The catalytic oxidation of propylene: VII. The use of temperature programmed reoxidation to characterize γ-bismuth molybdate

Temperature programmed reoxidation (TPR) was used to characterize the redox properties of γ-Bi2MoO6. Reoxidation of a prereduced catalyst by this technique yielded two well-defined temperature ranges for reoxidation. When the catalyst was prereduced by propylene, the maximum of a low-temperature peak occurred at 158 °C and the maximum of a high-temperature peak occurred at 340 °C. The physicochemical changes associated with these reoxidation temperatures were further investigated by X-ray diffraction, Auger spectroscopy, and kinetic measurements. The low-temperature reoxidation peak was found to be a result of the reoxidation of Mo4+ to Mo6+ and Bi0 to Bim+, where 0 < m < 3. The high-temperature reoxidation peak was found to be a result of the reoxidation of Bim+ to Bi3+. The high-temperature reoxidation may involve the direct oxidation of bismuth by gas phase oxidation or may involve shear structures. The high-temperature reoxidation process also appears to be related to the rate-limiting step for propylene oxidation to acrolein at temperatures below 400 °C.