal stability when used as catalyst support [23][24][25].…”

Emerging disorder in Gd2(Ti1−xZrx)2O7 pyrochlores matrices for radioactive waste disposal: symmetry lowering versus defect clustering

“…The Anti-Frenkel defect leaves the O1 site depleted, affecting the A 1g component of band M5 ultimately leading to a decrease in intensity of M5 (ref. 91, 105 and 106) (see Fig. 3c).…”