Platinum, microcrystals

Figure 20 demonstrates by transmission electron microscopy the distribution of platinum microcrystals on (transparent) graphitized soot. Soot particles form agglomerates that measure around 0.1 /u-m in diameter. 

Fig. 13.27. Potential vs. current density plots for state-of-the-art fuel cells, o, proton exchange membrane fuel cell , solid oxide fuel cell , pressurized phosphonic acid fuel cell (PAFC) a, direct methanol fuel cell, direct methanol PAFC , alkaline fuel cell. (Reprinted from M. A. Parthasarathy, S. Srinivasan, and A. J. Appleby, Electrode Kinetics of Oxygen Reduction at Carbon-Supported and Un-supported Platinum Microcrystal-lite/Nafion Interfaces, J. Electroanalytical Chem. 339 101-121, copyright 1992, p. 103, Fig. 1, with permission from Elsevier Science.)...

Pr4Ni2054C36H72, Praseodymium(III), tris(l,4,7,10,13,16-hexaoxacy cloocta-decane)dodecanitratotetra-, 23 155 Pt, Platinum, microcrystals, 24 238 PtBClF4S3C6Hig, Piatinum(II), chloro-tris(dimethyl sulfide)-, tetrafluorobor-ate(l-), 22 126... 

Once the platinum microcrystals begin to form, they act as highly active catalysts for the hydrolysis of NatBH,], so that a large excess of Na[BIl,] is needed to maximize the reduction of HzlPlCy. However, below a certain HJPtCle] concentration, the hydrolysis of Na[BH ] is predominant, and a concentration of about 4 X 10 M H2[PtCU, which was determined colorimetrically, is left after the reduction, regardless of initial HjlPtCy concentration. The yield of platinum is 66 5%, so diat about IS mgof platinum is produced by this procedure. 

Fig. 1. Transmission electron micrograph cf platinum microcrystals, with the cone- Hmding Pt diffraction rings in the lower left comer. The granular backgrouttd is the amorphous carbon film support.

Fig. 2. Histogram of the diameters of the platinum microcrystals. The sample population is 300.

A study has been reported by Glaunsinger and coworkers [39] on the reduction of chloroplatinic add by a variety of redudng agents inducting sodium borohy-dride, hydroxylamine hydrochloride, dimethylamine borane, sodium dtrate, hydrazine monohydrate, sodium formate, trimethylamine borane, sodium trime-thoxyborohydride, and formaldehyde. By careful variation of reaction temperature, protective polymer (PVP was found to be the most effective), time, pH, and reagent concentration, as well as the use of both direct and reverse addition of the reagents, it was shown that colloidal platinum could be produced as spheres, strands, or mosaic composites of platinum microcrystals. 

The reduction of metal salts with borohydride or trialkylborohydride is a widely used colloid synthesis method. The preparation of platinum microcrystals having a mean diameter of 28 A by the reduction of chloroplatinic add with sodium borohydride has been reported as a reprodudble standardized preparation. [52] PVP stabilized copper sols have been prepared by borohydride reduction of copper salts. [53, 54] In some cases, however, the formation of metal... 

The structure of a polycrystalline electrode depends on its preparation. Usually toe rough electrodes are prepared by electrochemical deposition of a given metal onto a suitable substrate. Microcrystals present in polycrystaUine samples are randomly oriented on the surface. Most likely, not only basal but also higher MiUer-index planes should be considered in anticipating toe final structure of the electrode surface. It was shown that the stmcture of the platinized platinum surface depends strongly on toe platinization conditions, e.g., on toe concentration of the platinization... 

A few other types of anion sensors have been mentioned recently in the literature. Tetrathiofulvalene microcrystals immobilised at a platinum electrode displayed electrochemical properties that were affected by the presence of anions in solution, with some selectivity for anions such as bromide [ 145]. A flow-injection analysis system using anion-exchange columns for separation and polyaniline electrodes as detectors could detect dichromate down to 0.004 ppb and could be used for seawater samples [146]. 

The many forms of so-called amorphous carbon, such as charcoals, soot, and lampblack, are all actually microcrystalline forms of graphite. In some soots the microcrystals are so small that they contain only a few unit cells of the graphite structure. The physical properties of such materials are mainly determined by the nature and magnitude of their surface areas. The finely divided forms, which present relatively vast surfaces with only partially saturated attractive forces, readily absorb large amounts of gases and solutes from solution.9 Active carbons impregnated with palladium, platinum, or other metal salts are widely used as industrial catalysts. 

The magnetic susceptibility of these microcrystals as well as commercial platinum foil (99.9%) [Johnson Matthey] has been measured at 300 K using a Faraday apparatus described elsewhere, and their susceptibilities are equal within experimental error (1.00 0.07 and 0.989 0.004 cm /g, respectively). However, low-temperature susceptibility measurements on these microcrystals have revealed a Curie-Weiss law behavior below about 150 K, which has been attributed to their very small size. ... 

A cell of this design was employed in studies of microcrystal solids attached to a platinum flag electrode [608]. The measured ESR spectrum of electrochem-ically reduced 7,7, 8,8 -tetracyanoquinodimethane was in perfect agreement with the respective spectrum of the chemically prepared compound and the simulated spectrum. 

Fig. 4.11a-d Consecutive cyclic voltammetric curves obtained for diphenylamine microcrystals attached to a platinum electrode (A = cm ) in the presence of aqueous solution containing 1 moldm H2SO4. (Reproduced from [134] with the permission of Elsevier Ltd.) Scan rate 100 mVs . Cycles a 1-5, b 11-30 (started after a 3 min delay at — 0.2 V)... 

The electrode reaction is rarely as simple as described above. In many cases the product is either insoluble, or partly adsorbed at the electrode surface. Besides, the reactants of many reactions are also surface active. Furthermore, the electrode reaction can either be preceded, or followed, by chemical reactions. Hence, the choice of the working electrode also depends on the reaction mechanism. For instance, the reduction of lead ions on a platinum electrode is complicated by nucleation and growth of lead microcrystals, while, on a mercury electrode, lead atoms are dissolved in mercury and the reduction is fast and reversible. Similarly, the well-known pigment alizarine-red S and the product of its reduction are both strongly adsorbed on the surface of mercury and carbon electrodes [17]. In this case the liquid mercury electrode is analytically much more useful because the adsorptive accumulation on the fresh electrode surface can be easily repeated by creating a new mercury drop. However, on the solid electrode, the film of irreversibly adsorbed substance is so stable that it can be formed in one solution and then transferred into another electrolyte for the measurement of the kinetics of the electrode reaction. After each experiment of this type, the surface of the carbon electrode needs careful cleaning and polish-... 

J. H. Jiang, A. Kucernak, An electrochemical impedance study of the electrochemical doping process of platinum phthalocyanine microcrystals in non-aqueous electrolytes, J. Electroanal. Chem., 2001,514,1-2, pp. 1-15. 

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