Perovskite catalysis study

So far, the bonding and surface structure aspects of electrocatalysis have been presented in a somewhat abstract sort of way. In order to make electrocatalysis a little more real, it is helpful to go through an example—that of the catalysis of the evolution of oxygen from alkaline solutions onto substances called perovskites. Such materials are given by the general formula RT03, where R is a rare earth element such as lanthanum, and T is a transition metal such as nickel. In the electron catalysis studied, the lattice of the perovskite crystal was replicated with various transition metals, i.e., Ni, Co, Fe, Mn, and Cr, the R remaining always La. 

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... 

These mixed oxides allow the introduction of a large variety of cations within the same structure to study their effect upon the catalytic behavior. This feature is particularly useful both in practical and fundamental catalytic studies. This is why many articles and several books have been published on perovskite catalysis. 

Studies in these laboratories have resulted in the synthesis and catalytic evaluations on a wide range of perovskites and ion modified homogeneous solid solutions for Fischer-Tropsch catalysis, copper modified spinels for higher alcohol synthesis, ion substituted perovskites for methane activation, alkali modified metal... 

The author of this book has been permanently active during his career in the held of materials science, studying diffusion, adsorption, ion exchange, cationic conduction, catalysis and permeation in metals, zeolites, silica, and perovskites. From his experience, the author considers that during the last years, a new held in materials science, that he calls the physical chemistry of materials, which emphasizes the study of materials for chemical, sustainable energy, and pollution abatement applications, has been developed. With regard to this development, the aim of this book is to teach the methods of syntheses and characterization of adsorbents, ion exchangers, cationic conductors, catalysts, and permeable porous and dense materials and their properties and applications. 

Isopova ct al. described the preparation of various perovskite-based monolithic catalysts for fuel combustion by extrusion of synthesized perovskite powders [14]. Blanco ct al. [25], Lachman and Williams [97], and del Valle et al. [98] reported titania-based and other monolithic catalysts by extrusion. The titania catalysts were tested in a coal-fired power pilot plant for electrostatic separation of fly-ash [98]. Lyakhova et al. studied the W03-doped titania-vanadia monolithic catalysis for selective catalytic reduction (SCR NOx conversion) by extrusion [26]. The rheological properties of the paste for extrusion and the effect of various organic plastisizers on catalytic activity in SCR were discussed. 

The defect perovskites that have been more studied in heterogeneous catalysis were those having in position A an alkaline, alkaline-earth, or lanthanide element and in position B a first-row transition metal. We will discuss here some examples of nonstoichiometric perovskites, paying attention preferentially to the concentration and type of defects that are formed. The influence of these defects in the catalytic performance of these oxides has been clearly established in a number of cases. Some relevant examples will be discussed in Section VII. 

LnMOj perovskites in which the lanthanide (Ln) ions are essentially inactive in catalysis and the active transition-metal (M) ions are placed at relatively large distances (ca. 0.4 nm) from each other are excellent catalytic models for study of the interaction of CO and 02 on single surface sites. It must be stressed, however, that idealized correlations between catalytic activity that is confined to the surface, and a single collective... 

Despite the fact that perovskite-type oxides have been suggested as substitutes for noble metals in automotive exhaust catalysis [1], relatively few studies were devoted to the synthesis, characterization and catalytic activity of these catalysts. 

In a book of modest size, it is necessary to be somewhat selective in material content. For this reason, two areas have been omitted. The first of these concerns preparation techniques. In the main these are the normal techniques of solid-state chemistry, physics and ceramic science and are not unique to perovskites. Secondly catalysis has also been omitted. Again, the bulk of the catal5mc reactions studied are not unique to perovskites and are better described and discussed within the broader perspective of catalysis rather than via the narrower standpoint of perovskites. 

Tablet, C., Grubert, G Wang, H. et al. (2005) Oxygen permeation study of perovskite hollow fiber membranes. Catalysis Today, 104,126-130. 

Since 1970, the perovskite type oxides, typically rare earth oxides with a (ABO3) formula, have been suggested as substitutes for noble metals in automotive exhaust catalysis (1). The most studied perovskites are LaM03 ( M = first row transition metal ) (2,3,4), where M is considered as the active site of the catalyst. The cobaltites show good activity as oxidation catalysts, the reactivity seems to depend on the facility of cobalt to undergo the transition Co Co m, which may be correlated to an oxygen non stoichiometry, and to the spin state of the cation (5). Furthermore, series of LaM03 oxides revealed similar profiles for CO adsorption studies as for NO adsorption, with NO adsorption maxima for M = Mn and Co (6). The reactivity of these catalysts has been shown not only to depend on the surface area, but also on the preparation process (7). 

For applications in heterogeneous catalysis, perovskites generally comprise a lanthanide (La is the most common) in the A site and a transition metal (Mn, Co, etc.) in the B site. The efficiency of such perovskite oxides, with or without cationic substitution, is well documented for a variety of catalytic reactions [2-9]. Actually, the specific catalytic activities of perovskites were sometimes found to be comparable to that of noble metals for various oxidation reactions. Early on, Arai et al. illustrated the activity of strontium-substituted LaMnOs, which was found to be superior to that of Pt/alumina catalysts at a conversion level below 80% [5]. Several authors have also discussed the application of La-based perovskite oxides as catalysts for volatile organic compound (VOC) oxidation (see, for example. Refs [10-14]). Zhang et al. have also shown that some perovskite oxides substituted with Pd or Cu are also good catalysts for the reduction of NO by CsHg [15-18] and by CO [19,20]. More recently, Kim et al. studied the effect of Sr substitution in LaCoOs and LaMnOs perovskites for diesel oxidation (DOC) and lean NO, trap (LNT) processes [9]. The observations made by these authors clearly indicate that the perovskites used in their study could efficiently outperform Pt-based catalysts. Typically, Lai. Sr cCoOs catalysts achieved higher... 

Besides the size of the ionic radii of A and B, there is another condition for the formation of a perovskite structure, which is electroneutrality the sum of charge of cations must be equal to the sum of oxygen anion charge [41]. In heterogeneous catalysis, the most studied systems are those with an alkali element or alkaline earth lanthanide in position A, and a transition metal of the first set in position B (Ti, V, Cr, Mn, Fe, Co, Ni, Cu). 

Among the various catalytic reactions studied, those applicable to environmental catalysis (e.g., automobile exhaust gas cleaning catalyst) attract particular attention. Initially, it was reported that perovskite oxide consisting of Cu, Co, Mn, or Fe exhibited superior activity to NO direct decomposition at higher temperatures [16 18]. The direct NO decomposition reaction (2NO = N2 -b O2) is one of the dream reactions in the catalysis field. In this reaction, the ease of removal of surface oxygen as a product of... 

Application of perovskite oxides have been extensively studied [6-13] in sueh as, solid state chemistry, physics, advanced materials, and catalysis. Because of these diverse applications and special requirements in each application, perovskite-type oxides with speeial properties are required depending on the ultimate end use. For example, materials-oriented applications require densification by high temperature sintering to minimize both surfaee area and surface free energy in order to maximize meehanieal strength. In eontrast, eatalytie materials have to maintain sufficiently high surface area in order to maximize their participation and activity in chemical reactions.[14]... 

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