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

Abstract

An investigation was conducted of the extended x-ray absorption fine structure (EXAFS) associated with the Liii absorption edge of osmium and the K absorption edge of copper in silica supported osmium–copper catalysts with a 1:1 atomic ratio of copper to osmium. The effect of the copper on the Liii x-ray absorption threshold resonance of the osmium was also investigated. Chemisorption and catalysis studies had earlier indicated a strong interaction between osmium and copper in such catalysts, and led to the proposal that bimetallic clusters of osmium and copper were present on the silica. Information on the structure of the osmium–copper clusters has been obtained from the EXAFS data of the present investigation. The osmium atoms in the clusters are coordinated to a large extent to other osmium atoms, whereas the copper atoms are coordinated extensively to osmium atoms as well as to copper atoms. The results are consistent with a model in which the copper in the clusters is present at the surface and are very similar in this regard to our earlier reported results on ruthenium–copper clusters. In addition to the structural information derived from the EXAFS data, some indication of electronic interaction between osmium and copper was obtained from a comparison of the osmium Liii x-ray absorption threshold resonances in osmium and osmium–copper clusters. The copper appears to contribute electrons to unfilled d states of the osmium.