Supported bimetallics

Z. Knor, and J. Sotola, Supported Bimetallic Catalysts, Coll. Czech. Chemical Comm. 

Supported bimetallic Re—Pt catalysts are important in selective reforming of petroleum. It is believed that sulhding the catalyst before use gives ReS units which act as inert diluents to reduce the size of a local ensemble of platinum atoms. Selectivity for desirable dehydrocyclization and isomerization reactions... 

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. 

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. 

All of the supported bimetallic catalysts studied show a sharp drop in methanatlon activity when the Ru/M ratio falls below four. 

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. 

Supported bimetallic catalysts find many industrial applications. Examples include Pt and Rh in automobile exhaust conversion catalysts and Pt and Re (or Pt and Sn or Pt and Ir) in naphtha reforming catalysts. 

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. 

Information on the chemical state of iridium on going from the molecular precursors, and its adsorption on the surface of the support can be obtained by Ir Mossbauer spectroscopy. It allows to estimate the composition of the Ir-containing alloys that are possibly formed during the activation treatment of supported bimetallic systems. The main results obtained in the application of Ir Mossbauer spectroscopy to characterize two Ir-containing bimetallic supported nanoparticles, i.e., Pt-Ir on amorphous silica and Fe-Ir on magnesia are presented and discussed... 

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... 

TPR of supported bimetallic catalysts often reveals whether the two metals are in contact or not. The TPR pattern of the 1 1 FeRh/SiOi catalyst in Fig. 2.4 shows that the bimetallic combination reduces largely in the same temperature range as the rhodium catalyst does, indicating that rhodium catalyzes the reduction of the less noble iron. This forms evidence that rhodium and iron are well mixed in the fresh catalyst. The reduction mechanism is as follows. As soon as rhodium becomes metallic it causes hydrogen to dissociate atomic hydrogen migrates to iron oxide in contact with metallic rhodium and reduces the oxide instantaneously. 

Schmid, G., West, H., Mehles, H., and Lehnert, A., Hydrosilation reactions catalyzed by supported bimetallic colloids, Inorg. Chem., 36, 891, 1997. 

A. Sachdev and J. Schwank, Microstructure and reactivity of supported bimetallic platinum-gold catalysts, J. Catal. 120, 353-369 (1989). 

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). 

C. W. Hills, N. H. Mack, and R. G. Nuzzo, The size-dependent structural phase behaviors of supported bimetallic (Pt-Ru) nanoparticles, J. Phys. Chem. B 107, 2626-2636 (2003). 

J. Margitfalvi, S. Szabo, andF. Nagy, Supported bimetallic catalysts prepared by controlled surface reactions. Stud. Surf. Sci. Catal. 27, 373-409 (1983). 

Supported bimetallic catalysts have gained unquestionable importance in subjects such as refining, petrochemistry and fine chemistry since their earliest use in the 1950s [1, 2]. The catalytic behavior of such a system is influenced by the size of the metal particles and by the interactions among them and with the support and other catalyst components. The second metal may influence the first metal through electronic interactions or by modifying the architecture of the active site. Very often, the interactions between the two metals are complex and largely unknown, and consequently the preparation procedure critically influences the nature of the catalytic system obtained. 

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