Studies on the reduction-reoxidation of bismuth molybdate catalysts by temperature programmed reoxidation method

Abstract

A new method of temperature programmed reoxidation (TPR) which gives a correlation of reoxidation rates with temperature in curved lines was devised and applied to slightly reduced MoO 3, Bi 2O 3 and bismuth molybdate catalysts. Those oxide catalysts are found to be characterized by peaks in TPR. That is, MoO 3 and Bi 2O 3 give TPR peaks at 410 and 180 °C, respectively. An increase in the extent of prereduction results in another peak with both oxides, indicating that reoxidation takes place in two steps. Bismuth molybdate catalysts of different compositions give different peaks which may be identified as composites of three TPR peaks. Every bismuth molybdate catalyst which gives the TPR peak at 320 °C is invariably active for selective oxidation of propylene to acrolein. The addition of phosphoric acid to bismuth molybdate catalysts increases the peak area at 320 °C and consistently enhances the formation of acrolein from propylene. The activation energies were determined for reduction of bismuth molybdate catalysts, reoxidation of slightly reduced catalysts and catalytic oxidation of propylene to acrolein. Those values are correlated to the TPR peak.