Perovskites oxides

Many perovskite-structured oxides exhibit high oxide-ion conductivities at elevated temperatures, and have attracted significant interest for use as sohd electrolytes in, for example, SOFCs (see Chapters 9, 12 and 13). The compounds can be divided into camps with compositions or A + B + O3, of which LaGaO3 and 

---

Catalysts include oxides, mixed oxides (perovskites) and zeolites [3]. The latter, transition metal ion-exchanged systems, have been shown to exhibit high activities for the decomposition reaction [4-9]. Most studies deal with Fe-zeolites [5-8,10,11], but also Co- and Cu-systems exhibit high activities [4,5]. Especially ZSM-5 catalysts are quite active [3]. Detailed kinetic studies, and those accounting for the influence of other components that may be present, like O2, H2O, NO and SO2, have hardly been reported. For Fe-zeolites mainly a first order in N2O and a zero order in O2 is reported [7,8], although also a positive influence of O2 has been found [11]. Mechanistic studies mainly concern Fe-systems, too [5,7,8,10]. Generally, the reaction can be described by an oxidation of active sites, followed by a removal of the deposited oxygen, either by N2O itself or by recombination, eqs. (2)-(4). 

Ciambelli, P Cimino, S Lisi, L Faticanti, M Minelli, G Pettiti, 1 Porta, P. La, Ca and Fe oxide perovskites preparation, characterization and catalytic properties for methane combustion. Appl Catal, B Environmental, 2001, Volume 33, Issue 3, 193-203. 

Belessi, VC Costa, CN Bakas, TV Anastasiadou, T Pomonis, PJ Efstathiou, AM. Catalytic behavior of La-Sr-Ce-Fe-0 mixed oxidic/perovskitic systems for the NO+CO and NO+CH4+O2 (lean-NOx) reactions. Catal. Today, 2000, Volume 59, Issues 3-4, 347-363. 

There exists quite a number of hexagonal oxidic perovskites 183, 332), but there seem to be only three types in the case of ternary fluorides. Their occurrence again clearly depends on the tolerance factor wich thus proves to be useful in classifying the hexagonal perovskites also. After having described their structures in detail they will be further discussed under a common point of view. 

The structure of the hexagonal oxide perovskite BaRuOa, recently described by Donohue et al. 84), is also adapted by the ternary fluoride CsCoFs 11). The positional parameters (not listed above) are almost the same in both compounds. 

Blasse (37) on the other hand found linear superexchange effective along Me—O—A—0—Me and distances of about 8 A in oxide perovskites. 

Conducting polymers (PANI, PPy and others as well as manganous oxides, perovskite and some other materials are used as non-noble metal catalysts. 

For oxygen reduction N4-organic metal complexes and conducting polymers, transition metals oxides, perovskites, etc. 

Mixed oxides perovskite type catalytic combustion... 

Oxygen bacancies are commonly encountered in oxide perovskites. In AB03 unlike in W03, and Ti02 crystallographic shear planes are not found. Instead, a variety of superstructures are seen due to the ordering of vacancies. The brownmillerite phase of... 

A typical powder diffraction apparatus is shown in Fig. 14. The desirable requirements of high beam intensity and high resolution are rather incompatible and conventional instruments often have fairly poor resolution. This is advantageous in experiments where peaks are well separated as count rates can be high, and was the situation for example, in the measurements on Mn2+ and Ni2+ rock-salt compounds (56, 61) and Cr2+, Fe + and Mn + oxide perovskites (62), where only a few low-angle peaks were measured. High resolution is essential in profile analysis refinement, however, or when many intensities must be measured to... 

Some dense inorganic membranes made of metals and metal oxides are oxygen specific. Notable ones include silver, zirconia stabilized by yttria or calcia, lead oxide, perovskite-type oxides and some mixed oxides such as yttria stabilized titania-zirconia. Their usage as a membrane reactor is profiled in Table 8.4 for a number of reactions decomposition of carbon dioxide to form carbon monoxide and oxygen, oxidation of ammonia to nitrogen and nitrous oxide, oxidation of methane to syngas and oxidative coupling of methane to form C2 hydrocarbons, and oxidation of other hydrocarbons such as ethylene, methanol, ethanol, propylene and butene. 

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 ... 

It is generally held that metal oxide perovskites with extrinsic oxygen vacancies react with atmospheric water. This entrains hydrogen into the lattice and leads to their significant proton conducting properties. The location of these hydrogen atoms was studied as a function of a series of dopants in a cerium based ceramic. The dopants were niobium, holmium... 

Fig. 15. Activities of first-row transition-metal oxide perovskites for CO oxidation in a 2 1 mixture of CO and 02 at atmospheric pressure (a) or in a 1 1 mixture of CO and 02 at 227°C at atmospheric pressure (b). The activities of vanadates ( ), chromates ( ), manga-nates (A), ferrates (O), cobaltates ( ), and nickelates ( ) are plotted at the appropriate d-orbital occupation corresponding to the average valence of the transition-metal ion. (Redrawn by permission from Refs. 14 and 176.)...

---