Chemistry of ethylidene moieties on platinum surfaces: 1,1-diiodoethane on Pt(111)

Reflection-absorption infrared spectroscopy (RAIRS) and temperature-programmed desorption (TPD) were used to study the thermal chemistry of 1,1-diiodoethane adsorbed on clean and deuterium-covered Pt(111) surfaces. The RAIRS spectra of condensed 1,1-diiodoethane, obtained after large doses of the compound on clean Pt(111) at 95 K, resemble that of the liquid. At submonolayer coverages, on the other hand, only the peaks for the delta(CH) (1197 cm(-1)), delta(s)(CH3) (1371 cm(-1)), nu(s)(CH3) (2919 cm(-1)), and nu(a)(CH3) (2976 cm(-1)) modes can be resolved. A more detailed study of the latter symmetric and asymmetric C-H stretch modes of 1,1-diiodoethane shows a change in the tilt angle of the C-C axis with respect to the surface normal, from 53 +/- 6 degrees at 2.0 langmuirs (20% of saturation) to 20 +/- 4 degrees at 5.0 langmuirs (half saturation). The weak C-I bonds in the adsorbed 1,1-diiodoethane break first upon thermal activation, and the ethylidene groups that form on the surface determine the subsequent chemistry of this system. Ethylidene groups can in principle undergo four elementary reactions, namely, alpha-H elimination to ethylidyne, 1,2-H shift to ethylene, alpha-H incorporation to ethyl, and beta-H elimination to vinyl, but only the first two actually occur on the Pt(111) surface. The selective conversion of 1,1-diiodoethane to ethylidyne is indeed seen around 150 K, and proof that this occurs via a direct alpha-H elimination step comes from the fact that no deuterium is incorporated at low temperatures when deuterium is present on the surface. However, since that elimination reaction requires empty surface sites to accommodate the hydrogen atoms that are released, it is suppressed at high surface coverages, where ethylene is formed instead. Ethylene is most likely produced via a direct 1,2-H shift because, according to the data presented here, the alternative routes (alpha-H incorporation to ethyl followed by beta-H elimination or beta-H elimination to vinyl) do not seem very likely, All this is consistent with a mechanism for the conversion of ethylene to ethylidyne involving the interconversion between ethylene and ethylidene, These results also highlight the fact that the availability of empty surface sites plays a key role in the kinetics of ethylidyne formation: the isomerization to ethylidene is rate limiting at low coverages while the alpha-H elimination to ethylidyne is the slow step at high coverages, and a preequilibrium between ethylene and ethylidene exists in the latter case. Ethylene can also equilibrate with ethyl on the surface, a side reaction that accounts for hydrogen/deuterium exchange reactions.