Regresar

Studies on the behavior of mixed-metal oxides: Structural, electronic, and chemical properties of β-FeMoO<inf>4</inf>

Abstract:

The structural and electronic properties of iron molybdate (FeMoO4) have been studied using synchrotron-based time-resolved X-ray diffraction (TR-XRD), X-ray absorption near-edge spectroscopy (XANES), and first-principles density functional (DFT-GGA) calculations. The β phase of FeMoO4 is stable in a large range of temperatures (25-800 °C). For β-FeMoO4, the results of Reitveld refinement and DFT-GGA calculations give a crystal structure in which Fe is in an almost octahedral environment while Mo is tetracoordinated. The Mo LII-edge XANES spectra of β-FeMoO4 exhibit a line shape that is typical of oxides in which Mo is in a tetrahedral environment. The valence electronic structure of β-FeMoO4 is characterized by an intense peak with Fe 3d character near the top of the valence band. The iron molybdate displays a good chemical affinity for H2, H2S, and SO2. H2-TPR (temperature programmed reduction) spectra for β-FeMoO4 show consumption of hydrogen and evolution of water (β-FeMoO4+H2,gas→H2Ogas+FeMoOx) between 600 and 850 °C. Upon exposure to H2S at moderate temperatures (50-250 °C), there is formation of sulfides and sulfates on the oxide. H2S exposure at elevated temperatures (350-450 °C) leads to formation of FeMoSx compounds without SOx species (β-FeMoO4+H2,Sgas→H2Ogas+FeMoSx). β-FeMoO4 is a good sorbent for sulfur dioxide. The adsorption of SO2 on β-FeMoO4 at room temperature produces SO3 and SO4 groups without dissociation of SO2. A correlation is found between changes in the electronic and chemical properties of β-FeMoO4, β-NiMoO4, and β-MgMoO4, β-NiMoO4 exhibits a large density of metal states near the top of its valence band and a substantial reactivity toward H2 and H2S. β-MgMoO4 displays completely opposite trends, and β-FeMoO4 is an intermediate case between the two extremes.