Copper/ruthenium clusters

When the ruthenium EXAFS for the ruthenium-copper catalyst is compared with the EXAFS for a ruthenium reference catalyst containing no copper, it is found that they are not very different. This indicates that the environment about a ruthenium atom in the bimetallic catalyst is on the average not very different from that in the reference catalyst. This result is consistent with the view that a ruthenium-copper cluster consists of a central core of ruthenium atoms with the copper atoms present at the surface. 

If the degree of coverage of the ruthenium by the copper is very high, the copper atoms should be coordinated extensively to ruthenium atoms. It is emphasized that the ruthenium-copper clusters are of such a size (average diameter of 32A by electron microscopy (33)) that the surface metal atoms constitute almost half of the total. Hence for a Cu/Ru atomic ratio of one, the number of copper atoms would correspond roughly to that required to form a monolayer on the ruthenium. 

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). 

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. 

Since ruthenium and rhodium are neighboring elements in the periodic table, a closer comparison of the properties of ruthenium-copper and rhodium-copper clusters is of interest (17). When we compare EXAFS results on rhodium-copper and ruthenium-copper catalysts in which the Cu/Rh and Cu/Ru atomic ratios are both equal to one, we find some differences which can be related to the differences in miscibility of copper with ruthenium and rhodium. The extent of concentration of copper at the surface appears to be lower for the rhodium-copper clusters than for the ruthenium-copper clusters, as evidenced by the fact that rhodium exhibits a greater tendency than ruthenium to be coordinated to copper atoms in such clusters. The rhodium-copper clusters presumably contain some of the copper atoms in the interior of the clusters. 

Ruthenium compounds, 19 637-641 synthesis of, 19 640 uses for, 19 640—641 Ruthenium-copper clusters, 16 70 Ruthenium initiators, 26 934 Ruthenium plating, 9 823 Ruthenium-silica... 

Recent work conducted in the laboratory of G. Ertl in Munich has extended the investigations on the ruthenium-copper system to include single crystal specimens (18-20). The results of the work are in excellent accord with those obtained in our laboratory on unsupported ruthenium-copper aggregates and on supported ruthenium-copper clusters as well. Our work on supported bimetallic clusters of ruthenium and copper is discussed in detail in the following chapter. 

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... 

In contrast, the magnitude of the same peak in the transform of the ruthenium EXAFS data for the ruthenium-copper catalyst does not decrease when the data are obtained with oxygen present. This result suggests that the presence of copper tends to shield the ruthenium from the oxygen, as might be expected if the copper concentrated at the surface in the ruthenium-copper clusters (2,10-12). 

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. 

In view of the expectation that platinum will concentrate in the surface of platinum-iridium clusters, we might anticipate that platinum and iridium would segregate from one another to an increasingly greater extent as the clusters become smaller and the ratio of surface atoms to total metal atoms increases (48). When the ratio is equal to 0.5 for clusters containing 50% each of platinum and iridium, one can visualize a situation in which essentially all of the platinum is present in the surface and all of the iridium in the interior. There would then be a close resemblance to the ruthenium-copper clusters (2,31) discussed earlier. 

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