Chemisorption of organics on platinum. 2. Chemisorption of C2Hx and CHx onPt(111)

Using the interstitial electron surface model (IESM) developed in the accom panying part, we examined the structures and energetics of a number of orga nic fragments on Pt surfaces. Using nonlocal density functional methods (B3 LYP) we find that organics covalently bond to the Pt(lll) surface with loca lized sigma bands to the surface Pt atoms, leading to tetrahedral hybridiza tion of each carbon bonded to the surface. Thus, (i) CH3 prefers an on-top site (a bond energy of similar to 54 kcal/mol), (ii) CH2 prefers a 2-fold b ridge site (a bond energy of similar to 104 kcal/mol), and (iii) CH prefers the fee 3-fold bridge site (a bond energy of similar to 167 kcal/mol). Sim ilarly, C2H4 forms a strong (36 kcal/mal) di-sigma bond (the pi bond is BE = 8.5 kcal/mol), while CHCH2 forms a tri-sigma bond. The results for C2Hx/P t-8 are in good agreement with available experimental results on Pt(lll) (p i- and di-sigma-bonded ethylene and ethylidyne). These results are used to obtain heats of formation (Delta H-f) for chemisorbed intermediates useful in estimating the energetics of various hydrocarbon intermediates on Pt sur faces. The application of these Delta H-f values is illustrated by consider ing ethylene hydrogenation and the decomposition of C2H4 on Pt(111).