Electronically Induced Geometrical Catalytic Effects

Recent work has shown that small additions of chemical modifiers can cause major structural rearrangements on Ag, Cu (l-3), and Pt (4) surfaces. The first purpose of this letter is to explain how these electronically driven geometrical changes arise. Second, we suggest that in catalysis, the role of electron donor/acceptors may often be an indirect one, namely to cause such geometrical changes. We also point out that particle size effects may often be due to the reverse case of a geometrically driven electronic effect. Recent experimental (5-8) and theoretical investigations (6, 9) have established an understanding of what goes on at noble metal surfaces, which can be extended to explain the modifier effects on Ag, Cu, and Pt mentioned above. We start with a consideration of the tension between the pairwise d-electron forces and the more metallic sp bonding in the bulk metal. This is fairly well understood (20, 1Z). In the middle of the transition metal series, the d electrons provide the primary cohesion, and indeed would like to establish a shorter interatomic spacing than is observed in the metals. They cannot do so in the bulk metals because the sp electrons exert a balancing, outward pressure. (A bond shortening is observed in some molecules especially when the sp electrons are partially removed by electronegativity effects (II).) In the noble metals the situation is reversed, with the sp bonding favoring a smaller atomic volume but prevented from contracting the structure by the repulsion of the full d shells. There is a trend across the periodic table as the antibonding d orbitals becoming progressively filled, with