Size and Shape of Small Metal Particles

These questions lead on to further fundamental questions concerning the shapes and properties of small metal particles. For example, what is the stable shape for a small metal particle How is this affected by size, method of preparation, temperature, gaseous environment, precursor compound, support morphology, etc. Do small metal particles have different electronic properties from bulk metal Do surface electronic properties depend on particle size, and if so, do they vary in the same way as bulk electronic properties When, indeed, is a particle small enough to have unusual properties ... 

The catalytic behavior of small metal particles in heterogeneous catalysts varies with metallic particle size and shape a phenomenon referred as a structure-sensitivity. Simple alkanes such as ethane, propane, n-butane and isobutane can be used as archetype molecules for studying hydrogenolysis reactions as they... 

The structure and stability of small gold particles is a function of the chemical and physical nature of the support on which they reside.7,116,151>152 Jt is clear that the extent of the influence of the support on a metal particle will depend on the fraction of the metal atoms directly in contact with it for particles of the same shape this will increase as the size decreases, but it will also depend on the shape of the particle, which is conditioned by the chemical forces at the interface. In principle the particle shape is determined by the contact angle 0 defined by the equation... 

In the case of the downward air flow pattern, the belt is porous, whereas in the case of the cross flow pattern, the belt may be solid. The selection of air flow/belt design would depend on the particle size and shape of the product being dried. One would choose a porous belt when drying a large granular material which would not fall through the pores of the belt (which can be a wide metal or nylon wire mesh or, with a smaller particle size, could be a tight braided type screen). For very small particles, or those which... 

It is recognized, of course, that other arrangements can be employed than those shown and the uranium may be in one of a variety of forms. For example, the uranium may vary in size and shape from small particles to larger lumps or bodies in any convenient shape, such as spheres, tubes, or rods. The uranium may be in the form of metal, or it may be In a compound such as UO2 or U3O11. Unless otherwise. specified reference to uranium bodies is Intended generically to mean bodies including the metal or its compounds. If desired, the natural uranium in such bodies may be enriched as to its isotope content, or... 

Surface Area, Porosity, and Permeability. Some very interesting and important phenomena involve small particles and their surfaces. For example, SO2 produced from mining and smelting operations that extract metals such as Cu and Ph from heavy metal sulfide ores can be oxidized to SO3 in the atmosphere, thus contrihutingto acid rain problems. The reaction rate depends not only on the concentration of the SO2 bnt also on the siuTace area of any catalyst available, such as airborne dnst particles. The efficiency of a catalyst depends on its specific surface area, defined as the ratio of siuTace area to mass (17). The specific snrface area depends on both the size and shape, and is distinctively high for colloidalsized species. This is important in the catalytic processes nsed in many indnstries for which the rates of reactions occurring at the catalyst siuTace depend not only on the concentrations of the feed stream reactants bnt also on the sinface area of catalyst available. Since practical catalysts freqnently are snpported catalysts, some of the sinface area is more important than the rest. Since the supporting phase is usnally porous the size and shapes of the pores may influence the reaction rates as well. The final rate expressions for a catalytic process may contain all of these factors sinface area, porosity, and permeability. 

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