Bimetallic particles

The ions of a second metal can be reduced on the charged nano-cathode to form a shell around the primary particle. Numerous bimetallic combinations have been 

The deposition of indium on silver occurs in a more complex manner. When the irradiated silver sol contains In2(C104)3, an underpotential deposition of a thin indium layers occurs at the begiiming. Upon further reduction, the deposited indium undergoes partially the surface dismutation  

When two dissimilar metals are connected, there is momentary flow of electrons from the metal with the smaller work function to the other. The result is the build-up of a contact potential at the interface so that the Fenni level is the same in both metals. When 

The Fermi level in lead (which is a base metal) lies at an higher energy than in the silver particles. A mechanism has been proposed in which electron tunneling fi om lead to silver occurs in close encounters of the particles. Pb ions are then detached from the lead particles into solution. They are subsequently reduced on the surface of the negatively charged silver particles. The rate of Fermi level equilibration was found to be drastically increased by the presence of small concentrations of methyl viologen, MV .  

Adsorption of ions or neutral molecules on the surface of metal particles can lead to a change in optical absorption and chemical reactivity. The optical changes are most strongly produced in the wavelength range of the plasmon absorption band. Most remarkable effects are observed for silver particles whose plasmon absorption is especially intense, i.e. little damped. The adsorption of substances can lead to a destabilization of particles, i.e. to agglomeration. This effect generally is accompanied by a redshift of the plasmon absorption band due to dipole-dipole interaction between near-by 

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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. 

In order to verify the presence of bimetallic particles having mixed metal surface sites (i.e., true bimetallic clusters), the methanation reaction was used as a surface probe. Because Ru is an excellent methanation catalyst in comparison to Pt, Ir or Rh, the incorporation of mixed metal surface sites into the structure of a supported Ru catalyst should have the effect of drastically reducing the methanation activity. This observation has been attributed to an ensemble effect and has been previously reported for a series of silica-supported Pt-Ru bimetallic clusters ( ). 

A comment regarding the dispersion of the Ru-Rh/Si02 and the Ru-Ir/Si02 is in order. For the case of the supported Pt-Ru catalysts. Increases in dispersion as a result of clustering were very large ( ). This effect was particularly noticeable for bimetallic particles which conform to the cherry model. Evidently, the formation of an inner core enriched in one of the two metals, followed by an outer layer enriched in the other metal, inhibits further crystal growth. For the alumina-supported Pt-Ru bimetallic clusters, the effect, although present, is considerably smaller. 

These conclusions from the infrared reflectance spectra recorded with Pt and Pt-Ru bulk alloys were confirmed in electrocatalysis studies on small bimetallic particles dispersed on high surface area carbon powders.Concerning the structure of bimetallic Pt-Ru particles, in situ Extended X-Ray Absorption Fine Structure (EXAFS>XANES experiments showed that the particle is a true alloy. For practical application, it is very important to determine the optimum composition of the R-Ru alloys. Even if there are still some discrepancies, several recent studies have concluded that an optimum composition about 15 to 20 at.% in ruthenium gives the best results for the oxidation of methanol. This composition is different from that for the oxidation of dissolved CO (about 50 at.% Ru), confirming a different spatial distribution of the adsorbed species. 

For bimetallic Pd - Cu particles, the coordination of CO to surface Cu and Pd sites has been directly observed together with a surface reconstruction at room temperature [42]. Thus, under vacuum, there is an enrichment of the bimetallic particle surface in Cu. Upon addition of CO, the IR spectrum dis-... 

Amiridis followed that also in propene hydrogenation over Pt-Au/Ti02 a minimum ensemble of Pt atoms is necessary for a successful concomitant adsorption of olefin and hydrogen. However, beside this dilution effect of gold on the bimetallic particles, an electronic impact can not be excluded. An application of Amiridis ... 

However, it should be noted that the structure of those bimetallic particles (Ag-In) can be changed under the conditions of the catalytic reaction, that is, in presence of the reactants, as recently shown by in situ X-ray absorption spectroscopy of acrolein hydrogenation [100]. 

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 reaction system is a complex one variants used include also zeolites, pillard clays, bimetallic particles, and the use of promoters such as K. 

Hodoshima, S., T. Kubono, S. Asano, H. Arai, and Y. Saito, Preparation of nanosize bimetallic particles on activated carbon. Stud. Surf. Sci. Catal., 132, 323-326 (2001). 

Let us first consider what EXAFS might tell us in the case of bimetallic particles that are not too small - say a few nanometer in diameter. For a truly homogeneous alloy with a 50 50 composition, EXAFS should see a coordination shell of nearest neighbors with 50% Cu and 50% Ru around both ruthenium and copper atoms. If, on the other hand, the particle consists of an Ru core surrounded by a Cu shell of monatomic thickness, we expect that the Ru EXAFS shows Ru as the dominant neighbor, because only Ru atoms in the layer directly below the surface are in contact with Cu. The Cu EXAFS should see both Cu neighbors in the surface and Ru neighbors from the layer underneath, with a total coordination number smaller than that of the Ru atoms. The latter situation is indeed observed in Ru-Cu/Si02 catalysts, as we shall see below. 

This survey focuses on recent developments in catalysts for phosphoric acid fuel cells (PAFC), proton-exchange membrane fuel cells (PEMFC), and the direct methanol fuel cell (DMFC). In PAFC, operating at 160-220°C, orthophosphoric acid is used as the electrolyte, the anode catalyst is Pt and the cathode can be a bimetallic system like Pt/Cr/Co. For this purpose, a bimetallic colloidal precursor of the composition Pt50Co30Cr20 (size 3.8 nm) was prepared by the co-reduction of the corresponding metal salts [184-186], From XRD analysis, the bimetallic particles were found alloyed in an ordered fct-structure. The elecbocatalytic performance in a standard half-cell was compared with an industrial standard catalyst (bimetallic crystallites of 5.7 nm size) manufactured by co-precipitation and subsequent annealing to 900°C. The advantage of the bimetallic colloid catalysts lies in its improved durability, which is essential for PAFC applicabons. After 22 h it was found that the potential had decayed by less than 10 mV [187],... 

The SEA approach can be applied to a novel system in three steps (1) measure the PZC of the oxide (or carbon) and choose a metal cation for low-PZC materials and an anion for high-PZC materials, (2) perform an uptake-pH survey to determine the pH of the strongest interaction in the appropriate pH regime (high pH for low PZC and vice versa), and (3) tune the calcination/reduction steps to maintain high dispersion. Highly dispersed Pt materials have been prepared in this way over silica, alumina, and carbon. Other oxides can be employed similarly. For bimetallics, the idea is to first adsorb a well-dispersed metal that forms an oxide intermediate with a PZC very different to the support. In this way the second metal can be directed onto the first metal oxide by SEA. Reduction may then result in relatively homogeneous bimetallic particles. 

The dendrimer-assisted method can be extended to bimetallic clusters. The second metal ion can be introduced by partial displacement of the first metal, or sequentially after the first, or together with the first by cocomplexation. Although these methods succeed in making bimetallic particles, it is not obvious that they can generate uniform composition particles. Thus, new methods or variations of the existing method have to be developed. 

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. ... 

Various catalytic reactions are known to be structure sensitive as proposed by Boudart and studied by many authors. Examples are the selective hydrogenation of polyunsaturated hydrocarbons, hydrogenolysis of paraffins, and ammonia or Fischer-Tropsch synthesis. Controlled surface reactions such as oxidation-reduction reactions ° or surface organometallic chemistry (SOMC) " are two suitable methods for the synthesis of mono- or bimetallic particles. However, for these techniques. 

An alternative approach to the preparation of bulk Pt bimetallic particles has been the preparation of surface modification of Pt particles with second metals. This approach has been used to prepare partially coated and fully coated Pt particles to give "core-shell" structures. Core-shell structures of Pt or PtM bimetal-lies on alternative metal cores have also been prepared. This approach argues... 

The process of using organometallic compounds to prepare bimetallic particles is quite general, and we will review below the chemistry of organometallics reagents with metal surfaces, which are limited so far mainly to group VIII metals. 

Table 6.3 Mean particle size d) of metallic and bimetallic particles measured by TEM and H2 and CO chemisorption properties of selected catalysts. (Reproduced from Reference 147].)...

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. 

Figure 8.1 Simplified picture showing the steps leading to supported bimetallic particles from a heteronuclear neutral carbonyl complex.

Figure 8.2 Open routes for the preparation of supported bimetallic particles from carbonyl species generated on the surface of a support from an initial M L metal precursor.

The use of carbonyl complexes has enabled proper study of the role of rhenium in Pt-Re bimetallic catalysts used in the reforming of naphtha [57-60] and tailoring of the preparation of Pt-Ru bimetallic particles. Pt-Ru systems are of interest in developing electrodes for fuel cell applications [61]. 

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]. 

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