In situ spectroscopic investigation of molecular structures of highly dispersed vanadium oxide on silica under various conditions

Journal Article · Thu Dec 24 04:00:00 EST 1998 · Journal of Physical Chemistry B: Materials, Surfaces, Interfaces, amp Biophysical

DOI:https://doi.org/10.1021/jp9826367· OSTI ID:315998

Gao, X; Wachs, I R ^[1]; Bare, S R ^[2]; Weckhuysen, B M ^[3]

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1. Lehigh Univ., Bethlehem, PA (United States)
2. UOP Research, Des Plaines, IL (United States)
3. K.U. Leuven, Heverlee (Belgium). Centrum voor Oppervlaktechemie en Katalyse

The molecularly dispersed V{sub 2}O{sub 5}/SiO{sub 2} supported oxides were prepared by the incipient wetness impregnation of 2-propanol solutions of V-isopropoxide. The experimental maximum dispersion of surface vanadium oxide species on SiO{sub 2} was achieved at {approximately}12 wt% v{sub 2}O{sub 5} ({approximately}2.6 V atoms/nm{sup 2}). The surface structures of the molecularly dispersed V{sub 2}O{sub 5}/SiO{sub 2} samples under various conditions were extensively investigated by in situ Raman, UV-vis-NIR DRS and XANES spectroscopies. The combined characterization techniques revealed that in the dehydrated state only isolated VO{sub 4} species are present on the silica surface up to monolayer coverage. Interestingly, the three-member siloxane rings on the silica surface appear to be the most favorable sites for anchoring the isolated, three-legged (SiO){sub 3} V{double_bond}O species. Hydration dramatically changes the molecular structure of the surface vanadium oxide species. The specific structure of the hydrated surface vanadium oxide species is dependent on the degree of hydration. The molecular structure of the fully hydrated vanadium oxide species closely resembles V{sub 2}O{sub 5}{center_dot}nH{sub 2}O gels, rather than V{sub 2}O{sub 5} crystallites. The fully hydrated surface vanadium oxide species are proposed to be chain and/or two-dimensional polymers with highly distorted square-pyramidal VO{sub 5} connected by V-OH-V bridges, which are stabilized on the silica surface by the sixth neighbor of Si-OH hydroxyls via Si-OH{hor_ellipsis}V hydrogen bonds. In analogy to the hydration process, alcoholysis occurs during methanol chemisorption, and similar molecular structures are proposed to interpret the interaction between methanol molecules and the surface vanadium oxide species on silica.

• Sponsoring Organization: National Science Foundation, Washington, DC (United States); USDOE, Washington, DC (United States)
• DOE Contract Number: FG02-93ER14350
• OSTI ID: 315998
• Journal Information: Journal of Physical Chemistry B: Materials, Surfaces, Interfaces, amp Biophysical, Journal Name: Journal of Physical Chemistry B: Materials, Surfaces, Interfaces, amp Biophysical Journal Issue: 52 Vol. 102; ISSN 1089-5647; ISSN JPCBFK
• Country of Publication: United States
• Language: English

Related Subjects

36 MATERIALS SCIENCE
HYDRATION
MOLECULAR STRUCTURE
SILICA
SPECTROSCOPY
SUBSTRATES
VANADIUM OXIDES