EXAFS/ XAFS fine structure

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. 

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... 

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... 

Abbreviations CCD, charge-coupled device DEXAFS, dispersive extended X-ray absorption fine structure EXAFS, extended X-ray absorption fine structure QEXAFS, quick extended X-ray absorption fine structure TPR, temperature-programmed reaction XRD, X-ray diffraction XAFS, X-ray absorption fine structure. 

Extended X-ray absorption fine structure (EXAFS) spectrum Part of an X-ray absorption spectrum that is used to identify the coordination of atoms, estimate bond lengths, and determine the adsorption complexes on the surfaces of adsorbents. EXAFS spectra may provide useful information on the speciation (valence state), surface complexes, and the coordination of arsenic on adsorbents (e.g. (Randall, Sherman and Ragnarsdottir, 2001 Ladeira, et al. (2001) Teixeira and Ciminelli (2005) Kober, et al. (2005)) (compare with X-ray absorption spectroscopy (XAS), X-ray absorption near edge structure (XANES) spectra, and X-ray absorption fine structure spectroscopy (XAFS)). 

X-ray absorption spectroscopy (XAS) Methods that use X-rays to investigate the physical and chemical properties of materials on an atomic scale. XAS includes X-ray adsorption fine structure (XAFS) spectroscopy and its X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectra. 

Extended X-ray absorption fine structure (EXAFS) on the other hand, is due to the interference of electron waves between atoms, and provides local structure information that is limited to a few interatomic distances. Here, we talk about the distance and the number of nearest and next-nearest neighbors of atoms in the catalyst. The more uniform the environment is through the catalyst, the more meaningful is the EXAFS information. Related to this method is X-ray absorption near edge spectroscopy (XANES), which deals with the detailed shape of the absorption edge, and yields important information on the chemical state of the absorbing atom. Commonly, one uses nowadays the acronym XAFS to include both EXAFS and XANES. 

Clearly there are many methods that can be used to probe a catalyst s structure under reaction conditions, as documented in other chapters in this series (Advances in Catalysis, Volumes 50 and 51, and this volume). In this chapter we focus on X-ray absorption spectroscopy (XAS), including X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopy, sometimes simply referred to as X-ray absorption fine structure (XAFS) spectroscopy. In this review the term XAFS will be used generically, but EXAFS and XANES will be used when the information is specifically related to the extended or near edge structure, respectively. 

Lindau I, Spicer WE (1980) Photoemission as a tool to study solids and surfaces. In Winick H, Doniach S (eds) Synchrotron Radiation Research, Plenum Press, New York, p 159-221 Lindqvist-Reis P, Lamble K, Pattanaik S, Persson I, Sandstrocni. M (2000) Hydration of the yttrium(III) ion in aqueous solution. An X-ray diffraction and XAFS structural study. J Phys Chem 104 402-408 Lindqvist-Reis P, Munoz-Paez A, Diaz-Moreno S, Pattanaik S, Persson I, Sandstroem M (1998) The structure of the hydrated gallium(III), indium(HI), and chromium(III) ions in aqueous solution. A large angle X-ray scattering and EXAFS study. Inorg Chem 37 6675-6683 Liu C, Frenkel AI, Vairavamurthy A, Huang PM (2001) Sorption of cadmium on humic acid Mechanistic and kinetic studies with atomic force microscopy and X-ray absorption fine structure spectroscopy. Canadian J Soil Sci 81 (3, Spec. Issue) 337-348... 

Sayers DE, Hesterberg D, Zhou W, Robarge WP, Plummer GM (1997) XAFS characterization of copper contamination in the unsaturated and saturated zones of a soil profile. J de Physique IV 7 (Colloque C2, X-Ray Absorption Fine Structure, Vol. 2) 831-832 Sayers DE, Lytle FW, Stem EA (1972) Structure determination of amorphous germanium, germanium oxide, and germanium selenide by Fourier analysis of extended X-ray absorption fine structure (EXAFS). J Non-Crystal Solids 8-10 401-407... 

Further significant developments of the EXAFS technique are envisaged. Energy-dispersive X-ray absorption tine structure (DXAFS) allows collection of a spectrum in much less than 1 s, and its extension to the fj,s or even the ns time scale is not impossible. Polarisation-dependent total-reflection fluorescence X-ray absorption fine structure (PTRF-XAFS) permits high spatial resolution and has been applied to the copper trimer Cus on a Ti(llO) surface. The use of time-and space-resolved XAFS observation for studies of dynamic aspects of the local structure at catalyst surfaces under working conditions is forseen . Note that the acronym XAFS is used, rather than EXAFS. 

One of the new fields of actinide chemistry since the 1990s is an application of X-ray absorption spectroscopy (Teo 1986) especially by use of the synchrotron radiation. XAFS (X-ray absorption fine structure) is a powerful technique for characterization of the local structure of specific elements, even radioactive elements such as actinides, and their electronic states. XAFS contains two fundamental information, EXAFS (extended X-ray absorption fine structure) and XANES (X-ray absorption near-edge structure). O Figure 18.27 shows a fundamental pictorial view of the XAFS process (Koningsberger and Prins 1987). 

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