A RAIRS study on the surface chemistry of ethyl iodide on Pt(111)

The chemisorption and thermal decomposition of ethyl iodide on Pt(111) have been studied by reflection-absorption infrared spectroscopy (RAIRS). At 100 K the chemisorption is molecular, and a change in adsorption geometry occurs from a flat to an upright orientation as the surface coverage is increased. Ethyl, ethylene and ethylidyne were identified at higher temperatures by IR spectrometry as decomposition products. As a consequence of the coverage-dependent orientation of the ethyl iodide molecules on the surface, different reaction mechanisms are operative at low and high coverages. At low coverages, ethyl iodide decomposes quantitatively between 150 and 200 K through a C-I bond breaking step followed by immediate dehydrogenation to ethylene. At saturation coverages, some ethylene formation competes with molecular desorption around 200 K (analogous to the low coverage decomposition), but additional ethylene is produced between 200 and 240 K in a slower, reversible reaction which involves the formation of ethyl species as stable intermediates. Ethylene further decomposes around 300 K to form ethylidyne.