Local site symmetry of dispersed molybdenum oxide catalysts: XANES at the Mo L2,3-edges

Abstract

The local site symmetry of nanodispersed molybdenum oxide phases have been determined for the first time using X-ray absorption near edge spectroscopy (XANES) at the Mo L[sub 2,3]-edges. Local site symmetries varying between pure tetrahedral coordination through mixed symmetries to pure octahedral coordination were determined for species present at concentrations above and below a submonolayer. X-ray absorption near-edge spectra at the Mo L[sub 2,3]-edges were obtained for a series of supported molybdate catalysts and Mo[sup 6+] reference materials with a range of oxygen coordination types. Basic ligand field concepts can be used to interpret the spectra. Tetrahedral molybdates display d-orbital splitting in the 1.8-2.0-eV range, whereas octahedrally coordinated molybdates display split peaks in the 3.0-4.5-eV range. The direct comparison between a complete set of molybdenum reference compounds and dispersed magnesium oxide-supported molybdate catalysts has been used to determine the detailed symmetries of the dispersed molybdenum phases that are present as amorphous, crystalline, and mixed phases. 26 refs., 5 figs., 1 tab.