Ruthenium-copper clusters EXAFS studies

The results of the EXAFS studies on osmium-copper clusters lead to conclusions similar to those derived for ruthenium-copper clusters. That is, an osmium-copper cluster Is viewed as a central core of osmium atoms with the copper present at the surface. The results of the EXAFS investigations have provided excellent support for the conclusions deduced earlier (21,23,24) from studies of the chemisorption and catalytic properties of the clusters. Although copper is immiscible with both ruthenium and osmium in the bulk, it exhibits significant interaction with either metal at an interface. 

EXAFS Studies of Ruthenium-Copper Clusters (31). An X-ray absorption spectrum at 100°K showing the extended fine structure beyond the K absorption edge of ruthenium is given in Figure 4.6 for a catalyst containing 1.0 wt% ruthenium and 0.63 wt% copper in the form of small metal clusters... 

The discussion of EXAFS on ruthenium-copper clusters in the previous paragraphs emphasizes the qualitative aspects of the data analysis. A quantitative data analysis, yielding information on the various structural parameters of interest, has also been made and published (31). Of particular interest was the finding that the average composition of the first coordination shell of ruthenium and copper atoms about a ruthenium atom was about 90% ruthenium, while that about a copper atom was about 50% ruthenium. Details of the methods involved in the quantitative analysis of EXAFS data on ruthenium-copper clusters are not considered here, since the technique is described in the following discussion of EXAFS studies on osmium-copper clusters. 

EXAFS Studies of Osmium-Copper Clusters (32). Results of EXAFS studies on osmium-copper clusters dispersed on silica lead to conclusions similar to those derived for ruthenium-copper clusters. The discussion here is concerned with a catalyst with a 1 1 atomic ratio of copper to osmium. The clusters constituted 2.66 wt% of the total catalyst mass (2% Os, 0.66% Cu). The average diameter of the clusters was estimated to be about 15 A. 

In summary, the results of the EXAFS studies on osmium-copper clusters indicate that the osmium atoms in the clusters are coordinated predominantly to other osmium atoms, while the copper atoms are extensively coordinated to both copper and osmium atoms. The results are thus very similar to those obtained for ruthenium-copper clusters. Since the copper atoms appear to have essentially equal numbers of copper and osmium atoms as nearest neighbors, it seems reasonable to conclude that the osmium-copper clusters consist of small patches or multiplets of copper atoms located on the surface of the osmium. 

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