ADSORPTION ON ORDERED CU3PT(111) - SITE SELECTIVITY

The interaction of Xe, CO and H-2 with the ordered Cu3Pt(111) surface has been studied by thermal desorption spectroscopy (TDS), Auger electron spectroscopy (AES), low-energy electron diffraction (LEED), UV-photoelectron spectroscopy (UPS) and work-function measurements (DELTAPHI) in the temperature range 50-400 K. In contrast to the Xe results CO and H-2 adsorption shows a site selectivity suggesting a local character of the interaction. After adsorption at 50 K in the case of CO two different chemisorption states are observed, CO on Pt atoms and CO on Cu sites, whereas in the case of H-2, dissociative adsorption via Pt atoms is found with only one Pt-like adsorption state. In spite of this site selectivity the properties of CO and H adsorption on the Pt-like sites are completely different from those found on pure Pt(111), namely the desorption energy is significantly lower. In the case of CO this difference can only be explained in terms of a change of the electronic properties of Pt by alloying with Cu (ligand effect), whereas in the case of H adsorption additionally an ensemble effect (different adsorption sites) has to be included.