The strong peak at 1370–1375 cm−1,

The strong peak at 1370–1375 cm−1, present in the spectrum of the catalyst containing 1% V2O5, has been previously observed in the infrared spectra of sulfated metal oxides and has been assigned to the stretching frequency of a terminal S==O bond (11). Its presence is consistent with the formation of a surface sulfate species. More importantly, the intensity of this peak is significantly lower in the spectrum of the catalyst containing 3% V2O5, in agreement with the kinetic results, which suggest that the promoting effect of SO2, and hence, the concentration of surface sulfates, is significantly decreased on V2O5/TiO2 catalysts containing more than half a monolayer of surface vanadia.