CARBON-MONOXIDE ADSORPTION ON A CU3PT(111) SINGLE-CRYSTAL ALLOY

The adsorption of CO on Cu3Pt(111) was investigated by low energy Heion scattering. The alloy surface was characterised by low energy elec tron diffraction (LEED) and work function change (Delta phi) measureme nts. No surface ordering of the alloy was evidenced by LEED. The CO in duced work function changes were small. Under normal incidence of the beam, only the scattered He+ ions from O, Cu and Pt were detected, ind icating the orientation of the CO molecule with O facing away from the surface. In the initial stages of CO adsorption at 200 K, the Pt inte nsity in a Cu-rich surface was not detectable at an exposure of simila r to 5 L CO, indicating that all the Pt atoms were covered by CO. At a saturation coverage of CO, the Pt intensity in a Pt-rich surface, how ever, fell to similar to 12% of its clean surface intensity. This obse rvation strongly suggested that not all the Pt sites were covered by t he CO molecules. These results are discussed in relation with the stru cture and composition of the surface and compared to data for CO adsor ption on Pt(111).