Simulation of gas-surface interactions using ReaxFF reactive molecular dynamics: Oxygen adsorption on platinum

Atomistic simulations equipped with the ab initio based classical reactive force field ReaxFF are used to study adsorption of oxygen on a Pt(111) surface. Molecular Dynamics (MD) simulations are used to study the adsorption dynamics of O₂ on Pt(111) for both normal and oblique impacts, whereas Grand Canonical Monte Carlo (GCMC) calculations are employed to study the surface coverage of atomic oxygen on the same platinum surface. Overall, good quantitative agreement with the experimental data is found. Our MD simulations reproduce the characteristic minimum of the trapping probability at kinetic incident energies around 0.1 eV. This feature is determined by the presence of a physisorp-tion well in the ReaxFF Potential Energy Surface (PES) and the progressive suppression of a steering mechanism when increasing the translational kinetic energy (or the molecule's rotational energy) because of steric hindrance. In the energy range between 0.1 eV and 0.4 eV, the sticking probability increases, similarly to molecular beam sticking data. For very energetic impacts (above 0.4 eV), ReaxFF predicts sticking probabilities lower than experimental sticking data by almost a factor of 3, due to an overall less attractive ReaxFF PES compared to experiments and DFT. For oblique impacts, the trapping probability does not scale with the total incident kinetic energy, but is reduced by the non-zero parallel momentum because of the PES corrugation. Furthermore, our simulations predict quasi-specular (slightly supraspecular) distributions of angles of reflection, in accordance with molecular beam experiments. With GCMC simulations, a coverage of about 0.25 is determined at ultra-vacuum conditions (∼ 10^-10 atm), reproducing the experimental observations. Further refining of the potential parameters will be aimed to improve the agreement of sticking results at high E_i (by including direct dissociation pathways in the training set) and to reproduce the p(2 × 2) adsorbates surface structure at 0.25 coverage (by strengthening the lateral repulsion between adsorbed O atoms).