Platinum catalyst colloidal

Toshima et al. obtained colloidal dispersions of platinum by hydrogen- and photo-reduction of chloroplatinic acid in an aqueous solution in the presence of various types of surfactants such as dodecyltrimethylammonium (DTAC) and sodium dodecylsulfate (SDS) [60]. The nanoparticles produced by hydrogen reduction are bigger and more widely distributed in size than those resulting from the photo-irradiation method. Hydrogenation of vinylacetate was chosen as a catalytic reaction to test the activity of these surfactant-stabilized colloids. The reaction was performed in water under atmospheric pressure of hydrogen at 30 °C. The photo-reduced colloidal platinum catalysts proved to be best in terms of activity, a fact explained by their higher surface area as a consequence of their smaller size. 

In order to evaluate the catalytic characteristics of colloidal platinum, a comparison of the efficiency of Pt nanoparticles in the quasi-homogeneous reaction shown in Equation 3.7, with that of supported colloids of the same charge and of a conventional heterogeneous platinum catalyst was performed. The quasi-homogeneous colloidal system surpassed the conventional catalyst in turnover frequency by a factor of 3 [157], Enantioselectivity of the reaction (Equation 3.7) in the presence of polyvinyl-pyrrolidone as stabilizer has been studied by Bradley et al. [158,159], who observed that the presence of HC1 in as-prepared cinchona alkaloids modified Pt sols had a marked effect on the rate and reproducibility [158], Removal of HC1 by dialysis improved the performance of the catalysts in both rate and reproducibility. These purified colloidal catalysts can serve as reliable... 

Kohler, J.U. and Bradley, J.S., Enantio selective hydrogenation of ethyl pyruvate with colloidal platinum catalysts the effect of acidity on rate, Catal. Lett., 45, 203,1997. 

An aqueous solution containing 300 ng/L chloroform and colloidal platinum catalyst was irradiated with UV light. After 15 h, only 10 ng/L chloroform remained. A duplicate experiment was performed but 0.1 g zinc was added to the system. At approximately 2 h, 10 ng/L chloroform remained and 210 ng/L methane was produced (Wang and Tan, 1988). 

Chemical/Physical. The estimated hydrolysis half-life in water at 25 °C and pH 7 is 274 yr (Mabey and Mill, 1978). Hydrogen gas was bubbled in an aqueous solution containing 18.8 pmol dibromochloromethane. After 24 h, only 18% of the dibromochloromethane reacted to form methane and minor traces of ethane. In the presence of colloidal platinum catalyst, the reaction proceeded at a much faster rate forming the same end products (Wang et al., 1988). 

As described in the first part of this section, MVt can reduce protons to give H2 with a platinum catalyst. The presence of Pt colloid in the photoreaction mixture of Ru(bpy) + polymer complex (derived from water soluble homopolymer of Vbpy), MV2+ and EDTA gave H2 gas at almost the same rate as the mixture containing Ru(bpy)j + instead of polymer complex401 (see Scheme 1). The turnover number of the Ru polymer complex exceeded 25 in 1 h s irradiation. The water insoluble polymer complex (5) showed almost the same activity when used as suspensions in a mixture of MeOH/H20 =1/1. 

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) ... 

Platinum Catalyst for Reductions (CoU. Vol. i, 452) Directions are given for the preparation of a colloidal platinum (or palladium) catalyst in an anhydrous, alkaline medium. This catalyst is said to be particularly valuable for the reduction of nitriles, oximes, and nitrostyrenes to pure prinaary amines. Skiia and Keil, Ber. 63, 424 (r932). 

The ion-pair approach to the design of photosensitizers for electron transfer processes has been followed also in the case of [Co" (sep)] -oxalate system. In a deoxygenated solution, the excitation in the IPCT band of [Co" (sep)] +-HC204 causes the reduction of [Co" (sep)] + to [Co"(sep)] " and the oxidation of oxalate to carbon dioxide. The [Co"(sep)] + complex is a sufficiently strong reductant to reduce H+ to H2 at moderately acidic pH values. Thus, when the photoreaction is carried out in the presence of colloidal platinum catalyst, such a reaction indeed occurs, and H2 evolves from the solution in addition to carbon dioxide. Under such conditions, the overall reaction is the oxidation of oxalate, which plays the role of sacrificial agent, combined with the reduction of water to yield carbon dioxide and dihydrogen, according to Eq. 8. 

With a platinum catalyst, methylviologen is undoubtedly a better electron transfer agent than macrobicyclic cobalt complexes. To the contrary, when colloidal RU2O3 (5 x IQ- moM i) deposited on silicon surface is employed as a catalyst, the rate of hydrogen production in the case of [Co(Clsar)]3+ cation is 55 pmolmin-h whereas for methylviologen it is 11 pmol min-i under the same experimental conditions. 

In the second reaction, ethylene is reacted with the alkylaluminums to form n-a-olefins and to regenerate triethylaluminum (Figure 2). Although the reaction can be conducted without a catalyst by operating at high temperatures (3, 6), the reaction is normally carried out at lower temperatures in the presence of a nickel, cobalt, or platinum catalyst. Reaction conditions are moderate, 2500 p.s.i. and 200° F. when less than 0.01% of nickel catalyst is used. The catalyst is formed in situ by the addition of a nickel salt. A small amount of alkylaluminum will react with the nickel salt, reducing it to colloidal nickel. 

It was found that for enantioselective hydrogenation over modified platinum catalysts the most suitable SC-solvents are ethane and propane as indicated in Table 5.15. Application of SC-CO2 in hydrogenation on chiral modified Pt-catalysts proved to be less suitable because CO2 is partly hydrogenated into CO, which poisons the catalyst But polymer-supported colloidal Pd nanoparticles as catalysts in supercritical CO2 (SCCO2) revealed top s as high as 4 x 10 h" at 15 bar hydrogen, and 50°C (Niessen et al. 

Scheme 6-2. Schematic representation of selective catalytic hydrogenation of isomeric undecenoic adds in polymerized micelle encapsulated colloidal platinum catalysts. The position of the double bond in the substrate determines the necessary orientation for 3ccess to the hydrogenation site, and this is mediated by the ordered polar micelle walls. (Adapted from ref. [261].)...

Ikble 6-L Catalytic Activities of Colloidal Platinum Catalysts for Hydrogen Generation by Water Photolysis using EDTA/Ru(bipy)3 /MV /Pt(coll). (Adapted from ref. 285)... 

---