Ensemble vs. electronic effects on the reactivity of two-dimensional Pd alloys: a comparison of CO and CH₃OH adsorption on Zn/Pd(111) and Cu/Pd(111)

The adsorption of CO and CH₃OH on two-dimensional PdCu alloys on Pd(111) was studied using temperature-programmed desorption (TPD) and high-resolution electron energy loss spectroscopy (HREELS), and compared to results previously obtained for analogous PdZn alloys on Pd(111). Cu addition to the Pd(111) surface was found to alter the preferred adsorption sites for CO from threefold to bridge Pd sites and decrease the activity for the dehydrogenation of CH₃OH. However, the effect of Cu was much less dramatic than that observed for Zn on Zn-modified surfaces. Preliminary DFT calculations also show that Cu causes less perturbation of the electronic structure of nearby Pd sites relative to Zn. The experimental results for the surface PdCu alloys indicate that Cu predominantly has an ensemble effect on reactivity while more significant long-range electronic interactions in addition to ensemble effects appear to be important for PdZn.