Catalytic and surface-structural investigations of bismuth molybdate selective-oxidation catalysts

Abstract

In a project designed to study the relation between structural features and the activity with which metal oxides catalyse the oxidation of hydrocarbons the bismuth molybdate system has been examined in detail. Catalytic studies have been carried out using a variety of chromatographic methods on a microreactor scale. Catalysts were made by both low-temperature precipitation and high-temperature fusion of oxides, introducing deliberate stoichiometric changes particularly in the region of Bi:Mo ratio 2:1. X-ray diffraction and ESCA have been used to obtain detailed information on both the bulk and surface structures of the catalysts. Investigations into the catalytic oxidation of but-1-ene to butadiene and propene to acrolein have shown profound changes in activity with only slight changes in stoichiometry near the koechlinite phase (Bi₂MoO₆) composition. Reduction and reoxidation of samples containing a slight excess of bismuth in this composition region reveal dramatic changes in catalytic properties that can be directly related to changes in surface stoichiometry. The results show that the catalytic properties of bismuth molybdates are very sensitive to the composition of the surface which in turn is critically dependent on the bulk stoichiometry.