Microscopic metals dispersion

For catalysts reduced at 400°-500°C, average nickel particle diameters in the range 30-45 A (40,43), and 30-200 A (41) have been quoted. Coenen and Linsen (41) have assumed a roughly hemispherical shape for the nickel particles which expose (111), (100), and (110) planes, and this is at least consistent with the very limited electron microscopic evidence. On the whole, it appears to be more difficult to produce a very high degree of metal dispersion with nickel than with platinum, and it is very difficult to obtain an average nickel particle diameter <30 A, although not impossible. 

The metal dispersion of the Ni-Mg-Ce/Al203 catalyst was slightly improved after H2 reduction at high temperature, with Ni concentration increased from 2.80% to 2.96%. The Ni concentration declined to 2.06% after reaction. This was mainly the partial coverage of carbonaceous species on the surface active sites of the used catalyst. However, the carbon concentration of 26.7% was much less than that on Ni-Mg-Ce/Si02 catalyst. Fig. 1 and Fig.2 showed the microscopic observations of reduced and used Ni-Mg-Ce/A Os catalysts. 

Specific surface areas were determined by the BET method from the nitrogen adsorption at 77 K, using an automatic Micrometries ASAP 2000. Palladium metal dispersion was determined by the dynamic pulsed hydrogen chemisorption. The metallic average particle size of palladium was examined by a transmission electron microscope (JEOL 100 CX) with a resolution factor of 0.3 nm. Chemical analysis allowing the... 

Another example of reactions in which M MF catalysts exhibit exiting properties is the very selective olefin hydrogenation by palladium nanoparticles supported on magnesium fluoride phase. Transmission electron microscope (TEM) micrographs in Figure 6.14 display that the majority of Pd particles are approximately 5 nm in diameter, although the particle size distribution spreads up to 20 nm. These catalysts showed extremely high metal dispersion compared with examples reported in literature [73]. 

The high surface area of the charcoal used in this work prevented us from using gas chemisorption to determine metallic dispersion of our catalysts. We used an electron microscope (Jeol JEM 100 CX) to investigate the geometric appearance of the particles. At first sight, the micrographs show that in the pure platinum catalyst the metal crystallites form two populations whereas in the bimetallic preparations the size is much more uniform. 

Removal of deposits and corrosion products from internal surfaces revealed irregular metal loss. Additionally, surfaces in wasted areas showed patches of elemental copper (later confirmed by energy-dispersive spectroscopy) (Fig. 13.12). These denickelified areas were confined to regions showing metal loss. Microscopic analysis confirmed that dealloying, not just redeposition of copper onto the cupronickel from the acid bath used during deposit removal, had occurred. 

Close examination of the weld under a low-power stereoscopic microscope revealed small openings (Fig. 15.6). Probing these sites with a pin revealed a large pit that had been covered by a thin skin of weld metal. These sites contained fibrous, metallic remnants (Fig. 15.7). Examination under a scanning electron microscope further revealed the fibrous character of the material (Fig. 15.2) and also the convoluted shapes of the individual fibers (Fig. 15.21). Energy-dispersive spectrographic analysis of this material revealed the compositions in Table 15.1. 

There are no known examples of supported clusters dispersed in crystallo-graphically equivalent positions on a crystalline support. Thus, no structures have been determined by X-ray diffraction crystallography, and the best available methods for structure determination are various spectroscopies (with interpretations based on comparisons with spectra of known compoimds) and microscopy. The more nearly uniform the clusters and their bonding to a support, the more nearly definitive are the spectroscopic methods however, the uniformities of these samples are not easy to assess, and the best microscopic methods are limited by the smallness of the clusters and their tendency to be affected by the electron beam in a transmission electron microscope furthermore, most supported metal clusters are highly reactive and... 

In all microscopic methods, sample preparation is key. Powder particles are normally dispersed in a mounting medium on a glass slide. Allen [7] has recommended that the particles not be mixed using glass rods or metal spatulas, as this may lead to fracturing a small camel-hair brush is preferable. A variety of mounting fluids with different viscosities and refractive indices are available a more viscous fluid may be preferred to minimize Brownian motion of the particles. Care must be taken, however, that the refractive indices of sample and fluid do not coincide, as this will make the particles invisible. Selection of the appropriate mounting medium will also depend on the solubility of the analyte [9]. After the sample is well dispersed in the fluid, a cover slip is placed on top... 

Metal hydrogenation catalysts may be employed in any one of a variety of forms (a) macroscopic forms as wires, foils or granules (b) microscopic forms as powders obtained by chemical reduction, colloidal suspensions, blacks or evaporated metal films (c) supported catalysts where varying concentrations of metal are dispersed to a varying degree on a carrier such as alumina, silica or carbon. 

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