The structure and dynamics of water adsorbed at metal surfaces is of central importance not only from a fundamental but also from an applied point of view in corrosion processes, electrochemistry and catalysis, in which the formation and/or dissociation of water on metallic surfaces occur. Even though water/metal systems have been extensively investigated, particularly on flat surfaces, little is known about the chemistry of water on stepped surfaces, which commonly take place in realistic situations.
By means of first-principles simulations, we investigate the adsorption and dissociation of water clusters and one-dimensional chains of water on flat (111) and stepped (211) and (221) surfaces of several transition metals: Rh, Ir, Pd and Pt. Water binds preferentially to the edge of the steps than to terrace sites, forming isolated clusters or chains. The linear clusters formed at the step are stabilized by the cooperative effect of chemical bonds with the metal and hydrogen bonding. The adsorption of water on (221) surfaces is the most favorable, while on (111) it is the least favorable. The enhanced reactivity of metal atoms at the step edge and the cooperative effect of hydrogen bonding enhance the chances of partial dissociation of water clusters on stepped surfaces. Partial dissociation occurs only when the coverage is sufficient to produce trimers or larger clusters, and is promoted by the combination of O−Pt interaction and hydrogen-bonding. The proton transfer mechanism in trimer or 1D chains is determined as well. Dissociation is facilitated most on (211), whereas the least is on (111) surfaces. The identification of trends in the energetics of water dissociation at transition metals is expected to aid the design of better materials for catalysis and fuel cells, in which the density of steps at surfaces would be a relevant additional parameter.
We further explore the dynamics of water film on flat Pt(111) by means of first-principles molecular dynamics simulations. We discuss the structure of water adlayer, the protonation state of water interface and how proton hopping occurs in water/Pt system, and typical time scales of water dynamics.