Crystallite size distribution

Fig. 2. Platinum crystallite size distribution for 0.12 ng cm-1 ultrathin platinum film (Fig. 1). Full line, number distribution broken line, surface area distribution. 

Fig. 4. Platinum crystallite size distribution for 2.5% (w/w) platinum/silica catalyst (Fig. 3). Full line, number distribution broken line, surface area distribution. After T. A. Dorling, and R. L. Moss, J. Catal. 7, 378 (1967) and R. L. Moss, Platinum Metals Rev. 11 (4), 1 (1967).

The graphite lattice may show stacking faults or defects within the sheets, and, possibly, bending of the sheets (Fig. 2.25). Omission of a carbon atom (voids), or inclusions of noncarbon elements or molecules, disrupts the orderly configuration and inhibits crystallization of carbon as graphite. These impurities act as sites of local strain that directly influence crystallite size, distribution, and orientation within a sample, and in turn affect the physical and chemical characteristics of the material, especially its strength. 

The asymmetry of peak shape is preserved in anthraxolite heated to 1200°C. showing that turbostratic disorder persists in spite of a general enhancement of ordering. The band is also sharper and narrower. This may be interpreted to mean either that fewer class intervals are represented in the crystallite size distribution or that increased ordering of aromatic lamellae has reached the point where graphite (hid) planes are more common. Diffraction peaks of both (100) and (101) fall with the 2-A. band. 

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. 

The more detailed study of the crystallite size distribution and crystallite arrangement in PPX matrix was carried out for cryochemically synthesized nanocomposite PbS-PPX films in the range of PbS content from 4.7 to 10.2 vol.%. The distribution curves obtained from analysis of wide-angle X-ray scattering (WAXS) data is presented in Figure 10.5 [71, 86]. The curve for nanocomposite with 4.7 vol.% of PbS differs a little from histograms of the crystallite size distribution determined by TEM for nanocomposite films Ag-PPX and Pb-PPX with low metal content [75, 80]. It means that, as the average size of crystals, distribution in the sizes almost does not depend on nature of M/SC incorporated in polymer as a result of cryochemical synthesis. 

PbS inclusions in PPX matrix regardless of their internal structure. The size distribution of PbS inclusions calculated from SAXS data are similar to the WAXS crystallite size distribution in Figure 10.5. This result signifies that PbS nanoparticles do not aggregate in matrix but are distributed in PPX mostly as small crystallites [71]. 

While considerable rate data for sintering of supported nickel and platinum catalysis are available from previous studies, their interpretation, comparison, and correlation is complicated by experimental limitations which include (1) lack of systematic or statistical experimentation, and (2) nonstandard and difficult-to-compare measurements of dispersion, crystallite diameter and crystallite size distribution. 

Many polymeric solids consist largely of folded chain lamellae and that the breadth of X-ray diffraction lines is caused by the crystallite size distribution and by the disorder within the lamella. 

There are situations in which crystallites are readily visible, especially on supports which do not offer excessive electron scatter. In these cases, metal content can be quantitatively determined for areas which have highly dispersed metal and agglomerated metal. This information in conjunction with the crystallite size distribution provides the microscopist with the information required to make an estimate of metal dispersion (13). These estimates are valuable especially in situations where conventional gas adsorption measurements cannot be made on the metal, i.e., when the crystallites are contaminated, have multiple oxidation states, or are poisoned. 

J. R. Katzer You are correct in your statement as we indicated in our paper, the crystallite size distribution is responsible for the majority of the variation found for SK-500 under most conditions. However, your approach would not eliminate all of the variation found in the systems studied here and is a small correction relative to the far more significant variations found in the systems studied. 

Langford, J.I., Louer, D., and Scardi, R, Effect of a crystallite size distribution on x-ray diffraction line profiles and whole-powder-pattem fitting, J. Appl. Crystal-logr, 33, 964, 2000. 

Figure 8 Crystallite size distributions (histograms from TEM) for model-supported Pd catalysts sintered in at 150 to 800 <>C (a) PdHlOj and (b) Pd/Al203. ...

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. 

Jf-Ray Methods. - A -ray line-broadening is often used to estimate the mean size of supported metal crystallites and introductory accounts of the technique in relation to catalyst examination are available. Recently, more detailed information is being extracted, e.g., the distribution of crystallite size, the presence of strain, and an indication of crystallite shape. The crystallite size distribution obtained from X-ray line-profile analysis can be complicated by spurious oscillations. A method for the smoothing of crystallite size distribution by use of a least-squares procedure with a stabilization scheme has been described (with references to other methods). One such correction procedure was appUed to obtain crystallite size distributions in... 

The H2 chemisorption capacity of the washed sample decreased slightly in comparison with the fresh one, is consistent with poisoning or sintering. However, TEM analysis revealed no appreciable difference in the average crystallite dimension and crystallite size distribution of fresh and used samples. The S content (by chemical analysis) was very different in fresh and used samples, while that of the washed sample was comparable with that of the used one. 

No appreciable changes in the metal dispersion and crystallite size distribution between fresh and sulfid samples were observed by TEM analysis (Figure 5). This is consistent with a selective poisoning of the metallic sites, unable to chemisorb hydrogen. The behavior observed for high sulfiding coverages is not easily explained. 

This expression was used for the P ri) extraction directly from f x), but as this did not remove spurious oscillations, a smoothing procedure was applied, and the method was tested on composite specimens prepared by mixing known quantities of samples of nickel hydroxide, whose crystallite size distributions were previously determined. The Fourier method is in principle an exact... 

MudMaster MudMaster A program for calculating crystallite size distributions and strain from the shapes of X ray diffraction peaks. D. D. Eberl, V. Drits, J. Srodon, and R. Nuesch, U.S. Geological Survey Open File Report 96 171, (1996) 46 pp and XRD measurement of mean thickness, thickness distribution and strain for illite and illite/ smectite crystallites by the Bertaut Warren Averbach technique. 

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