Local order at the surface of binary alloys in relation to their chemical reactivity

The behavior of binary alloys with respect to catalysis can be understood in terms of the geometric ''ensemble effect'' and/or the electronic ''ligand effect.'' For understanding, one has consequently to know precisely the local concentration and arrangement of both components at the very surface (in contact with the reactants), and also in the sublayers which influence electronically the outer atoms. Two kinds of well-defined surface are considered: binary alloys and well-controlled deposits of a given metal on a foreign metallic substrate. After a brief description of the driving forces for surface segregation and of metal-on-metal growth modes, the relations between surface composition/structure and reactivity of binary alloys are illustrated by some striking examples. In order to limit the discussion, the presentation is focused on platinum-based and palladium-based alloys. The ensemble effect is discussed first on the basis of the influence of inactive Bi, Pb or Sn deposits on Pt single crystal surfaces. For alloys, the PtCu3(111), which exhibits a p(2 x 2) ordered surface structure in which the Pt surface atoms are all isolated by Cu atoms, offers a unique model sample for studying the dilution (of Pt by Cu) influence. The electronic influence of elements present in the sublayers is illustrated on PtNi(111) and Pt3Fe(111) samples, which present a quasicomplete Pt surface layer (with more or less Ni or Fe in the sublayers) and strong modifications of their chemisorptive properties and catalytic performances for some simple reactions. The properties of surfaces largely concentrated into Pd (either by large surface segregation in alloys containing low bulk Pd content, or by Pd deposition on various single crystal metal substrates) are discussed in the light of electronic modifications induced by the substrate atoms, with consequences on their chemical reactivity.