The Application of UV Raman Spectroscopy for the Characterization of Catalysts and Catalytic Reactions

Ultraviolet Raman spectroscopy is a powerful tool for the characterization of solid catalysts. When the excitation wavelength is below 260 nm, the Raman peaks typically appear at shorter wavelengths than fluorescence, which would otherwise obscure the spectrum. This advantage is particularly useful for zeolite materials, which have traditionally been difficult to characterize with Raman spectroscopy. With a fluidized-bed reactor, it is possible to perform experiments with thermally and photochemically sensitive adsorbed species or even under catalytic reaction conditions, without interference from laser-induced sample decomposition. Resonance enhancement provides the opportunity to probe specific components in a heterogeneous mixture and to increase band intensities above the detection limit for weak signals. This feature is demonstrated for VO_x/alumina as a function of vanadium loading and for Fe/MFI catalysts. The spectra of adsorbed benzene demonstrate the ability of resonance Raman spectroscopy to detect subtle distortions in the benzene structure. Furthermore, the UV Raman spectra of coke formed during catalytic hydrocarbon conversions are shown to be diagnostic of the coke topology.