Chemisorption of (CH _x and C ₂H _y) hydrocarbons on Pt(111) clusters and surfaces from DFT studies

We used the B3LYP flavor of density functional theory (DFT) to study the chemisorption of all CH, and C ₂H _y intermediates on the Pt(111) surface. The surface was modeled with the 35 atom Pt _14.13.8 cluster, which was found to be reliable for describing all adsorption sites. We find that these hydrocarbons all bind covalently (σ-bonds) to the surface, in agreement with the studies by Kua and Goddard on small Pt clusters. In nearly every case the structure of the adsorbed hydrocarbon achieves a saturated configuration in which each C is almost tetrahedral with the missing H atoms replaced by covalent bonds to the surface Pt atoms. Thus, (Pt ₃)-CH prefers a μ ₃ hollow site (fcc), (Pt ₂)CH ₂ prefers a μ ₂ bridge site, and PtCH ₃ prefers μ ₁ on-top sites. Vinyl leads to (Pt ₂)CH-CH ₂(Pt), which prefers a μ ₃ hollow site (fcc). The only exceptions to this model are ethynyl (CCH), which binds as (Pt ₂)C=CH(Pt), retaining a CC π-bond while binding at a μ ₃ hollow site (fcc), and HCCH, which binds as (Pt)HC=CH(Pt), retaining a π bond that coordinates to a third atom of a μ ₃ hollow site (fcc) to form an off center structure. These structures are in good agreement with available experimental data. For all species we calculated heats of formation (δH _f) to be used for considering various reaction pathways on Pt(111). For conditions of low coverage, the most strongly bound CH _x species is methylidyne (CH, BE = 146.61 kcal/mol), and ethylidyne (CCH ₃, BE = 134.83 kcal/mol) among the C ₂H _y molecules. We find that the net bond energy is nearly proportional to the number of C-Pt bonds (48.80 kcal/mol per bond) with the average bond energy decreasing slightly with the number of C ligands.