A comparative theoretical study of the hydrogen, methyl, and ethyl chemisorption on the Pt(111) surface

Chemisorbed hydrogen and various intermediate hydrocarbon fragments play an important role in the important reaction of ethylene hydrogenation to etha ne, which is catalyzed by Pt(lll). As a first step toward building a theore tical mechanism of the ethylene hydrogenation process, binding site prefere nces and geometries of chemisorbed hydrogen, methyl, and ethyl on the Pt(ll l) surface are presented and rationalized. State-of-the-art Pseudopotential Planewave Density Functional Theory is employed for calculating accurate b inding energies and geometries for the adsorbates. A comprehensive theory o f hydrogen and methyl chemisorption on Pt(lll) is developed with the help o f Crystal Orbital Hamilton Population formalism within the extended Huckel molecular orbital theory. The symmetry properties of the surface Pt orbital s as well as the mixing of Pt s, p, and d orbitals in pure Pt is shown to b e crucial in determining the strength of subsequent interaction with an ads orbate. It is suggested that hydrogen moves freely on the Pt(lll) surface w hile the methyl and ethyl groups are essentially pinned on the atop positio n. Strong agostic interactions between C-H bonds and surface Pt are propose d for methyl and ethyl on higher symmetry sites. The different nature of ch emisorption on Pt and Ni surfaces is speculated. Theoretical results presen ted in this paper are generally consistent with the available experimental data.