TRANSMISSION ELECTRON-MICROSCOPY OF SUPPORTED MOLYBDENUM AND VANADIUM-OXIDES

Transmission electron microscopy has been used to characterize dispersions of molybdena and vanadia on titania and silica supports. When silica spheres of controlled morphology were used as support, the dispersed "monolayer" phase of both oxides could be imaged due to characteristic changes in contrast. In addition to the dispersed phase, we could detect three-dimensional crystallites of V2O5 but in the case of MoO3 only two-dimensional islands were seen. On Degussa P-25 titania, there was no observable contrast change due to the presence of a monolayer of these dispersed oxides. However, exposure to the electron beam caused dramatic changes in the surface texture of the support. Such changes were not seen when blank TiO2 was similarly irradiated. These e-beam induced changes were more pronounced in the vanadia/titania catalysts leading to formation of 1-3 nm clusters of reduced VO(x). However, on the MoO3/TiO2 sample, e-beam exposure caused only a pronounced change in texture but no well defined clusters could be detected.