Microstructure and microanalysis of bismuth molybdates

Abstract

The microstructures of commercially important bismuth molybdate catalysts in relation to olefin oxidation reactions are examined by electron microscopy (EM) techniques. The microstructural characterization has been carried out using dynamic ( in situ) EM, high resolution EM, and microanalysis. The coprecipitated catalyst system Bi 2MoO 6 or γ, together with the γ phase, contains small amounts of tetragonal Bi 2MoO 6 phase, Bi 2Mo 3O 12 (α phase), Bi 2O 3, and MoO 3. In reduction with propylene, at catalyst operating temperatures of 400–500°C, in the dynamic experiments conducted on α- and γ-phase crystallites under reaction conditions no evidence for extended defects such as crystallographic shear planes has been obtained, instead an ordered intermediate phase similar to (101) Bi 2Mo 2O 9 (β phase) is observed which is found to be unstable. Observations by electron microscopy have been confirmed with parallel measurements made in a reactor connected to a gas chromatograph and mass spectrometer system. The possible influence of the microstructural changes on the catalytic behavior of the system is examined.