EXAFS description

Figure 4 Descriptive aspects of EXAFS Curves A-E are discussed in the text. Adapted from J. Stohr. In Emission and Scattering Techniques Studies of inorganic Molecules, Solids, and Surtees. (P. Day, ad.) Kluwer, Norwell, MA, 1981.

We will consider the theoretical description of EXAFS based on the single-scattering short-range order formalism. The EXAFS can... 

The data collected are subjected to Fourier transformation yielding a peak at the frequency of each sine wave component in the EXAFS. The sine wave frequencies are proportional to the absorber-scatterer (a-s) distance /7IS. Each peak in the display represents a particular shell of atoms. To answer the question of how many of what kind of atom, one must do curve fitting. This requires a reliance on chemical intuition, experience, and adherence to reasonable chemical bond distances expected for the molecule under study. In practice, two methods are used to determine what the back-scattered EXAFS data for a given system should look like. The first, an empirical method, compares the unknown system to known models the second, a theoretical method, calculates the expected behavior of the a-s pair. The empirical method depends on having information on a suitable model, whereas the theoretical method is dependent on having good wave function descriptions of both absorber and scatterer. 

Full derivations of the theory were presented by Lee and Pendry and Ashley and Doniach in 1975. They showed that a complete quantitative description of th EXAFS process was possible and that accurate bond lengths and coordination numbers could be extracted from the analysis of EXAFS data. Lee and Pendry also showed that at high photoelectron energies, the curvature of the electron wave can be neglected and thus the theory can be greatly simplified into what has beeome known as the plane-wave approximation. This approximation results in an expression equivalent to that derived by Stem semi-empirically ... 

A non exhaustive description of the history of X-ray Absorption Spectroscopy (XAS) can be found in Ref. 1. The modem EXAFS (Extended X-ray Absorption Fine Structure) technique began in the early seventies of the last century. It corresponds to the concomitance of both theoretical and experimental developments. Between 1969 and 1975, Stem, Sayers and Lytle succeeded in interpreting theoretically the X-ray Absorption Structures observed above an absorption edge [2], while during the same period, the advent of synchrotron radiation (SR) sources reduced drastically the acquisition time of a spectrum if compared to data obtained with conventional X-ray tubes. XAS provides essential information about the local atomic geometry and the electronic and chemical state of a specific atom, for almost any element of the periodic table (Z>5). This prime tool for... 

Local description of the arrangement around cations concludes unambiguously for [Cu-Cr-Cl] in a cationic ordering. The evidence of similar ordering for [Zn-Cr-Cl] was only obtained by a combined EXAFS and UV-Vis study of the formation of this LDH in solution [18], The structural pathway so-reported involves the heterocondensation between hexa-aquo zinc(II) complexes and deprotonated chromium monomers. 

The work has largely focused on the coordination chemistry of transition metal ions (i.e., on the description of the nature and symmetry of their environments) (Section 2.1), in line with other spectroscopies, mainly optical (UV-vis), magnetic (EPR and NMR), which take advantage of partly filled d orbitals, and structural (EXAFS) (Sojka and Che, 2009). It has even become possible with PL via well-resolved fine structures to determine the extent of distortion of the environment of tetrahedral species (e.g., vanadium species in zeolites (Section 2.1.2)). It is likely that such information combined with that derived from other spectroscopies, vibrational on one hand, such as IR and Raman, and electronic on the other hand, such as EPR, will be applied by theoreticians to further improve the existing models and our understanding of the nature and role of surface species involved in catalytic processes. 

FIGURE 13 Graded oxide nanoparticle. Mo02 was oxidized with air at 723 K to give a core-shell structure of molybdenum dioxide and possibly molybdenum trioxide that was identified by its different electron energy loss spectrum. No structural description of the highly disordered and catalytically relevant outer oxide shell could be determined, either with XRD or with TEM, (or even with EXAFS spectroscopy) as the signals are dominated by the core structure. 

XAS, and particularly its application to catalysis, has been the subject of several previous reviews and books. In 1988, Koningsberger and Prins published the book "X-ray absorption principles, applications, techniques of EXAFS, SEXAFS and XANES" (Koningsberger and Prins, 1988). In this monograph there is a thorough description of the technique together with a chapter on its application to catalysis. Iwasawa in 1996 published "XAFS for catalysts and surfaces" (Iwasawa, 1996), which focused solely on XAFS spectroscopy as applied to catalyst characterization. This volume includes a chapter by Bazin, Dexpert, and Lynch about measurements of catalysts in reactive atmospheres, and several other chapters allude to examples of such characterization. Recently a book entitled In situ Spectroscopy of Catalysts" (Weckhuysen, 2004) was published that contains three chapters focused on XAFS of catalysts in reactive atmospheres one on XANES, one on EXAFS, and one on time-resolved XAFS. 

Although originally mainly the more pronounced near-edge features were observed and compared or interpreted (usually empirically), it is evident from a recent review (18) that today considerably greater emphasis is given to the study of the extended fine structure. This stands in relation to the improved experimental methods for detection of weak modulations and to the currently more advanced theoretical description of the EXAFS part of the spectrum. Complete understanding of the Kossel structure at the threshold part of an element s inner-shell spectrum, which contains among others valence orbital, ionization, and chemical shift information, is relatively slow due to the... 

The most appropriate description of the Ni oxidation state in all forms of the enzyme appears to be Ni(II), based on a valence bond sum analysis of the best EXAFS model (Thorp, H. H., University of North Carolina, unpublished data). This model is in agreement with studies of the magnetic properties of the EPR silent intermediate of the H2ase from D. baculatus that demonstrated that the Ni site was diamagnetic, albeit in an enzyme that has a selenocysteinate Ni ligand (29). 

The spherical-wave description has its own great advantages. It is best adapted to the scattering and emission by the spherical ion cores and it does not require the presence of any structural periodicity. In particular, it is well suited to the treatment of multiple scattering between different atoms within any cluster of atoms, in particular within a periodic unit cell as in LEED. It is also convenient for the treatment of fine structure arising from back-scattering by nearby atoms, as in (S)EXAFS, NEXAFS, EAPFS, etc. (i.e. in step 3 in our four-step description). 

The imaginary FT spectra in Figure 1 (b) show the separate Co-S and Co-Co(1) coordinations which cannot be discerned in the absolute FT spectra. It appears that the Co-S peak amplitude increases in the order COgSg Co/C Co-Mo/C, whereas the Co-Co(1) peak amplitude decreases in the same order. Data analysis was carried out by isolating the first and second peak in the absolute FT spectra and fitting the resulting EXAFS functions using Co-S and Co-Co EXAFS contributions. For a detailed description of the data analysis procedure we refer to (16). The structural parameters (N,R and A ) obtained in this way are collected in Table I. 

The k-space spectrum (as depicted Fig. 4b) represents the interference between outgoing photoelectrons and the backscattered ones. A physical description of the EXAFS - similar to a radial distribution function - can be extracted from the EXAFS by making use of the Fourier Transform ... 

As yet, no X-ray crystal structures are available for any of the molybdenum enzymes in Table I. Therefore, present descriptions of the coordination environment of the molybdenum centers of the enzymes rest primarily upon comparisons of the spectra of the enzymes with the spectra of well-characterized molybdenum complexes. The two most powerful techniques for directly probing the molybdenum centers of enzymes are electron paramagnetic resonance (EPR) spectroscopy and X-ray absorption spectroscopy (XAS), especially the extended X-ray absorption fine structure (EXAFS) from experiments at the Mo K-absorption edge. Brief summaries of techniques are presented in this section, followed by specific results for sulfite oxidase (Section III.B), xanthine oxidase (Section III.C), and model compounds (Section IV). 

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