Chemisorption of organics on platinum. 2. Chemisorption of C₂H_x and CH_x on Pt(111)

Using the interstitial electron surface model (IESM) developed in the accompanying part, we examined the structures and energetics of a number of organic fragments on Pt surfaces. Using nonlocal density functional methods (B3LYP) we find that organics covalently bond to the Pt(111) surface with localized σ bonds to the surface Pt atoms, leading to tetrahedral hybridization of each carbon bonded to the surface. Thus, (i) CH₃ prefers an on-top site (a bond energy of ∼54 kcal/mol), (ii) CH₂ prefers a 2-fold bridge site (a bond energy of ∼104 kcal/mol), and (iii) CH prefers the fee 3-fold bridge site (a bond energy of ∼167 kcal/mol). Similarly, C₂H₄ forms a strong (36 kcal/mol) di-σ bond (the π bond is BE = 8.5 kcal/mol), while CHCH₂ forms a tri-σ bond. The results for C₂H_x/Pt₈ are in good agreement with available experimental results on Pt(111) (π- and di-σ-bonded ethylene and ethylidyne). These results are used to obtain heats of formation (ΔH_f) for chemisorbed intermediates useful in estimating the energetics of various hydrocarbon intermediates on Pt surfaces. The application of these ΔH_f values is illustrated by considering ethylene hydrogenation and the decomposition of C₂H₄ on Pt(111).