Water and CO (co-)adsorption on pseudomorphic Pt films on Ru(0001) - a low-temperature scanning tunneling microscopy study.

Coadsorption of CO and water under ultrahigh vacuum (UHV) conditions can be considered as a model system for the interaction of metal surfaces with CO in an aqueous electrochemical environment. Nevertheless, this has rarely been investigated, and in particular for catalytically relevant bimetallic systems, there is hardly any information available. Here we report results of a low-temperature scanning tunneling microscopy (STM) study on the adsorption and coadsorption of CO and water on a Ru(0001) surface covered with a pseudomorphic Pt film of 2 or 3 monolayers thickness. The role of kinetic effects introduced by the sequence of adsorption, either pre-adsorption of CO followed by water adsorption or pre-adsorption of water followed by CO adsorption, on the adlayer structure formation will be demonstrated and discussed. Furthermore, the data show a distinct influence of the thickness of the Pt film, reflecting changes in the chemistry of the Pt surface due to electronic interactions with the underlying Ru(0001) substrate ('vertical ligand effects'). Implications of the present findings on the interaction of CO with these bimetallic PtRu surfaces under electrochemical conditions will be discussed.