Metal-metal bonding on surfaces: electronic and chemical properties of Ag on Ru(001)

Abstract

The interaction between Ag and Ru(001) has been studied using TDS, XPS and XAES. Submonolayer coverages of Ag desorbed in the temperature range from 900 to 1050 K. The desorption of the Ag monolayer followed zero-order kinetics with an activation energy of 65 kcal/mol. By comparing the desorption temperatures of Cu, Ag and Au from W(110) and Ru(001), a direct relation was found between the bulk cohesive energy of a noble metal and its adsorption energy on a transition metal. A monolayer of Ag on Ru(001) shows Ag 3d levels and Auger MVV transitions that are shifted ∼ 0.1 eV toward lower binding energy with respect to those of the bulk atoms in pure Ag. In CO/Ag/Ru(001) surfaces, the interaction between Ag and Ru enhances the strength of the AgCO bond and weakens the RuCO bond. The CO-desorption temperature from one monolayer of Ag supported on Ru(001) is ∼ 90 K higher than that found on Ag(111). This corresponds to an increase of ∼ 5 kcal/mol in the strength of the AgCO bond. The results of CO-TDS and O(1s)-XPS show that the presence of Ag induces a reduction in the charge transfer from Ru into the CO(2π ∗) orbitals, which weakens the RuCO bond by ∼ 6 kcal/mol and shifts the O(1s) peak position 0.4 eV toward higher binding energy. Oxygen molecules adsorbed on Ag atoms bonded to Ru(001) exhibit non-equivalent oxygen atoms (probably a “superoxo-like” species), a desorption temperature of 230–240 K and a very low probability for dissociation (0.05–0.15 at T < 250 K).