Platinum spherical

Bead, see Platinum, spherical bead Black, see Acetylene black Carbon black ... 

Solution displacement, periodic, see Platinum with periodic displacement of solution Sphere see alsoMercury-coated platinum sphere Mercury, hanging drop Mercury, rotating dropping Mercury, vibrating drop Platinum, spherical bead... 

To add surface area, the supports are uniformly coated with a slurry of gamma-alumina and recalcined under moderate conditions. The wash coat acts to accept the active metals, typically low levels of platinum and palladium, in a conventional impregnation process. In the United States in passenger car apphcations the spherical catalyst is used almost exclusively, and methods have been developed to replace the catalyst without removing the converter shell when vehicle inspection reveals that emission standards are not met. 

Solla-Gullon J, Solla-Gullon J, Vidal-Iglesias FJ, Herrero E, Feliu JM, Aldaz A. 2006. CO monolayer oxidation on semi-spherical and preferentially oriented (100) and (111) platinum nanoparticles. Electrochem Commun 8 189-194. 

The differential capacity can be measured primarily with a capacity bridge, as originally proposed by W. Wien (see Section 5.5.3). The first precise experiments with this method were carried out by M. Proskurnin and A. N. Frumkin. D. C. Grahame perfected the apparatus, which employed a dropping mercury electrode located inside a spherical screen of platinized platinum. This platinum electrode has a high capacitance compared to a mercury drop and thus does not affect the meaurement, as the two capacitances are in series. The capacity component is measured for this system. As the flow rate of mercury is known, then the surface of the electrode A (square centimetres) is known at each instant ... 

The platinum single crystals treated in this May are small spherical platinum beads Hith a diameter ranging from l.S to 1.9 am obtained by melting of a Mire. They are oriented, cut and polished according to the technique described in (10.) Mith an accuracy uithin 3 minutes of the nominal orientation. After polishing, the samples are annealed at 1300°-1500 C to eliminate the perturbed surface layer. 

Figure 7.10 An aerial view of a catalytic reforming processing plant. The reactors are the 21-ft spherical objects in the middle. These contain platinum and are in a series so that the octane is increased a little more in each reactor. (Courtesy of BP Amoco, Texas City, TX)...

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). 

Lee et al. (61) measured the equilibrium shape of clean platinum by monitoring the changes in the shape of a series of micrometer-sized platinum droplets during annealing at I200°C in 10 7 Torr of oxygen. Consistent with the work of Schmidt, their results showed that the equilibrium particle shape is influenced by what gas is present. As shown in Fig. 4, at equilibrium, the clean particle shape is nearly spherical, with distinct (100) and (111) facets. The facets occupy only 16% of the surface. However, if the particle is contaminated with carbon, the particle is a cubooctahedron with large (111), (100), and (110) facets. 

Fig. 4. Scanning electron micrographs of the equilibrium shapes of platinum particles show that both the gas phase and impurity in the metal can influence equilibrium shape, (a) A clean Pt particle is nearly spherical with distinct (100) and (111) facets after treatment in IO 7Torr of oxygen at 1200°C. (b) A carbon-covered Pt particle is cubo-octahedral (61).

For post-type DTA s in which thermocouple junctions measure the temperature of the container of the sample (e.g. platinum or poly crystalline alumina crucibles), good mechanical contact between the sample and the bottom of the crucible will improve instrument sensitivity to transformations. Surface contact may be optimized by using samples shaped to match the crucible, or finely crushed granules, as opposed to more spherical or odd-shaped chunks. Optimum mechanical contact minimizes the lag time between when a reaction occurs and when heat propagates to/from the point of temperature measurement, and the reaction is recorded. 

There are a number of different ways to prepare metal overlayers in practical nanoscale fuel cell electrocatalysts. One approach is schematically shown in Figure 3.3.13A, where a spherical alloy nanoparticle consisting of a noble metal (grey), platinum, and a nonnoble metal (red), such as copper, is subject to a corrosive electrochemical... 

Sharma G, Mei Y, Lu Y, Ballauff M, Irrgang T, Proch S, Kempe R (2007) Spherical polyelectrolyte brushes as carriers for platinum nanoparticles in heterogeneous hydrogenation reactions. J Catal 246 10-14... 

Mei Y, Sharma G, Lu Y, Drechsler M, Ballauff M, Irrgang T, Kempe R (2005) High catalytic activity of platinum nanoparticles immobilized on spherical polyelectrolyte brushes. Langmuir 21 12229-12234... 

Initially, crystallite growth occurs rapidly either by ion dissolution/reprecipitation (Ostwald ripening) or by surface atom diffusion, due to the requirement for lowering the surface energy of any individual crystallite. This thermodynamic driving force will tend to eliminate the incomplete faces but with the drive to lower the surface energy, the crystallites also will strive towards sphericity. This means, that to all intents and purposes, the ratios ofthe (111) and (100) faces should be approximately the same. Bett et al.1 noted that as the platinum crystallite sizes grew, the size distribution increased. If this is so, then... 

Figure 5 shows the influence for taking the logarithm ofthe specific activity versus the logarithm of 1/X, so that the slope defines the value of the exponent, m, which is of course, -3. All ofthe data fall in a linear band, limited at the upper level by the results of Buchanan et al.,s and at the lower by the results of Giordano et al.15,16 The results ofWatanabe-Stonehart fit between the boundaries with a directive that indicates a transition from spherical to semi-infinite diffusion due to closeness of approach of the crystallites. The size of an individual platinum crystallite also is indicated in Fig. 5. Extrapolation of the high points to zero crystallite separation gives 20 pA cm 2 Pt, which is close to that value obtained by Bett et al.1 in sulphuric acid.

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