While for Pd(111) CO adsorption on bridge and threefold sites is strongest, CO adsorption on Pt(111) is dominated by adsorption on on-top and bridge sites. Furthermore, adsorption on Pd(111) also exhibits pronounced repulsive interactions, as indicated by the very broad TPD peaks at high coverages, while this is much less the case for CO desorption on Pt(111). mainly in catalysis studies performed under ambient conditions. They were often proposed to form on undercoordinated sites such as edge or corner sites on metal nanoparticles, where steric repulsion between these species can be reduced, in addition to the generally stronger binding of adsorbates on these sites. Repulsion between closely neighbored COad species can also be reduced for adsorption on individual active surface atoms or small ensembles of surface atoms embedded in a matrix of inert surface atoms.