The Raman spectra of bismuth molybdate catalysts were recorded under a variety of conditions. At 200°C under evacuation, a peak at 1575 cm−1 was observed in the spectra of alpha, beta, and gamma bismuth molybdates and Fe2Mo3O12. There was no peak in this location in the spectra of MoO3 and Bi2O3. For alpha bismuth molybdate, this peak could be generated by room-temperature evacuation or by reduction of a fully oxidized catalyst with propylene at 400°C. This latter treatment had the additional effect of causing a dramatic decrease in the intensities of bands in the fundamental region (<1000 cm−1), although no changes in the relative intensities or locations of these bands were observed. The intensity decreases were attributed to the concomitant generation of a large number of free electrons within the catalyst. Reoxidation reversed this effect. The 1575-cm−1 peak could be removed by contact with O2 at 350°C. In addition, a band at 1375 cm−1 was sometimes observed; its intensity relative to the 1575-cm−1 band was temperature-dependent. It was not plausible to assign these bands to fundamental, overtone, or combination bands associated with Mo-O or Bi-O bonds. It appears that these bands are related to the presence of an electron donor level which can inelastically scatter light during an electronic transition.
William D. Kovats and Charles G. Hill, "Raman Spectroscopy of Bulk Bismuth Molybdate Catalysts," Appl. Spectrosc. 40, 1215-1223 (1986)