Particular perovskite structure, oxides

Various strategies were developed in the past for the synthesis of perovskite-structured oxides (Table 3.1). Of these, the choice of a particular method depends on the type of application expected. For catalytic applications, specific surfece area and crystal structure play crucial roles. Hence, the synthesis of these materials for catalytic applications always focused on obtaining crystalline materials with high values of specific surface area. The oldest method for the synthesis of perovskite-structured mixed metal oxides is the ceramic method. In this method, thoroughly mixed precursors (oxides, hydroxides, or carbonates) of the metals are calcined at elevated temperatures (>800 °C) for several hours. The surfece area of thus synthesized perovskites was, however, found to be less than 5m /g [5,30]. The high temperature used in solid-state reactions, for perovskite crystallization, results in the sintering of particles, which in turn leads to a large... 

Good results are obtained with oxide-coated valve metals as anode materials. These electrically conducting ceramic coatings of p-conducting spinel-ferrite (e.g., cobalt, nickel and lithium ferrites) have very low consumption rates. Lithium ferrite has proved particularly effective because it possesses excellent adhesion on titanium and niobium [26]. In addition, doping the perovskite structure with monovalent lithium ions provides good electrical conductivity for anodic reactions. Anodes produced in this way are distributed under the trade name Lida [27]. The consumption rate in seawater is given as 10 g A ar and in fresh water is... 

The relatively high cost and lack of domestic supply of noble metals has spurred considerable efforts toward the development of nonnoble metal catalysts for automobile exhaust control. A very large number of base metal oxides and mixtures of oxides have been considered, especially the transition metals, such as copper, chromium, nickel, manganese, cobalt vanadium, and iron. Particularly prominent are the copper chromites, which are mixtures of the oxides of copper and chromium, with various promoters added. These materials are active in the oxidation of CO and hydrocarbons, as well as in the reduction of NO in the presence of CO (55-59). Rare earth oxides, such as lanthanum cobaltate and lanthanum lead manganite with Perovskite structure, have been investigated for CO oxidation, but have not been tested and shown to be sufficiently active under realistic and demanding conditions (60-63). Hopcalities are out-... 

In this chapter the technological development in cathode materials, particularly the advances being made in the material s composition, fabrication, microstructure optimization, electrocatalytic activity, and stability of perovskite-based cathodes will be reviewed. The emphasis will be on the defect structure, conductivity, thermal expansion coefficient, and electrocatalytic activity of the extensively studied man-ganite-, cobaltite-, and ferrite-based perovskites. Alterative mixed ionic and electronic conducting perovskite-related oxides are discussed in relation to their potential application as cathodes for ITSOFCs. The interfacial reaction and compatibility of the perovskite-based cathode materials with electrolyte and metallic interconnect is also examined. Finally the degradation and performance stability of cathodes under SOFC operating conditions are described. 

The incorporation of Cu ions in the perovskite structure is known for only a few examples since this particular structure is normally stabilized by or requires a B atom in a high formal oxidation state such as Ti4+ in BaTiOs, or Rhs+ in LaRhOs. Further, since Cu can not be readily stabilized in its Cu(m) state, and is unknown in the tetravalent state, the simple formation of ternary compounds such as LaCuOg or BaCuOs is not expected. Even in the K2NiF4 structure, the stabilization of Cu4+ as in Ba2Cu04 is not expected, but the formation of a stable Cu(II) state is a distinct possibility, as in La2Cu04. Copper(II), however, has been introduced in the doubled-or tripled-perovskite structure. Examples of these, which include structural distortions from cubic symmetry, are listed ... 

The calculations also suggest that the electropositive cations (alkaline earth or rare earth metals) have little influence on the electronic structure near the Fermi level. I believe that their role is two fold. First, they help "enforce" a particular structure that is, the large cations are responsible for the compounds adopting the perovskite structure. Second, the electropositive metals effectively increase the oxidizing power of... 

Catalysts with perovskitic structure guarantee a good compromise between stability and activity and have a relatively low cost, so they can constitute a valid alternative to supported noble metals, with particular reference to the reactions of partial or total oxidation of hydrocarbons (catalytic combustion). The traditional route used to synthesize perovskites was first introduced by Delmon and co-workers in the late nineteen sixties [1]. It enables to obtain i) mixed oxides over a wide range of composition ii) good control of the stoichiometry iii) an excellent interspersion of the elements in the final product iv) very small grain size materials. 

A class of oxide catalysts which have been employed for combustion reactions, particularly hydrocarbon combustion are oxides with the perovskite structure, possessing the general formula ABO3 [60]. The activities of several unsubstituted component B oxides (BO3) have been compared with perovskite oxides for the catalytic oxidation of propylene [61], this is shown in figure 5. 

Despite its simplicity, the tolerance factor has reasonable predictive power, especially for oxides, where ionic radii are known with greatest precision. Ideally t should be equal to 1.0 and it has been found empirically that if t lies in the approximate range 0.9-1.0 a cubic perovskite structure is a reasonable possibility. If t>l, that is, large A and small B, a hexagonal packing of the AXj layers is preferred and hexagonal phases of the BaNiOj type form (Chapter 3). In cases where t of the order of 0.71-0.9, the structure, particularly the octahedral framework, distorts to close down the cuboctahedral coordination polyhedron, which results in a crystal structure of lower symmetry than cubic. For even lower values of t, the A and B cations are of similar size and are associated with the ilmenite, FeTiOj, structure or the C-type rare earth Ln Oj stmcture. 

The perovskite structure (ABO3) offers au opportunity for a material scientist to selectively substitute either the A or the B ion by introducing isovalent or aliova-lent cations. The compound (La, Sr)(Mg, Ga)03 (LSMG) has been developed as an oxide ion conductor. The use of LSMG is attractive because it has reasonable oxide ion conductivity and is compatible with a variety of cathodes, in particular the highly active ones. Other interesting materials, such as Bi4V20n (BIMEVOX (bismuth metal vanadium oxide)), have also been mentioned in the literature. 

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