Composites ceria-based

The performance of acceptor-doped ceria membranes, composite ceria-based membranes, and membranes with redox active dopants was reviewed in Sections 12.6.4.2, 12.6.4.3, 12.6.4.4, and 12.6.4.5, respectively. Variations in sample geometry, membrane thickness, as well as test conditions make comparison of performance somewhat difficult. Also for some studies, catalysts have been added to the surface of the samples to enhance the flux, whereas in other studies the as-prepared surface is used in the experiments. However, in particular, studies of thin-film membranes, both the... 

A marked effect of the Ce02/Zr02 composition (in samples containing 40 wt.% NiO) on the catalytic activity was noticed. The catalysts with Ce Zr =1 1 (6A) were not only more active (than 7A and 8A) but were also stable during the reaction. Sample 8A containing no zirconia in the support showed a low activity. The NiO crystallite size (Table 11.2) in these compositions varied in the order 7A < 6A < 8A. It may be recalled that on ceria-based catalysts the crystallite size of nickel metal was similar to that of NiO. The higher activity for 6A than 7A indicates that in addition to accessibility of... 

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. 

In the past decade, controlled synthesis of ceria based nanomaterials, such as obtaining the pure phase, doping for desirable composition, controlling uniform size, shape, and nanostructure, tuning surfaces, fabricating composites, assemblies and mesostructures, have been the targets of active research since ceria-based materials exhibits unique properties when their sizes are reduced to nanometer scale. 

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). 

Ceria-based systems showed mixed effects for methane oxidation. Composite catalysts of Ag/Ce02 fall apart, forming large silver metal aggregates and deactivating the catalyst system (38). The only system in which silver-modified ceria found any promise is in solid oxide fuel cells utilizing yttira-stabilized zirconia however, the silver-based system was not the optimum one in this case (39). 

TPR is a routinaiy technique in the redox characterization of M/Ce02 and related catalysts. It has been very extensively used in comparative studies aimed at establishing the influence of variables like the chemical composition (281,341,343,344,350-353), or the high temperature ageing treatments (187,288,337,347,354), on the reducibility of ceria-based mixed oxides, both in the presence and in the absence of a supported noble metal. 

When water salts solutions are used in OPCM synthesis, provided the Me CA ratio is equal to 1 6 in all the composition range studied here (up to Zr relative content equal to 0.9), single phase samples with a cubic fluorite-like structure are formed. Thus, for ceria-zirconia samples with zirconia content up to 50 mol. %, ceria-based cubic solid solution is formed. For those samples, the lattice parameter is decreased as compared with that of a pure ceria phase, due to cerium cations substitution by zirconium cations with a smaller... 

Ceria first, and since the mid 1990s ceria-zirconia mixed oxides, are key components in the formulation of TWCs [18,19]. A variety of functions are attributed to them, those related to their redox properties being particularly relevant [247]. Under the usual TWC operation conditions, the chemical composition of the exhaust gases rapidly oscillates between net reducing and net oxidizing conditions [8]. This implies deviations from the optimum stoichiometric air to fuel (A/F) ratio (A/F = 14.63), and therefore, loss of efficiency in the auto-exhaust catalyst. To attenuate these oscillations, oxygen buffer materials are required, ceria-based oxides constituting the best option at present available [19]. 

Lapa C M, Figueiredo F M L, de Souza D P F, Song L and Marques F M B (2010), Synthesis and characterization of composite electrolytes based on samaria-doped ceria and Na/Li carbonates , Int J Hydrogen Energy, 35, 2953-2957. 

To further increase the oxygen flux density of 7O2, laminated membrane stmctures were examined. From our previous work, it is found that higher 7O2 can be obtained with the use of CoFe204 (CFO) instead of MFO as an electronic conductive phase however, ceria-based composites with CFO are not chemically stable enough under a methane conversion atmosphere. Based on these observations, laminated membrane stmctures, in which CSO—30CFO layers were laminated on CSO—15MFO layers, were fabricated (Takamura et al., 2006). Prior to the lamination process. 

Takamura, H., Kobayashi, T., Kasahara, T., Kamegawa, A., Okada, M. (2006). Oxygen permeation and methane reforming properties of ceria-based composite membranes. Journal of Alloys and Compounds, 408—412, 1084. 

SOFC anodes are fabricated from composite powdered mixtures of electrol5de materials (YSZ, GDC, or SDC) and nickel oxide. The nickel oxide is subsequently reduced to nickel metal prior to operation. The NiO/YSZ anode material is suited for applications with YSZ material, whereas NiO/SDC and NiO/GDC anode materials are best used with ceria-based electrolyte materials. Typical anode materials have nickel content of approximately 40 volume percent after reduction of the nickel oxide to nickel. Depending upon the application, powders have surface areas of 15 to 20 m /g for screen-printing and 5 to 10 m /g for tape casting. 

Lohsoontorn P, Brett DJL, Brandon NP (2008) Thermodynamic predictions of the impact of fuel composition on the propensity of sulphur to interact with Ni and ceria-based anodes for solid oxide fuel cells. J Power Sources 175 60-67... 

For the particular cases of zirconia- and ceria-based electrolytes, the ionic conductivity is mostly related to dopant nature, composition, microstructure, local structure, impurity and processing, and so on [19,20]. In many cases, the... 

In a review by Hui et al. (2007), various approaches to enhancing the ionic conductivity of zirconia- and ceria-based solid electrolytes in the light of composition, microstructure, and processing are described. 

Cerium exhibits both the +4 and +3 oxidation states. The change of oxidation state between +4 and +3 is the key for the catalysis of ceria-based materials. Thus, the phase diagram of the Ce-O system is important for ceria-based catalysts. The Ce-O system contains a number of crystalline phases (Fig. 1.1). Table 1.1 shows some of the phases and their space groups in the Ce-O system. There is a disagreement in the literature concerning both the composition and the structure of intermediate phases. 

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