EFFECT OF LANTHANUM MANGANITE MODIFICATION BY CALCIUM AND/OR FLUORINE ON THE BONDING STRENGTH, MOBILITY AND REACTIVITY O
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EFFECT OF LANTHANUM MANGANITE MODIFICATION BY CALCIUM AND/OR FLUORINE ON THE BONDING STRENGTH, MOBILITY AND REACTIVITY OF THE LATTICE AND SURFACE OXYGEN V. A. Sadykov*’**, T. G. Kuznetsova*, A. V. Simakov*, V. A. Rogov*, V. I. Zaikovskii*, E. M. Moroz*, D. I. Kochubei*, B. N. Novgorodov*, V. P. Ivanov*, S. N. Trukhan*, G. S. Litvak*, N. N. Bulgakov*, V. V. Lunin***, E. Kemnitz****. *-
Boreskov Institute of Catalysis, Siberian Branch of the Russian Academy of Sciences, Novosibirsk, 630090, Russia; [email protected] ** Novosibirsk State University, Novosibirsk, 630090, Russia. *** Lomonosov Moscow State University, Moscow, Russia ****-Institute for Chemistry, Humboldt- University, Berlin, Germany ABSTRACT Ca and/or F-modified samples of LaMnO3 have been prepared by the Pechini method. The bulk structure of samples was characterized by TEM, EXAFS and XRD, while the surface composition was studied by SIMS. Thermal analysis, O2 TPD, H2 TPR and isothermal pulse/flow samples reduction by CO were applied to characterize the accessible surface/bulk oxygen mobility and reactivity. A reasonable description of the experimental energetic spectrum of the surface oxygen for various types of regular and defect surface sites on the perovskite faces was achieved by using semiempirical Interacting Bonds Method in the slab approximation with a due regard for the surface face termination and relaxation. Fluorine was found to decrease the surface coverage by reactive weakly bound oxygen forms while increasing the bulk oxygen excess and mobility. Calcium generated reactive weakly bound oxygen forms while decreasing the oxygen excess in the lattice and converting the regular M-O oxygen forms into the bridging ones through migration to the surface. INTRODUCTION Lanthanum manganite based systems including those substituted by the alkaline-earth cations are known to possess a high activity in catalytic oxidation reactions, including CO oxidation, methane combustion etc [1, 2]. They are also promising as cathodes for high temperature fuel cells [3]. Optimization of their performance by tuning the chemical composition and methods of synthesis requires knowledge of factors determining mobility and reactivity of the surface and bulk oxygen. Despite numerous studies [4-8], satisfactory description of those properties has not yet been achieved. This work aims of addressing this problem by using non-isothermal temperature – programmed methods such as O2 desorption (O2 TPD) and reduction by hydrogen (H2 TPR) combined with the isothermal pulse/flow reduction of samples by CO. The results thus obtained were analyzed with a due regard for the real structure and the surface composition of samples. To ensure homogeneity of the components spatial distribution within the particles, a polymerized complex precursor method (Pechini route [9]) was used. To affect the samples stoichiometry and the real structure, their modification by calcium and fluorine was used. The choice of fluorine was determined by its alleged ability to enhance the latti
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