CHEMISTRY OF ETHYLIDENE MOIETIES ON PLATINUM SURFACES - 1,1-DIIODOETHANE ON PT(111)

Reflection-absorption infrared spectroscopy (RAIRS) and temperature-pr ogrammed desorption (TPD) were used to study the thermal chemistry of 1,1-diiodoethane adsorbed on clean and deuterium-covered Pt(111) surfa ces. The RAIRS spectra of condensed 1,1-diiodoethane, obtained after l arge 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 pea ks 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 resolve d. A more detailed study of the latter symmetric and asymmetric C-H st retch modes of 1,1-diiodoethane shows a change in the tilt angle of th e C-C axis with respect to the surface normal, from 53 +/- 6 degrees a t 2.0 langmuirs (20% of saturation) to 20 +/- 4 degrees at 5.0 langmui rs (half saturation). The weak C-I bonds in the adsorbed 1,1-diiodoeth ane break first upon thermal activation, and the ethylidene groups tha t form on the surface determine the subsequent chemistry of this syste m. Ethylidene groups can in principle undergo four elementary reaction s, 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 selectiv e conversion of 1,1-diiodoethane to ethylidyne is indeed seen around 1 50 K, and proof that this occurs via a direct alpha-H elimination step comes from the fact that no deuterium is incorporated at low temperat ures when deuterium is present on the surface. However, since that eli mination reaction requires empty surface sites to accommodate the hydr ogen atoms that are released, it is suppressed at high surface coverag es, where ethylene is formed instead. Ethylene is most likely produced via a direct 1,2-H shift because, according to the data presented her e, the alternative routes (alpha-H incorporation to ethyl followed by beta-H elimination or beta-H elimination to vinyl) do not seem very li kely, All this is consistent with a mechanism for the conversion of et hylene to ethylidyne involving the interconversion between ethylene an d ethylidene, These results also highlight the fact that the availabil ity of empty surface sites plays a key role in the kinetics of ethylid yne formation: the isomerization to ethylidene is rate limiting at low coverages while the alpha-H elimination to ethylidyne is the slow ste p at high coverages, and a preequilibrium between ethylene and ethylid ene exists in the latter case. Ethylene can also equilibrate with ethy l on the surface, a side reaction that accounts for hydrogen/deuterium exchange reactions.