Ruthenium-copper clusters structure

The copper EXAFS of the ruthenium-copper clusters might be expected to differ substantially from the copper EXAFS of a copper on silica catalyst, since the copper atoms have very different environments. This expectation is indeed borne out by experiment, as shown in Figure 2 by the plots of the function K x(K) vs. K at 100 K for the extended fine structure beyond the copper K edge for the ruthenium-copper catalyst and a copper on silica reference catalyst ( ). The difference is also evident from the Fourier transforms and first coordination shell inverse transforms in the middle and right-hand sections of Figure 2. The inverse transforms were taken over the range of distances 1.7 to 3.1A to isolate the contribution to EXAFS arising from the first coordination shell of metal atoms about a copper absorber atom. This shell consists of copper atoms alone in the copper catalyst and of both copper and ruthenium atoms in the ruthenium-copper catalyst. 

This discussion of EXAFS on ruthenium-copper clusters has emphasized 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 (8). Of particular Interest was the finding that the average compo tion 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 bimetallic clusters can be obtained from our original papers (8.12-15). 

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... 

Ruthenium-copper and osmium-copper clusters (21) are of particular interest because the components are immiscible in the bulk (32). Studies of the chemisorption and catalytic properties of the clusters suggested a structure in which the copper was present on the surface of the ruthenium or osmium (23,24). The clusters were dispersed on a silica carrier (21). They were prepared by wetting the silica with an aqueous solution of ruthenium and copper, or osmium and copper, salts. After a drying step, the metal salts on the silica were reduced to form the bimetallic clusters. The reduction was accomplished by heating the material in a stream of hydrogen. 

The ruthenium-copper and osmium-copper systems represent extreme cases in view of the very limited miscibility of either ruthenium or osmium with copper. It may also be noted that the crystal structure of ruthenium or osmium is different from that of copper, the former metals possessing the hep structure and the latter the fee structure. A system which is less extreme in these respects is the rhodium-copper system, since the components both possess the face centered cubic structure and also exhibit at least some miscibility at conditions of interest in catalysis. Recent EXAFS results from our group on rhodium-copper clusters (14) are similar to the earlier results on ruthenium-copper ( ) and osmium-copper (12) clusters, in that the rhodium atoms are coordinated predominantly to other rhodium atoms while the copper atoms are coordinated extensively to both copper and rhodium atoms. Also, we conclude that the copper concentrates in the surface of rhodium-copper clusters, as in the case of the ruthenium-copper and osmium-copper clusters. 

Ruthenium-copper and osmium-copper clusters are examples of bimetallic clusters in which one component is from Group VIII and the other from Group IB of the periodic table. These clusters are of particular interest because copper is virtually completely immiscible with either ruthenium or osmium in the bulk (7). Copper has the face-centered cubic structure in the metallic state, whereas ruthenium and osmium both exhibit hexagonal close-packed structures (8). 

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. In recent years, however, advances in X-ray absorption spectroscopy have changed the situation markedly. In particular, improvements in methods of obtaining extended X-ray absorption fine structure (EXAFS) data with the use of synchrotron radiation (13), in conjunction with advances in methods of data analysis (14), have made EXAFS a valuable tool for obtaining structural information on bimetallic clusters. 

Similar structural changes of the copper layer on ruthenium are observed for the ethane hydrogenolysls reaction shown In Figure 10 (12). The effect of copper at low coverages Is to simply block active ruthenium sites on a one to one basis with three dimensional cluster growth occurring at roughly a third of a monolayer. 

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