Perovskites chemical/catalytic properties

The catalytic properties of the perovskite-type oxides depend on aspects such as the morphology, the particle size, and the crystalline structure, among others. The chemical composition also results determinant, Just as it is the nature of the A and B site ions and their valence states [8]. The A site ions, in contrast to B site ones, are generally proposed to be catalytically inactive, although their nature influence the stability of the solid [9,10]. 

The conversion and selectivities to toluene and benzene were determined after Ih of reaction, when nearly steady-states were reached. Table 1 shows the influence of chemical composition on catalytic properties of CaTiOs and substituted perovskites on benzaldehyde conversion reaction. Compared to CaTiOs, the substituted perovskites are less active under H2 flow and the benzaldehyde conversion decreases with the increasing of Ca substitution by Mg in the following order ... 

Our results showed that the hydrogenation of benzaldehyde over perovskite catalysts depended on chemical compositions. CaTiOs is more active than the substituted perovskites. Benzene, hydrogenolysis product, is the main product formed over CaTiOs catalyst and toluene, hydrogenation product, over substituted perovskites. In conclusion, the perovkite oxides can be used as catalysts in reduction reactions and their catalytic properties depend of the subtitution degree of Ca, Ti and oxygen by Mg, Li and F respectively. 

An important characteristic of perovskites, mentioned in the preceding sections, is their susceptibility of partial substitution in both A and B positions. This provides a wealth of isomorphic compounds that can easily be synthesized. Given the extensive range of possibilities in the tailoring of their chemical and physical properties, there is no doubt that new reactions will be studied, where these oxides can participate as catalytic agents. 

Because the majority of industrial heterogeneous catalysts are based on mixed-metal oxides, it is reasonable that perovskites have also been examined. The preparation of specific, tailor-made mixed oxides able to perform complex physico-chemical functions is one of the main topics of research in the field of catalysis. To achieve this goal, ample information on the physical and solid-state chemical properties of the catalytic materials should be accumulated. 

The larger A cations, usually alkaline or rare earth elements, with inert d" or f electronic structure, act as structural stabilizers and do not offer much to the redox catalytic activity. The smaller B cations can be 3d, 4d, or 5d transition metal elements and are the main catalytic sites/centers in such solids due to their ability to undergo reversed redox cycles without destruction of the structure. In the vast majority of catalytic studies, 3d cations are employed due to obvious economic reasons. Nevertheless, almost 95% of the elements of the periodic table can participate in perovskites and these many possible combinations result in a plethora of diverse, and sometimes imexpected, properties of such solids, which have been nick-named chemical chameleons. 

The perovskite-type oxides have unique characteristics in response to a wide range of properties that are assigned to the cation substitution capacity in its structure, generating isostructural solid with formula Ai- AyBi-yByOsi s. These substitutions can lead to the stabilization of the stmcture with an unusual oxidation state for one of the cations and the creation of anionic and cationic vacancies. This has a significant influence on the catalytic activity of these materials compared to the typical supported materials. Another important feature is the thermal stability of these materials and mechanical and chemically stable reaction conditions [41,148]. 

Perovskite-type Sr-doped LaMnOs (Lanthanum Strontimn Manganites- LSM) has particularly attracted substantial interest as a promising material for cathode in SOFCs. This material has good properties such as chemical and thermal stability, and high catalytic activity for oxygen reduction. Additionally, it has a thermal expansion coefficient similar to that of a solid electrolyte (YSZ), and high electronic conductivity [2]. 

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. 

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