Crystallite size distribution, various

Examination of automotive catalysts by various chemisorption techniques has shown that a loss in noble metal surface area caused by higher temperatures correlates monotonically with various activity indices (62, 63). Moreover, Dalla Betta and co-workers (64) were able to separate the additional effect of poisons on the surface of the precious metal by painstaking attention to detail. They developed techniques for accurately measuring the crystallite-size distribution in used automotive catalysts by... 

X-ray diffraction patterns from fibres generally contain a few closely overlapping peaks, each broadened by the contributions of crystallite size, crystallite-size distribution, and lattice distortion. In order to achieve complete characterisation of a fibre by X-ray methods, it is first necessary to separate the individual peaks, and then to separate the various profile-broadening contributions. Subsequently, we can obtain measures of crystallite size, lattice distortion and peak area crystallinity, to add to estimates of other characteristics obtained in complementary experiments. 

Metal Dispersion by Chemisorption and Titration Selective Chemisorption. - This is the most frequently used technique for determining the metal area in a supported catalyst and depends on finding conditions under which the gas will chemisorb to monolayer coverage on the metal but to a negligible extent on the support. Various experimental methods, conditions, and adsorbates have been tried and studies made of catalyst pre-treatment and adsorption stoicheiometry, viz, the (surface metal atom)/(gas adsorbate) ratio, written here as Pts/H, Bh jQO,etc., and reviews to about 1975 are available. A summary is given in Table IV of ref. 2 of methods used to confirm the various adsorption stoicheiometries proposed, sometimes from infrared studies. These include chemisorption on metal powders of known BET area or, more satisfactorily, one of the instrumental methods reviewed in Section 3 for the determination of crystallite size distributions. For many purposes, a relative measurement of metal dispersion is sufficient, conveniently expressed as the ratio (number of atoms or molecules adsorbed)/(totfl/ number of metal atoms in the catalyst), e.g., H/Ptt. 

Shape and size are two important features of morphology. Of secondary interest is crystallite size distribution. Information of this nature is necessary for complete knowledge of all catalytic components. It is especially significant for highly dispersed systems. Instrumentation for this purpose includes various types of electron microscopy and x-ray devices. 

It has often been observed that catalytic samples having the same qualitative composition (for instance, platinum on alumina) but differing in preparation mode, show pronounced dissimilarities in catalytic behavior. Different methods of preparation will yield catalysts differing in crystallite size, crystallite size distribution and shape of the metal crystals. Since 1969, an important work on the statistics of surface atoms and surface sites on metal crystals was undertaken by van Hardeveld and Hartog [14]. In the mean time, a considerable amount of work started to determine the specific rates, rates per unit surface area of metal, for various reactions and in parallel, improvement of chemisorption techniques to determine the surface area of a metal component was achieved. 

Furthermore, the synthesized a-Al203 powders possessed up to 100 % phase purity, 99.98 % chemical purity, equiaxed morphology, low aggregation levels, narrow crystallite size distributions, with primary particle sizes ranging between 100 and 40 pm, as well as high reproducibility. It was clarified that, various parameters, including types of precursors, the use and properties of seeds, chemical additives, as well as reaction temperature and time duration of the hydrothermal synthesis, could be used to govern the properties of the final powders. 

The unconventional applications of SEC usually produce estimated values of various characteristics, which are valuable for further analyses. These embrace assessment of theta conditions for given polymer (mixed solvent-eluent composition and temperature Section 16.2.2), second virial coefficients A2 [109], coefficients of preferential solvation of macromolecules in mixed solvents (eluents) [40], as well as estimation of pore size distribution within porous bodies (inverse SEC) [136-140] and rates of diffusion of macromolecules within porous bodies. Some semiquantitative information on polymer samples can be obtained from the SEC results indirectly, for example, the assessment of the polymer stereoregularity from the stability of macromolecular aggregates (PVC [140]), of the segment lengths in polymer crystallites after their controlled partial degradation [141], and of the enthalpic interactions between unlike polymers in solution (in eluent) [142], as well as between polymer and column packing [123,143]. 

Crystals of the synthetic zeolites are usually quite small and often exhibit various forms of twinning and intergrowth. With some zeolites, individual crystallites (e.g. cube-shaped NaA) of size < SO nm have been identified by electron microscopy, but the agglomerate sizes are generally in the approximate range 1-10 pm. For example, the particle size distribution over this range of a typical NaA powder was reported to be of a broad log-normal character (Breck, 1974, p. 388). 

An alternative explanation provides the "surface state mode 1" of Koch et al. [15]. Accordingly, the photogenerated electron-hole pair is trapped in surface states (which have an energy lower than the band gap of the nanocrysfedlite) and recombine, with some delay from such states. This model can explain various observed data, such as the polarization memory of the PL [17] and the dependence of the measured PL decay time on the phonon energy in terms of a distribution of the trap states as in a-Si [16]. In particular, the model can also explain the observation of the absence of any blue shift with decreasing crystallite size in compact nc-Si/Si02 films [18]. 

The size and shape of the channels and cavities, the location of cations and the distribution of A1 or other T-atoms affect the adsorption, rate of diffusion and the size of the catalyzed product. The Si/Al ratio, the concentration of the various ring sizes and the nature and location of cations determine the thermal and hydrothermal stabilities of the framework. The ability to determine these structural features is necessary in order to understand the physical and catalytic properties which can also be affected by faulting crystallite size and loading. 

Figure 19 Electron microscopy and size distribution of silver crystallites synthesized in AgAOT-NaAOT-water-isooctane reverse micelles at various water contents. [AgAOT] = 3 X 10 2 M, [NaAOT] = 7 x 10 2 m, [NaBH4] = 2.5 x lO" M, and Wq = 5 (top), 7.5 (middle), and 50 (bottom). The marks represent 20.8 nm. (From Ref. 177.)...

---