Heterogeneous crystallization

Radionuclides that are able to form normal or anomalous mixed erystals with the macrocomponent are ineorporated at lattice sites. In most cases the distribution in the lattice is heterogeneous, i.e. the concentration of the microcomponent varies with the depth. If the solubility of the microcomponent is lower than that of the maero-eomponent, it is enriehed in the inner parts of the crystals. Heterogeneous distribution may even out over longer periods of time by diffusion or recystallization. 

In brief, the catalysts under consideration are highly porous, poorly crystallized, heterogeneous mixtures of various oxides of alumina and chromia whose structures are of considerable complexity. In the following sections we will briefly describe the physical and chemical techniques one can use to elucidate the structures of these catalysts, and then we will discuss some relevant properties of chromia, alumina, and chromia-alumina solid solutions, before turning to a consideration of the catalysts themselves. 

The study of nucleation is fundamental to the understanding of crystallization. Heterogeneous and homogeneous nucleation, are terms proposed [5] to differentiate nucleation within a receptive and an inert environment. In the context of electrocrystallization, the terms can be apphed to phase formation at preferred sites on the electrode surface and phase formation at surfaces without such sites, respectively. Figure 3.1 [6] illustrates heterogeneous nucleation and shows a scanning electron microscopy (SEM) image of the nuclei of nickel formed on a scratched surface, and on indents. 

Phase separation may lead to the formation of a highly mobile phase demonstrating homogeneous crystallization, while the matrix crystallizes heterogenously, either in parallel, or later in the process. 

The weight fraction /(t ) of dispersed Ny6 crystallizing heterogeneously in the blends (at the usual undercooling conditions) was related to the volume of Ny6 particles by the relation ... 

Figure 10.27 Fraction of dispersed Ny6 particles crystallizing heterogeneously in blends of Ny6 with polyolefins (LDPE, PP) and functionahzed polyolefins (PE-AA, PP-AA), as a function of particle diameter. Calculated according to Equation (10.27) for N - 7.4 cm" (a) and A = 1.0 10 cm (b) SEM micrographs of fracture surfaces of (left) Ny6/PP-AA 40/60 (scale bar 1 pm) and (right) Ny6/PP 40/60 (scale bar 10 pm) blends.

In the in situ method the support is put in contact with a synthesis solution or a gel under hydrothermal conditions. At the same time, under appropriate conditions, zeolite nuclei are formed on the support and they will grow by forming a zeolite layer. Zeolite crystals heterogeneously nucleated on the support surface are also present. This technique does not give the possibility of controlling the nucleation of the crystals which is necessary to form a very uniform and compact zeolite layer (Li et al., 2003). 

A somewhat subtle point of difficulty is the following. Adsorption isotherms are quite often entirely reversible in that adsorption and desorption curves are identical. On the other hand, the solid will not generally be an equilibrium crystal and, in fact, will often have quite a heterogeneous surface. The quantities ys and ysv are therefore not very well defined as separate quantities. It seems preferable to regard t, which is well defined in the case of reversible adsorption, as simply the change in interfacial free energy and to leave its further identification to treatments accepted as modelistic. 

Surface heterogeneity may merely be a reflection of different types of chemisorption and chemisorption sites, as in the examples of Figs. XVIII-9 and XVIII-10. The presence of various crystal planes, as in powders, leads to heterogeneous adsorption behavior the effect may vary with particle size, as in the case of O2 on Pd [107]. Heterogeneity may be deliberate many catalysts consist of combinations of active surfaces, such as bimetallic alloys. In this last case, the surface properties may be intermediate between those of the pure metals (but one component may be in surface excess as with any solution) or they may be distinctly different. In this last case, one speaks of various effects ensemble, dilution, ligand, and kinetic (see Ref. 108 for details). 

Single molecules also have promise as probes for local stmcture when doped into materials tliat are tliemselves nonfluorescent. Rlrodamine dyes in botli silicate and polymer tliin films exliibit a distribution of fluorescence maxima indicative of considerable heterogeneity in local environments, particularly for the silicate material [159]. A bimodal distribution of fluorescence intensities observed for single molecules of crystal violet in a PMMA film has been suggested to result from high and low viscosity local sites witliin tire polymer tliat give rise to slow and fast internal conversion, respectively [160]. 

Benzoylpropionitrile. To a mixture of 21 4 g. of p dimethylamino propiophenone hydrochloride, 13 0 g. of potassium cyanide in a 500 ml. flask, add 260 ml. of boiling water heat the heterogeneous mixture under reflux for 30 minutes. Part of the dimethylamine, which is eliminated in the reaction, distils collect this in dilute hydrochloric acid. Cool the reaction mixture in ice the oil sohdifies and crystals form from the aqueous layer. Collect the solid (crude p benzoylpropiouitrile, 10-5 g.) by suction filtration and recrystallise it from benzene - light petroleum (b.p. 40-60°) it separates as almost colourless blades, m.p. 76°. 

Sofid sodium permanganate monohydrate has been shown to be a selective synthetic reagent (156). It is typically used in hexane for the heterogeneous oxidation of aldehydes, alcohols, and sulfides. Synthetic methodology based on crystal surfaces exhibited greater selectivity, higher yield, and easier work-up as compared to aqueous permanganate reactions. 

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