Clusters supported bimetallic

Hi ly dispersed supported bimetallic catalysts with bimetallic contributions have been prepared from molecular cluster precursors containing preformed bimetallic bond [1-2]. For examples, extremely high dispersion Pt-Ru/y-AUOa could be prepared successfully by adsorption of Pt2Ru4(CO)ison alumina [2]. By similar method, Pt-Ru cluster with carbonyl and hydride ligands, Pt3Ru6(CO)2i(p3-H)(p-H)3 (A) was used in this work to adsorb on MgO support. The ligands were expectedly removable from the metal framework at mild conditions without breaking the cluster metal core. 

Recently we reported EXAFS results on bimetallic clusters of iridium and rhodium, supported on silica and on alumina (15). The components of this system both possess the fee structure in Efie metallic state, as do the components of the platinum-iridium system. The nearest neighbor interatomic distances in metallic iridium and rhodium are not very different (2.714A vs. 2.690A). From the results of the EXAFS measurements, we concluded that the interatomic distances corresponding to the various atomic pairs (i.e., iridium-iridium, rhodium-rhodium, and iridium-rhodium) in the clusters supported on either silica or alumina were equal within experimental error. Since the Interatomic distances of the pure metals differ by only 0.024A, the conclusion is not surprising. 

The results of the EXAFS studies on supported bimetallic catalysts have provided excellent confirmation of earlier conclusions (21-24) regarding the existence of bimetallic clusters in these catalysts. Moreover, major structural features of bimetallic clusters deduced from chemisorption and catalytic data (21-24), or anticipated from considerations of the miscibility or surface energies of the components (13-15), received additional support from the EXAFS data. From another point of view, it can also be said that the bimetallic catalyst systems provided a critical test of the EXAFS method for investigations of catalyst structure (17). The application of EXAFS in conjunction with studies employing ( mical probes and other types of physical probes was an important feature of the work (25). 

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. 

It has generally been assumed that the most important consideration in the surface enrichment of one metal in preference to another in a supported bimetallic cluster is based on differences in the enthalpies of sublimation of the metals which comprise the cluster. In most cases, the surface composition is enriched in the metal having the lower enthalpy of sublimation (1 ). 

The role played by the support of influencing the surface composition of supported bimetallic clusters has only recently begun to receive some attention. Miura, a ( ) have shown that the nature of the support can play an important role in determining not only the surface composition of the supported bimetallic clusters but also the morphology of the particles. For silica-supported Pt-Ru... 

Methanatlon Studies. Because the most effective way to determine the existence of true bimetallic clusters having mixed metal surface sites Is to use a demanding catalytic reaction as a surface probe, the rate of the CO methanatlon reaction was studied over each series of supported bimetallic clusters. Turnover frequencies for methane formation are shown In Fig. 2. Pt, Ir and Rh are all poor CO methanatlon catalysts In comparison with Ru which Is, of course, an excellent methanatlon catalyst. Pt and Ir are completely inactive for methanatlon In the 493-498K temperature range, while Rh shows only moderate activity. 

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. 

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... 

B. D. Chandler, A. B. Schabel, C. F. Blanford, and L. H. Pignolet, The preparation and characterization of supported bimetallic Pt-Au particle catalysts from molecular cluster and chloride salt precursors, J. Catal. 367-383 (1999). 

Synthesis of Supported Bimetallic Catalysts from Molecular Cluster Precursors... 

Alumina-supported catalysts prepared using the bimetallic carbonyl precursors showed a better performance in alkene hydroformylation than conventional Co-Rh catalysts. This was related to the presence of highly dispersed Rh-Co clusters with frames corresponding to that of the parent carbonyl-precursor that were characterized by EXAFS [140, 183]. Silica-supported bimetallic entities RhCo3,... 

A brief overview of bimetallic catalysts is presented. Electronic vs. ensemble effects are discussed, and literature is reviewed on single crystal bimetallics, and supported bimetallic clusters. Bimetallic cluster compounds are considered as models. Structural considerations, effects of potential poisons, particles from bimetallic cluster compounds, and catalytic activity/selectivity studies are briefly reviewed and discussed. 

Supported bimetallic catalysts can be made by adsorption of a bimetallic precursor such as molecular cluster compounds, colloidal particles or dendrimer-stabilised particles. In several cases, homogeneous bimetallic particles have been found where the compositions lie within the miscibility gap of the bulk alloy (e.g. with PtAu particles). This suggests that when the particles are small enough and do not possess metallic properties, the normal rules do not apply. 

Of particular interest was the fact that the bimetallic cluster catalysts, i.e., RuNiY and RuCuY, had considerably better metal dispersions than the pure NiY and CuY catalysts. Further, the zeolite-supported bimetallic catalysts were more resistant to sintering during methanation than those supported on alumina. Particle-size measurement indicated, however, that most of the bimetallic clusters were too large to be located inside the zeolite pores. 

Fig. 1 Molecular graphic representation of a bimetallic cluster supported on MCM41 by R.G, Bell...

Bimetallic composition Precursor cluster" Support Applied reaction Ref. 

Table XV gives a summary of some bimetallic catalysts derived from the different bimetal clusters supported on metal oxides and applications to catalytic reactions.

M=Rh, Pd, or Pt) bondings. The highly dispersed RhFe and PdFe bimetallic ensembles were found to retain metal compositions fairly similar to those of the precursors. They still existed preferentially (> 85%) in the Fe state even after H2 reduction at 400°C and CO + Hj reaction at 250°C. The results imply that Fe in RhFe bimetallic structures acts not only as an anchor to fix the Rh, Pd, and Pt particles at the cluster-support interface, but also to provide M—Fe —OSi= species (M=Rh, Pd, Pt, and Ir) which are highly... 

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