| 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), andfirst-principles density functional (DFT-GGA) calculations. The β phase of FeMoO4 is stable in a large rangeof temperatures (25−800 °C). For β-FeMoO4, the results of Reitveld refinement and DFT-GGA calculationsgive a crystal structure in which Fe is in an almost octahedral environment while Mo is tetracoordinated. TheMo LII-edge XANES spectra of β-FeMoO4 exhibit a line shape that is typical of oxides in which Mo is in atetrahedral environment. The valence electronic structure of β-FeMoO4 is characterized by an intense peakwith Fe 3d character near the top of the valence band. The iron molybdate displays a good chemical affinityfor H2, H2S, and SO2. H2-TPR (temperature programmed reduction) spectra for β-FeMoO4 show consumptionof 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 onthe oxide. H2S exposure at elevated temperatures (350−450 °C) leads to formation of FeMoSx compoundswithout SOx species (β-FeMoO4 + H2,Sgas → H2Ogas + FeMoSx). β-FeMoO4 is a good sorbent for sulfurdioxide. The adsorption of SO2 on β-FeMoO4 at room temperature produces SO3 and SO4 groups withoutdissociation of SO2. A correlation is found between changes in the electronic and chemical properties ofβ-FeMoO4, β-NiMoO4, and β-MgMoO4. β-NiMoO4 exhibits a large density of metal states near the top of itsvalence 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.
|
