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In a combined variable temperature STM-HREELS study we could identify Au edge atoms in the first, second and higher Au layers on a Pd(111) substrate as sites of a strongly enhanced Au−CO interaction compared with CO adsorption on the respective terrace sites. CO molecules adsorbed on these Au edge atoms give rise to a C−O stretch frequency of 263 meV, indicative of linearly bound CO. Adsorption on Au terrace sites is not observed at 110 K. The increased Au−CO interaction is attributed to a modification in the electronic properties of Au edge atoms, related to their reduced coordination.The activity of supported metal catalysts is decisively influenced by various factors, such as the nature of the support material, the presence of chemical promoters, the addition of a second metal ("bimetallic catalysts") and, in particular, by the size of the chemically active particles [1,2]. For the latter case not only is the increased surface area important, but electronic modifications due to the increased fraction of low-coordination surface atoms ("defect atoms") on particles of a few nanometers in diameter may also play an important role. Significant modifications in the chemical properties caused by each of these effects have been verified in model studies, e.g. on highly stepped vicinal surfaces [3] or on bimetallic surfaces of single-crystal metal substrates [4,5]. The correct interpretation and assignment of the observed changes in the chemical properties, on a microscopic scale, however, depends critically on the detailed knowledge of the local surface structure and composition, in particular for more complex substrates, which in most studies so far has not been available.In an effort to overcome these problems we have recently started a detailed study on the local chemical properties of structurally well-defined bimetallic Au/Pd(111) surfaces, using CO as a probe molecule. By employing a combination of atomic resolution scanning tunneling microscopy (STM) as a local structural probe and high-resolution electron energy loss spectroscopy (HREELS) as an area-integrating chemical probe that is sensitive to the internal bonding properties of the adsorption complex, we can determine the local chemical properties of specific structural elements by systematically varying the surface structure. The first results, demonstrating the existence of two new weakly adsorbed CO states in a linear on-top configuration on mixed AuPd sites, were published recently [6]. It was shown that these states result from CO adsorption on an (intermixed) AuPd(111) surface alloy obtained by annealing of Au monolayer-covered Pd(111) substrates and from CO adsorption at the edges of Au monolayer islands on the same substrate, respectively. Further information on the geometry and nature of the "island edge" adsorption site, however, was not available.In this contribution we will demonstrate that CO adsorption at the Au island edges most likely takes place on the higher terrace edge of the Au islands and that the enhanced Au−CO interaction is mainly d...

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