UV Raman spectroscopy of adsorbed SOx on γ-alumina and Pt/γ-alumina catalysts

Raman spectra of SOx adsorbed on gamma-alumina and Pt/gamma-alumina model catalysts have been obtained with a 244 nm Raman spectrometer. Strong peaks in the 980-1380 cm(-1) region characteristic of adsorbed sulfates are clearly portrayed in the spectra, which contrast with fluorescence-dominated scans obtained using visible excitation. Broad bands are also observed in the 3500-3700 cm(-1) O-H stretch region on the gamma-alumina, which belong to weakly-bound physisorbed water and more strongly-bound surface hydroxyls. These features are monitored as the samples are heated up to 600 degrees C in the presence of nitrogen. The sulfate peaks vary in position depending on whether or not the gamma-alumina is loaded with platinum, hydrated, or dehydrated. Platinum appears to inhibit the physisorption of water and the formation of hydroxyls on the gamma-alumina surface, as evidenced by the absence of O-H stretch vibrations on the Pt-loaded sample. Our spectral data demonstrate that UV Raman spectroscopy is a useful technique for analytical studies of adsorbed SOx on gamma-alumina, and holds promise for future in situ investigations of other adsorbed species on catalytic substrates of automotive interest.