Studies on the Behavior of Mixed-Metal Oxides: Structural, Electronic, and Chemical Properties of β-FeMoO4
- 1 August 2000
- journal article
- research article
- Published by American Chemical Society (ACS) in The Journal of Physical Chemistry B
- Vol. 104 (34), 8145-8152
- https://doi.org/10.1021/jp001652c
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.Keywords
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