(5f) Water Interaction and Diffusion on Platinum Surfaces
Density functional theory (DFT PW91) calculations were used to study diffusion of water molecule on flat Pt(111) terrace as well as on kinks and steps edges. Diffusion on the (111) terrace is predicted to become active on the time scale of seconds at 74 K in reasonable agreement with experimental results of Kay and coworkers (Daschbach et al. J. Chem. Phys. 120 (2004) 1516). The binding at steps and kinks on the Pt(111) surface is substantially stronger than at the flat terrace, by 0.16 eV and 0.25 eV, respectively.
The diffusion barrier of a single water molecule moving on the flat terrace, up to a kinks side or along the step edge are all comparable while the barrier from the flat terrace up to a step is significantly lower then all the other barriers calculates. Consequently the barrier for diffusion from the step edge down to the terrace is substantially larger then on the flat terrace. At low coverage, clusters of water molecules would, therefore, be expected to line the upper side of step edges. This is consistent with experimental STM images taken by Morgenstern et al. (Morgenstern et al. Phys. Rev. Lett. 77 (1996) 703).