The Raman spectra of bismuth molybdate catalysts were recorded under a variety of conditions. At 200°C under evacuation, a peak at 1575 cm<sup>−1</sup> was observed in the spectra of alpha, beta, and gamma bismuth molybdates and Fe<sub>2</sub>Mo<sub>3</sub>O<sub>12</sub>. There was no peak in this location in the spectra of MoO<sub>3</sub> and Bi<sub>2</sub>O<sub>3</sub>. 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<sup>−1</sup>), 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<sup>−1</sup> peak could be removed by contact with O<sub>2</sub> at 350°C. In addition, a band at 1375 cm<sup>−1</sup> was sometimes observed; its intensity relative to the 1575-cm<sup>−1</sup> 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.

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