Study of the Local Structure of Molybdenum−Magnesium Binary Oxides by Means of Mo L₃-Edge XANES and UV−Vis Spectroscopy

Abstract

Mo L3-edge XANES and diffuse reflectance UV−vis spectra have been recorded for structural studies of Mo−Mg binary oxides which catalyze metathesis reactions of olefins after pretreatment with H2 at 773 K. The XANES of reference compounds revealed that the local symmetry around the Mo ion as well as the valence state affects the spectral features. From the XANES of Mo−Mg binary oxides in oxidized/reduced states, it is concluded that MoO4 tetrahedra are the main component in the near-surface region for samples of x (Mo/(Mo + Mg)) ≤ 0.7 in the oxidized state. For samples in the reduced state, the tetrahedral species remain, but reduced Mo ions including MoO2 species are also formed, while the MoO2 phase is formed in the bulk phase at x = 0.5−0.8. In the latter samples, Mo ions are easily reduced by treatment with H2. The active species for metathesis reactions relates to formation of the MoO2 phase not only in the bulk but also in the near-surface region.

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

“…In this case, the white line at higher energy is less intense than that at lower energy, which is in contrast to that observed in Td symmetry. Mo(IV) ion in MoO 2 is also known to be almost in Oh symmetry, but the reduction from Mo(VI) causes a lower absorption edge and narrower splitting of the energy gap (Aritani et al, 1996). Edge energy E 0 , defined as the first inflection point, is lower than the other spectra, indicating that Mo in MoO 2 is present as Mo(IV) ( Fig.…”

“…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.…”

Catalytic abatement of trichloroethylene over Mo and/or W-based bronzes

“…Mo 6+ (250-400 nm) [40,41], V 5+ (250-450 nm) [42], and Nb 5+ (235-310 nm) [43]. Moreover, the band at ca.…”