Epitaxially grown model catalyst particles of platinum, rhodium, iridium, palladium and rhenium studied by electron microscopy

In order to obtain oriented thin film model catalysts, small particles of Pt, Rh, Ir, Pd and Re (2-20 nm in size) were grown by high vacuum evaporation on NaCl cleavage faces or on in situ deposited NaCl films at 523-673 K. The particles were covered with a supporting film of Al₂O₃ or carbon and removed from the substrate. High resolution electron microscopy, selected area electron diffraction and weak-beam dark-field imaging were applied to determine the particular morphology, microstructure and orientation of the observed particles. Special attention was paid to Rh particles which appear in a variety of shapes. Pt, Ir and Pd model catalysts consist mainly of (001) oriented half octahedra which may exhibit truncations at the corners or on the top. This was also the dominant shape of Rh particles but in addition half tetrahedra in (011) epitaxy and multiply-twinned particles like decahedra in (001), (011) and (111) orientation were evident. These habits provide a definite "initial state" for study of the changes in structure and morphology of the particles during activating heat treatments necessary to induce catalytic activity of the Al₂O₃ supported metal films. Although Re films consisted of irregularly shaped particles, electron diffraction revealed a partial epitaxial alignment of both c.p.h. and f.c.c. Re.