Saturation of the surface with CO led to formation of CO adsorbed at Pd atop sites, but not at Pd2 bridge sites, indicating that the Pd atoms are isolated from each other. These Pd monomers are not stable with respect to subsurface alloy formation or Pd diffusion into the bulk, as indicated by decay of the related IR band upon annealing the surface to 350 K and more (complete disappearance after annealing the surface to 450 K). For Pt/Ag , formation of Pt monomers was concluded from the presence of a lower-temperature CO desorption peak at 380 K in temperature programmed desorption (TPD) spectra, which were recorded after Pt deposition at 380 K and saturation of the surface with CO at 90 K . This peak, which was observed up to rather high Pt coverages (> 0.22 monolayers), was associated with a weakly bound CO species on surface Pt monomers (see also the discussion in section 3.1).