and/or small differences between the C1s BEs of CO adsorbed on bridge or threefold sites. Finally, these calculations also revealed that the simultaneous adsorption of 2 CO on a Pt1 monomer is instable compared to 1 adsorbed CO and 1 gas phase . This agrees with experimental findings in so far as the formation of multicarbonyl species, which is often discussed in the literature for Ru nanoparticles [129-131], has rarely been proposed for Pt nanoparticle catalysts . Despite the very similar structural and electronic properties of the two substrate surfaces, the situation is very different for CO adsorption on PdAg/Pd surface alloys . Saturation CO-TPD spectra recorded on surface alloys prepared by deposition of 1.2 monolayers of Ag on Pd and subsequent annealing to increasing temperatures show a continuous shift of the CO desorption peak up to 800 K annealing . Annealing to even higher temperatures leads to the appearance of new peaks on the low-temperature and high-temperature side of the main peak, until upon annealing to 1100 K Ag is completely desorbed / dissolved in the bulk and the TPD spectrum resembles that of CO desorption from Pd(111) . More information is obtained from CO TPD series recorded on PdAg/Pd(111) monolayer surface alloys with different Pd surface contents, which at lowest Pd concentrations (2%) show a peak at 316 K, which was associated with CO desorption from Pt1 monomers surrounded by Ag surface atoms . At higher Pd contents, between 7% and 10%, this upshifted to 364 -380 K, with a shoulder at about 320 K. These features, which became more pronounced for 25% surface Pd, were attributed to desorption of 2 CO from Pd2 dimers, with the first one desorbing at 320 K and the second one at 380 K (see discussion of CO coverage effects with Figure ). At even higher Pd surface concentration ( 39%), a peak at about 460 K appeared, which was related to CO desorption from threefold hollow on Pd3 or larger ensembles . Pronounced differences compared to the CO adsorption behavior on PtAg/Pt(111) surface alloys were obtained also in DFT calculations performed for CO adsorption on PdAg/Pd(111) surface alloys . The resulting CO adsorption energies obtained for the same adsorption ensembles and sites as in Figure for PtAg/Pt(111) are presented in Figure . These calculations also show a strong preference for adsorption on threefold hollow sites, similar to CO adsorption on Pd [134,135], with the CO adsorption energy increasing in the order Ead,on-top < Ead,bridge < Ead,hollow site. On the other hand, there is very little change in the adsorption energy on the same site when going to larger ensembles, e.g., for on-top adsorption when going from Pd1 via Pd2 to Pd3, indicating that similar to CO adsorption on PtAg/Pt(111) lateral ligand and strain effects largely compensate each other. This conclusion is supported also by the calculated result that Pd3 and Pd4 (not shown) exhibit similar CO adsorption energies on the threefold adsorption sites, which suggests that adsorption on Pd3 sites largely determines the highest adsorption energy and that the effect of additional surrounding Pd ligands is small. In this sense, experimental results (TPD peaks, infrared bands) attributed to Pd3 ensembles may originate not only from Pd3, but also from larger ensembles . Finally we would like to add that these desorption temperatures are very close to those reported for CO desorption from Pd1 monomers (300 K) and Pd2 dimers (360 K) in a PdAu surface alloy on Pd , indicating rather similar ligand and strain effects in both cases . reduced CO adsorption energy on on-top sites of Pd monomers surrounded by Ag atoms also in the second layer . The trend seems to differ from expectations based on a simple bond order effect, since one would expect a stronger Pd-CO binding on such sites, due to the weaker Pd bonding to the underlying Ag atoms instead of Pd atoms. Looking more closely, however, we have to consider that also the interaction between Ag atoms in the surface layer and those in the Ag under-layer top layer is weaker than the interactions with a Pd second layer. This results in a stronger interaction between Pd surface atom and neighboring Ag surface atoms (Ag ligands), which leads to a weaker Pd-CO bond.