MOLECULAR MODELING OF ETHYLIDYNE ADSORPTION AND DIFFUSION ON PT(111)

The adsorption geometry and diffusion behavior of ethylidyne (CCH3) on Pt(111) is studied in order to help elucidate the role of this hydroc arbon species during ethylene hydrogenation and dehydrogenation over P t catalysts. A variant of the extended Huckel method is used, which al lows bond-specific parametrization in molecules. It is adapted to geom etrically infinite periodic systems, and empirical parameter values su itable for hydrocarbons on Pt surfaces are developed. The 3-fold fee a nd hcp hollows are found to be the most stable adsorption sites, with an energy barrier of about 0.11 eV between them. This barrier suggests relatively easy diffusion of ethylidyne along the Pt(111) surface (su bject only to steric intermolecular constraints), allowing unimpeded e thylene approach by opening up sites where subsequent hydrogenation re actions could take place with preadsorbed hydrogen.