Bimetallic clusters Interactions

MgO-supported model Mo—Pd catalysts have been prepared from the bimetallic cluster [Mo2Pd2 /z3-CO)2(/r-CO)4(PPh3)2() -C2H )2 (Fig. 70) and monometallic precursors. Each supported sample was treated in H2 at various temperatures to form metallic palladium, and characterized by chemisorption of H2, CO, and O2, transmission electron microscopy, TPD of adsorbed CO, and EXAFS. The data showed that the presence of molybdenum in the bimetallic precursor helped to maintain the palladium in a highly dispersed form. In contrast, the sample prepared from the monometallie precursors was characterized by larger palladium particles and by weaker Mo—Pd interactions. ... 

The Effect of Support-Metal Precursor Interactions on the Surface Composition of Supported Bimetallic Clusters... 

The effect of precursor-support interactions on the surface composition of supported bimetallic clusters has been studied. In contrast to Pt-Ru bimetallic clusters, silica-supported Ru-Rh and Ru-Ir bimetallic clusters showed no surface enrichment in either metal. Metal particle nucleation in the case of the Pt-Ru bimetallic clusters is suggested to occtir by a mechanism in which the relatively mobile Pt phase is deposited atop a Ru core during reduction. On the other hand, Ru and Rh, which exhibit rather similar precursor support interactions, have similar surface mobilities and do not, therefore, nucleate preferentially in a cherry model configuration. The existence of true bimetallic clusters having mixed metal surface sites is verified using the formation of methane as a catalytic probe. An ensemble requirement of four adjacent Ru surface sites is suggested. 

The results of this study suggest that the dynamics of the nucleatlon process are of the utmost Importance In determining the structure and the surface composition of supported bimetallic clusters. Because the surface mobility of the metal phase during pretreatment is strongly influenced by the nature of the precursor-support Interaction, it is useful to consider this Interaction in some detail. 

The rhenium interacts strongly with the oxygen atoms of the support and also with platinum platinum interacts less strongly with the support than rhenium. One is tempted to generalize that when one of the metals in a supported bimetallic cluster is noble and the other oxophihc, the oxophUic metal interacts more strongly with the support than the noble metal if the bimetalhc frame of the precursor is maintained nearly intact, then this metal-support interaction helps keep the noble metal highly dispersed. 

The surface reaction of impregnated mixed metal cluster complexes may be analogous to that of homometallic clusters on hydrated and dehydrated metal oxides as described in Sections III and IV. Bimetallic clusters are converted to anionic surface species by simple deprotonation via 0 on dehydrated MgO or AI2O3 surfaces these species have been characterized by IR spectroscopy (119). The ionic interaction with surface cations such as AF and Mg is demonstrated by IR and NMR measurements. The surface polynuclear carbonyl anions are stable up to about 373 K. If heated in vacuo at higher temperature, extensive decomposition takes place to give a mixture of Ru (or Os) metal particles and Fe oxides, accompanied by the evolution of H2, CO, and CO2. 

Christensen et al.103 studied bimetallic clusters using a Monte-Carlo method together with the effective-medium approximation for the description of the interatomic interactions (this method is closely related to the embedded-atom method). As initial structures they use ones that were derived from the fee... 

In two recent papers Ferrando and coworkers106,107 studied a whole series of bimetallic clusters using a parameterized potential for describing the interatomic interactions and genetic algorithms in optimizing the structures. In particular they studied clusters with 34 and 38 atoms and found that the structures of those bear no resemblance to those of the corresponding pure clusters of the same size. 

Other mixed catalysts (79), indicating that the lowest exchange current density (and catalyst activity) is reached when the d band is filled. Currently, little information exists on the surface structure, metal interactions, and adsorption characteristics of bimetallic clusters in an electric field. Recently developed electron spectroscopic techniques and comparison with similar studies on conventional mixed catalysts (774, 775) could shed some light on the catalytic action of bimetallic electrodes. 

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. 

Direct experimental verification of very highly dispersed bimetallic clusters is complicated by limitations in the ability of physical methods to obtain structural information on such systems. In such a system, however, a catalytic reaction can serve as a sensitive probe to obtain evidence of interaction between the atoms of the two metallic components. For supported bimetallic combinations of a Group VIII and a Group IB metal, the hydrogenolysis of ethane to methane is a useful reaction for this purpose. In the case of unsupported bimetallic systems of this type, as discussed previously, the interaction between the Group VIII metal and the Group IB metal results in a marked suppression of the hydrogenolysis activity of the former. 

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