STM STUDIES OF THE EFFECT OF PRETREATMENT ON THE MORPHOLOGY AND ASPECT RATIO DISTRIBUTION OF CRYSTALLITES ON PT/HOPG

STM studies of chemically impregnated model Pt catalysts supported on HOPG substrate reveal the dynamics of morphological transformation from catalyst-intermediate to metal crystallites of near-equilibrium shape. Both the morphology and aspect ratio distribution of the crystallites undergo significant changes for catalysts reduced for 0.5, 2, and 60 h at similar pretreatment conditions. The aspect ratio distribution shows that majority (60-70%) of the crystallites have metastable shapes. Another model Pt catalyst was prepared by vacuum deposition of Pt metals onto the graphite substrate. Prior to H-2 pretreatment, the Pt particles have random, irregular morphologies which transform to regular spherical shapes after H-2 treatment at 250-degrees-C for 30 min.