Structure of bimetallic clusters. Extended x-ray absorption fine structure (EXAFS) studies of Ag–Cu and Au–Cu clusters

Abstract

Extended x-ray absorption fine structure (EXAFS) studies were conducted on silica supported silver–copper and gold–copper bimetallic clusters to obtain information on their structures. The atomic ratio of copper to either silver or gold was close to one in the materials investigated. The EXAFS results, which were obtained in the presence of hydrogen, indicate extensive segregation of the components in both the silver–copper and gold–copper clusters, although it is much more pronounced in the former. The greater segregation in the silver–copper clusters is readily understandable, since silver and copper are only slightly miscible in the bulk, whereas gold and copper are completely miscible. The EXAFS results on the silver–copper clusters suggest that the copper-rich region is in the interior of the clusters, with the silver concentrating at the surface. The location of the gold-rich region in the gold–copper clusters is less clear from the EXAFS data, but there is a slight indication that it is present at the surface. However, for air exposed gold–copper clusters, x-ray diffraction data suggest that the gold-rich region is in the interior.