A comparison of ultraviolet and visible Raman spectra of supported metal oxide catalysts

The recent emergence of ultraviolet-wavelength-excited Raman spectroscopy a s a tool for catalyst characterization has motivated the question of how UV Raman spectra compare to visible-wavelength-excited Raman spectra on the s ame catalyst system. Measurements of Raman spectra from five supported meta l oxide systems (Al2O3-supported Cr2O3, V2O5, and MoO3 as well as TiO2-supp orted MoO3 and Re2O7), using visible (514.5 nm) and ultraviolet (244 nm) wa velength excitation have been compared to determine the similarities and di fferences in Raman spectra produced at the two wavelengths. The samples wer e in the form of self-supporting disks. Spectra from the oxides, both hydra ted as a result of contact with ambient air and dehydrated as a result of c alcination or laser-induced heating, were recorded. A combination of sample spinning and translation to produce a spiral pattern of laser beam exposur e to the catalyst disk was found to be most effective in minimizing dehydra tion caused by laser-induced heating. Strong absorption by the samples in t he ultraviolet significantly reduced the number of scatterers contributing to the Raman spectrum while producing only modest increases in the Raman sc attering cross section due to resonance enhancement. The result was much lo wer signal levels with ultraviolet excitation compared to visible wavelengt h excited spectra. The absence of strong resonance enhancement effects in t he ultraviolet also resulted in Raman spectra that were remarkably similar in terms of the vibrational bands observed, their Raman shift, and their pa ttern of intensities. Generally, the UV Raman spectra appear to be more sen sitive to the out-of-plane bending and symmetric stretching vibrations of b ridging oxygen species (M-O-M), whereas the visible Raman spectra are more sensitive to terminal oxygen vibrations (M=O). These differences suggest th at a more complete characterization of the supported metal oxide species ca n be obtained by Raman measurements at several excitation wavelengths. Subt le differences in the spectra due to the extent of dehydration were also ev ident. These results indicate both that the Raman spectra are very sensitiv e to the nature of the supported metal oxide species and their environments , which requires that substantial care must be taken in the method of sampl e treatment during the measurements to obtain meaningful spectra.