Perovskite-type oxides preparation

Catalytic combustion of diesel soot particulates over LaMnOs perovskite-type oxides prepared by malic acid method has been studied. In the LaMn03 catalyst, the partial substitution of alkali metal ions into A site enhanced the catalytic activity in the combustion of diesel soot particulates and the activity was shown in following order Cs>K>Na. In the LarxCs MnOj catalyst, the catalytic activity increased with an increase of X value and showed constant activity at the substitution of x>0.3... 

Table 1. Perovskite-type oxides prepared by malic acid method and their catalytic performances...

In conclusion, the few examples provided above show that powder perovskite-type oxides prepared by USS demonstrate in general enhanced structural properties compared to more conventional synthesis methods. However, a rational synthesis by design is desirable that should be based on a systematic research of the effect of synthesis parameters on structural and morphological properties of the product... 

Merino, NA., Barbero, B.P., Grange, P., and Cadiis, L.E. (2005) Lai ,C ri Co03 perovskite-type oxides preparation, characterisation, stability, and catalytic potentiality for the total oxidation of propane. J. Catal, 231, 232-244. 

Stathopoulos, V.N., Belessi, V.C., Bakas, T.V., Neophytides, S.G., Costa, C.N., Pomonis, P.J., and Efstathiou, A.M. (2009) Comparative study of La-Sr-Fe-O perovskite-type oxides prepared by ceramic and surftictant methods over the CH4 and H2 Lean-deNO. Appl. Catal. B Environ., 93 (1-2), 1-11. 

In this paper, we prepared LaMnOa perovskite-type oxides using the malic acid method and investigated their physical properties. It has been also investigated the effect of partial substitution of metal iorrs into La and Mn sites and the reaction conditions on the activity for the combustion of soot particulates. 

The preparation method of perovskite-type oxides was taken from the previous paper[4]. Malic acid was added into mixed aqueous solution of metal nitrates in a desired proportion so as for the molar ratio of malic... 

Kaliaguine, S., Van Neste, A., Szabo, V. et al. (2001) Perovskite-type oxides synthesized by reactive grinding Part I. Preparation and characterization, Appl. Catal. A 209, 345. 

Cui X. and Liu Y. New methods to prepare ultrafine particles of some perovskite-type oxides, Chem. Eng. J., 2000, Volmne 78, Issues 2-3, 205-209. 

Suzuki, M., Enomoto, Y. Murakami, T. and Inamura, T., Thin Film Preparation of Superconducting Perovskite-Type Oxides by rf Sputtering. Jpn. J. Appl. Phys. 20(s 20-4) 13 (1981). 

Siemons, M. Weirich, Th. Mayer, J. Simon, U., Preparation of nanosized perovskite-type oxides via polyol method, Z. Anorg. Allg. Chem. 2004, 630, 2083-2089... 

Manganese oxides have long been known to be catalysts for a variety of gas clean-up reactions. Manganese/copper mbced oxide (Hopcalite) is the catalytically active component in gas mask filters for CO CO is converted to CO2 at room temperature [4]. Further applications of manganese oxide catalysts are the NH3 oxidation to N2 [5], the combustion of VOC [6,7] and methane [8], the oxidation of methanol [7], the O3 decomposition [9] and the NOx reduction [14]. Perovskite-type oxide catalysts (e.g. LaMnOs) have been proven to be effective catalysts for the total oxidation of chlorinated hydrocarbons [10]. Several studies have shown that besides preparation method and calcination temperature the kind... 

A primary characterization of perovskite-type oxides must include textural analysis and X-ray identification of the phase(s) present. For a more detailed characterization, structural analysis for establishing the lattice position of cations and surface analysis (by means of techniques such as XPS) for defining the surface concentration and oxidation states of cations are desirable. Consequently, information provided by these techniques will furnish the essential criteria for comparing the different preparation methods. For convenience, we will classify the methods used to date for the preparation of pure perovskite phases according to the scheme proposed by Courty and Marcilly (29) for the whole field of mixed oxides. Table I gives a survey of methods used as a function of the phenomena on which they are based. 

In recent years, much attention has been focused on hydrocarbons total oxidation over mixed oxides. It was reported that perovskite type oxides remarkably oxidise carbon monoxide, light alkanes and also methane at low temperatures [1]. However, the major obstacles to the successful application of these materials in a large scale are both then-low resistance to sulphur poisoning and also their scarce BET surface area which is often linked to the catalytic activity. For this, development of more active catalysts has become a challenge to be overcome. Many attempts have been made to develop new preparation methods to improve... 

Perovskite-type oxides with A and/or B sites partially substituted present properties such as structural defects and reactivity of adsorbed and lattice oxygen that play a central role in catalytic combustion. However, preparation methods as well as temperature of calcination could affect the surface area, and most important, changes on the surface composition that will be reviewed in the following section. 

These perovskite-type oxides are prepared by solid state reaction of cerium dioxide, strontium or barium carbonate and dopant oxide at about... 

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). 

Zhang, H.-M., Teraoka, Y and Yamazoe, N. (1987) Preparation of perovskite-type oxides with large surfece area by citrate process. Chem. Lett, 16, 665-668. 

The ideal control on the structural and textural properties of perovskite-type oxides can be better achieved by exploiting aerosol spray synthesis methods to prepare highly dispersed and nanostructured materials from metal salt precursors. High-specific-surface-area (above 20m /g) crystalline perovskite-type oxides can be then obtained, which are suitable for a variety of applications. A major advantage of spray methods is that the material is directiy processed from the precursor solution with a reduced number of processing steps during powder synthesis (one-step approach), thus making them ideally suited for... 

Pd-doped catalysts have been produced by USS [82]. The fingerprint of Pd adopting the octahedral coordination of Fe in LaFeo,95Pdo,o503 has been observed in the XANES spectra of the material prepared by spray synthesis (27m /g) similarly to the preparation by the amorphous citrate method (14m /g) [17,82]. In contrast, the flame-made material of the same composition (22m /g) exposed metallic Pd particles on LaFeOs similarly to preparation by solution combustion. The different nature of the Pd species obtained by changing the synthesis method dramatically influences their catalytic performance, since PdO nanoparticles exposed at the surface of the mixed oxide exhibit catalytic activity, whereas Pd—O species in the bulk of the mixed oxide are inactive, at least in the case of methane oxidation [27]. In contrast to LaFeOs, LaMnOs did not allow Pd to adopt the octahedral coordination irrespective of synthesis method. Therefore, the coordination of Pd strongly depends on both the composition of the perovskite-type oxide and the synthesis method. 

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