Investigation of the Reducibility of Supported Oxomolybdate Species for Mapping of Active Centers of Partial Oxidation Reaction: In Situ Mo K-Edge XAS and DFT Study

Abstract

We have investigated the molecular and electronic structure of a TiO 2supported oxomolybdate phase upon reduction under methanol or dihydrogen atmosphere. In situ Mo K-edge X-ray absorption spectroscopy, supported by density functional theory (DFT) calculations and ab initio X-ray absorption spectroscopy (XAS) spectra simulation showed that the reducibility of supported molybdate species is closely related to the geometric environment of Mo atoms. Indeed, Mo atoms not involved in terminal oxo sites are easily reducible whereas Mo dioxo species are not under the conditions we have applied. Between those extreme cases, Mo atoms involved in monooxo terminal sites exhibit intermediate reducibility, which varies according to their local environment and their interaction with the support. Spin density calculations showed that under CH 3 OH flow, the surface of the catalyst is partially reduced. The structure of the reduced phase after reaction with methanol was compared to that afforded after reduction by dihydrogen. Our results show that the reduction under H 2 flow does not increase the number of reduced Mo atoms but causes a higher spin density per Mo atom. On the basis of the individual mapping of the electronic structure of Mo species as drawn by DFT and XAS spectra simulation, a model that details the structure of the Mo active sites in the selective oxidation of methanol is proposed.

Observation of Low-γ Quadrupolar Nuclei by Surface-Enhanced NMR Spectroscopy

“…The peaks at 420 and 285 ppm are assigned to OMo 2 and OMo 3 sites . These sites are present in [Mo 6 O 21 (Mo 4 O 12 ) n ] 6– clusters with n ≥ 2, which have been identified by XANES spectroscopy as the predominant molybdenum oxide structures in MoO 3 /TiO 2 and are active in catalytic oxidation reaction. , These signals can also be produced by α-MoO 3 crystals, which are formed for such loading of MoO 3 . , The signals of OMoTi and OMo 2 Ti local environments are probably masked by those of OTi 3 , OMo 2 , and OMo 3 sites. The additional small peaks at 150 and 20 ppm can be ascribed to OMo 4 and OMo 5 sites, which may indicate the adsorption of polyoxomolybdates, such as [Mo 7 O 24 ] 6– and β-[Mo 8 O 26 ] 4– , onto the particle surfaces, beside the [Mo 6 O 21 (Mo 4 O 12 ) n ] 6– clusters .…”

“…95 These sites are present in [Mo 6 O 21 (Mo 4 O 12 ) n ] 6− clusters with n ≥ 2, which have been identified by XANES spectroscopy as the predominant molybdenum oxide structures in MoO 3 /TiO 2 and are active in catalytic oxidation reaction. 88,96 These signals can also be produced by α-MoO 3 crystals, which are formed for such loading of MoO 3 . 89,97 The signals of OMoTi and OMo 2 Ti local environments are probably masked by those of OTi 3 , OMo 2 , and OMo 3 sites.…”

Ultradispersed Mo/TiO2catalysts for CO2hydrogenation to methanol

“…The stability of these particles, even after re-exposure to ambient air, can be related to the epitaxial growth of the partially reduced Mo oxide on TiO 2 . [54][55][56][57][58] This sintering could lead to a change of active site from the MoO x -TiO 2 interface to the MoO x surface. A reducing treatment at 700 °C induces the formation of even larger MoO x particles (2.7 ± 0.5 nm) and the sintering of titania (Figure S9), which loses part of its specific surface area (from 78 m 2 /g for R450 catalysts to 58 and 20 m 2 /g for their R600 et R700 counterparts, respectively).…”

Isolated Molybdenum(VI) and Tungsten(VI) Oxide Species on Partly Dehydroxylated Silica: A Computational Perspective

“…Computational chemistry methods can provide complementary insight into supported transition metal oxides, which is helpful in the interpretation of the spectroscopic data and allows for better understanding the nature of the surface metal oxide species. − Adequate modeling is crucial for a realistic description of such catalytic systems, particularly in the case of amorphous supports, like silica, with inhomogeneous distribution of metal sites. ,,, In the past, small and medium-size cluster models, more or less arbitrarily constructed, were used to represent the MoO x /SiO 2 catalyst. ,,− More realistic modeling of this system was possible by employing an advanced cluster approach, including models developed from the amorphous silica structure, and by using periodic slab models . The latter, however, were based on the β-cristobalite framework.…”