Chemisorption of Organics on Platinum. 2. Chemisorption of C2Hx and CHx on Pt(111)

Abstract

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) CH_3 prefers an on-top site (a bond energy of ∼54 kcal/mol), (ii) CH_2 prefers a 2-fold bridge site (a bond energy of ∼104 kcal/mol), and (iii) CH prefers the fcc 3-fold bridge site (a bond energy of ∼167 kcal/mol). Similarly, C_2H_4 forms a strong (36 kcal/mol) di-σ bond (the π bond is BE = 8.5 kcal/mol), while CHCH_2 forms a tri-σ bond. The results for C_2H_x/Pt_8 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 C_2H_4 on Pt(111).