ELECTRONIC-PROPERTIES OF BIMETALLIC SURFACES

The formation of a surface metal-metal bond can produce large perturba tions in the electronic and chemical properties of a metal, In this re view, we examine the electronic and chemical properties of bimetallic systems formed by vapor depositing metals from groups 10-12 of the per iodic table onto single-crystal faces of transition or s,p metals, It is shown that the bonds in these systems are best described as 'metall ic' with a small degree of 'ionic' character, In many cases, bimetalli c bonding induces a redistribution of charge that affects the populati on of the d orbitals of the metals, producing binding-energy shifts in their core levels and valence d band, This may be viewed as a result of an optimization of the bonding capabilities of the metals, A very g ood correlation is found between the electronic perturbations in a bim etallic system and its cohesive energy, Bimetallic bonds that display a large stability usually involve a significant redistribution of char ge around the metal centers, This redistribution of charge also affect s the ability of the metals to adsorb CO, Some examples that illustrat e this phenomenon are briefly discussed, For Pd and Pt atoms supported on surfaces of early-transition or s,p metals, there is a shift of el ectrons from the adatoms toward the metal-metal interface that weakens the strength of the Pd-CO and Pt-CO bonds, On the other hand, Cu atom s deposited on surfaces of late-transition metals exhibit a gain of d electrons that enhances the strength of the Cu-CO bond.