Charge and coordination states of iron cations in La 1-x Me x FeO 3-y (Me=Ca, Sr, Ba) prepared by mechanochemical route
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Charge and coordination states of iron cations in La1-xMexFeO3-y (Me=Ca, Sr, Ba) prepared by mechanochemical route as related to oxygen reactivity Lyubov A. Isupova, Yurii T. Pavlyukhin*, Vladimir A. Rogov, Galina M. Alikina, Sergei V. Tsybulya, Irina S. Yakovleva and Vladislav A. Sadykov Boreskov Institute of Catalysis SB RAS, Pr. Lavrent’eva 5, Novosibirsk, Russia; *Institute of Solid State Chemistry SB RAS, ul. Kutateladze,18, Novosibirsk, Rusia; e-mail:[email protected]
ABSTRACT This work presents results of investigation of the charge and coordination state of Fe cations in La1-xMexFeO3-y perovskites prepared by mechanochemical route by using MÖssbauer spectroscopy as related to bulk oxygen reactivity in H2 or in CH4 temperature-programmed reduction (TPR). INTRODUCTION Mixed oxides with perovskite structure are known to be promising for such hightemperature applications as cathodes in SOFC, oxygen-conducting membranes, catalysts for hydrocarbons combustion or their selective oxidation into syngas by gas phase or lattice oxygen [1-5]. Their surface/lattice oxygen mobility and reactivity may depend upon the coordination and charge state of transition metal cations as well as the specificity of their defect structure varied due to substitution in the cation sublattices or change of the preparation route [6, 7]. Substituted ferrites-perovskites La1-xMexFeO3-y (Me=Ca, Sr, Ba) are among the most promising for these high-temperature processes. At the same time La1xCaxFeO3-y and La1-x(Sr/Ba)xFeO3-y perovskite systems are known to be characterised by different types of charge compensation required to maintain electroneutrality (namely, oxygen vacancies and low-coordinated Fe cations appear in the former and Fe4+ cations in the latter systems). For La1-xCaxFeO3-y system synthesis parameters affect not only the coordination and charge of Fe cations but even the phase composition of samples [8-9]. The mechanochemical activation of the mixture of solid reagents is known as an efficient approach to strongly accelerate the perovskite synthesis as compared with the traditional ceramic approach. The method allows to synthesize products with a developed surface area without water wastes [10-12]. Due to mild condition of sintering, the mechanochemical (MC) route may influence the phase composition, charge and coordination state of 3d cations in perovskites, thus affecting the bulk oxygen mobility and reactivity. These studies are aimed at the elucidation of the effect of the type and content of dopant (Ca, Sr, Ba) on the coordination and charge state of the iron cation as well as on the reactivity of bulk oxygen forms in lanthanum ferrite prepared by mechanochemical route. EXPERIMENTAL Samples were prepared from the mixture of starting compounds (simple oxides) after preliminary mechanical treatment (MT) in the high power planetary ball mill APF-5 for 3 minutes followed by calcinations at 1100 oC for 4 hours in air. The ratio of the weights of
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milling balls and powder was equal to 10:1, acceleration
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