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

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 L_II-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+H_2,gas→H₂O_gas+FeMoO_x) 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 FeMoS_x compounds without SO_x species (β-FeMoO₄+H_2,S_gas→H₂O_gas+FeMoS_x). 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.