Bimetallic particle size distribution

Covering monometallic (Pd, Sn) and multimetallic (Pd-Sn, Pd-Ag) systems, several examples are presented in this chapter to illustrate the possibility offered by this chemistry to control the particle size distribution and the bimetallic interaction at a molecular level. This work is supported by a multitechnique characterization approachusing SnM6ssbauerspectroscopy,X-rayphotoelectron spectroscopy (XPS), low-energy ion spectroscopy (LEIS), and transmission electron microscopy (TEM). Catalytic performances in hydrogenation of different unsaturated hydrocarbons (phenylacetylene, butadiene) are finally discussed in order to establish structure-reactivity relationships. 

Table 6.3 includes particle sizes for bimetallic catalysts determined by TEM. Figure 6.4 shows particle size distribution histograms of PtSn-BM and NiSn-BM bimetallic catalysts and the corresponding Pt and Ni monometallic ones. In all the cases, the sohds were subjected to a hydrogen reduction pretreatment. These... 

Fig. 7.10 TEM image of a bimetallic PdAu/C catalyst, along with particle size distribution and energy dispersive X-ray analysis of the composition, showing the X-ray emission lines of the carbon support, and the metals Au and Pd. The Cu lines are from the sample holder. (Adapted from [23]).

Abstract We review the preparation, characterization, and properties of dendrimer-templated bimetallic nanoparticles. Polyamidoamine (PAMAM) dendrimers can be used to template and stabilize a wide variety of mono- and bimetallic nanoparticles. Depending on the specific requirements of the metal system, a variety of synthetic methodologies are available for preparing nanoparticles with diameters on the order of 1-3 nm with narrow particle size distributions. The resulting dendrimer-encapsulated nanoparticles, or DENs, have been physically characterized with electron microscopy techniques, as well as UV-visible and X-ray photoelectron spectroscopies. 

TEM micrographs of bimetallic catalysts revealed the presence of randomly accessible ordered domains as well as the partly disordered mesostructure of silica films (Figure 4). The nanoparticles of Ru-Pt have a mean size of 1.4 nm with a narrow particle size distribution (Figure 5). However, the TEM image also shows the small agglomeration of nanoparticles due to the absence of a local mesostructure. 

Similar synergic Au-Pd interactions were reported for bimetallic Au-Pd catalysts supported on polyaniline (PANI) [79] for benzyl alcohol oxidation. Here, the colloidal preparative route provided a narrow particle size distribution ( 3nm) with a Pd-rich shell encapsulating an Au-rich core. In this instance, the optimal composition was Au Pd =1 9. Benzyl alcohol oxidation has likewise been studied over an Au-Pd/Ti02 catalyst in which the deposition-precipitation method was used to improve the particle size distribution and activity versus wet impregnation [80]. In contrast, liquid-phase oxidation of cinnamyl, benzylic, octenol, and octenal... 

The first two stages of the synthesis of catalysts prepared by dendrimers are inextricably linked. Proper incorporation of the metal precursor within the PAMAM dendrimer is essential for the formation of dendrimer-metal nanocomposites and, eventually, nanoparticles with controlled particle sizes. Complications in the complexation stage, such as incomplete or inadequate incorporation of the metal precursor, will leave free metal cations or colloidal particles in the impregnating solution, resulting in the formation of supported catalysts that exhibit wide particle size distributions. In the case of bimetallic catalysts, the loss is twofold In addition to an array of metal particle sizes, there will also be a significant loss of compositional control in the active phase. In short, if the complexation step is not tightly controlled, the dendrimer-prepared catalyst will not differ substantially from a catalyst prepared by wet impregnation. 

Using silver(I)-bis(oxalato)palladate(II) complex, Rhee et al. reported synthesis of ultrafine Ag-Pd bimetallic nanoparticles. The advantage of this complex, which contains both metal ions, is prevention of formation of silver halides. Moreover, the oxalate ligand rapidly decomposes by light irradiation or chemical reduction. It was demonstrated that the particle size distribution is dependent on the Ag/Pd ratio [46],... 

The electronic properties of bimetallic (PdAu, PdPt, and PdZn) NPs were studied using XRD, TEM, XPS, and FTIR of the adsorbed CO. Bimetalhc based catalysts contained 1.5-2 nm NPs with a narrow particle size distribution, but with different NP morphology cluster-in-cluster for PdR and PdZn and core-shell for PdAu [15]. Adchtion of a modifying metal (Au, Pt and Zn) leads to a change of the NP electronic properties as well. 

CuNPs) in Fig. 7 shows the monodisperse and uniformly distributed spherical particles of 10+5 nm diameter. The solution containing nanoparticles of silver was found to be transparent and stable for 6 months with no significant change in the surface plasmon and average particle size. However, in the absence of starch, the nanoparticles formed were observed to be immediately aggregated into black precipitate. The hydroxyl groups of the starch polymer act as passivation contacts for the stabilization of the metallic nanoparticles in the aqueous solution. The method can be extended for synthesis of various other metallic and bimetallic particles as well. 

The alkaline EG synthesis method has been successfully applied to the preparation of unprotected bimetallic nanocluster colloids with controllable composition. Figure 3 shows the TEM image of bimetallic Pt/Ru nanoclusters (Pt/Ru molar ratio = 1 1.9, total metal concentration 1.85 g/1) with an average particle size of 1.9 nm and a size distribution from 1.4 to 2.4 nm. XRD pattern of the bimetallic nanoclusters is shown in Figure... 

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... 

Bimetallic particles with a very narrow size distribution of circa 1.5 nm have been prepared by decarbonylation under H2 at 400 °C of the impregnated Ru5PtC(CO)i6 on carbon black. EXAFS data indicate that a surface segregation of Pt on the fee Ru structure occurs in the bimetallic nanoparticles. Moreover, they undergo reversible oxidation, forming a MO surface and a core of metal [62]. 

Au and Pd. The Cu signal is due to the TEM grid. In order to prove that the particles are truly bimetallic, the authors took EDX images of 50 single particles, which appeared to have a similar composition as the overall sample. Hence, the catalyst contains Pd-Au particles with a distribution of particle sizes, but a rather uniform composition. 

Just as DENs particle sizes have some distribution (albeit relatively narrow), there is surely some distribution in particle compositions for bimetallic DENs. This is a fundamentally important aspect of DENs, particularly with regard to their catalytic properties however, there are presently no reliable characterization methods for evaluating particle composition distributions. One method that has been applied to PdAu [21] and PtPd [19] DENs, as well as dendrimer-templated PtAu [24] is to collect single particle EDS spectra from several (15-20) nanoparticles. These experiments indicate that individual particle composition distributions may vary widely, but the difficulty in obtaining data from the smallest particles may skew the results somewhat. EDS spectra collected over large areas, which sample tens or hundreds of particles, generally agree well with the bulk composition measurements [24] and with stoichiometries set in nanoparticle synthesis [19,21,24]. 

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