Structure and Properties of Perovskite Oxides

La2Cu04, Sr2Cu04. As we show in chapter 6, when a perovskite forms a composite or intergrowth with other structures, new compounds of interest in catalysis can be formed (such as in high-temperature superconducting copper oxides) and EM is used to determine the structures and properties of these complex compounds. The merits of using perovskites in steam reforming, membrane catalysis and fuel cells are discussed in chapter 6. 

Many of the new tasks would be at the boundary with materials science. There are some that are obviously applications-oriented, like the electronic theory of high temperature superconduction in the layered copper-oxide perovskites, and other aspects of nanotechnology. There are also fundamental valence problems, such as accounting for the structures and properties of quasiciystals. Why is the association of transition metals and aluminium apparently of central importance How do we deal with the valence properties of systems where the free energy of formation or phase transition is dominated by the entropy term ... 

Besides the ionic radii requirements, the other condition to be fulfilled is electroneutrality, i.e., that the sum of charges of A and B ions equals the total charge of X anions. This is attained in the case of oxides by means of charge distribution of the form Al+B5+03, A2+B4+03, or A3+B3+03. Moreover, partial substitution of A and B ions giving rise to complex oxides is possible while keeping the perovskite structure. Figure 3, elaborated from some comprehensive compilations of data on the structure and properties of this type of compound (4,15,17,21,22), shows that almost all the stable elements have been included in the perovskite framework, many of them in both the A and B positions. In what follows we will... 

It is far beyond the scope of this chapter to review the electronic structures and properties of all metal oxides, or even all of the important metal oxide stmc-ture types. Instead, this section covers some featnres of one stmctural family, perovskite, in some detail. In doing so, it is hoped that the important concepts will be illnstrated in snch a way that they can be widely appUed. Of course, the choice of the perovskite stmctnre as an illnstrative example is not a random choice. The perovskite family of componnds is very extensive, encompassing most of the periodic table. Fnrthermore, perovskites exhibit nearly every type of interesting electronic or magnetic behavior seen in oxides (ferromagnetism, ferroelectricity, piezoelectricity, nonlinear optical behavior, metaUic condnctivity, snpercondnct-ivity, colossal magnetoresistance, ionic conductivity, photoluminescence, etc.). One important property that is not readily found among perovskites, transparent conductivity, is the focus of Section 6.7. 

Search for HMF and HMAFM properties in other classes of materials led to the emergence of double perovskite oxides as potential candidates. Computational searches based on density functional theory predicted several new double perovskites with HMAFM properties (details on structure and properties of double perovskites are in Section 5.5). 

Because of the variety of structures and chemical compositions, perovskite oxides exhibit a large variety of properties. Well-known properties of the perovskite oxides are ferroelectricity in BaTiOs-based oxides and superconductivity in Ba2YCu307, etc. In addition to these well-known properties, several perovskite oxides exhibit good electrical conductivity, which is are close to that of metals, and ionic conductivity, as well as mixed ionic and electronic conductivity. Based on these variations in electrical conducting property, perovskite oxides are chosen as the components for SOFC. It is also well known that several perovskite oxides exhibit high catalytic activity with respect to various reactions, in particular, oxidation reactions [10]. Table 1.2 provides examples of the typical properties of perovskite oxides. In this section, several typical properties of the perovskite oxides, namely, ferroelectricity, magnetism, superconductivity, and catalytic activity, are briefly discussed. 

The previous discussion has focused on the properties of perovskite materials rather than on their performance as anodes. The number of actual fuel-cell studies is more limited, but this literature has been reviewed recently by Irvine. Various perovskites have been investigated as potential SOFC anode materials however, these early efforts were hampered by low electrochemical activity toward methane oxidation,poor anode structure,or insufficient electrode conductivity. Most recently, Tao and Irvine demonstrated that an anode based on (Lao.75Sro.25)o.9Cro.5Mno.503 can provide reasonable power densities at 1173 K in 3% humidified CH4. Barnett and co-workers also reported stable power generation with methane and propane fuels on an anode based on LaCr03 however, they reported that the addition of Ni, in levels too small to affect the conductivity, was crucial in providing activity for the electrochemical oxidation reactions. 

The special electric, magnetic, optical, superconductive and catalytic properties of perovskite-typed oxides make this group of materials attracting and widely used. Perovskites were named according to the similarity of their structure with the CaTiOs compoimd. The... 

In recent years, research on catalysts for the ATR of hydrocarbons has paid considerable attention to perovskite systems of general formula ABO3. In the perovskite stmcture, both A and B ions can be partially substituted, leading to a wide variety of mixed oxides, characterized by structural and electronic defects. The oxidation activity of perovskites has been ascribed to ionic conductivity, oxygen mobility within the lattice [64], reducibility and oxygen sorption properties [65, 66]. 

Five aspects of the preparation of solids can be distinguished (i) preparation of a series of compounds in order to investigate a specific property, as exemplified by a series of perovskite oxides to examine their electrical properties or by a series of spinel ferrites to screen their magnetic properties (ii) preparation of unknown members of a structurally related class of solids to extend (or extrapolate) structure-property relations, as exemplified by the synthesis of layered chalcogenides and their intercalates or derivatives of TTF-TCNQ to study their superconductivity (iii) synthesis of a new class of compounds (e.g. sialons, (Si, Al)3(0, N)4, or doped polyacetylenes), with novel structural properties (iv) preparation of known solids of prescribed specifications (crystallinity, shape, purity, etc.) as in the case of crystals of Si, III-V compounds and... 

Helmut Ullmann, Nikolai Trofimenko, Composition, structure and transport properties of perovskite-type oxides , Solid State Ionics 119,1-8 (1999). 

The most numerous and most interesting compounds with the perovskite structure are oxides. Some hydrides, carbides, halides, and nitrides also crystallize with this structure (4). This review will refer only to the study of oxides and their behavior in the gas-solid interface and in heterogeneous catalysis. It will not cover, however, electric, magnetic, and optical properties of perovskites. Comprehensive studies on these... 

In Section II the perovskite and related structures are briefly introduced. In Section III the methods most frequently used for perovskites preparation are described comparatively. Sections IV, V, and VI refer to the bulk and surface properties of perovskites. Some of these properties will facilitate understanding of the catalytic action of these compounds. Section VII includes a review of the reactions where perovskite oxides were used as catalysts. Some of them are described separately (Sections VII,A-H). These include reactions that were more extensively studied or reactions that may have an increasing interest in the near future. Section VII,I includes less studied reactions such as oxygen homomolecular exchange, hydrogen and NH3 oxidations, N20 decomposition and dehydro-... 

Since 1970 perovskite-type oxides (ABO3) have been suggested as substitutes for noble metals in automotive exhaust catalysts [1]. These oxides are efficient for oxidation reactions when for reduction the results obtained from the literature are dissimilar [2], mainly due to huge differences in the experimental conditions. The properties of perovskite-based catalysts are a flmction of the spin and the valence state of the metal in the B site cation, which is surrounded octahedrally by oxygen. The A site cation is located in the cavity made by these octahedra. For some perovskite-type oxides, their electronic structures have been pointed out to be similar to those of transition metals on the basis of theoretical... 

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