Effect of the reaction medium on the structure of the La 1 − x Ca x MnO 3 ( x = 0-1) solid solutions prepared by the pechini method

Author： Isupova L. Gerasimov E. Zaikovskii V. Tsybulya S.

Publisher： MAIK Nauka/Interperiodica

ISSN： 0023-1584

Source： Kinetics and Catalysis, Vol.52, Iss.1, 2011-02, pp. : 104-110

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Abstract

The phase composition and microstructure of La_{1 − x}Ca_xMnO₃ (x = 0-1) materials prepared by the Pechini method from polymer-salt stocks were studied after testing these materials in methane oxidation. According to X-ray diffraction data, the reaction medium causes no significant changes in the samples, while high-resolution transmission electron microscopy indicates that the x > 0.3 samples are unstable. Under the action of the reaction medium, the perovskite structure of these samples undergoes partial decomposition accompanied by the formation of planar defects having a lower manganese content. The number and degree of segregation of these defects increase with increasing calcium content. The calcium oxide and manganese oxide phases as segregated nanoparticles are observed on the particle surface. These changes are caused by the decrease in the oxygen content of the manganites under the action of the reaction medium , by the formation of vacancies, and by the variation of the charge of the manganese cations, as well as by the charge ordering tendency of the manganese cations. Therefore, the observed changes in catalytic activity under the action of the reaction medium for x > 0.3 can be due to perovskite decomposition accompanied by the formation of planar defects, the release of the manganese oxide and calcium oxide phases, and their subsequent sintering.