Surface potential measurements of CO chemisorption on clean palladium-silver alloy films

Abstract

Surface potentials (S.P.) due to CO chemisorption at 0 °C were measured for a series of evaporated PdAg alloy films prepared in ultrahigh vacuum. The S.P. increased from zero (for pure Ag) with increasing Pd content but then increased sharply at ~60% Pd, remaining constant for higher Pd contents up to pure Pd (−0.62 V at 2 × 10 −6 Torr CO). Results for Ag-rich films require a special explanation, e.g., electron activation, if the concept of relating chemisorption capacity to d-band vacancies, a collective property, is to be maintained but the concept of surface Pd atoms acting as individual adsorption centers in an Ag diluent is difficult to generalize across the whole composition range. It is suggested that the viewpoint from which the surface properties of an alloy system such as PdAg are discussed, may depend on the concentration of the Group IB additive because, e.g., the band of surface electron states may not overlap the normal crystal band in Ag-rich alloys (unlike Pd-rich alloys), so that the surface properties need not relate to the electronic structure of the corresponding bulk alloy.