Segregated metallic particles

On a pure Pt electrode, H20ads(Pt) dissociation is difficult [6,7] and relatively high positive electrode potentials are needed to activate H2O. Nanoscale binary Pt-M catalysts are often employed to increase the efficiency of CO electro-oxidation to combine the catalytic effects of both metals. The binary catalysts may be alloys or segregated metal particles. Pt-Ru and Pt-Sn alloys have received considerable attention as promising catalysts for the direct electro-oxidation of methanol (EOM) it has been shown that the catalytic activity of Pt for EOM can be enhanced by alloying with Ru or Sn. Other metals, e.g. Mo [17-21], Re [22], and Rh [23], have also been used as allo5dng metals. 

The segregation process of graphite on the surface of a metal particle is similar to that proposed by Ober-lin and Endo[35] for carbon fibers prepared by thermal decomposition of hydrocarbons. Flowever, the... 

Using this method, homogeneous alloys, segregated alloys, layered bi-metallics, and decorated particles are all readily accessible. An obvious advantage of the precursor concept over the conventional salt-impregnation method is that both the size and the composition of the colloidal metal precursors may be tailored independent of the support. Further, the metal particle surface may be modified by lipophilic or hydrophilic protective shells and coated by intermediate layers, e.g., of oxide. The modification of the precursor by dopants is also possible. 

It is also possible to reinforce polymers with metallic particles. D. T. Turner and one of his students observed that good electrical conductivity can be measured even at very low fillings, such as only 6% by volume. Microscopic examinations showed that the metallic particles formed continuous chains segregated around zones of unpenetrated polymer. 

Mixtures of powders of poly(vinyl chloride) (FVC) and various metals were compacted at a pressure of 10,000 psig at 120-130°C. The compacts appear to be strong, and density measurements show the porosity to be <1.5%, Electrical resistivity is reduced, from a value for unloaded FVC of about JO25 Clem, to < JO"1 Clem by a fractional volume loading of nickel or copper as low as 0.06. Microscopic examination of polished sections of the compacts show the metallic particles to be segregated around zones of unpenetrated polymer which correspond in size to the initial particles of FVC. The pattern of segregation favors the formation of continuous chains of metallic particles at unusually low volume loadings. 

A reason for the difference in behavior noted above becomes apparent by comparing photomicrographs of sections. In Gurland s system the particles of silver are distributed more or less randomly throughout the polymer as may be seen from his results quoted in Figure 2. By contrast, in the present system the metallic particles are excluded from certain elements of volume and segregated into others. This was apparent in all... 

The observed pattern of segregation might well be a consequence of the accommodation of metallic particles in the free volume available in an assembly of large approximately spherical particles of PVC. For example, close packing of spheres in a face centered cubic system would provide a continuous network of open channels which in the (100) planes would provide spaces of the kind shown in Figure 7. Some features corresponding to the filling of such spaces by small metallic particles... 

In general, for an alloy AB, the phenomenon of segregation is controlled by a range of parameters including the differences in bond strengths A-A, B-B and A-B, atomic size, enthalpies of sublimation and surface energies, temperature, the exposed crystal plane, and metal particle size. 

Thus, the deactivation observed on the used catalyst may be explained as follows. First, Pt is sintered and Rh is segregated on the surface of noble metal particles by the thermal effect [8, 9], and so, on only thermally deaetivated catalysts, the reactions due to Rh sueh as NO reduction are not slowed, although methane oxidation due to Pt is considerably slowed [3, 4]. However, when Pb is adsorbed on Rh, the catalytic activity of Rh is suppressed and the reaction rate of NO reduction is decreased to the same order as the rate of CH4 oxidation. Further, the surfaee of the wash eoat layer is covered with compounds consisting of Ca, P, Zn and Fe, and the effeetive surface area of the catalyst which the exhaust gas ean reach decreases, causing a considerable decrease in NO conversion and the disappearance of the window, accompanied wdth decreases in CH4 conversion and CO conversion on the rich side. 

Physical processes separate contaminants from uncontaminated material by exploiting differences in their physical properties (e.g. density, particle size, volatility, by applying some external force (e.g. abrasion) or by altering some physical characteristic to enable separation to occur (e.g. flotation). Depending on the nature and distribution of the contamination within the soil, physical processes may result in the segregation of differentially contaminated fractions (for example a relatively uncontaminated material and a contaminant concentrate based on a size separation) or separation of the contaminants (for example oil or metal particles) from the soil particles. Table 6 summarises the main advantages and disadvantages of physical processes. 

With active components supported on activated carbon the adhesion of the active particles to the carbon is important. The interaction between precious metal particles or clusters of small particles and carbon surfaces can be low, which might lead to segregation of very small precious metal particles or slightly larger clusters of small particles into the liquid. Separation of such small particles or... 

Another Important concept introduced by Taylor was that of heterogeneity of surface-active centers.(25-26) This stemmed from observation of R. N. Pease that minute amounts of carbon monoxide, much smaller than the amount necessary to cover the surface, were sufficient to poison the surface of a copper catalyst. Taylor proposed that there were active centers on the surface while others argued that nickel impurities segregated preferentially on the surface and acted as catalyst. The variation of the heats of adsorption with surface coverage as determined by R. Beebe was used as evidence supporting the concept of active centers. In spite of the contradictory interpretation of the same experimental data, the concept of active centers has been a fruitful one. It inspired Imaginative research in the field of metal and oxide catalysis and has its present day expression in sophisticated surface physics studies. Subsequent work by coworkers of Turkevich at Princeton refined the nature of active centers in monodisperse metal particles and crystalline oxide catalysts. 

Sintering, segregation, and redispersion of metal particles as a result of oxidative treatment... 

The size of the metal particles relative to any structure present in the polymer matrix can also affect the value of pc. Segregated composites have been prepared by compression molding a mixture of metal and polymer particles [35]. When the radius of the polymer... 

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