Scanning tunneling microscopy study of supported Pt and Pd catalysts: Microstructure and morphology

Abstract

Supported Pt crystallites were prepared by two different methods, physical deposition by vacuum evaporation of Pt metal, and chemical impregnation from a precursor solution. Physically deposited Pt forms large elongated crystallites of approximately pillbox or inverted boat shape with an irregular, highly striated surface, while Pt crystallites from impregnated catalysts are small and have regular boxlike morphology. H2 treatment of the physically deposited Pt results in fragmentation of the Pt crystallites into smaller but irregularly shaped particles. Pd crystallites prepared from a tetraammine precursor are large and ellipsoidal in shape, while the crystallites prepared from a tetrapyridine precursor are small and have emeraldlike morphology. The shape distribution of the supported particles for Pt and Pd impregnated catalysts shows that only 30% to 40% of the crystallites have an equilibrium aspect ratio of 1.0 and 60%–70% have an aspect ratio ≳1.0 characteristic of metastable morphology.