Density functional theory calculations on the interaction of ethene with the {111} surface of platinum

We have performed density functional theory calculations on the adsorption of ethene onto the {111} surface of platinum. We find that the adsorption e nergy is sensitive to the k-point sampling used, with low k-point sampling giving rise to overestimated adsorption energies, close to the values predi cted from small cluster calculations. Six adsorption modes (bridge, fee hol low, hcp hollow, atop bridge, atop hollow, and cross bridge) were investiga ted on a rigid Pt {111} surface. The most stable site was the bridge site ( di-sigma type adsorption) with an adsorption energy of 108.7 kJ mol(-1) and C-C bond length of to 1.483 Angstrom, which is significantly longer than t he calculated gas-phase ethene bond length of 1.334 Angstrom. The recently proposed fee hollow site adsorption was found to be significantly less stab le (63.6 kJ mol(-1)) although slightly more favorable than the atop (pi ads orbed) modes. The effect of surface relaxation on the adsorption energy and structure was investigated by allowing the entire Pt {111) slab to relax, giving rise to large changes in the positions of the coordinating Pt atoms. The bridge site shows displacements of 0.235 Angstrom out of the surface f or the two Pt atoms directly coordinated to the ethene C atoms with an incr ease in the adsorption energy of 18.6 kJ mol(-1) compared to the rigid surf ace case from 108.7 to 127.3 kJ mol(-1). The effect of Pt relaxation was gr eatest on the atop sites with the single Pt atoms coordinated to the ethene moving 0.356 Angstrom out of the surface for both adsorption modes. This w as accompanied by an increase of the adsorption energy of 26 kJ mol(-1) wit h the atop bridge (85.8 kJ mol(-1)) slightly more stable than the atop holl ow (84.8 kJ mol(-1)). The hollow sites were affected by surface relaxation so much so that the energetic order of the atop and hollow sites is reverse d when surface relaxation is included, indicating that the latter are unlik ely to be observed. We conclude that the large effect of both the k-point s ampling and surface relaxation on the adsorption energy is based on a compr omise between the extended electronic states and localized bonding. The eff ect of periodic calculations with converged k-point sampling is to accurate ly treat the repulsion between the extended electronic states and the molec ule. The effect of surface relaxation is to allow the atoms involved in loc alized bonding to move out of the surface, reducing the repulsion due to th e extended electronic states and so increasing the adsorption energy. As su ch, the use of cluster calculations, especially for molecules with weak int eractions with the surface, would be expected to result in significant over estimation of the adsorption energies.