A Raman and ultraviolet diffuse reflectance spectroscopic investigation of silica-supported molybdenum oxide

Journal Article · Wed Oct 30 23:00:00 EST 1991 · Journal of Physical Chemistry; (United States)

DOI:https://doi.org/10.1021/j100175a067· OSTI ID:7252914

Williams, C C; Ekerdt, J G ^[1]; Jehng, Jihmirn; Hardcastle, F D; Turek, A M; Wachs, I E ^[2]

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1. Univ. of Texas, Austin (United States)
2. Lehigh Univ., Bethlehem, PA (United States)

Laser Raman spectroscopy (LRS) and ultraviolet-visible diffuse reflectance spectroscopy (UVDRS) were used to characterize silica-supported, hydrated molybdenum oxide. Molybdenum loadings of 0.1-10 wt % were studied. LRS was also used to characterize dehydrated molybdenum oxide, under in situ conditions, at a loading of 6.4 wt. % Mo. Fumed silica and silica gels were used as supports. Raman spectra were recorded either in ambient air following calcination or in an in situ cell under oxygen purge. For the hydrated, ambient samples, all precursors were found, by LRS, to form octahedrally coordinated surface polymolybdate species at Mo loadings above 0.4 wt %. No Raman evidence was found for the existence of isolated, surface molybdenum oxide tetrahedra. In situ Raman showed that the octahedrally coordinated, hydrated polymolybdate species could be reversibly transformed into an isolated structure upon dehydration. Four crystalline compounds were also observed: MoO{sub 3}, CaMoO{sub 4}, Na{sub 2}Mo{sub 2}O{sub 7}, and Na{sub 2}MoO{sub 4}. The formation of these crystalline compounds depended upon the Mo loading, the method of preparation, and the presence of impurities on the support. UVDRS, without supplementary evidence, is shown to be ineffective in differentiating tetrahedrally and octahedrally coordinated molybdenum for the systems studied. The formation of the hydrated, octahedrally coordinated surface polymolybdate species is shown to be related to the aqueous chemistry of Mo{sup 6+}.

• OSTI ID: 7252914
• Journal Information: Journal of Physical Chemistry; (United States), Journal Name: Journal of Physical Chemistry; (United States) Vol. 95:22; ISSN JPCHA; ISSN 0022-3654
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
400102 -- Chemical & Spectral Procedures
400201* -- Chemical & Physicochemical Properties
CATIONS
CHALCOGENIDES
CHARGED PARTICLES
DATA
EXPERIMENTAL DATA
HYDRATES
IMPURITIES
INFORMATION
IONS
MECHANICAL STRUCTURES
MOLYBDENUM COMPOUNDS
MOLYBDENUM OXIDES
NUMERICAL DATA
OXIDES
OXYGEN COMPOUNDS
RAMAN SPECTRA
REFLECTION
REFRACTORY METAL COMPOUNDS
SILICON COMPOUNDS
SILICON OXIDES
SPECTRA
SUPPORTS
TRANSITION ELEMENT COMPOUNDS
ULTRAVIOLET SPECTRA