Perovskite-based compounds

Ordering of vacancies also plays a key role in selective oxidation catalysis over perovskite-based catalysts such as CaMnOs oxides. CaMnOs has a CaTiOs (AMO3) perovskite structure which is made up of cations coordinated to 12 0 anions. They, in turn, are connected to corner-sharing MoOe octahedra. CaMnOs was used as a model catalyst on a laboratory scale by Thomas et al (1982) in propene oxidation to benzene and 2-methyl propene to paraxylene. In such reactions the compounds are found to undergo reduction to form anion-deficient metastable phases of the type CaMnOs-x where 0 < x < 0.5, forming several distinct phases. 

The large number of compounds found with perovskite based structures arises from several factors other than the great stability implied by the permitted variation in t and the subsequent distortions that take place with the maintenance of the basic framework. One factor is that the A and B sites are... 

Franke and Winnick [105], using a K2S2O7 based electrolyte dispersed within the interstices of an inert K2Mg2(S04)3 matrix, were able to achieve removal efficiencies of SO2 greater than 99% at current efficiencies near 100%. A porous electrode constructed of a perovskite-type compound, Lao 8Sro.2Co03, was found to be conductive and stable in the corrosive cell environment. 

As an attempt to simulate real operating conditions of automotive converters, a laboratory bench has been designed and ageing procedures determined to reproduce simultaneous chemical and thermal modifications encountered by catalysts in the exhaust line. Characterization of commercial samples after ageing according to different temperature cycles evidences formation of both platinum/rhodium alloys and cubic perovskite-type compound, CeA103. Simultaneously with the formation of cerium aluminate, a thermal stabilization of catalysts is observed, in terms of mean noble metal particles size and concentration of rhodium in alloyed phases. An interpretation based on the crystallographic adaptation of alumina, cerium aluminate and ceria is proposed. 

LaGaOs-based compounds, doped with alcaline-earth cations and Mg show very high oxygen-ion conductivity and have a great importance for applications in high-temperature electrochemical devices, such as solid oxide fuel cells [1,2]. Perovskite-like EaGaOs lattice tolerates a substitution for... 

Solid solutions based on RAIO3 perovskite-type compounds... 

The LaGaOs family has been suggested (Ishihara et a/., 1994) as a potential base compound for intermediate temperature SOFCs (IT-SOFCs), operating close to 700°C. This compound crystallises in the perovskite structure, even if substituted on A- or/and B-sites, which considerably improves the oxide ion conductivity. Nevertheless, these materials suffer from Ga volatilisation at high temperature, mainly during sintering step, and a non-sufficient stability under very low oxygen partial pressure. 

LaNisHg). Perovskite hydrides of ABH3 structure (A is a monovalent alkali metal like K, Sr, Cs, and Rb, whereas B is a divalent alkaline earth metal like Ca and Mg), particularly Mg-based compounds, receive particular attention because of their lightweight characteristic and low-cost production. There are plenty of open areas of research that are yet to be explored both experimentally and theoretically for perovskite hydrides and their possible application in hydrogen storage [8],... 

In the last decade, some authors have studied the application of perovskite catalysts to the oxidation of VOCs. Transition metal perovskites based on lanthanum (LaM03, where M = Mn or Co) have demonstrated that they are very good oxidation catalysts for the removal of aromatic compounds. The redox properties of the M cation, the availability of weakly-bonded oxygen at the surface and the presence of... 

Transport behavior of mobile ions in perovskite-type compounds has been investigated by many researchers using various experimental methods and computer modeling. It is acceptable that the transport mechanism of mobile host ions in the perovskite is based on hopping via vacant sites of the ionic species. In that case, tolerance factor t and free volume v of the crystal are thought to be important factors affecting the mobility of the ions. The tolerance factor is a measure of symmetry of the crystal structure of the perovskite and is written as follows ... 

As we know, among the perovskites used as catalyst, LaMOs (M = Co, Mn, Fe, Cr, Ni)-based compounds have been received much attention due to their satisfactory catalytic performance [2]. In the past years, our group have developed a novel strategy, citric acid complexing-hydrothermal coupled method, to prepare nanoscale perovskites [30,31]. 

These copper-oxide compounds crystallize in the perovskite structure and superconductivity is based on the (hole or electron) doping in the copper-oxide planes. This is the reason why these materials can be regarded as being 2D. The first compound of the family was La2 i Sr i Cu04 with Tc 38 K, which soon led to YBa2Cu307 5 with Tc — 92 K for 5 < 1 (Bums, 1993). The non-copper oxide electron-doped perovskite Bai-jcK cBiOa exhibits superconductivity near 30 K for 0.3 < X < 0.5 (Cavaeta/., 1988). 

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