Abstract
Effects of O 2, N 2, H 2O, and Cl 2 atmospheres on sintering rates and morphologies of Pt on amorphous SiO 2 were studied by observation with transmission electron microscopy following heat treatment in flowing gases at 1 atm. An initial Pt film 10 to 20 Å thick produced crystallites 10 to 200 Å in diameter when heated to 400 to 700 °C. The average crystallite size was larger by a factor of up to 2 in air than in N 2, and H 2O in these gases significantly inhibited particle growth. No crystallite migration was observed in any gases at any temperature. Atomic diffusion is therefore the exclusive sintering mechanism even though the size distribution exhibited a “tail” on the large diameter side which had been predicted by the particle coalescence model. Oxygen enhancement could occur through vapor transport as PtO 2 or through surface diffusion, although it is suggested that the latter is more plausible. Tilt experiments showed that all particles were three dimensional. Bright- and dark-field microscopy revealed that at least 20% of the particles heated to 600 °C exhibited twinning, and multiple twins were observed in at least 2% of the particles. Good correspondence is observed between these results and behavior of supported catalysts.
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