Line-broadening analysis

Bulk characterization of calcined precursors and reduced catalysts was carried out by X-ray diffractometry using Cu K radiation. Reduced catalysts were first passivated by exposure to N2O as described above. Line-broadening analysis was carried out on the Fe(llO) reflection to obtain the iron particle size. Overlap with the MgO(200) reflection limited its usefulness to the more highly-loaded catalysts. 

Transmission electron micrographs (TEM) of submicrometer-size particles show faceted particles, and selected area electron diffraction (SAED) patterns of isolated particles show that they are formed by a small number of crystallites (Fig. 9.2.14a), This result is consistent with the mean size of the crystallites, which can be inferred from the x-ray diffraction lines broadening analysis using a William-son-Hall plot (35) in order to take into account the contribution of microstrains to the line broadening. Over the whole composition range, the mean crystallite size is in the range 40-60 nm for particles with a mean diameter in the range 200-300 nm (Table 9.2.5) (33). 

Table 1 gives the average sizes of nickel crystallites measured by X-ray line broadening analysis on (111) reflections, before and after the five hydrogenation runs. They increase moderately and even decrease for RNiFe. This confirms that the BET area loss could be due in part to a poisoning which reduces the capacity of nitrogen adsorption. However, measurements of the metallic surface area should also be done to confirm possible surface poisoning. 

The technique can be complemented by line-broadening analysis which gives valuable information on the size of individual crystallites. Variations of ratios between lines indicate cither order imperfections along... 

For supported metal catalysts, no simple calculation is possible. A direct measurement of the metal crystallite size or a titration of surface metal atoms is required (see Example 1.3.1). TWo common methods to estimate the size of supported crystallites are transmission electron microscopy and X-ray diffraction line broadening analysis. Transmission electron microscopy is excellent for imaging the crystallites, as illustrated in Figure 5.1.5. However, depending on the contrast difference with the support, very small crystallites may not be detected. X-ray diffraction is usually ineffective for estimating the size of very small particles, smaller than about 2 nm. Perhaps the most common method for measuring the number density of exposed metal atoms is selective chemisorption of a probe molecule like H2, CO, or O2. 

G.39 Harold P. Klug and Leroy E. Alexander. X-Ray Diffraction Procedures, 2nd ed. (New York Wiley, 1974). Contains a great deal of useful detail on the theory and operation of powder cameras and diffractometers. Covers the following topics in depth chemical analysis by diffraction, parameter measurement, line-broadening analysis, texture determination, stress measurement, and studies of amorphous materials. Single-crystal methods are not included. 

T. Ungar and J. Gubicza, J. Appl. Crystallogr., 2001, 34, 669 676 Fullprof Line broadening analysis using FullProf Determination of... 

On the basis of the paracrystalllne theory, the Cauchy-plot method can be employed in line-broadening analysis (12). In this method, the integral breadth of the first order reflection is approximated by... 

N.C., STEPHENS P.W., TOBY B.H., Size-strain line-broadening analysis of the ceria round-robin sample ,/. Appl. Cryst, vol. 37, p. 911-924,2004. 

The ceria surface area of a commercial Pt-Rh three-way catalyst was determined after laboratory hydrothermal aging at 1173-1373 K and after 200 h on engine bench. It was measured by X-ray diffraction (XRD) line broadening analysis and by a method based on the exploitation of the hydrogen temperature programmed reduction (TPR) profiles. In this case, the hydrogen uptakes below about 900 K include the ceria surface reduction and that of the oxidized noble metals. They are analyzed and discussed, assuming two possiblities for the metals oxidation state. 

The results on hydrogenation of p-IBAP using supported Ru catalysts is presented in Table 3. It can be observed that both 2%Kuland 2%Ru/HY showed an optimum activity for the present case. XRD line broadening analysis indicated the presence of veiy small Ru particles that are highly dispersed on the support. As the dimension of the metal crystalline sites are well... 

There are two very general X-ray techniques for study of the metal particle size distribution the line-broadening analysis (LBA) and the small angle X-ray scattering (SAXS) (lb, 170). Other methods include the radial electron distribution (RED) and the extended X-ray absorption fine structure (EXAFS), which are aimed primarily at studying the structure of catalysts (Section IV,G). 

The weight-average crystallite size D, and the maximum distortion parameter g are obtained by line broadening analysis. The results are summarized in Table 4.2. In PTeOX obtained at 105 °C, g decreases by annealing at 170 °C for 2 hours. In PTOX no special change is observed excq>t when annealed at 180 °C for 30 min. In this case, the folded chain crystal is formed inbetween the extended chain crystals, or main crystals, to give larger g values. 

M50 Steel Produced by Chemical Reduction. Nanopowders of M50 steel produced by the chemical reduction technique were characterized by SEM/EDAX. The SEM of the powders showed a similar porous coral-like morphology as shown in Figure 1 for powders synthesized by the sonochemical method. In Figures 9(a) and (b) is shown the EDAX spectrum of the powders before and after the vacuum heat treatment at 650 °C. It indicates that the lithium chloride by-product has been successfully removed by vacuum sublimation. The average particle size, calculated from X-ray line broadening analysis was 34 nm. Current efforts are under way to compact these powders and to study their structural and physical properties. 

Sonochemically Prepared Nanostructured Iron. The consolidated iron pellet had a homogenous microstructure as confirmed by SEM taken at lOOX magnification (Figure 10) and it had a density of 100%. The carbon and oxygen contents were determined to be 0.05% and 1.1% respectively. In the XRD spectra the major peaks were assigned to the a-Fe phase and line broadening analysis revealed the average crystallite size in the consolidated specimen to be 40 nm. 

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