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Studies on the behavior of mixed-metal oxides: Structural, electronic, and chemical properties of β-FeMoO4

Abstract:

The structural and electronic properties of iron molybdate (FeMoO<inf>4</inf>) 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 FeMoO<inf>4</inf> is stable in a large range of temperatures (25-800 °C). For β-FeMoO<inf>4</inf>, 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 L<inf>II</inf>-edge XANES spectra of β-FeMoO<inf>4</inf> exhibit a line shape that is typical of oxides in which Mo is in a tetrahedral environment. The valence electronic structure of β-FeMoO<inf>4</inf> 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 H<inf>2</inf>, H<inf>2</inf>S, and SO<inf>2</inf>. H<inf>2</inf>-TPR (temperature programmed reduction) spectra for β-FeMoO<inf>4</inf> show consumption of hydrogen and evolution of water (β-FeMoO<inf>4</inf>+H<inf>2,gas</inf>→H<inf>2</inf>O<inf>gas</inf>+FeMoO<inf>x</inf>) between 600 and 850 °C. Upon exposure to H<inf>2</inf>S at moderate temperatures (50-250 °C), there is formation of sulfides and sulfates on the oxide. H<inf>2</inf>S exposure at elevated temperatures (350-450 °C) leads to formation of FeMoS<inf>x</inf> compounds without SO<inf>x</inf> species (β-FeMoO<inf>4</inf>+H<inf>2,</inf>S<inf>gas</inf>→H<inf>2</inf>O<inf>gas</inf>+FeMoS<inf>x</inf>). β-FeMoO<inf>4</inf> is a good sorbent for sulfur dioxide. The adsorption of SO<inf>2</inf> on β-FeMoO<inf>4</inf> at room temperature produces SO<inf>3</inf> and SO<inf>4</inf> groups without dissociation of SO<inf>2</inf>. A correlation is found between changes in the electronic and chemical properties of β-FeMoO<inf>4</inf>, β-NiMoO<inf>4</inf>, and β-MgMoO<inf>4</inf>, β-NiMoO<inf>4</inf> exhibits a large density of metal states near the top of its valence band and a substantial reactivity toward H<inf>2</inf> and H<inf>2</inf>S. β-MgMoO<inf>4</inf> displays completely opposite trends, and β-FeMoO<inf>4</inf> is an intermediate case between the two extremes.