Platinum, colloidal

A substitute may be prepared thus 0 05 gram palladous chloride is placed in a special shaking flask with 50 c.c, of 50 per cent, alcohol and 1 or 2 c c. of 1 per cent, aqueous solution of gum-arabic, the weight of gum being about one-fourth the weight of the palladous chloride. On shaking this mixture in an atmosphere of hydrogen the chloride is reduced with formation of a black solution of colloidal platinum, which is rendered stable by the small quantity of gum present. 

Although Ru(bipy)2+ alone will not split water into hydrogen and oxygen, it has been accomplished with Ru(bipy)2+ using various catalysts or radical carriers. Perhaps the most studied system for the photoreduction of water involves using methyl viologen as the quencher, EDTA as an electron donor (decomposed in the reaction) and colloidal platinum as a redox catalyst (Figure 1.19). 

Ru(bipy)3 formed in this reaction is reduced by the sacrificial electron donor sodium ethylenediaminetetra-acetic acid, EDTA. Cat is the colloidal catalyst. With platinum, the quantum yield of hydrogenation was 9.9 x 10 . The yield for C H hydrogenation was much lower. However, it could substantially be improv l by using a Pt colloid which was covered by palladium This example demonstrates that complex colloidal metal catalysts may have specific actions. Bimetalic alloys of high specific area often can prepared by radiolytic reduction of metal ions 3.44) Reactions of oxidizing radicals with colloidal metals have been investigated less thoroughly. OH radicals react with colloidal platinum to form a thin oxide layer which increases the optical absorbance in the UV and protects the colloid from further radical attack. Complexed halide atoms, such as Cl , Br, and I, also react... 

Henglein A, Ershov BG, Malow M (1995) Absorption spectrum and some chemical reactions of colloidal platinum in aqueous solution. J Phys Chem 99 14129-14136... 

This value can be compared with those reported for a aqueous colloidal particles, eg colloidal gold 30-M0, nm particle diameter), colloidal platinum - irk... 

Balthis and Bailar6 obtained tris (ethylenediamine) chromium-(III) complexes by the oxidation of chromium(II) solutions, using a procedure somewhat similar to that used for the synthesis of cobalt (III) com plexes. Mori7 described the preparation of hexaamminechromium(III) salts from the oxidation of chromium (II) salts in the presence of ammonia. The results obtained in both syntheses have been erratic.8,9 Berman noted that the foregoing syntheses are rendered dependable by the use of a catalyst of activated platinum on asbestos. Schaeffer,100 in a subsequent study, independently used colloidal platinum as a catalyst but reported some difficulty in separating it from the product.106 The procedures recommended and described here are based on the use of platinized asbestos as the catalyst. 

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... 

Duff, D.G. et al., Structural characterization of colloidal platinum by high resolution electron microscopy and EXAFS analysis, Angew. Chem. 101, 610, 1989 Angew. Chem. Int. Ed. Engl., 28, 590,1989. 

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. 

The rapid back-electron transfer between these products is deliberately impeded by reducing Ru(bpy)3+ back to Ru(bpy)j+ with the sacrificial triethanolamine (TEOA), and in the presence of colloidal platinum MV+ reduces water to H2. 

Successful systems have used colloidal platinum as an efficient catalyst for the multi-electron reduction process by which hydrogen is produced. The platinum acts as a charge pool in that electrons from one-electron processes are trapped, to be later delivered to the substrate in a concerted manner, thus avoiding formation of high-energy intermediates (Figure 12.12). 

R. Narayanan and M. A. El-Sayed, Changing catalytic activity during colloidal platinum nanocrystals due to shape changes Electron-transfer reaction, J. Am. Chem. Soc. 126, 7194-7195 (2004). 

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). 

The phenylhydrazone of acetone gave on hydrogenation over colloidal platinum a 90% yield of A(-isopropyl-A(-phenylhydrazine [950], and the semi-carbazone of benzil on electroreduction a 70% yield of A(-aminocarbonyl-N - 1,2-diphenyl-2-ketoethyl)hydrazine [95/]. 

Acetylenic aromatic acids having the triple bond Hanked by carboxyl and an aromatic ring were partially reduced to olefinic aromatic acids by chromous sulfate in aqueous dimethylformamide at room temperature in high yields. Phenylpropiolic acid afforded irani -cinnamic acid in 91% yield [195]. Its sodium salt in aqueous solution gave on catalytic hydrogenation over colloidal platinum at room temperature and atmospheric pressure 80% yield of cis-cinnamic acid if the reaction was stopped after absorption of 1 mol of hydrogen. Otherwise phenylpropanoic acid was obtained in 75-80% yield [992]. 

Reactions. w-Hydroxybenzoic acid affords a variety of products, depending on the catalyst and conditions employed. Catalytic reduction over platinum black or platinum oxide in alkaline solution gives 3-hydroxycyclohexanecarboxylic acid [22267-35-2]. Reduction of a warm aqueous solution over platinum oxide or over colloidal platinum yields cyclohexanecarboxylic acid. w-Hydroxybenzaldehyde can be prepared by reducing ///-hydroxybenzoic acid with sodium amalgam. Finally, reduction over Raney nickel gives cydohexanol. 

As described in Section 3 of Chapter 2, multi-electron processes are important for designing conversion systems. Noble metals are most potent catalysts to realize a multi-electron catalytic reaction. It is well known that the activity of a metal catalyst increases remarkably in a colloidal dispersion. Synthetic polymers have often been used to stabilize the colloids. Colloidal platinum supported on synthetic polymers is attracting notice in the field of photochemical solar energy conversion, because it reduces protons by MV to evolve H2 gas.la)... 

Fig. 7.189. Colloidal platinum is sometimes regarded as a catalyst for the reduction of nitrobenzene in solution. A mixed potential alternative is shown.

Figure 6.8 Schematic representation of a cytochrome P450 mimic in which catalytic manganese porphyrins are captured in the bilayer of polymerized vesicles. Colloidal platinum encapsulated in the vesicles in combination with molecular hydrogen serves as a reductant.

Colloidal platinum may be prepared very easily by arcing under distilled water between platinum electrodes (Fig. 24). The current is taken from an ordinary 110-volt lighting circuit through an ammeter and an adjustable resistance in series or simply through a known... 

To a solution of hydrogen peroxide, add some of the colloidal platinum solution. The catalytic action of the finely divided platinum is plainly evidenced by the evolution of oxygen. 

The poisoning of a catalyst may be shown by adding some hydrogen sulfide solution to the hydrogen peroxide before the colloidal platinum is introduced. No decomposition of the peroxide is observed in this case, since the platinum has been poisoned by the presence of the hydrogen sulfide. This method is applicable to most of the metals below hydrogen in the electrochemical series. 

Preparation of stabilized highly dispersed colloidal platinum, that is, Pt particles of from 1 to 10 nm size in aqueous solution by chemical reduction followed by adsorptive precipitation of the dispersed Pt particles on the outer and inner surface of soot particles. 

The ultimate goal of catalyst preparation is to obtain very small platinum particles to increase the surface to volume ratio (S/V = 3/r for a spherical particle). Platinization by ion exchange and impregnation with colloidal platinum yield the best results in this respect (135). 

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