Surface stress and chemical reactivity of Pt and Pd overlayers

Chemical properties of metals can be strongly modified by alloying. Whereas for association of transition metals having very different d band filling, i.e. located very far apart in the periodic table, the modified properties can be explained by direct electronic interaction, other effects have to be considered when considering metals in the same column of the periodic table, Like Ni, Pd and Pt: viz the stress induced on the outer atoms by the substrate when their respective geometric parameters are very different. We will consider associations of Ni, Pd and Pt couples, the latter two having atomic radii more than 10% greater than Ni. Surfaces that were highly concentrated in one component were obtained either by surface segregation or by atomic beam deposition. Whatever the preparation method, the surface properties were comparable. PtxNi1-x(1 1 1) alloys of respectively, 78 and 50 at.% Pt both present a well ordered (1 x 1) quasi complete Pt outer layer, the second layer mainly consisting of Ni. Nevertheless, they show very different chemisorptive properties with respect to Hz, CO,... and different reactivity for the 1,3-butadiene hydrogenation reaction. The interatomic distance in the substrate, which is respectively 2% and 5% lower than thai of pure Pt, is th;only parameter which is changed. Pd deposited on Ni(1 1 1), or Pd segregated at the surface of a Pd8Ni92(1 1 1) alloy, also exhibit a large enhancement of catalytic activity (for 1,3-butadiene hydrogenation) compared to pure Pd(1 1 1). We can again suggest that the Pd outer atoms, which are pseudomorphic with the substrate sites, have to sustain a noticeable stress. Pd deposited on Ni(1 1 0), or Pd segregated at the surface of a Pd8Ni92(1 1 0) alloy, also show a highly increased activity for the catalytic reaction considered, when compared to Pd(1 1 0). But, for this system a strain-induced buckling is observed, probably due to partial relaxation of strain at the surface. Pd segregates to a large and similar extent on the surfaces of PdNi and PdPt alloys having low Pd bulk content. But, while the catalytic reactivity of strained Pd atoms present at the surface of the PdNi alloy is greatly enhanced, the reactivity of unstressed Pd atoms present on the PdPt alloy is comparable to that of pure Pd. In conclusion, chemisorptive and catalytic properties of outer atoms can be more or less strongly modified with respect to the pure component depending upon the degree of stress they retain at the surface. (C) 2000 Elsevier Science B.V. All rights reserved.