X-ray XAFS

For bulk structural detemiination (see chapter B 1.9). the main teclmique used has been x-ray diffraction (XRD). Several other teclmiques are also available for more specialized applications, including electron diffraction (ED) for thin film structures and gas-phase molecules neutron diffraction (ND) and nuclear magnetic resonance (NMR) for magnetic studies (see chapter B1.12 and chapter B1.13) x-ray absorption fine structure (XAFS) for local structures in small or unstable samples and other spectroscopies to examine local structures in molecules. Electron microscopy also plays an important role, primarily tlirough unaging (see chapter B1.17). 

EXAFS is part of the field of X-ray absorption spectroscopy (XAS), in which a number of acronyms abound. An X-ray absorption spectrum contains EXAFS data as well as the X-ray absorption near-edge structure, XANES (alternatively called the near-edge X-ray absorption fine structure, NEXAFS). The combination of XANES (NEXAFS) and EXAFS is commonly referred to as X-ray absorption fine structure, or XAFS. In applications of EXAFS to surface science, the acronym SEXAFS, for surface-EXAFS, is used. The principles and analysis of EXAFS and SEXAFS are the same. See the article following this one for a discussion of SEXAFS and NEXAFS. 

XAFS V, Proceeding of the Fifth International Conference on X-ray Absorption Fine Structure. (J. M. de Leon, E. A. Stern, D. E. Sayers, Y. Ma, and J. J. Rehr, eds.) North-Holland, Amsterdam, 1989. Also in Physica B. 158, 1989. Report of the International Workshop on Standards and Criteria in X-ray Absorption Spectroscopy (pp. 701-722) is essential reading. 

XAFS (EXAFS and XANES) methods X-ray diffraction method Biological investigations Flydrolysis of [OrganotinllV)]" Cations Interactions of [OrganotinllV)]" with Biological Molecules... 

The X-ray absorption fine structure (XAFS) methods (EXAFS and X-ray absorption near-edge structure (XANES)) are suitable techniques for determination of the local structure of metal complexes. Of these methods, the former provides structural information relating to the radial distribution of atom pairs in systems studied the number of neighboring atoms (coordination number) around a central atom in the first, second, and sometimes third coordination spheres the... 

Ylwasawa (ed.). X-ray Absorption Fine Structure(XAFS) for Catalysts and Surfaces, World Scientific, Singapore, 1996. 

A XAS experiment involves the irradiation of a sample with a tuneable source of monochromatic X-rays, usually from a synchrotron facility (high brilliance). Third-generation synchrotrons have sufficient intensity to observe XAFS spectra up to 100 keV. Nevertheless, laboratory-scale XAFS spectroscopy is of importance, despite the vast availability of synchrotron beam time [305]. 

XANES spectroscopy is also the basis of chemically sensitive X-ray imaging, as well as qualitative and quantitative microspectroscopy [306], ptXANES is attractive for chemical analysis, with its spatial resolution down to 10 ptm. Variations on the theme are surface EXAFS (SEXAFS), grazing incidence XAS and in situ time-resolved XAS investigations. Grazing angle XAFS can be used for the study of ultrathin multilayer systems. 

Groothaert et al., using operando UV-vis spectroscopy combined with online GC analysis [176] and operando X-ray absorption fine structure (XAFS) [177], presented the first experimental evidence for the formation of the bis( x-oxo)dicopper core in Cu-ZSM-5 and for its key role of intermediate in the sustained high activity of Cu-ZSM-5 in the direct decomposition of NO into N2 and 02. In particular, monitoring the catalytic conversion of NO and N20 above 673 K, they found that the bis( x-oxo)dicopper core is formed by the O abstraction of the intermediate N20 (Figure 4.14). Subsequently,... 

X-ray Diffraction, X-ray Photoelectron Spectroscopy, and XAFS Spectroscopy Study 741... 

The most prevalent technique exploiting synchrotron radiation is X-ray absorption spectroscopy (XAS, also called X-ray absorption fine structure, XAFS). Two related types of experiments are conducted X-ray absorption near-edge spectroscopy (XANES), which probes the initial absorption edge and related nearby structure, and... 

Temperature-programmed reduction combined with x-ray absorption fine-structure (XAFS) spectroscopy provided clear evidence that the doping of Fischer-Tropsch synthesis catalysts with Cu and alkali (e.g., K) promotes the carburization rate relative to the undoped catalyst. Since XAFS provides information about the local atomic environment, it can be a powerful tool to aid in catalyst characterization. While XAFS should probably not be used exclusively to characterize the types of iron carbide present in catalysts, it may be, as this example shows, a useful complement to verify results from Mossbauer spectroscopy and other temperature-programmed methods. The EXAFS results suggest that either the Hagg or s-carbides were formed during the reduction process over the cementite form. There appears to be a correlation between the a-value of the product distribution and the carburization rate. 

The aim of this work was to apply combined temperature-programmed reduction (TPR)/x-ray absorption fine-structure (XAFS) spectroscopy to provide clear evidence regarding the manner in which common promoters (e.g., Cu and alkali, like K) operate during the activation of iron-based Fischer-Tropsch synthesis catalysts. In addition, it was of interest to compare results obtained by EXAFS with earlier ones obtained by Mossbauer spectroscopy to shed light on the possible types of iron carbides formed. To that end, model spectra were generated based on the existing crystallography literature for four carbide compounds of... 

X-ray absorption fine structure (XAFS) technique, 74 464-465 X-ray absorption near edge structure (XANES), 24 72... 

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