Ceria-based solid solutions

The introduction of isovalent and aliovalent elements into a ceria lattice can induce important modifications of its textural, structural and redox properties and this can deeply affect its catalytic properties. Ceria-zirconia still represents the best example showing how the incorporation of an isovalent element like Zr into the ceria structure can enhance the thermal, redox and catalytic properties. The state-of-the-art material for oxygen storage in TWC is in fact appropriate amounts of zirconia-modified ceria mixed oxides including small amounts of other rare-earth elements, which are added to further enhance the redox and textural features of ceria.  

The introduction of Zr (and in general the substitution of Ce for other transition metals and rare-earth elements) induces structural modifications into the fluorite lattice of which is 

A better synergism of all these factors can be achieved with ternary oxides/ based mainly on Ce02-Zr02-M0 (M = Pr, Y, La,. ..), which can give near-optimal reactivity for oxygen storage and release when coupled with higher thermal resistance. 

In several studies the key roles of surface area and oxygen storage capacity have been taken into account but their relative importance is still a matter of discussion. In a recent study we investigated in more detail the activity of a series of ceria-zirconia-based catalysts.Ceria-zirconia solid solutions over the whole 

Therefore, in order to enhance activity, it is very important to maximize the number of active surface oxygen species, but oxygen storage is important because it provides an alternative route for the oxidation of large soot particles in contact with ceria or it can provide oxygen for oxidation in a rich environment. 

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Another way to decrease the anodic overpotential is to intercalate a mixed conductor between the yttria stabilized zirconia electrolyte and the metallic anode. Such a combination enlarges the reaction area which theoretically lowers the anodic overpotential. Tedmon et al. [93] pointed out a significant decrease of polarization when ceria-based solid solutions like (Ce02)o.6 (LaO, 5)04 are used as anode materials for SOFCs. This effect is generally attributed to the mixed conductivity resulting from the partial reduction of Ce4+ to Ce3+ in the reducing fuel atmosphere. A similar behaviour was observed in water vapor electrolysis at high temperature when the surface zirconia electrolyte is doped with ceria [94, 95]. 

K. El Adham and A. Hammou [1983] Grain Boundary Effect on Ceria-Based Solid Solutions, Solid State Ionics 9-10, 905-912. 

Adham, K. E., and Hammou, H. (1983). Grain boundary effect on ceria based solid solutions. Solid State Ionics 9-10 905-912. 

Dudek, M., Rapacz-Kmita, A., Mroczkowska, M., Mosialek, M., and Mordarski, G. (2010) Co-doped ceria-based solid solution in the Ce02-M203-CaO, M=Sm, Gd system. Electrochim. Acta, 55 (14), 4387 394. 

This review is organized in four main sections the first section discusses bare ceria systems, in the second section ceria-based solid solutions are reviewed, while the third part covers other metal/ metal oxide doped cerias finally, before the concluding section, soot oxidation in an Oa/NO atmosphere is summarized. 

A shortcoming of the interlayer is that doped ceria reacts easily with stabilized zir-conia, forming (Zr,Ce)02-based solid solutions [206], The product is formed at the... 

Ceria affords a number of important applications, such as catalysts in redox reactions (Kaspar et al., 1999, 2000 Trovarelli, 2002), electrode and electrolyte materials in fuel cells, optical films, polishing materials, and gas sensors. In order to improve the performance and/or stability of ceria materials, the doped materials, solid solutions and composites based on ceria are fabricated. For example, the ceria-zirconia solid solution is used in the three way catalyst, rare earth (such as Sm, Gd, or Y) doped ceria is used in solid state fuel cells, and ceria-noble metal or ceria-metal oxide composite catalysts are used for water-gas-shift (WGS) reaction and selective CO oxidation. 

Related systems It should be noted that specific properties for applications could be enhanced by using solid solutions, doped materials, and composites, instead of pure ceria. For example, ceria-zirconia solid solution is a well known ceria based material for enhanced OSC and high ionic conductivity for solid state fuel cell components. It is also used in the three way catalysts for automobile waste gas cleaning, because of the improved thermal stability, surface area, and reducibility. The synthesis, structure, and properties of ceria-zirconia have been actively studied for a long time. Di Monte and Kaspar et al. presented feature articles on the nanostructured ceria—zirconia-mixed oxides. The studies on phase, structures, as well as the microstructures are discussed and reviewed (Di Monte et al., 2004). 

The defective structure in nanocrystalline ceria based catalysts proved to have strong effect on the OSC. Mamontov et al. (2000) reported the neutron diffraction studies of the atomic structures of nanocrystalline powder of ceria and ceria-zirconia solid solution. They found that the concentration of vacancy-interstitial oxygen defects has a direct correlation with the OSC. This effect is stronger than the correlation of surface area with OSC. Zirconia reduces ceria and preserves oxygen defects to retard the degradation of ceria-zirconia in OSC. Yan et al. observed the strong correlation between OSC and the lattice strain in nanosized ceria-zirconia, which could be measured via XRD (Si et al., 2004 Figure 11). 

The acid-base properties of ceria-zirconia solid solutions [82] and ceria-lanthana co-precipitated mixed oxides have been investigated by Cutrufello and... 

The cubic pyrochlore-based ceria-zirconia solid solution is a good material. However, it has been reported that the reduction temperature has become higher after the high-temperature oxidation.This is due to the transformation from pyrochlore-based-cubic to r -tetragonal phase which is more stable than it, and r ... 

Recently one of the solutions to overcome this problem has been proposed.This does concern surface modification of the pyrochlore-based oxide.s. It is known that cerium and zirconium chlorides provide vapor phase complexes with aluminum chloride at elevated temperatures.The new surface modification technique utilizes the formation of these vapor complexes to remove and modify the top surface of the pyrochlore ceria-zirconia solid solution. This method is named "chemical filing". Application of the above complexes formation has already been demonstrated for the vapor phase extraction and mutual separation of rare earths based on the so-called chemical vapor transport (CVT). ... 

Development of a modified co-precipitation route for thermally resistant, high surface area ceria-zirconia based solid solutions... 

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. 

Ceria-based catalysts [219] and Ce02 Zr02 solid solution catalysts [220] have each been reported to be very efficient catalysts for the synthesis of EC and PC by reaction of C02 with ethene glycol and propene glycol, respectively. The catalytic activity has been shown to depend heavily on the composition and calcination temperature of the catalysts. 

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