genolysis and these other reactions, it has frequently been suggested that hydrogenolysis requires surface sites consisting of arrays of active metal atoms that are larger than the arrays required for the other reactions. The availability of large arrays of active metal atoms relative to small arrays decreases sharply when an inactive component such as copper is dispersed in, or on top of, the surface layer of the active metal. The possible role of an electronic interaction between the Group VIII and Group IB metal has also been considered.

Bimetallic Clusters

For industrial application of bimetallic catalysts, high metal surface areas are desirable. Highly dispersed bimetallic clusters can be prepared by impregnating a carrier with an aqueous solution of salts of the two metals of interest. The material is dried and then brought in contact with a stream of H2 at elevated temperature to reduce the metal salts. This procedure results in the formation of bimetallic clusters even where individual metal components exhibit very low miscibility in the bulk.23,32,33 Examples of such metal clusters that have been investigated are ruthenium-copper and osmium-copper supported on silica, in which the metal clusters cover about 1 percent of the surface of the silica. Size of the clusters ranges from about 10 to 30 angstroms in these systems.

As copper is incorporated with ruthenium or osmium in bimetallic clusters, the selectivity for conversion of cyclohexane to benzene is improved greatly (Figure 10); hydrogenolysis to alkanes is inhibited markedly, whereas dehydrogenation to benzene is relatively unaffected.5,33 The behavior is similar to that described for unsupported Ru-Cu aggregates and therefore provides clear evidence for the interaction between Cu and the Group VIII metal on the carrier. As in the case of the unsupported materials, the copper in the bimetallic clusters is present at the surface.

When the initial research on bimetallic clusters such as ruthenium-copper and osmium-copper was conducted, the characterization of the clusters was limited to methods involving chemical probes because of the difficulty of obtaining information with physical probes. However, the situation changed markedly when it became evident that x-ray absorption spectroscopy was effective for investigating the structures of catalysts. Results of EXAFS studies on Ru-Cu and Os-Cu bimetallic clusters have provided strong evidence in support of the conclusions about structure derived from the studies with chemical probes.

The quantitative analysis of EXAFS data on bimetallic cluster catalysts has been limited to consideration of contributions of nearest-neighbor atoms to EXAFS.3439 In Figure 11, the EXAFS fluctuations represented by the solid line in all three fields of the figure are due to contributions from nearest-

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