A FT-IRAS study of the vibrational properties of CO adsorbed on Cu/Ru(001). II. The dispersion of copper

Abstract

The dispersion of copper adsorbed on a Ru(001) substrate has been investigated by using Fourier transform-infrared reflection absorption spectroscopy (FT-IRAS) and carbon monoxide as a molecular probe. Copper films evaporated at 85 K show a drastically different CO adsorption behavior compared to annealed films and exhibit a variety of adsorption sites. Characteristic C–O stretching frequencies allow us to identify small copper clusters of 1–4 atoms (2138–2123 cm−1), two-dimensional (2120–2110 cm−1) and three-dimensional (2098 cm−1) copper aggregates. After annealing to 250 K copper films at sub- and monolayer coverages form well-ordered small two- and three-dimensional copper aggregates. Formation of the epitaxial monolayer or islands of copper (2082 cm−1) requires a surprizingly mild annealing temperature of 350 K. Further annealing to 540 K results in increasing domain size of the copper islands or annealing of defect sites of the epitaxial monolayer. Multilayer coverages of copper evaporated at 85 K exhibit C–O stretching frequencies found for high-index copper single crystal surfaces, e.g., (211) and (755). This indicates a large number of surface steps and protruding copper atoms associated with rough films. Annealing to 540 K results in a smooth copper layer with preferential (111) orientation (2075 cm−1). The vibrational data presented here for Cu–Ru(001) agree well with previous reports of CO adsorption on copper single crystals, supported or evaporated films, and matrix-isolated clusters. They further allow us to determine the dispersion of supported Cu–Pt and Cu–Ni catalysts from data in the literature.