CO chemisorption on Cu adlayers on Pt(111)

Photoemission spectroscopy and thermal desorption have been used to study the interactions between CO and Cu adlayers on Pt(111). At the Cooper minimum in the $\mathrm{Pt}5d$ photoionization cross section ($h\nu =150\mathrm{}$ eV), the $\mathrm{Cu}3d$ states are observed to undergo a preferential attenuation when CO—Cu bonding takes place (at a sample temperature of ∼155 K). The occurrence of significant Cu valence-electronic rearrangement, induced by CO—Cu bonding, is also reflected in the $\mathrm{Cu}2p_{\frac{3}{2}}$ core-level shift of 0.88-0.80 eV towards larger binding energy. In contrast, room-temperature adsorption of CO on the exposed Pt sites does not affect the electronic structure of the Cu adlayer. CO—Pt bonding selectively attenuates the $\mathrm{Pt}5d$ states near the Fermi level, and leads to a higher binding-energy shift of ∼1.3 eV in the $\mathrm{Pt}4f_{\frac{7}{2}}$ surface core level. For CO chemisorbed on Cu adlayers which are in immediate contact with the Pt substrate, the CO desorption spectra show a chemisorption bond strength varying between 15.8-17.0 kcal/mole and 12.6-13.5 kcal/mole, in contrast to the bond strength of only 11.8-12.7 kcal/mole on Cu substrates without Pt neighbors.