Raman spectroscopic study of supported chromium(VI) oxide catalysts

Raman spectroscopy has been used to characterise Cr(VI) oxide species supported on the surfaces of SiO2, Al2O3, SiO2-Al2O3 and TiO2, as a function of Cr loading and calcination temperature. The interpretation of the spectra has been facilitated by Lorentzian band deconvolution and ab initio calculations employing the hybrid SCF-DFT method B3-LYP, in conjunction with the effective core potential LanL2DZ basis set. These calculations have enabled the elucidation of the geometries of surface chromate species by comparison with reference Cr(VI) compounds, and also detailed force constant calculations and vibrational assignments in terms of potential energy distributions. For all of the surface chromate(VI) species only Raman bands due to stretching vibrations involving Cr=O bonds are observed. Other chromate vibrations are predicted to be strongly mixed with vibrational coordinates of the support, giving rise to composite modes which have much lower Raman intensity.