First-Principles Simulations of an Aqueous CO/Pt(111) Interface

We discuss the structural, electronic, and vibrational properties of an adsorbed CO monolayer in the presence of liquid water at room temperature, as obtained by ab initio molecular dynamics simulations. The water molecules at closest distance from the substrate form a bilayer, where they are coadsorbed at empty surface sites and with relatively rigid orientation, at the difference with the bulk liquid above. Coadsorbed water strengthens the back-bonding to CO, which leads to the red shift of about 40 cm–1. The synergy between the first bilayer and the bulk liquid induces further polarization of the CO bond, which changes the transition dipole and results in the CO intensity enhancement. It is also verified that no hydrogen bond is effectively present between CO and bulk water. These findings provide new insight into the physics of the surface/adsorbate/solvent interface, thus clarifying the experimental observation to be exploited for the design of improved catalysts.