Possibility of VOx/SiO2 complexes speciation: comparative multi-wavelength Raman and DR UV-vis study

Raman spectroscopy is one of the very often used spectroscopic methods for characterization of vanadium surface species. However, Raman spectra of VOx-silica systems are more complex and interpretation is more difficult in comparison with other supports (like Al2O3, ZrO2, TiO2 or Nb2O5) because there is strong vibrational coupling between the vanadia species and the silica support. Therefore, assignment and interpretation of some vibrational bands is still subject of controversy. This fact results in incongruity of suggested molecular structure and population of individual vanadium surface complexes. In this contribution, we present systematic comparative study of diffuse reflectance UV-vis spectra and Raman spectra excited by 325 and 514.5 nm lasers obtained on set of dehydrated vanadium modifed hexagonal mesoporous silica (VOx-HMS) samples with vanadium loading from 2 up to 12 wt. %. We prove that changes in population of oligomeric and monomeric VOx species in individual samples are not manifested by significant changes in the character of Raman signals. On the other hand it is evident that with increasing of vanadium loadings the UV-vis spectra show systematic changes. Raman spectroscopy is useful characterization technique for detection presence of very small amount of V2O5 microcrystallites, especially if suitable wavelength of laser is used for remarkable resonant enhancement of Raman intensity of its bands (e.g. 514.5 nm). (C) 2013 The Authors. Published by Elsevier B.V. Open access under CC BY-NC-ND license.