In Situ Spectroscopic Investigation of Molecular Structures of Highly Dispersed Vanadium Oxide on Silica under Various Conditions

Abstract

The molecularly dispersed V 2 O 5 /SiO 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 2 was achieved at ∼12 wt % V 2 O 5 (∼2.6 V atoms/nm 2 ). The surface structures of the molecularly dispersed V 2 O 5 /SiO 2 samples under various conditions were extensively investigated by in situ Raman, UVvis-NIR DRS and XANES spectroscopies. The combined characterization techniques revealed that in the dehydrated state only isolated VO 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) 3 VdO 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 2 O 5 ‚nH 2 O gels, rather than V 2 O 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 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‚‚‚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.

Partial Oxidation of Methanol Over Highly Dispersed Vanadia Supported on Silica SBA-15

“…nH 2 O gel-like structure. The blue-shift of the band at 1023 cm )1 can be explained by the variation of the water content n in the gel [27]. In the same study it has been shown, that the variation of the water content in the gel can also lead to the presence of an additional band at $915 cm )1 , which has been attributed to the stretching of V@O double bond coordinated with water.…”

“…[22] and Mg 3 V 2 O 8 [23,24]. Also, the corresponding Raman spectra are dominated by a band at 1040 cm )1 (see inset), which has been assigned to isolated tetrahedral VO 4 with three V-O support bonds and one V@O bond in the literature [25][26][27][28][29]. It should be mentioned, that recent studies have challenged the above assignment [30,31].…”

“…The corresponding spectra after air treatment at 500°C for l h are shown as spectra c and e [41]. Spectrum a is characterized by bands at 518, 706, and 1023 cm )1 , which can be assigned to hydrated vanadium oxide forming a V 2 O 5 AEnH 2 O gel-like structure [26,27]. Exposure to the MeOH/O 2 /He gas mixture at temperatures, at which no reaction was observed, leads to dramatic changes in the Raman spectrum (b).…”

Elucidation of the molecular structure of hydrated vanadium oxide species by X-ray absorption spectroscopy: correlation between the V⋯V coordination number and distance and the point of zero charge of the support oxide

“…[3] Apparently, variations in the vanadium oxide loading, support oxide and measurement conditions complicate the determination of the molecular structure of hydrated supported vanadium oxide catalysts making use of X-ray absorption spectroscopy. Although clustering of vanadium oxide species upon hydration of the sample has often been suggested, [4,8,18,19] only a limited number of research groups have investigated the presence of vanadium atoms in the second coordination shell. [14,20] Silversmit et al detect additional cations in their hydrated sample (CN = 2, R = 3.5 Å), however, they were not able to distinguish between the support cation (Ti) and a neighbouring vanadium atom.…”

Cyclohexene photo-oxidation over vanadia catalyst analyzed by time resolved ATR-FT-IR spectroscopy

“…A large extent of hydration of the catalyst under ambient conditions is also apparent from the UV-Vis spectrum of the catalyst, shown as an insert in Figure 1. The assignments of the absorption bands in the UV-Vis spectrum are discussed in detail elsewhere [6,7]. Changes in the Infrared spectrum upon dehydration, induced by evacuation to 100 mTorr, are shown in the difference spectrum Figure 1b.…”