Particle shape plane

Particle morphology and crystallinity influence the position of the symmetric Fe-O stretch at ca. 630 cm because this vibration, which corresponds to a transverse moment, lies in the (010) plane as the crystallinity of the sample falls and the unit cell a length increases (see Chap. 2), the frequency of this band decreases (Cambier, 1986 a). The band at 397 cm which corresponds to an antisymmetric Fe-O vibration parallel to (100) is insensitive to particle shape and crystallinity. 

Here we concentrate on cylindrical containing walls, although there is some work on particles near plane boundaries and surfaces of arbitrary shape. Most of the work on rigid particles refers to spheres, and it is then convenient to use the diameter ratio... 

More important than the particle size are the purity and the particle shape. The effect of bad milling on a large lamellar particle is a particle having far smaller "shape factor" or aspect ratio than the original material. Since it is the basal plane surface which provides low friction and the edges which cause abrasion, it is therefore important in milling to maintain the highest possible aspect ratio consistent with the required particle size. 

Nevertheless, it is clear that any anisotropic - growth that results from the preferential binding of organic species to certain crystal faces relies on the crystal structure of the seed nanoparticles. Whether the seeds are single crystalline or whether they possess any twin planes or other defects, will determine the type and orientation of the crystal faces that are exposed to the growth medium in the first place. This is all the more apparent when we consider that in most syntheses a range of particle shapes are observed and yet the same shaped particle can be the major product of very different syntheses. 

If the metal catalyst particles were present only in the form of these idealized crystals, then the number of active comer atoms present would be very low. However, STO evaluations of dispersed metal catalysts have shown that these active atoms are present in rather large amounts, at times as high as 30%-35% of the total metal atoms present. Such high surface concentrations of the highly unsaturated atoms can only be accounted for by the presence of the irregular particle shapes that were observed using dark field TEM imaging techniques. Additional active sites are probably present as adatoms on the 111 (M) and 100 (K) planes as shown in Fig. 4.4. 

In summary, wc conclude tluu particle shape control in supported metal catalysis is feasible. When a supported platinum catalyst is annealed under conditions where the platinum particles stay dean, the platinum particles assume a spherical shape with flat facets in the (100) and (111) directions. All of the particles in the catalyst assume the same general shape. In contrast, different shapes are observed when the catalyst is annealed in various udgasscs. Shi[ll] showed theoretically that only a few planes can be produced by eciuilibratiug the particles in simple adsorbates. However, w e speculate that one may be able to produce a wide distribution of panicle siiapes. with appropriate adsorbates,... 

However, the INi sample exhibits a mean metal particle diameter (dm = 40 A) smaller than the DPNi one (dm = 57 A). Assuming the same particle shape, it may be deduced that the ratio of the number of low coordinated Ni atoms to the number of Ni atoms on surface planes is higher for the INi sample than for the DPNi sample. In other words, the particles of INi exhibit higher roughness than those of DPNi. 

Morphology effects appear for reactions that are structure sensitive. A good example is the reduction of NO by CO on Pd, which shows the highest activity on (Iff) planes and lower activity on open surfaces [67]. However, in the case of supported particles it is difficult to control the particle shape independently of particle size [68]. However, by using a careful TEM characterization it is possible to disentangle size and morphology effects. An example is given by the NO reduction of CO on Pd/MgO(100) model catalysts [64]. 

This relationship has important implications. As shown in Figure 12.23, permeation must be extremely sensitive to particle shape, as expressed in the values of L/W. Thus cubes (or spheres, which are in this sense equivalent) are an order of magnitude less effective than thin plates lying parallel to the plane of the membrane. This trend is in accord with experience (Michaels and Bixler, 1961 Nielsen, 1967b). A second, practical implication is that processing techniques such as extrusion can lead to a significant degree of filler orientation. 

In Sections 3.1 and 3.2 the effect of size on IR spectra was discussed solely in the context of ultrathin Aims with plane-parallel boundaries. However, this size effect can be seen for all particles whose size is small relative to the wavelength and can lead to additional, abnormal absorption by both the particles and ultrathin Aims coating such particles. This phenomenon is well known for metals and causes metallic ultrathin films to have different colors than bulk metals. In 1857, Faraday proposed that such a color transformation is associated with the intrinsic aggregating nature of metallic films. His hypothesis has since been confirmed and understood based on Maxwell electrodynamics, and these effects have subsequently been found in the IR range for metals, dielectrics, and semiconductors. Moreover, it has been established that the particle shape also affects the IR spectrum of an ultrathin film in the closest vicinity of a system of particles that are small compared to the wavelength of irradiation. The abnormal absorption of inhomogeneous films remains the subject of intense theoretical investigations, due to the wide practical implications. However, the purpose of this section is not to review this theory in depth but rather to concentrate on the practical aspects of... 

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