Molecular Structures and Reactivity of Supported Molybdenum Oxide Catalysts

doi:10.1006/jcat.1994.1295

Journal of Catalysis

Volume 149, Issue 2, October 1994, Pages 268-277

https://doi.org/10.1006/jcat.1994.1295 Get rights and content

Abstract

Supported molybdenum oxide catalysts were prepared by an equilibrium adsorption method. The molecular structures of the molybdenum oxide overlayers on different oxide supports (Al₂O₃, TiO₂, ZrO₂, SiO₂, and MgO), under in situ conditions, were investigated by Raman spectroscopy. The molybdenum oxide species on TiO₂, ZrO₂, and Al₂O₃ possess a highly distorted, octahedrally coordinated surface molybdenum oxide species with one short MoO bond regardless of the molybdenum oxide content. The MoO₃/SiO₂, catalysts primarily contain crystalline MoO₃ because of the lower density and reactivity of the silica surface OH groups. The MoO₃/MgO catalysts possess MgMoO₄ and CaMoO₄ compounds due to the high aqueous solubility of MgO and CaO (an impurity in the MgO support) and the strong acid-base interaction between molybdenum oxide and MgO/CaO. The methanol oxidation reaction studies revealed that the MoO₃/TiO₂ (anatase and rutile) and MoO₃/ZrO₂ catalysts are the most active catalysts and that their activities (TOFs) are at least 1-2 orders of magnitude higher than those of the MoO₃/Al₂O₃, MoO₃SiO₂, and MoO₃/MgO catalysts. It was also found that the catalytic activities correlate with the reducibility of the surface molybdenum oxide species on the oxide supports as well as their surface morphology (e.g., molybdenum oxide dispersion and molybdate compound formation). These studies demonstrate that the specific oxide support controls the reactivity of the supported molybdenum oxide phases.

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Citation Excerpt :
Similarly, the peak at around 848 cm−1 can be associated to the Mo–O–Mo asymmetric stretching mode of the octahedral molybdate species [59]. Noticeably, the NMW/S16 and NMW/Ti/S16 are unique catalysts showing the band at about 903 cm−1 ascribed to the stretching modes of MoO bond of the tetrahedral molybdate species [60,61]. Opposite to the NMW/S16 and NMW/Ti/S16 catalysts, both catalysts prepared by sol-gel method exhibit the band at about 962 cm−1, which should be assigned to Mo(W) oxides species having octahedral coordination (Mo(Oh)).
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Citation Excerpt :
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Regular ArticleMolecular Structures and Reactivity of Supported Molybdenum Oxide Catalysts

Abstract

Regular Article
Molecular Structures and Reactivity of Supported Molybdenum Oxide Catalysts