Dynamic studies of metal oxide catalysts: MoO₃

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The dynamic microstructural changes of the metal oxide catalyst MoO3 in reducing atmospheres of carbon monoxide, hydrogen and propylene diluted with helium as well as mixtures of propylene and oxygen have been studied up to temperatures of ∼600°C in a gas-reaction eell fitted to a high-voltage electron microscope. On reduction, apart from partial screw dislocations at ∼80°C, two sets of surface domains with predominantly pure shear displacement across them are generated at ∼ 120°C: and lie parallel to ⟨304⟩. Up to 380°C, the domains grow in size, appear to change their character, and extend into the bulk. At operating temperatures of ∼400°C, crystallographic shear planes (CSPs) are formed at the surface bounded by partial edge dislocations with Burgers vector b =[α/2, b/7, 0 01 bounding a stacking fault extending from the surface to a depth of ∼300 Å. Observations of nucleation and growth confirm the model of collapsed vacancy discs expanding by dislocation climb. The response of the defects in propylene (C3H6)-oxygen mixtures as well as in oxygen at various temperatures is described. In C3H6−O2 reactions it appears that extraction of oxygen from the solid is followed by replenishment from the oxygen gas phase. Estimates for various thermodynamical quantities are also presented.