Enhanced Ab Initio Molecular Dynamics Simulation of the Temperature-Dependent Thermodynamics for the Diffusion of Carbon Monoxide on Ru(0001) Surface

CO diffusion on the metal surface is an elementary process in many heterogeneous catalytic reactions. The thermodynamics and the molecular mechanism of the diffusion process are key factors contributing to the kinetics of the catalysis. Theoretical study based on computer simulations can complement experimental studies to provide much needed thermodynamic and mechanistic information. Here, we report direct ab initio molecular dynamics (MD) simulation to investigate the temperature-dependent thermodynamics of CO diffusion on the Ru(0001) surface combined with an efficient sampling method based on integrated tempering to speed up phase space sampling. We show that reliable and smooth two-dimensional potential of mean force surfaces of CO diffusion can be obtained at different temperatures. As expected, with increasing temperature, the distribution of CO adsorbate at different surface sites becomes more and more uniform, while the height of the free energy barrier to CO diffusion decreases. The simulation results were used to elucidate the physics of the temperature-dependent in-plane diffusion. The good agreement between the results of current simulations and previous theoretical studies demonstrates the effectiveness and reliability of free energy simulation with the enhanced ab initio molecular dynamics in heterogeneous surface processes. For complex situations where a simple harmonic model becomes inappropriate and convergent phase space sampling is required, direct MD simulation with enhanced sampling methods can make important contributions to our understanding of the thermodynamics of the surface catalytic processes.