Chemisorption of CO on ultrathin films of Pd on Mo(100)

The altered bonding that exists at bimetallic interfaces can affect the properties of metal monolayers and thin films. We have probed the chemisorptive properties of ultrathin films of Pd on Mo(100) by studies of CO adsorption on these surfaces. Our investigations were carried out using Auger electron spectroscopy (AES), low energy electron diffraction (LEED), temperature programmed desorption (TPD), and high resolution electron energy loss spectroscopy (HREELS). The heat of adsorption of CO is reduced from 36.5 kcal/mol on Pd(100) single crystal surfaces to 20 kcal/mol on the pseudomorphic Pd monolayer. In addition, a 34% reduction in the initial sticking probability of CO occurs on the Pd monolayer at 150 K relative to thick (20 layers) Pd films. Both the heat of adsorption and the sticking probability of CO increase with Pd film thickness indicating that the chemistry of these surfaces can be "tuned". However, the origin of this tuning is not clear. On the Pd monolayer, HREELS shows that hollow and bridging sites are populated first, followed by atop sites at higher CO coverages. This is consistent with the site preference on bulk Pd(100) surfaces. The bonding of CO at bridging sites is clearly destabilized on the Pd monolayer and to a lesser extent on thicker films. The vibrational spectra of chemisorbed CO are shown to be a sensitive probe of the structure of Pd films of this type. The alterations in CO adsorption on the Pd monolayer and thin films on Mo(100) compared with that on bulk Pd surfaces are consistent with those observed for ultrathin Pd films on other early transition metal substrates such as Nb, Ta, and W. CO bonding on the pseudomorphic monolayer of Pd on Mo(100), which has a highly strained PdPd lattice, is not weakened as much as on the relatively less strained Pd monolayers on Nb(110) and Ta(110), indicating that lattice strain is not sufficient to account for the origin of the weakened CO interactions.