A Magnetically Ordered Non-Stoichiometric Zinc Ferrite for the Oxidative Dehydrogenation Reactions.
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A Magnetically Ordered Non-Stoichiometric Zinc Ferrite for the Oxidative Dehydrogenation Reactions. J. A. Toledo*, N. Nava, X.C. Sun and X. Bokhimi1 Prog. Simulación Molecular, Instituto Mexicano del Petróleo, Lázaro Cárdenas # 152, 07730, México, D.F., México. *E-mail: [email protected] Fax: (+525) 333 -6239 1 Institute of Physics, The National University of Mexico (UNAM) 01000, México, D.F. México ABSTRACT ZnFe2O4 nanoparticles were prepared by hydrothermal reduction approach. A considerable amount of α-Fe2O3 was segregated in the as-synthesized sample, which diffused into the tetrahedral and octahedral sites of the ZnFe2O4 spinel structure with increasing the annealing temperature. The introduction of Fe3+ into the tetrahedral positions was observed by Mössbauer spectra. Magnetization measurements showed an unusual ferrimagnetic behavior of the ZnFe2O4 phase, even at room temperature, confirming the introduction of Fe3+ into the tetrahedral sites of the spinel structure. Catalytic activity measured in the oxidative dehydrogenation of 1-butene reaction increased with increasing annealing temperature, indicating that those interactions of Fe3+ in tetrahedral and octahedral positions also promotes the activity and selectivity to butadiene formation. INTRODUCTION Zinc ferrite, possesses spinel structure with a general formula A2+ B 32+ O4. Where the tetrahedral (A) sites are occupied by Zn2+ ions and the octahedral (B) positions are occupied by Fe3+ ions [1]. ZnFe2O4 normally presents a long-range antiferromagnetic order above the Néel temperature (TN ≈ 10 K) [2], since the A sites are occupied by nonmagnetic ions, as Zn2+, that having no unpaired electronic spins, produce no antiferromagnetic interaction with the Fe3+ ions on the B sites. From this analysis, it derives, then, that the preferential energy of the divalent ion (Me2+) for occupying A and B sites is decisive as far as the magnetic behavior is concerned [3]. ferrimagnetic materials are technologically important since they have been widely used for several applications [4]. Recently, progress in synthesis techniques has initiated a new surge of interest in ferrites focusing to improve their physical properties and expand their application [5]. In fact, the preparation method could modify the state of the chemical order, the cation sites distribution in the material, and hence, the magnetic properties. In this work, we presented the synthesis, magnetic and catalytic studies of nonstoichiometric ZnFe2O4. The cation distribution around A and B sites in the spinel structure and their implication in the catalytic and magnetic applications were also investigated. EXPERIMENTAL DETAILS ZnFe2O4 nanoparticles were synthesized by hydrothermal method, which was carried out in a 500 ml autoclave reactor, in which 100 ml of a 1.0 M aqueous solution of FeCl3 (Baker) were mixed with an (50 vol. %) aqueous solution of ammonia NH4(OH) (Baker) until the pH value reached 8.0 and a brown slurry was formed. Then, the stoichiometric amount of metallic zinc Y3.5.1
powder (Bake
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