Potential energy of H2 dissociation and adsorption on Pt(111) surface:: First-principles calculation

We investigate the interaction of molecular hydrogen with the Pt surface. In particular, we calculate the potential energy surfaces (PESs) corresponding to the dissociative adsorption of H-2 at different symmetric sites on the Pt(1 1 1) surface and for parallel and perpendicular orientations of H-2 with respect to the surface, on the basis of the density functional theory. The PES results show the dependence of the activation barriers on the H-2 adsorption site and orientation relative to the surface. For parallel orientations. the barrier is lowest (almost zero) when the H-2 center of mass (CM) is directly above the top site while the H atoms are directed towards the hcp and fee hollow sites. The activation barriers for the perpendicular orientation are always much higher than those for the parallel orientation, which indicates that the parallel orientation is favored for H-2 adsorption. This result also suggests that an incident H-2 initially in a nonparallel (e.g., perpendicular) orientation tends to reorient itself towards a parallel orientation where it is easier for H, to be adsorbed.