Behaviour of Platinum and Palladium Microclusters on Alumina Support Films
Microclusters (typical diameter~4–12 nm) of platinum and palladium have been dispersed over alumina support films by vacuum deposition of metal at substrate temperature 350°C under vacuum (~10-5Torr) and characterized in detail by transmission electron microscopy and diffraction techniques. An interesting observation encountered is that clusters of platinum and palladium are highly reactive and lead to hydride formation during heat treatment in hydrogen. The phases formed are PdHx (x<l) possessing f. c. c. structure having a~0.403 nm, ɤ-Pd3H4 simple cubic with a ~0.299 nm and a new hydride of platinum based on a body centred cubic unit cell having a~0.310 nm. In view of the catalytic applications of platinum and palladium-alumina systems, coarsening and redispersion of microclusters have been examined and discussed with specific reference to deactivation and rejuvenation. It is illustrated that deactivation is caused not just by coarsening of clusters but by interaction between the catalyst and gaseous components as well (i. e. through hydride or oxide formation in reducing or oxidizing conditions, described as reactive coarsening or reactive redispersion).
KeywordsTransmission Electron Micro Vacuum Deposition Anodic Oxide Film Metallic Cluster Hydride Formation
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