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 b e considered when considering metals in the same column of the periodic tab le, Like Ni, Pd and Pt: viz the stress induced on the outer atoms by the su bstrate when their respective geometric parameters are very different. We will consider associations of Ni, Pd and Pt couples, the latter two havi ng atomic radii more than 10% greater than Ni. Surfaces that were highly co ncentrated in one component were obtained either by surface segregation or by atomic beam deposition. Whatever the preparation method, the surface pro perties were comparable. PtxNi1-x(1 1 1) alloys of respectively, 78 and 50 at.% Pt both present a we ll ordered (1 x 1) quasi complete Pt outer layer, the second layer mainly c onsisting of Ni. Nevertheless, they show very different chemisorptive prope rties with respect to Hz, CO,... and different reactivity for the 1,3-butad iene hydrogenation reaction. The interatomic distance in the substrate, whi ch is respectively 2% and 5% lower than thai of pure Pt, is th;only paramet er 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 t hat the Pd outer atoms, which are pseudomorphic with the substrate sites, h ave 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-indu ced buckling is observed, probably due to partial relaxation of strain at t he surface. Pd segregates to a large and similar extent on the surfaces of PdNi and PdP t 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 enha nced, the reactivity of unstressed Pd atoms present on the PdPt alloy is co mparable 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 dependin g upon the degree of stress they retain at the surface. (C) 2000 Elsevier S cience B.V. All rights reserved.