Colloids bimetallic

Many of the preparative methods described above can be applied to the formation of bimetallic colloids. The simultaneous reduction of two metal salts has been reported to lead to homogeneous bimetallic partides in which a range of product compositions is possible. The synthesis of bimetallic PtAu sols by the 

Some Pd-Pt bimetallic colloids protected by polymers [162-165] have been prepared by the coreduction of the metal salts with aqueous alcohols. Similar bimetallic sols in nonpolar organic solvents were obtained by N2H4 or NaBH4 reduction of palladium and platinum salt mixtures after their extraction into the organic phase (cyclohexane or chloroform) with such surfactants as trioctylphos-phine oxide or distearyldimethylammonium chloride. [166] 

Uniform bimetallic copper-palladium colloids (the metals are completely miscible) have been prepared by thermolysis of mixtures of the acetates in high boiling solvents such as bromobenzene, xylenes, and methyl- o-butyl ketone [86] in the absence of polymer stabilizers. The resulting agglomerated bimetallic particles often contained CuO in addition to the metals. Smaller particle sizes and narrower size distributions without oxide formation were reported to result from the analogous reaction in 2-ethoxyethanol in the presence of PVR [38] In both cases the homogeneous composition of the particles was established during the electron microprobe analysis used to determine the Cu Pd ratio in the individual particles. 

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Colloids embedded in a silica sol-gel matrix were prepared by using fully alloyed Pd-Au particles. The Mossbauer data have yielded evidence that alloying Pd with Au in bimetallic colloids leads to enhanced catalytic hydrogenation and also to improved selectivity [426]. 

Lynne K, Maritza G, Arnim H (1996) Bimetallic colloids Silver and Mercury. J Phys Chem 100(27) 11203-11206... 

In 1996, Liu et al. reported the selective hydrogenation of cinnamaldehyde, an a,/ -unsaturated aldehyde, to cinnamyl alcohol, an a,/ -unsaturated alcohol, by means of PVP-protected Pt/Co bimetallic colloids prepared by the polyol process [111]. The colloids were obtained as a dark-brown homogeneous dispersion in a mixture of ethylene glycol and diethylene glycol, and characterized by TEM and XRD. These authors prepared different samples of nanoparticles with Pt Co ratios of 3 1 and 1 1, the mean diameters of which measured 1.7 and 2.2 nm, respectively. These colloidal systems were also compared with the single metal-... 

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],... 

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

Various metallic nanoparticles have been prepared in the case of PS-P2(4) VP reverse micelles, as illustrated by the works of Moller et al. [ 109-111 ] and Antonietti and coworkers [112]. Very recently, Bronstein et al. reported on the synthesis of bimetallic colloids formed in PS-P4VP micelles and their catalytic behavior for the selective hydrogenation of dehydrolinalool [113]. 

Wang Y, Toshima N (1997) Preparation of Pd-Pt bimetallic colloids with controllable core/ shell structures. J Phys Chem B 101 5301-5306... 

A new method to synthesis nanoparticles of group VIII-X elements has been developed by Choukroun et al. [178] by reduction of metalUc precursors with CP2V. Mono- and bimetallic colloids of different metals (stabilized by polymers) have been prepared in this way (Fe, Pd, Rh, Rh/Pd). These colloids are then used as catalysts in various reactions such as hydrogenation of CC, CO, NO or CN multiple bonds, hydroformylation, carbonylation, etc. 

Bimetallic colloids (Pd-Au, Pd-Pt and Pd-Zn) stabilized into the micelle cores of PS-b-P4VP were investigated to determine the influence of the second metal (18). In this case, during the preparation of the catalyst, the addition of both metals salts occurs simultaneously, followed by reduction of metal compounds. 

Ag-Pd Alloy Powders (7,27). Bimetallic colloids, namely, Ag-Pd and Au-Pt, can be obtained by the polyol process. The composition Ag7oPdx> is of particular interest to make the internal electrodes of multilayer ceramic capacitors (MLCC). Polymer-protected, monodisperse, nanoscale Ag70Pd30 particles have been obtained... 

Studies on the immobilization of Pt-based hydrosilylation catalysts have resulted in the development of polymer-supported Pt catalysts that exhibit high hydrosilylation and low isomerization activity, high selectivity, and stability in solventless alkene hydrosilylation at room temperature.627 Results with Rh(I) and Pt(II) complexes supported on polyamides628 and Mn-based carbonyl complexes immobilized on aminated poly(siloxane) have also been published.629 A supported Pt-Pd bimetallic colloid containing Pd as the core metal with Pt on the surface showed a remarkable shift in activity in the hydrosilylation of 1-octene.630... 

Bimetallic colloids containing gold and palladium or platinum have been deposited onto carbon or graphite for use as catalysts for the selective oxidation of organic compounds52 (Section 8.3), in the same way as for pure gold colloids (Section 4.6). 

Methods for preparing colloidal gold and also bimetallic colloids containing gold have been described in Section 3.2.3. The gold particles may be immobilised on supports by dipping the supports in the colloidal suspension,... 

Methods for preparing bimetallic colloids containing gold have been described in Section 3.2.3. Their composition may be easily tuned since solutions of the mixed chloride precursors are normally used, and are reduced in a variety of ways after addition of a stabiliser. As already mentioned in Section 4.3.6, for gas-phase catalytic reactions, after depositing the colloid on an oxide support the stabilisers must be removed, but for liquid-phase reactions they may be retained providing access to the metal is still possible. 

The catalytic activity toward hydrogenation reactions has been indeed observed in such bimetallic colloidal systems. The SERS spectrum of p-nitrobenzoate (PNBA) adsorbed on colloidal silver and the SERS spectra observed on Ag/Pd nanoparticles, immediately after the addition of PNBA and after 1 week, are shown in Fig. 20.7 A, B, and C, respectively. In the bimetallic colloid, instead of the SERS spectrum of PNBA (spectrum A), a different spectrum is obtained (spectrum B) that slowly evolves toward a different spectral feature, which becomes predominant after a week (spectrum C). This modification may be related to the initial formation of p-aminobenzoate as a result of the catalytic reduction of the nitro group, followed by slow oxidation to azodibenzoate by atmospheric oxygen (see Fig. 20.8). 

Supported mixed metal catalysts are also prepared by other means such as the deposition of bimetallic colloids onto a support O and the decomposition of supported bimetallic cluster compounds.208 The photocatalytic codeposition of metals onto titania was also attempted with mixed results.209 with a mixture of chloroplatinic acid and rhodium chloride, very little rhodium was deposited on the titania. With aqueous solutions of silver nitrate and rhodium chloride, more rhodium was deposited but deposition was not complete. In aqueous ammonia, though, deposition of both silver and rhodium was complete but the titania surface was covered with small rhodium crystallites and larger silver particles containing some rhodium. With a mixture of chloroplatinic acid and palladium nitrate both metals were deposited but, while most of the resulting crystallites were bimetallic, the composition varied from particle to particle.209... 

This conclusion was supported by measurements on AuPd colloidal catalysts where the selectivity of the catalysts showed a strong correlation with the extent of alloying in the particles [10]. X-ray diffraction data showed that the bimetallic colloids consisted of a ( 7.5 nm) Au core with a thin (<1 nm) Pd shell. When the bimetallic colloids were annealed in the temperature range 300-600 K, no particle size increase was observed, but a change in the lattice parameter of the bulk phases present was detected indicative of alloy formation. 

Bimetallic particles with layered structures have opened fascinating prospects for the design of new catalysts. Schmid et al. [10m] have applied the classical seed-growth method [20] to synthesize layered bimetallic Au/Pd and Pd/Au colloids in the size range of 20-56 nm. The sequential reduction of gold salts and palladium salts with sodium citrate allows the gold core to be coated with Pd. This layered bimetallic colloid is stabilized by trisulfonated triphenylphosphane and sodium sulfanilate. More than 90% metal can be isolated in the solid state and is redispersable in water in high concentrations. 

This very versatile preparation route for nano structured mono- and bimetallic colloids has been further developed by Reetz and his group since 1994 [2e,12a,b]. The overall process of electrochemical synthesis [Eq. (7)] can be divided into six elemental steps (see Figure 6). 

Layered bimetallic nanocolloids (e.g., Pt/Pd) have been synthesized by modifying the electrochemical method [12c,i]. Finally, the preparation of bimetallic colloids (Pt/Pt) electrochemically using a new strategy based on the use of a preformed colloid, e.g., ( -CgHi7)4N Br stabilized Pt particles and a sacrificial anode, e.g., Pd sheet, has recently been reported [12J] (Figure 7). 

Upon the addition of CO or H2 in the presence of appropriate stabilizers, the controlled chemical decomposition of zerovalent transition metal complexes yields isolable products in multigram amounts [49]. The growth of metallic Ru particles from Ru(COT) (COD) (COT = cyclooctatetraene, COD = cycloocta-1,5-diene) with low-pressure dihydrogen was first reported by Ciardelli et al. [49a]. This material was, however, not well characterized, and the colloidal aspect of the ill-defined material seems to have been neglected in this work. Bradley and Chaudret [49b-l] have demonstrated the use of low-valent transition metal olefin complexes as a very clean source for the preparation of nanostructured mono- and bimetallic colloids. 

Indeed, when using an excess of (n-C4H9)4N OAc , this worked well. Surfactant-stabilized Pt colloids in THF having a size of 2.5 nm or 5 nm were obtained, the specific size depending upon the current density used. Similarly, RhCh, RuCf, OsCh, Pd(OAc)2, Mo2(OAc)4 were employed with formation of the corresponding nanosized colloids, although size-selectivity was not studied in these exploratory experiments. Moreover, bimetallic colloids of the type Pt/Sn, Pd/Cu and Pd/Pt were also obtained [38]. 

Finally, most of the recent studies on the hydrosilylation reaction are based on Pt colloids [100] or Pt/Au or Pt/Pd bimetallic colloids supported on alumina [101],... 

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