Raman Spectroscopy of Bulk Bismuth Molybdate Catalysts

Abstract

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.