Resonance Raman Spectroscopy of Discrete Silica-Supported Vanadium Oxide

Abstract

Vanadium oxide deposited as discrete oxovanadium groups, [(-O) 3 VdO], in transparent silica xerogels were investigated by resonance Raman spectroscopy. Spectra were collected at 351 and 257 nm excitation into two distinct absorption bands of the oxovanadium site. Three new bands associated with vibrations of the vanadium oxide site were observed at 496, 568, and 720 cm -1 . From these additional modes and the previously known vibrations at 1064, 1033, and 923 cm -1 an empirical force field was determined from which a normal-mode analysis of the primary stretching vibrations of the vanadium oxo group was carried out. This analysis indicates that for most of the observed bands the interfacial Si-O-V stretches are the primary component, and in fact, only the weak band at 923 cm -1 was dominated by the terminal VdO stretch. Shifts in the band positions with 18 O isotopic enrichment are in general agreement with the normal-mode analysis, moreover, the enrichment indicates that the bridging groups are generally quite labile to substitution.

A unified view on heterogeneous and homogeneous catalysts through a combination of spectroscopy and quantum chemistry

“…This signal has been assigned to a variety of V, O and Si functionalities ranging between monomeric to oligomeric vanadia species as are shown in Scheme 1. 14,25,[27][28][29][30][31][32][33] However, no consensus has emerged. It is clear that based on these data, this signal can only be assigned to a mixture of overlapping features of monomeric and oligomeric vanadyl species coexisting on SBA-15 already at small vanadium loadings (e.g.…”

“…The calculated rRaman spectra reproduce the observed signals around 1000 cm À1 and they are in very good agreement with previous studies. 25,26,30 (Fig. 8, top-right).…”

In situ diagnostics of catalytic materials using tunable confocal Raman spectroscopy

“…Prior to discussion of the Raman features originating from dispersed vanadium oxide, it should be mentioned that according to previous studies, the normal modes cannot be associated with single bonds but rather depend on various bonds of the vanadium oxide‐silica network . Hence, it is understood that assignment of a feature as ‘vanadyl’ (V = O) stretching vibration means that the V = O internal coordinate makes the largest contribution to the potential energy of the normal mode discussed.…”

“…Variation of excitation wavelength may strongly affect the Raman spectra of catalytic materials due to a variety of factors including the wavelength‐dependent scattering intensity, the appearance of fluorescence and the presence of resonance effects . Figure illustrates the influence of excitation wavelength on the Raman spectra of dispersed vanadium oxide.…”

“…Industrially important examples for such oxide‐based catalysts include supported chromium, molybdenum and vanadium oxides. While supported vanadium oxide catalysts have been studied extensively with Raman spectroscopy employing standard Raman excitation wavelengths, there are only few studies using tunable Raman spectroscopy . The latter work was devoted to alumina‐supported vanadium oxide employing 220 nm, 287 nm and 430–435 nm excitation.…”