Ruthenium bimetallic 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. 

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. 

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). 

Shephard, D. S., Maschmeyer, T., Sankar, G., Thomas, J. M., Ozkaya, D., Johnson, B. F. G., Raja, R., Oldroyd, R. D. and Bell, R. G. Preparation, characterization and performance of encapsulated copper-ruthenium bimetallic catalysts derived from molecular cluster carbonyl precursors, Chem. Eur. J., 1998, 4, 1214-1224. 

The same workers (234) also studied the methanation behavior of bimetallic clusters of Ru/Ni and Ru/Cu in zeolite Y. Such clusters can be formed by metals, such as ruthenium and copper which are immiscible as bulk metals (235, 236). The turnover numbers versus bimetallic cluster composition are shown in Fig. 22. Dilution of ruthenium with copper clearly causes a marked decrease in specific activity. This decrease in activity is also accompanied by a decrease in methanation selectivity. This was attributed to an inhibiting effect of copper on the ruthenium hydrogenolysis activity. 

Fig. 1 shows clearly that only one hydrogen consumption peak was found for the bimetallic precursor prepared by coadsorption, which has been assigned to hydrogen conjointly consumed during the reduction of both metals due to the formation of a kind of alloy between platinum and ruthenium. The bimetallic clusters thus formed will be richer in platinum for the CAD series than for the CAC series, due to the metal contents shown in Table 1. 

Also it is remarkable that this activity is due to a greater extent to deep hydrogenolysis in CAD than in CAC series. Thus, more segregation of particles of ruthenium from bimetallic clusters occurs with drying. 

Since the ability to form bulk alloys was not a necessary condition for a system to be of interest as a catalyst, it was decided not to use the term alloy in referring to bimetallic catalysts in general. Instead, terms such as bimetallic aggregates or bimetallic clusters have been adopted in preference to alloys. In particular, bimetallic clusters refer to bimetallic entities which are highly dispersed on the surface of a carrier. For systems such as ruthenium-copper, it appears that the two components can interact strongly at an interface, despite the fact that they do not form solid solutions in the bulk. In this system the copper is present at the surface of the ruthenium, much like a chemisorbed species. 

The surface properties of unsupported ruthenium-copper aggregates are considered in this chapter. In a subsequent chapter on bimetallic cluster catalysts, the properties of supported ruthenium-copper and osmium-copper catalysts are considered in detail. 

For highly dispersed ruthenium-copper catalysts (bimetallic clusters), to... 

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. 

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). 

Figure 4.5 Selectivity of conversion of cyclohexane over silica-supported bimetallic clusters of ruthenium-copper and osmium-copper at 316°C, as represented by the ratio D/H (1,12), (D is rate of dehydrogenation of cyclohexane to benzene, and H is rate of hydrogenolysis to alkanes.) (Reprinted with permission from Academic Press, Inc.)...

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. 

There has been great interest in the preparation of bimetallic transition metal cluster complexes containing palladium.899-902 Bimetallic palladium-ruthenium clusters have been shown to be good precursors to supported bimetallic catalysts.903,904... 

To clarify the mechanism of propylene adsorption on Ru-Co clusters the quantum-chemical calculation of interaction between it and Ru-Co, Ru-Ru, and Co-Co clusters were carried out. During the calculation it was assumed that carbon atoms of C-C bond are situated parallel to metal-metal bond. The distance at which the cluster and absorbable molecule begin to interact is characterized by the nature of active center. Full optimization of C3H6 molecule geometry confirms that propylene adsorbs associatively on Co-Co cluster and forms Jt-type complex. In other cases the dissociate adsorption of propylene is occurred. The presence of Ru atom provides significant electron density transfer from olefin molecule orbitals to d-orbitals of ruthenium in bimetallic Ru-Co- or monometallic Ru-Ru-clasters (independently on either the tertiary carbon atom is located on ruthenium or cobalt atom.). At the same time the olefin C-C bond loosens substantially down to their break. 

However, only alkyl formates are formed in the conventional reactions of alcohols, CO2 and H2 using transition metal complexes, because intermediary hydride complexes generally react with CO2 to give formate complexes. On the other hand, we have found that mthenium cluster anions effectively catalyze the hydrogenation of CO2 to CO, methanol, and methane without forming formate derivatives [2-4]. Ethanol was also directly formed from CO2 and H2 with ruthenium-cobalt bimetallic catalyst [5]. In this paper, we report that this bimetallic catalytic... 

An infrared study of CO adsorption on Ru-Au supported on magnesia suggested that this bimetallic behaves differently from Ru-Cu, with no evidence of Au segregation at the cluster surface, (nor separate Au clusters although ruthenium and gold are practically immiscible in the bulk). At temperatures below 383 K where the reaction between cyclopropane and hydrogen adopted routes to propane or methane + ethane, no interaction between Au and Ru containing up to 36% Au was evident from the kinetic parameters.However, a more complete examination (unpublished) of these catalysts by XPS, EXAFS, SAXS, and other techniques has been made and it is believed that the surface contained Ru atoms only. 

Reduction temperature seems to have positive influence on metal dispersion for both mono- and bimetallic catalysts. There was no reduction temperature that improved the activity in all catalysts. Cracking or deep hydrogenolysis was directly related to higher ruthenium content and to higher mean cluster size. Catalysts reduced at 773 K are more suitable to produce isoparaffins. 

Co-decomposition of Ru(77 -C8H]o)( -C8Hi2) and Pt(dba)2 leads to the formation of bimetallic Ru .-Pti j, particles. Platinum-rich particles are fee, whereas ruthenium-rich ones are hep. There is a critical composition Ru3-Pt, for which most of the particles are twinned. In this case, the particles are monodisperse and very small (1.1 nm). This composition corresponds roughly to the limit of solubility of ruthenium in the platinum lattice for bulk alloys. The particles adopt a twinned fee structure with the twinning wall lying in a (111) plane located in the middle of the particle. The homogeneity in the size and shape of the twinned particles suggests a well-defined atomic organization, namely, a twinned truncated octahedron for the particles which can also be described as well-defined clusters. 

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