Ultraviolet Raman spectroscopy characterization of sulfated zirconia catalysts: Fresh, deactivated and regenerated

Ultraviolet Raman spectroscopy (UVRS) has been demonstrated to be a powerful new tool for catalysis and surface science studies. UVRS can successfully avoid the surface fluorescence which frequently occurs in normal Raman spectra of many catalysts. Fresh, deactivated and regenerated sulfated-zirconia catalysts have been characterized using this new method. The UV Raman spectrum of the fresh sample is dominated by the tetragonal phase, however, the spectrum of deactivated sulfated zirconia is nearly identical to that of the monoclinic phase of pure zirconia. A regeneration of the deactivated catalyst restores the spectrum back to that of the fresh sample. In addition, a new band at 750 cm^-1 associated with the fresh sample attenuates with the deactivation process. This band is tentatively assigned to the surface [ZrO₄]^4- unit which is assumed to bond to the surface sulfate group. The results indicate that the surface phase of sulfated zirconia is reconstructed from tetragonal to monoclinic phase during the deactivation process while the bulk remains in the tetragonal phase after the deactivation. It is proposed that the surface tetragonal phase is stabilized by sulfate groups, and is associated with the catalytic activity.