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 L2,3-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 L2,3-edges were obtained for a series of supported molybdate catalysts and Mo6+ 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.

X-ray absorption and magnetic circular dichroism characterization of Mo1–xFexO2 (x = 0–0.05) thin films grown by pulsed laser ablation

“…We note a strong feature at A 3 followed by a shoulder-like peak (marked as B 3 ) at higher energy side for the undoped film, while upon Fe-doping a fine crystal-field splitting is observed between these two peaks and the relative intensity of the peak B 2 is enhanced significantly. These spectral variations at Mo M 3 -edge of the studied films led to identify the effects of the crystal field of the surrounding O atoms on the d levels of the Mo atom and hence, the signature of symmetry-breaking distortion of the MoO 6 octahedra [15]. However, these effects are quite difficult to quantify in these compounds with their almost empty 4d shell and mixed-valent nature of Mo ions.…”

A XAFS study on the mechanism of isotopic fractionation of molybdenum during its adsorption on ferromanganese oxides

“…Molybdenum L 3 -edge XANES spectroscopy is a useful probe for Mo local symmetry because it has widely been reported for Mo(VI) compounds that the variable splitting in the L 3 -edges reflects the ligand field splitting of d-orbitals, where smaller splitting (1.8-2.6 eV) was observed for Mo(VI) in tetrahedral symmetry compared to that of octahedral symmetry (3.1-4.5 eV) (Aritani et al, 1996;Bare et al, 1993). The splitting and edge energy of the spectra summarized in Fig.…”

Molecular-scale mechanisms of distribution and isotopic fractionation of molybdenum between seawater and ferromanganese oxides

“…Mo L 3 -edge XANES Molybdenum L 3 -edge XANES spectroscopy is a useful probe for Mo local symmetry (Bare et al, 1993;Aritani et al, 1996;Lede et al, 2002;George et al, 2009). The origin of Mo L 3 -edge XANES is mostly the electron transition from 2p 3/2 to a vacant 4d state, and both the splitting and relative intensities of the white line can be rationalized using ligand field theory.…”

“…The structural analyses of Mo on the surface of various oxides, such as MgO, Al 2 O 3 , SiO 2 , and TiO 2 , have vigorously been conducted by XAFS analysis in the field of catalysis science (e.g., Mensch et al, 1988;Bare et al, 1993;Aritani et al, 1996Aritani et al, , 2001Takenaka et al, 1998;Radhakrishnan et al, 2001;Lede et al, 2002). This method is also a powerful tool in the geochemistry field because it enables in situ analysis of the surface complex structure of trace elements at the solid/water interface.…”