ALLOYING EFFECT ON THE ADSORPTION PROPERTIES OF PD50CU50(111) SINGLE-CRYSTAL SURFACE

The surface composition of the Pd50Cu50{111} single crystal as measure d by LEIS corresponds to Pd45Cu55 in the very first layer. This small Cu segregation did not induce a special surface structure as observed by LEED. The classical (1 x 1) LEED pattern was observed both after an nealing and during gas adsorption. The chemical properties of Pd-Cu al loy have been investigated by adsorption of CO, NO and H-2 and also by XPS. The amount of CO adsorbed on the surface at 160 K is at least fo ur times higher than for NO or H-2. Furthermore, the adsorption energi es decrease on Pd and increase significantly on Cu in alloy with respe ct to adsorption on pure metals. The significant shifts of core and va lence bands observed by XPS and, moreover, the change in adsorption en ergies of CO and NO were attributed to a strong electronic interaction between Pd and Cu upon alloying. This Pd-Cu interaction has been inte rpreted in terms of bond formation between the almost full valence ''s d'' band of Pd and the resonant ''dsp'' band of Cu near the Fermi leve l. An explanation using back-donation capabilities of the Pd-Cu alloy surface to CO and NO is also given to support the formation of a hybri dized molecular orbital.