Evidence for the formation of stable alkylidyne structures from C3 and C4 unsaturated hydrocarbons adsorbed on the pt(111) single crystal surface

C3 and C4 hydrocarbon (methylacetylene, propylene, and the 2-butenes) adsorption on the Pt(111) face was studied by observing the LEED patterns that formed and by measuring the intensity versus voltage spectra for each structure. Two phases exist for each of these molecules adsorbed on the Pt(111) surface. At low temperatures, the unsaturated C-C group forms a di-σ bond to two Pt atoms. Upon warming to about room temperature, and in the presence of hydrogen for the alkynes, a conversion takes place to an alkylidyne species that is bonded to three Pt atoms and has its C-C bond nearest to the metal substrate oriented perpendicularly to the surface. The butylidyne species is shown to order its ethyl group into an (8×8) or (2323)R30° superlattice when the hydrocarbon exposure is increased; this ordering is probably a natural consequence of the steric hindrance among neighboring ethyl groups as the hydrocarbon coverage increases slightly with larger exposures.