Surface defects play a critical role in many surface processes, for example in the formation of a water layer on Pt(111). In this DFT study, we describe water adsorption, diffusion, dissociation and early cluster formation on terrace, steps and kinks on Pt(111). The adsorption energy of a single water molecule increase as it moves from the flat terrace up to the step edge and then atop a kink atom. The highest activation barrier along this path is only 0.22 eV. The stronger binding on the step and kinks makes the reverse barriers much larger and, therefore, the diffusion from the step or kink back to the flat terrace less likely. This suggests that at approximately 100 K, water will diffuse to lower energy adsorption sites on the steps and kinks and form chains on step edges before wetting the lower terrace, in agreement with STM measurements by Morgenstern, M. et al. [1].

Surface defects also play an important role in the dissociation of water molecules. The calculated reaction energy for water dissociation, H2Oads → OHads + Hads, on flat terrace, step and kink shows an interesting trend where the reaction energy on the flat terrace is almost twice the energy at a kink and three times the energy on a step. Water molecule dissociation on flat Pt(111) terrace is therefore less likely than at defects. The reaction energy is 0.21 eV on the (221) step and 0.37 eV on the (854) kink, making the defect sites significantly more promising for water dissociation.

The activation energy for dissociation is also lowest at a step site, 0.80 eV. An oxygen assisted dissociation on the step (H2Oads + Oads → 2 OHads) has even lower reaction energy but the activation barrier is similar. Although the reaction energy at the kink was not so high, the activation barrier for dissociation is very high, 1.4 eV. The energy landscape is quite complicated around the kink but we consistently get this high barrier for the various dissociative pathways that have been tried. The high barrier may be the result of the strong adsorption at the kink sites.

1. Morgenstern, M., Michely, T. and Comsa, G., Phys. Rev. Lett.77 703 (1996).