Abinitio simulation of ‘liquid’ water on a Pd surface

Abstract

We report on the application of the Car–Parrinello molecular dynamics method (CPMD) to the interaction of water ensembles with Pd(100) model surfaces. Periodic Pd₅₀O₈ and Pd₁₈O₂ slabs covered with nine and 18 water molecules, respectively, were employed to analyze the balance of adsorbate–adsorbate and adsorbate–surface interactions as a function of water ensemble size, in the limiting cases of a clean surface and high surface coverage with atomic oxygen. Our simulations suggest that ‘liquid’ water molecules are distributed, for the most part, randomly on Pd(100) at 300 K, with a characteristic intermolecular distance distribution in distinct regions of the water/metal interface. The structure of low-mobility local water clusters which stay close to the surface resembles that of earlier proposals based on thin film experiments. In addition, our results corroborate the experimental finding that a high surface coverage with oxygen reduces the water–Pd interaction compared to that on the clean solid.