CO CHEMISORPTION ON DIFFERENTLY PREPARED CU3PT(111) ALLOY SURFACES

We have investigated the interaction of CO with single-crystal Cu3Pt(1 11) alloy surfaces between 100 and 900 K by means of LEED, Auger elect ron spectroscopy (AES), and temperature-programmed thermal desorption (TPD). The surface composition and long-range order of single-crystall ine Cu3Pt(111) faces can be varied within wide limits by appropriate s puttering and annealing. For the clean alloy system, we found, in addi tion to the previously reported p(2 X 2) superstructure, a novel (2 ro ot 3 X 2 root 3)R30 degrees phase at still higher Cu surface concentra tions, formed upon annealing close to the critical temperature. In agr eement with previous investigations, Pt and Cu adsorption sites are we ll separated on Cu3Pt(111) surfaces and could be distinguished also by CO chemisorption-titration. We find that the CO binding energy on Pt sites is markedly reduced even at small Cu concentrations, and invoke electronic (ligand) effects exerted on the Pt atoms by neighboring Cu atoms; the Cu sites, on the other hand, are much less affected by Pt.