A covalent model for the bonding of adsorbed hydrocarbon fragments on the (111) face of platinum

A systematic molecular orbital study of the location of CH_n and C-CH_n (n = 1, 2, 3) species on a Pt(111) single-crystal surface has been performed by using both a cluster model and a band structure calculation within the framework of the Extended Hückel Theory (EHT). These species may be involved in the hydrogenation/dehydrogenation processes of hydrocarbons. The observed dependence of the adsorption site on the number of hydrogens in the CH_n fragments suggests that any C-H bond breaking in CH_n species must involve a change of adsorbate bonding site. The carbon is found to be located on the surface in such a way as to complete its tetravalancy. Thus, CH occupies a three-fold coordinated hollow site, CH₂ a two-fold coordinated bridge site and CH₃ a one-fold coordinated top site. C-CH₃ is found to be perpendicular to the surface in a three-fold hollow site in agreement with experimental observations. It is also found that a displacement of C-CH₂-R to a top site makes a β C-R cleavage easier.