Bimetallic particles, functions

Sathish Kumar et al. [45] prepared bimetallic Au-Ru nanoparticles by the simultaneous reduction of both Au3+ and Ru3+ ions by ultrasound irradiation at three different molar ratios (Au3+ Ru3+ 1 1, 1 3 and 1 5) in 4 h in the presence of PEG. A significant change in the absorption as a function of sonication time was observed for Au-Ru bimetallic particles (Fig. 6.10), which indicated the... 

The use of hetero-metallic (MM )carbonyl complexes as precursors can lead to the preparation of supported catalysts having weU-defined bimetallic entities in which the intimate contact between M and M remains in the final catalyst and the atomic ratio M/M of the aggregates is that of the bimetallic carbonyl precursor used. This is illustrated in Figure 8.1, in which the definite interaction of the MjM (CO) complex with the functional group (F) of a surface (S) produces a new anchored surface species. This new surface species could evolve with an appropriate treatment producing tailored bimetallic particles. 

Deiters, E., Song, B., Chauvin, A.S., et al. (2009) Luminescent bimetallic lanthanide bioprobes for cellular imaging with excitation in the visible-Ught range. Chemistry —A European Journal, 15, 885—900. Charbonniere, L.J., Weibel, N., Estoumes, C., et al. (2004) Spatial and temporal discrimination of siUca particles functionalized with luminescent lanthanide markers using time-resolved luminescence microscopy. New Journal of Chemistry, 28, 777—781. 

Such a reaction of Fe(CO)5 (at 293-363 K, PVP) without ultrasonic radiation proceeds very slowly and only after few days there, a material is formed with very low Fe content (2%, the isolated particles 2-5 nm in size). It is of interest that the sonochemical decomposition of Fe(CO)5 does not proceed in the presence of PVP if THF is used as the solvent, but the reaction is very effective when anisole is used as the solvent and PFO is used as the polymer matrix [93]. A black product formed contains up to 10% (in mass) of the spheric particles of nonoxidized Fe (mainly y-Fe, with little content of a-Fe) with 1-12 nm in size (the mean diameter is 3nm, as shown in Figure 3.7). It is likely that the big particles present the flocks of little ones ( 2-2.5nm). The sonochemical synthesis allows us to produce the functionalized amorphous nanoparticles of ferric oxide with 5-16 nm in diameter [94]. The ultrasonic irradiation in the PFO presence allows us to also produce the stabilized nanoparticles of copper, gold, and so on. In the literature the findings are not about the bimetallic particle formation in the ultrasonic fields by carbonyl metal reduction in the polymer matrices presence (as, for example, in the case of the carbon-supported Pt-Ru from PtRu5C(CO)i6 reduced clusters [95]). 

PAMAM dendrimers can also be used as templating agents and nanoparticle stabilizers for the synthesis of bimetallic particles. The unique ability of dendrimers to host various metal precursors enables the simultaneous complexation of multiple metallic species at its various internal functional groups. The three primary methods of bimetallic nanoparticle synthesis through dendrimer stabilization are partial displacement, co-complexation, and sequential complexation. 

The synthesis of bimetallic nanoparticles is mainly divided into two methods, i.e., chemical and physical method, or bottom-up and top-down method. The chemical method involves (1) simultaneous or co-reduction, (2) successive or two-stepped reduction of two kinds of metal ions, and (3) self-organization of bimetallic nanoparticle by physically mixing two kinds of already-prepared monometallic nanoparticles with or without after-treatments. Bimetallic nanoparticle alloys are prepared usually by the simultaneous reduction while bimetallic nanoparticles with core/shell structures are prepared usually by the successive reduction. In the preparation of bimetallic nanoparticles, one of the most interesting aspects is a core/shell structure. The surface element plays an important role in the functions of metal nanoparticles like catal5dic and optical properties, but these properties can be tuned by addition of the second element which may be located on the surface or in the center of the particles adjacent to the surface element. So, we would like to use following marks to inscribe the bimetallic nanoparticles composed of metal 1, Mi and metal 2, M2. 

Methylcyclopentane is a powerful probe molecule for the study of metal surfaces. The product distribution on platinum depends on the following factors particle size 491 reaction conditions 492-494 carbonaceous residues,492,493,495 and the extent of the interface between the metal and the support.492,493,495 The hydrogenolysis rate of methylcyclopentane depends on the hydrogen pressure.496,497 The rate exhibits a maximal value as a function of hydrogen pressure on EuroPt catalysts.498 The hydrogenolysis of methylcyclopentane has also been studied over Pt-Ru bimetallic catalysts.499... 

Homogeneous catalysis is, of course, a major field in it s own right, as catalytic transformations are important synthetic tools. However, catalysis is also a potentially sensitive probe for nanoparticle properties and surface chemistry, since catalytic reactions are ultimately carried out on the particle surface. In the case of bimetallic DENs, catalytic test reactions have provided clear evidence for the modification of one metal by another. DENs also provide the opportunity to undertake rational control experiments not previously possible to evaluate changes in catalytic activity as a function of particle composition. 

Figure 5 shows a plot of TOFs for the partial hydrogenation of 1,3-COD by PdRh DENs compared to TOFs for physical mixtures of Pd and Rh monometallic DENs as a function of mol % Rh. As the mol % of Rh in the bimetallic DENs was increased, an increase in the TOF was observed that was greater than that of the physical mixtures. Importantly, the average particle size and distribution did not change as the mol % Rh increased, which was used to rule out the possibility that the TOF enhancement was a consequence of a systematic decrease in particle size. This allows for the conclusion that the bimetallic DENs are truly intimately mixed bimetallic nanoparticles and that a synergistic effect is responsible for the catalytic rate enhancement. 

Such higher residual activity could be in accordance with the model proposed by Sachtler, which assumes that S is preferentially adsorbed on Re sites in bimetallic Pt-Re particles [20]. Indeed, as the Pt-Re interaction is the highest, sulfur ought to divide the Pt-Re surface into very small Pt ensembles. Consequently, the reorganization of the carbonaceous overlayer into pseudo-graphitic entities, which are detrimental to the metallic function, could be impeded. 

After drying and reduction, the Pd-Ag/C catalysts are composed of bimetallic Eilloy nanoparticles ( 3 nm). CO chemisorption coupled to TEM and XRD analysis showed that that, for catalysts 1.5% wt. in each metal, the bulk composition of the alloy is close to 50% in each metal, whereas the surface is 90% in Ag and 10% in Pd [9]. Mass transfer limitations can be detected by testing the same catalyst with various pellet sizes [18]. Indeed, if the reactants diffusion is slow due to small pore sizes, the longer the distance between the pellet surface and the metal particle, the larger the influence of the difiusion rate on the apparent reaction rate. Pd-Ag catalysts with various pellet sizes were thus tested in hydrodechlorination of 1,2-dichloroethane. Results were compared to those obtained with a Pd-Ag/activated charcoal catalyst. Fig. 4 shows the evolution of the effectiveness factor of the catalysts, i.e. the ratio between the apparent reaction rate and the intrinsic reaction rate, as a function of the pellet size. The intrinsic reaction rate was considered equal to the reaction rate obtained with the smallest pellet size. When rf = 1, no diffusional limitations occur, and the catalyst works in chemical regime. When j < 1, the observed reaction rate is lower than the intrinsic reaction rate due to a slow diffusion of the reactants and products and the catalyst works in diffusional regime [18]. 

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