Dispersion of metals

As was found in Ref. [13], the method of catalytic decomposition of acetylene on graphite-supported catalysts provides the formation of very long (50 fim) tubes. We also observed the formation of filaments up to 60 fim length on Fe- and Co-graphite. In all cases these long tubules were rather thick. The thickness varied from 40 to 100 nm. Note that the dispersion of metal particles varied in the same range. Some metal aggregates of around 500 nm in diameter were also found after the procedure of catalyst pretreatment (Fig. 2). Only a very small amount of thin (20-40 nm diameter) tubules was observed. 

More than half a century ago the first observations were made on photochemical reactions in dispersions of metal oxides. Baur and Ferret found that oxygen was produced and silver deposited in ZnO suspensions illuminated in the presence of... 

Finally, it has been reported that carbon electrodes modified with thin polymeric films of polypyridyl metal complexes containing a dispersion of metal particles (Rh° or Pd°) can be used as electrocatalyst for reduction of C02 to hydrocarbons in MeCN. Apparently CH4 is the dominant reduction product (up to 18% of faradaic efficiency).123,124 It should be noted that the product distribution is reminiscent of a Fischer-Tropsch process since C2, C3, and C4 hydrocarbons are also formed. 

Effects of CyDTA on the Reducibility of Co and Dispersion of Metallic Co Species... 

Element distribution patterns in till around the MFN deposit are most likely the result of concentration of anionic species in the gossan, glacial dispersal of metal-rich bedrock, and mobilization of... 

Metallothionein was first discovered in 1957 as a cadmium-binding cysteine-rich protein (481). Since then the metallothionein proteins (MTs) have become a superfamily characterized as low molecular weight (6-7 kDa) and cysteine rich (20 residues) polypeptides. Mammalian MTs can be divided into three subgroups, MT-I, MT-II, and MT-III (482, 483, 491). The biological functions of MTs include the sequestration and dispersal of metal ions, primarily in zinc and copper homeostasis, and regulation of the biosynthesis and activity of zinc metalloproteins. 

The simplest cases of disintegration to a colloidal solution are those produced by immersion of a solid in a liquid, e.g. the disintegration of nitrocelluloses by means of amyl acetate. The solvent action of the medium is frequently greater at high temperatures than at low as noted by Lorenz in the dispersion of metals in contact with fused salts,... 

The general structure of this class of materials can, therefore, be summarized as a fine dispersion of metal oxide in a polymer matrix very similar to plasma polytetrafluoroethylene and in principle any metal should be able to be incorporated. Clearly, if the films are protected from the atmosphere, for metals which form involatile fluorides having a relatively weak metal-fluorine bond strength, it should be possible to produce films having metal atoms dispersed in the matrix. It is expected that these films will have many interesting chemical, optical, electrical and magnetic properties., ... 

EM techniques provide important information in the characterization of the dispersion of metallic catalysts. Surface areas of catalysts are measured by the standard BET method described previously. An isotherm is produced using nitrogen as the adsorbate chemisorption of certain gases (e.g. H2 or CO) is also used, including for particle size distributions. We give some examples in chapter 5. 

Particle size distributions (PSD) measurement of dispersion of metal particles on supports... 

Finally, Table 8 compares the extent of metal loss into the environment as a result of (1) the corrosion of our hypothetical HT material, (2) the anthropogenic dispersion of metal-containing materials used as fertilizers on Swiss soils, and... 

Colloidal dispersions of fine metal particles can usually be prepared by reduction of metal ions. The first scientific report to synthesize colloidal dispersion of metals was presented by Faraday, who prepared metal colloids without stabilizers (5). In his case counteranions may have played the role of the stabilizer. In most recent cases, however, stabilizers are usually added to the system to stabilize the colloidal dispersions. 

Other Methods. Other reductants like hydrazine, sodium metal, etc. can be used for the reduction of metal ions. Decomposition of metal salts or complexes by heat treatment is sometimes used for synthesis of fine particles as well. In this case the valence of metals in the fine particles should be carefully examined. Recently, irradiation of ultrasonic wave was applied to the synthesis of colloidal dispersions of metal fine particles. 

The composition of the surface-bound species must be considered they contribute to the stability of the dispersions of metal nanoparticles. In the case of electrostatically stabilized dispersions, the techniques to measure the interfacial electronic phenomena, including electrophoresis, electroosmosis, etc., are useful (54). In order to understand the composition (as well as structures) of the chemical species bound in the surface of metal particles, spectroscopic measurements used for common organic substances are used as well as the elemental analysis. 

For example, the aggregated structures of the solutions containing polymer-metal complexes and the colloidal dispersions of metal nanoparticles stabilized by polymers have been analyzed quantitatively (64). SAXS analyses of colloidal dispersions of Pi, Rh, and Pt/Rh (1/1) nanoparticles stabilized by PVP have indicated that spatial distributions of metal nanoparticles in colloidal dispersions are different from each other. The superstructure (greater than 10.0 nm in diameter), with average size highly dependent on the metal element employed, is proposed. These superstructures are composed of several fundamental clusters with a diameter of 2.0-4.0 nm, as shown in Figure 9.1.13 for PVP-stabilized Pt nanoparticles. 

Aggregation of Metal Atoms to Form Metal Nuclei. After the reduction of metal ions, the solution colored by metal ions becomes colorless. At this stage, metal nanoparticles are not produced because the color of the dispersions of metal nanoparticles does not appear at all. If coin metal is used, this color change is much more clear than for the other precious metals. 

Thus, the color change from pale to colorless means the reduction of metal ions to metal atoms or microclusters that have no color at all. There is no evidence of whether the solution consists of single atoms or microclusters. However, it is clear at least that further treatment of this solution with heat or photons can produce a colloidal dispersion of metal nanoparticles, and that, in contrast, the treatment of this solution with oxygen at room temperature provides the colored solution of metal ions. These observations again support the presence of atoms or microclusters in the solution at this stage. 

Fig. 9.4.29 Comparison of stability of metallic nanoparticles in bulk liquid with a droplet on a metal surface, (a) Wetting of a droplet on a metal surface, (b) Coagulation and dispersion of metallic particles in liquid. Figures on the left-hand side stand for weak interaction in case A causing coagulation in case B. Those on the right-hand side are a strong interaction between metal and liquid, suggesting good dispersion and good contact.

The photoconductivity of organic dyes, especially phthalocyanine pigments, could be utilized to provide reusable or non-reusable electrophotographic plates having sensitivities that extend over the entire visible spectrum and produce high-contrast images 167,168) Microcrystalline dispersions of metal-free phthalocyanine in suitable binders have a white-light sensitivity equal to that of selenium 169> and can be incorporated into coated papers and drums. 

The chemical compositions of the different samples calculated from elemental analyses are in good accordance with those obtained by thermogravimetry On the other hand, the low amount of Mn is in agreement with the dispersion of metal complexes revealed by the EPR pattern. 

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