Geometry of the Adsorption Sites in Metal Nanoparticles and Surfaces

We present a study of 18 adsorption sites found on metal nanoparticles and surfaces that have either a Face-Centred Cubic (FCC) or Hexagonal Close-Packed (HCP) structure. Most metals in the periodic table have these structures, and we determine using a geometric approach the adsorption site geometry on a nanoparticle made using physical magnetic ball-and-stick models. These geometric models include the existence of an octahedral or tetrahedral hole beneath the adsorption site, as these can affect the adsorption site strength of adsorbates. Furthermore, these adsorption sites are a combination of the three-fold hollows and four-fold hollows, which are adsorption sites known to activate diatomic molecules (e.g. N2, CO). In addition, adsorption of larger molecular weight adsorbates can be defined on these sites as they provide multiple contact points in contrast to the typical, four-fold hollow, three-fold hollow, bridge and atop adsorption. We find that there are 9 geometrically distinct adsorption site topologies that are composed of square (i.e. 100) and triangular (i.e. 111) motifs. These adsorption site topologies when combined with a characteristic angle (ζ) result in 18 distinct adsorption site geometries that can be found on metal nanoparticles and surfaces. A systematic naming system for these adsorption sites is provided that explicitly defines the adsorption site geometry. Using this approach we find that there are five different type of B5 sites, an adsorption site that has been previously found to activate dinitrogen on ruthenium for the ammonia synthesis reaction.