Ceria oxidation

The redox properties of ceria-zirconia mixed oxides are interesting, because these materials find applications as electrolytes for solid oxide fuel cells, supports for catalysts for H2 production, and components in three-way automobile exhaust conversion catalysts. The group of Kaspar and Fornasiero (Montini et al., 2004, 2005) used TPR/TPO-Raman spectroscopy to identify the structural features of more easily reducible zirconia-ceria oxides and the best method for their preparation by suitable treatments. TPR/TPO experiments and Raman spectra recorded during redox cycles demonstrated that a pyrochlore-type cation ordering in Ce2Zr2Og facilitates low temperature reduction. 

Figure 9,5. Spectra demonstrate three independem methods to determine ceria oxidation state. Spectra from oxidized (black) and highly reduced (gray) ceria are shown.

We prepared by sol-gel some thermally stable Zro.io(Cei-xPrx)o.9o02 mixed oxides (x between 0 and 0.75) with a fluorite-type structure. This structure was confirmed by the presence, in the Raman spectrum, of a single band ca. 460 cm characteristic of the M-0 vibration in the fluorite-type structure. Moreover, the band position and the shoulder at 570 cm indicate the presence of oxygen vacancies probably associated with praseodymium cations. Consequently, high OSCs appears to be the result of the presence of both cerium and praseodymium atoms. Thus, addition of praseodymium atoms into zirconia-ceria oxides appears to be very promising for the design of new automotive catalysts. 

The hydrogen chemisorption on two Rh/Ce02 catalysts prepared respectively from rhodium nitrate and rhodium chloride has been studied. The evolution of the ceria oxidation state with the series of treatments applied was monitored by means of a magnetic balance. Temperature Programmed Reduction and Oxidation (TPR/TPO) as well as Volumetric Adsorption techniques have also been used. 

Recently, combining ceramics with molten salts has been of growing interest for innovative high-temperature fuel cell applications. In the last ten years, doped-ceria oxides mixed with molten salts, such as chlorides, fluorides, carbonates and sulphates (Zhu and Mat, 2006 Di et al, 2010 Lapa et al, 2010 ... 

Raman spectra were recorded with a HORIBA Raman or T-64000 spectrometer equipped with an optical alignment and monochromator with AABSPEC 2000-A. The excitation source was a 532 nm SHG of Nd YVO4 laser with a power of 20 mW at the sample point. In order to check the stability of each sample, differential thermal analysis (DTA) measurements were carried out with Rigaku Thermo Plus under CO2 gas during all the measurements. Aliquots of 10 mg of ceria oxides/carbonate salts were loaded into an Au pan ( = 5 mm). The temperature range was 573-873 K and the scanning rate was 10 K/min in all cases. 

DeCaluwe SC, Grass ME, Zhang CJ, El Gabaly F, Bluhm H, Liu Z, Jackson GS, McDaniel AH, McCarty KF, Farrow RL, Linne MA, Hussain Z, Eichhorn BW (2010) In situ characterization of ceria oxidation states in high-temperature electrochemical cells with ambient pressure XPS. J Phys Chem C 114 19853-19861... 

A modification of the pseudo-sol-gel method based on the thermal decomposition of metallic propionates, was used to synthesize ceria oxides doped with 10 mol% of Zr, Zn, and Fe. Briefly, calculated amounts of Ge(lll) acetate, Zr(lV), Zn(ll), and Fe(III) acetyl acetonates were dissolved in propionic acid. After mixing by stirring, the propionic acid excess was evaporated until a kind of resin was obtained that was calcined at 500 °G for 2h. The formation of a solid solution was observed after doping with Zr and Fe, and there was ZnO surface segregation with Zn. It is interesting to consider the differing effects of the dopants. Doping with Zr enhances the... 

PdO(lOO). The theoretical study was in agreement with experimental studies establishing the importance of the Pd-O-Ce linkages and more generally oxidized Pd species in dictating the catalytic activity of Pd/ceria. The model illustrated how mixed noble-metal-ceria oxides could provide enhanced activity for hydrocarbon oxidation by tuning the reducibility of the surface. ... 

In the present work, we report the synthesis of nanosized ceria oxides, starting from cerium nitrate and cerinm chloride, as precursors and using a high surface area silica xerogel, as template. After calcination of the template material impregnated with the... 

Table 1. Structural properties of silica xerogel and synthesized ceria oxides.

Fig. 1. XRD patterns in the angular range 20-50 26 for ceria oxides obtained through template method.

Fig.2. Nitrogen adsorption/desorption isotherms for ceria oxides.

Fig. 4. Hj-TPR profiles for ceria oxides obtained through templatemethod.

When ceria oxides are used in anode materials in eombination with zireonia-based electrolytes, the calcination temperatures should be set as low as possible to prevent the formation of ceria-zirconia solid solutions which have a lower oxide ion conductivity. The detailed properties of those solid solutions are discussed in section 3.7 concerning the ehemieal stability of SOFC materials. 

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