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

The recent emergence of ultraviolet-wavelength-excited Raman spectroscopy as a tool for catalyst characterization has motivated the question of how UV Raman spectra compare to visible-wavelength-excited Raman spectra on the same catalyst system. Measurements of Raman spectra from five supported metal oxide systems (Al2O3-supported Cr2O3, V2O5, and MoO3 as well as TiO2-supported MoO3 and Re2O7), using visible (514.5 nm) and ultraviolet (244 nm) wavelength excitation have been compared to determine the similarities and differences in Raman spectra produced at the two wavelengths. The samples were in the form of self-supporting disks. Spectra from the oxides, both hydrated as a result of contact with ambient air and dehydrated as a result of calcination or laser-induced heating, were recorded. A combination of sample spinning and translation to produce a spiral pattern of laser beam exposure to the catalyst disk was found to be most effective in minimizing dehydration caused by laser-induced heating. Strong absorption by the samples in the ultraviolet significantly reduced the number of scatterers contributing to the Raman spectrum while producing only modest increases in the Raman scattering cross section due to resonance enhancement. The result was much lower signal levels with ultraviolet excitation compared to visible wavelength excited spectra. The absence of strong resonance enhancement effects in the ultraviolet also resulted in Raman spectra that were remarkably similar in terms of the vibrational bands observed, their Raman shift, and their pattern of intensities. Generally, the UV Raman spectra appear to be more sensitive to the out-of-plane bending and symmetric stretching vibrations of bridging oxygen species (M-O-M), whereas the visible Raman spectra are more sensitive to terminal oxygen vibrations (M=O). These differences suggest that a more complete characterization of the supported metal oxide species can be obtained by Raman measurements at several excitation wavelengths. Subtle differences in the spectra due to the extent of dehydration were also evident. These results indicate both that the Raman spectra are very sensitive to the nature of the supported metal oxide species and their environments, which requires that substantial care must be taken in the method of sample treatment during the measurements to obtain meaningful spectra.