Bimetallic clusters preparation

Harada M, Asakura K, Toshima N (1993) Catalytic activity and structural analysis of polymer-protected gold/palladium bimetallic clusters prepared by the successive reduction of hydrogen tetrachloroaurate(ffl) and palladium dichloride. J Phys Chem 97 5103-5114... 

Toshima, N., Yonezawa, T., and Kushihashi, K., Polymer protected Pd-Pt bimetallic clusters preparation, catalytic properties and structural considerations, J. Chem. Soc. Faraday Trans., 89, 2537, 1993. 

Infrared spectroscopy has been widely used to identify the ligands on supported bimetallic clusters prepared from organometallic precursors [60, 65]. Additionally, two of the most commonly used techniques to determine structural data of small bimetallic clusters are XRD and EXAFS spectroscopy, the latter offering the advantage that it does not reqnire long-range order for characterization. A brief summary of supported bimetallic clusters with postulated structural models based on EXAFS characterization is shown in Table 19.3. 

Sulfided bimetallic clusters which mimic the metal composition of commercial hydrodesulfurization (HDS) catalysts have been prepared and their homogeneous catalytic behavior studied. Reaction of thiophenol with [Mo2Co2(/z4-S)... 

A MgO-supported W—Pt catalyst has been prepared from IWsPttCOIotNCPh) (i -C5H5)2l (Fig. 70), reduced under a Hs stream at 400 C, and characterized by IR, EXAFS, TEM and chemisorption of Hs, CO, and O2. Activity in toluene hydrogenation at 1 atm and 60 C was more than an order of magnitude less for the bimetallic cluster-derived catalyst, than for a catalyst prepared from the two monometallic precursors. 

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. ... 

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. 

Bimetallic clusters of platinum and iridium can be prepared by coimpregnating a carrier such as silica or alumina with an aqueous solution of chloroplatinic and chloroiridic acids (22,34). After the Impregnated carrier is dried and possibly calcined at mild conditions (250°-270 C), subsequent treatment in flowing hydrogen at elevated temperatures (300 -500°C) leads to formation of the bimetallic clusters. 

Toshima N, Yonezawa T, Harada M, Asakuara K, Iwasawa Y (1990) The polymer-protected Pd-Pt bimetallic clusters having catalytic activity for selective hydrogenation of diene. Preparation and EXAFS investigation on the structure. Chem Lett 19 815-818... 

Lu, P. et al., Polymer-protected Ni/Pd bimetallic nano-clusters preparation, characterization and catalysis for Hydrogenation of Nitrobenzene, J. Phys. Chem. B, 103, 9673, 1999. 

Nanoparticles of various metals and metal oxides have been prepared on nanocarbons including Pt [139], Pd [206,207], Ag [208,209], Au [210], MnOx [211], TiOx [212], NiOx [213], CoOx [213], Ru02 [214] and various bimetallic clusters [215]. For example,... 

As an aside, we should mention that the same principles apply to the formation of bimetallic clusters on a support. In the case of Pt-Re on AI2O3 it has been shown that hydroxylation of the surface favors the ability of Re ions to migrate toward the Pt nuclei and thus the formation of alloy particles, whereas fixing the Re ions onto a dehydroxylated alumina surface creates mainly separated Re particles. As catalytic activity and selectivity of the bimetallic particles differ vastly from those of a physical mixture of monometallic particles, the catalytic performance of the reduced catalyst depends significantly on the protocol used during its formation. The bimetallic Pt-Re catalysts have been identified by comparison with preparations in which gaseous Re carbonyl was decomposed on conventionally prepared Pt/Al203 catalysts. ... 

Preparation of mixed-metal intradendrimer clusters by partial displacement is a straightforward extension of the complete displacement approach for forming single-metal clusters described in the previous section. If less than a stoichiometric amount of Ag+, Au +, Pd +, or Pt + is added to a Gh-OHlCujj) solution, or if less than a stoichiometric amount of Au +,Pd +, or Pt + is added to G6-OH(Agiio) solution, it is possible to form Ag/Cu,Au/Cu (Au/Ag),Pd/Cu (Pd/Ag), and Pt/Cu (Pt/Ag) bimetallic clusters inside dendrimers. 

Dendrimer-encapsulated bimetallic clusters can also be prepared by simultaneous co-complexation of two different metal ions, followed by a single reduction step. For example, the absorption spectrum of a solution containing G6-OH, PtCl , and PdCl is essentially the sum of the spectra of a solution containing G6-OH + PtCl and a second solution containing G6-OH -i- PdCl, which strongly suggests co-complexation of Pt + and Pd + within individual dendrimers. After reduction of these co-complexed materials, a new interband transition, which has an intensity different from that of either a pure Pt or Pd cluster, is observed. 

Fig. 17. Schematic illustration of the preparation of dendrimer-encapsulated bimetallic clusters by three different methods. Displacement reaction, co-complexation, and sequential loading...

Fig. 9.1.5 Preparation process of Pd-core/Pt-shell (inverted core/shell) structured bimetallic clusters by a sacrificial hydrogen reduction. (From Ref. 69.)...

Two bimetallic clusters derived from [Ru6C(CO),6]2- have been prepared, in which eight and thirteen metal atoms are present. The reaction of Cu(MeCN)JBF4 with 18 results in the addition of two copper vertices to the Ru6C core [Eq. (14)] (50). 

Iron-only hydrogenase, dithiolate-bridged compounds as biomimetic models, 6, 239 Iron oxide films, synthesis, 12, 51 Iron-palladium nanoparticles, preparation, 12, 74 Iron-platinum bimetallic clusters, with isocyanide clustes,... 

A few illustrative examples dealing with bimetallic cluster compounds will now be presented. For example, mixed cluster compounds [MjRu -CO CO qCPPI J where M = Cu, Ag, and Au have been prepared and structurally characterized. ) Metal core structures consist of a Ru4 tetrahedron with M-PPh3 groups capping two... 

Indeed there are numerous bimetallic clusters now prepared and characterized and the recent book by Mingos and Whales summarizes the field very well. (3)... 

Another non-conventional preparative route to bimetallic catalysts has been developed where metal atoms (vapors) have been trapped at low temperature in solvating media. (A review has recently appeared).(17) By solvating two metals at the same time (eg. Co in toluene and Mn in toluene), followed by warming, bimetallic clusters/particles form. In the presence of a catalyst support, surface -OH groups can have a dramitic affect on the structure of the small bimetallic cluster produced. For example, with Co and Mn, a layered structure of MnOx covered by Co° in a particle of about 25 A was formed.(iS) With Fe and Co combinations, a layer of FeOx followed by Fe°Co° alloy and a surface rich in Co° was formed. (19)... 

Although typical catalyst preparation procedures vary slightly from one laboratory to another, the "conventional approach" is to deposit a metal salt on a support, convert this salt to the oxide, and then reduce to the metallic state. When two metals are simultaneously so treated and reduced, bimetallic clusters may form. However, it cannot be assumed that bimetallic clusters are produced since monometallic separate particles may predominate. In fact, it is perhaps the unusual case where bimetallics do form since there are many possible paths, both thermodynamic and kinetic, that can lead to (1) separate monometallic s, (2) one metal not reduced, (3) thermal segregation of bimetallic precursor particle, and/or (4) volatilization or migration of one metal. 

Bimetallic clusters of gold were reviewed by Braunstein and Rose (6) in 1985, and Hall and Mingos (7) surveyed both homonuclear and hetero-nuclear gold clusters in the previous year. In addition, Steggerda et al. (8) reviewed the preparation and properties of gold clusters in 1982, Jones (9) has listed gold clusters studied by single-crystal X-ray diffraction up to mid... 

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