Platinum colloids

Fig. 8 TEM (a) and HREM (b) micrographs of octanethiol stabilized platinum colloid showing nearby particles with no coalescence...

Multipods and Dendritic Nanoparticles of Platinum Colloidal Synthesis and Electrocatalytic Property... 

Scheme 9.11 Enantioselective hydrogenation of ethyl pyruvate with platinum colloids stabilized by protonated-dihydrocinchonidine.

Although several noble-metal nanoparticles have been investigated for the enantiomeric catalysis of prochiral substrates, platinum colloids remain the most widely studied. PVP-stabilized platinum modified with cinchonidine showed ee-values >95%. Several stabilizers have been also investigated such as surfactants, cinchonidinium salts and solvents, and promising ee-values have been observed. Details of a comparison of various catalytic systems are listed in Table 9.16 in one case, the colloid suspension was reused without any loss in enantioselectiv-ity. Clearly, the development of convenient two-phase liquid-liquid systems for the recycling of chiral colloids remains a future challenge. 

Witek, G. et al., Interaction of platinum colloids with single crystalline oxide and graphite substrates a combined AFM, STM and XPS study, Catal. Lett., 37, 35, 1996. 

Kohler, J.U. and Bradley, J.S., A kinetic probe of the effect of a stabilizing polymer on a colloidal catalyst accelerated enantio selective hydrogenation of ethyl pyruvate catalyzed by poly(vinylpyrroli-done)-stabilized platinum colloids, Langmuir, 14, 2730,1998. 

TEM images of the as-prepared tin oxide and platinum colloid are shown in Fig. 15.2. It can be seen that both the tin oxide and platinum nanoparticles are uniform and well distributed. Based on the measurements of 300 particles in randomregions, the... 

Occurrence and History of Platinum--Proparatioi) -Punlioulimi Physical Properties—Volatilisation -Diffusion of Cases Solubility of (laMes Chemical Properties - -Catalytic Activity -Passivity Crystalline Platinum —Colloidal Platinum — Platinum Black Platinum Mpongo Explosive Platinum —Atomic Weight — Uses Buhstitutes Alloys Platinum Amalgam. 

All synthesized polymers are completely soluble in usual solvents. The effect of platinum colloid forms used in polyaddition reaction on molecular structure of synthesized cyclolinear polyalkylcar-bosiloxane polymers has been studied. In accordance with the opinion [93], application of plati-num-hydrochloric acid [80] to polyaddition of dihydromethyl-... 

Finally, metal colloids can adsorb chiral molecules such surface-modified particles can catalyze hydrogenations in high optical yields. An example is platinum colloids treated with cinchona alkaloids.18... 

The following mechanism of polymerization was proposed platinum catalyst is reduced by Si-H compound to platinum colloid, which activates another molecule of Si-H compound and facilitates a nucleophilic attack of oxygen atom in the monomer on silicon atom (e.g., for substituted oxirane) ... 

In contrast to standard borohydride reductive nanoparticle synthesis, we have developed an alternative strategy to amino acid encapsulated nanoparticles by utilizing a metal nanoparticle (M°-(Ligand))/metal ion (M"+) precursor redox pair with matched oxidation/reduction potentials. Simply, a metal nanoparticle such as Pt°-(Cys) acts as the principal reductant to a complimentary selected metal ion of Au + resulting in a new stabilized metal nanoparticle of Au°-(Cys) and the oxidation product of the original nanoparticle Pt"+. Malow et al. have reported a metathesis/transmetallation type reaction between a platinum colloid and a Au cyanide compound. Similarly, we employed a Pt°-(Cys)/AuCl4 pair and 0.5-2.0 equivalents of Au to Pt -(Cys). XRD analysis of the nanoparticle products revealed differences in crystallinity... 

The linear CO stretching frequency for the carbonylated platinum colloid while lower than that found for surface bound CO, is in the range reported for the platinum carbonyl clusters [Pt 3 (CO) 6 ] n / sind we find that the carbonylated colloid is easily transformed into the molecular cluster [Pt 12 (CO) 24 ] (10) reaction with water. The cluster was isolated in 50 yield based on platinum content of the precipitate by extraction with tetraethylammonium bromide in methanol from the aluminum hydroxide precipitated when water is added to the aluminoxane solution. The isolation of the platinum carbonyl cluster reveals nothing about the size or structure of the colloidal platinum particles, but merely emphasizes the high reactivity of metals in this highly dispersed state. The cluster isolated is presumably more a reflection of the stability of the [Pt3(CO)6]n family of clusters than a clue to the nuclearity of the colloidal metal particles - in a similar series of experiments with colloidal cobalt with a mean particle size of 20A carbonylation results in the direct formation of Co2(CO)8. 

Platinum salts were incorporated into water in an oil emulsion, e.g. penta-ethylene glycol dodecyl ether in hexadecane/water (= inverted micelle) with well-defined cavities. The platinum colloids which are then produced by hydrazine hydrate are uniform. No measurable particles fell outside the limit of... 

There have been fewer reports on the particle size dependence of catalysis by platinum-catalyzed redox reactions. A report by Sharma et al. [21] showed that platinum colloidal nanoparticles do not demonstrate the same dependence on particle size as gold nanoparticles do for the reduction of hexacyanoferrate (III) by thiosulfate [19]. Platinum nanoparticles protected by sodium di(2-ethylhexyl) sulfosuccinate (synthesized by a reverse micelle technique) exhibit an optimum size ( 38 nm) for the reduction of ferricyanide by thiosulfate (Fig. 18.2). The reason for an optimum particle size is not fully understood however, they proposed the following explanation a shift in the Fermi level occurs as the diameter is increased. 

The noble metals were the favorite metals for demonstrating the usefulness of the microwave operation in conducting the polyol reaction. Among the noble metals platinum was synthesized most frequently. Polymer-stabilized platinum colloids with nearly uniform spherical shape were prepared by Yu and coworkers by microwave dielectric heating [172]. The average diameters of the as-prepared platinum colloids were 2-4 nm with a narrow size distribution in regard to the preparation conditions. 

Platinum colloids (3% Pt) were incorporated in Si02 and Si02-Ta205 (15% Ta20s) polymeric networks. TEOS and Ta(OC2H5)5 (PET), respectively, were used as precursors. Since Pt colloids were soluble in ethanol, the sol composition (molar ratio) was TEOS EtOH H20 HCl of 1 4.5 4.0.02 for silica, and TEOS PET EtOH H20 HC1 of 1 0.05 10 4.5 0.0.3 for Si02-Ta20s mixed oxide. For mixed oxide, because of the differences in the hydrolysis rate of TEOS and PET, the silica sol was prepared in the first step. The pre-hydrolysis and condensation step was carried out at 80 °C for 2h, then the mixture was cooled to room temperature. PET was separately solved in ethanol and added at room temperature to the preformed silica sol. 

The control of the gelification rate was achieved by the parameters of the sol-gel process (pH and quantity of the solvent). The gelification was conducted at room temperature. Platinum colloids (see Table 1) were solved in ethanol. 

Benner et found a small (less than 20) enhancement effect for platinum cyanide on a platinum colloid. 

Manners first proposed that the transition-metal-catalyzed ROP occurred via a homogenous mechanism.157 However, a heterogenous catalytic cycle has been reported.158 The proposed mechanism for the Pt(l,5-cod)2 (cod-cyclooctadiene) catalyzed reaction is shown in Scheme 2.24. The Pt(l,5-cod)2 forms a [2]platinasilaferrocenophane through oxidative addition to the zero-valent Pt complex via elimination of a 1,5-cod ligand. Platinum colloids are then formed by the elimination of the second 1,5-cod ligand these platinum colloids are proposed to be the active catalysts. The polymers are then formed by subsequent oxidative addition and reductive eliminations at the colloid surface. 

Karstedt solution, a complex compound of H2PtCl6 (H20)6) and vinyl-substituted disiloxanes are well known and very active catalysts [19]. Several other catalytic systems, e.g., Pt(cod)2, leading to the formation of platinum colloids have been examined [20]. More recently, hydrosilylation with the Speier catalyst has been tested both under single- and two-phase conditions. The hydrosilylation reaction was thereby optimized for technical realization [21]. 

Fig. 7. Transmission electron micrograph of porous silica templated from a mixture of platinum-loaded microgels, SElOlO, and tetramethoxysilane. Micrograph illustrates the retention of discreet platinum colloids following sihca formation and calcination under elevated temperatures. Scale bar=50 nm. Reproduced by permission of Wiley-VCH from [65]...

Figure 1 Scheme of the heterogenization procedure for chirally stabilized platinum colloids 1) mixing the solutions, 2) dropping the mixed solutions on a suitable surftice e. g. a PE film, and drying the droplets afterwaids. 

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