Dynamics of water adsorption on Pt{110}-(1×2): A molecular dynamics study

Abstract

Recent experimental studies of water adsorption on Pt{110}-(1x2) using supersonic molecular beams [F. R. Laffir et al., J. Chem. Phys. 128, 114717 (2008)] have revealed that the translational energy dependence of the initial sticking probability is a stepwise function with a threshold energy of 5 kJ/mol. The initial sticking probability increases sixfold from approximately 0.1 (at translational energies less than 5 kJ/mol) to approximately 0.64 (at translational energies greater than 10 kJ/mol). The aim of this work is to study the adsorption dynamics of water using classical molecular dynamics simulation in order to assess what physical factors are responsible for the observed behavior of the initial sticking probability. The simulations were performed using a purpose-designed code; water molecules were modeled using the well-known TIP4P water model, whereas the water-platinum potential energy function was determined using the ab initio density functional theory calculations. We conclude that the main factor controlling the initial sticking probability is a relatively weak energy transfer between the water molecule and the surface substrate during collision. This energy transfer is enhanced when the total energy of the water molecule increases. The assumption of an exponential increase of the probability of the energy transfer as a function of total energy of water molecule gives initial sticking probabilities very similar to those experimentally obtained. The same model was applied for the simulation of the coverage dependent sticking probability using a hybrid method comprising molecular dynamics and kinetic Monte Carlo approaches. We found a reasonable agreement between our results and the experimental data. The sticking probability as a function of coverage initially increases due to an increasing amount of the adsorbate island edges; it reaches a maximum and finally decreases as the islands merge together at high coverage. The saturation coverage was determined to be 2.8 ML at surface temperature 165 K, where water forms a puckered almost regular lattice with each water molecule having four nearest neighbors. At the studied temperature we did not observe the existence of stable water multilayers on the surface which is consistent with the experimental findings.