Chemisorption of CO on particulate deposits of platinum

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Abstract
The morphology and CO chemisorption properties of particulate Pt films have been studied as a function of particle size. The Pt films were vapor deposited onto UHV-cleaved and heat treated mica surfaces at a 200 °C substrate temperature. The average particle sizes ranged from 1.5 to 4.0 nm in diameter as measured by TEM. The particles were found to be irregular in shape and randomly oriented except for larger particles which were more epitaxial. After CO adsorption at 60 °C, two desorption peaks were observed; one at 120 °C (α) attributed to smooth particle surfaces, and another at about 175 °C (β) attributed to particle edges and steps. The ratio of areas under these peaks varied significantly with particle size. The β peak from small particles was a large fraction of the total CO desorption flux, but for a continuous film it was only a minor contributor. With successive desorptions the area under the β peak slowly decreased. TEM showed that major changes in the particle morphology occur during adsorption–desorption cycling. Thus, the decrease in the CO desorption flux is interpreted as due to particle restructuring rather than contamination by CO decomposition as in the case of Ni or Pd on mica. AES measurements corroborate the lack of CO decomposition, even for very small Pt particles.

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