Applications EXAFS

Applications EXAFS spectroscopy is obviously well suited to speciate and quantify the state of heavy metals, e.g. in soils [310]. Similarly, it allows differentiation of ZnO and Zn stearate on the basis of the X-ray absorption structure of zinc. 

Figure 8.39 shows some results of EXAFS following absorption by iron atoms in proteins with three prototype iron-sulphur active sites. In the example in Figure 8.39(a) application of a 0.9-3.5 A filter window before Fourier retransformation shows a single wave resulting... 

Figure 8.40 The k ySk) extended X-ray absorption fine structure (EXAFS) signal, Fourier transformed and then retransformed after application of the filter window indicated, in (a) osmium metal and (b) a 1% osmium catalyst supported on silica. (Reproduced, with permission, Ifom Winnick, FI. and Doniach, S. (Eds), Synchrotron Radiation Research, p. 413, Plenum, New York, 1980)...

EXAFS is a nondestructive, element-specific spectroscopic technique with application to all elements from lithium to uranium. It is employed as a direct probe of the atomic environment of an X-ray absorbing element and provides chemical bonding information. Although EXAFS is primarily used to determine the local structure of bulk solids (e.g., crystalline and amorphous materials), solid surfaces, and interfaces, its use is not limited to the solid state. As a structural tool, EXAFS complements the familiar X-ray diffraction technique, which is applicable only to crystalline solids. EXAFS provides an atomic-scale perspective about the X-ray absorbing element in terms of the numbers, types, and interatomic distances of neighboring atoms. 

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. 

EXAFS Spectroscopy Techniques and Applications. (B. K. Teo and D. C. Joy, eds.) Plenum, New York, 1981. Contains historical items and treatments of EXELFS, the electron-scattering counterpart of EXAFS. 

X-Ray Absorption. Principles, Applications, Techniques of EXAFS, SEXAFS andXANES. (D. C. Koningsberger and R. Prins, eds.) Wiley, New York, 1988. 

Extended X-ray absorption fine structure (EXAFS) studies have been very useful for obtaining structural information on bimetallic cluster catalysts. The application to bimetallic systems is a particularly good one for illustrating the various factors which have an influence on EXAFS. Moreover, the applicability of EXAFS to this area has been very timely, in view of the enormous interest in bimetallic systems in both catalytic science and technology. 

The results of the EXAFS studies on supported bimetallic catalysts have provided excellent confirmation of earlier conclusions (21-24) regarding the existence of bimetallic clusters in these catalysts. Moreover, major structural features of bimetallic clusters deduced from chemisorption and catalytic data (21-24), or anticipated from considerations of the miscibility or surface energies of the components (13-15), received additional support from the EXAFS data. From another point of view, it can also be said that the bimetallic catalyst systems provided a critical test of the EXAFS method for investigations of catalyst structure (17). The application of EXAFS in conjunction with studies employing ( mical probes and other types of physical probes was an important feature of the work (25). 

Investigations utilizing EXAFS have the very important feature of yielding information in an environment of the kind actually encountered in catalysis. We have recently demonstrated the feasibility of making measurements while a catalytic reaction is actually occurring. One can anticipate that measurements of this type will receive Increased emphasis in the future. For studies of the structures of highly dispersed metal catalysts, EXAFS may well be the most generally applicable physical probe currently available. 

These conclusions from the infrared reflectance spectra recorded with Pt and Pt-Ru bulk alloys were confirmed in electrocatalysis studies on small bimetallic particles dispersed on high surface area carbon powders.Concerning the structure of bimetallic Pt-Ru particles, in situ Extended X-Ray Absorption Fine Structure (EXAFS>XANES experiments showed that the particle is a true alloy. For practical application, it is very important to determine the optimum composition of the R-Ru alloys. Even if there are still some discrepancies, several recent studies have concluded that an optimum composition about 15 to 20 at.% in ruthenium gives the best results for the oxidation of methanol. This composition is different from that for the oxidation of dissolved CO (about 50 at.% Ru), confirming a different spatial distribution of the adsorbed species. 

This paper describes the successful incorporation of molybdenum and molybdenum-nickel clusters into zeolites with 12-membered ring by aqueous ion exchange and application of the resulting materials to HDS reaction of benzothiophene. Stoichiometry of the ion exchange was examined by elemental analysis. UV-visible spectroscopy and EXAFS measurements were carried out to investigate the structure of molybdenum species loaded on zeolites. 

Spin-state transitions have been studied by the application of numerous physical techniques such as the measurement of magnetic susceptibility, optical and vibrational spectroscopy, the Fe-Mbssbauer effect, EPR, NMR, and EXAFS spectroscopy, the measurement of heat capacity, and others. Most of these studies have been adequately reviewed. The somewhat older surveys [3, 19] cover the complete field of spin-state transitions. Several more recent review articles [20, 21, 22, 23, 24, 25] have been devoted exclusively to spin-state transitions in compounds of iron(II). Two reviews [26, 27] have considered inter alia the available theoretical models of spin-state transitions. Of particular interest is the determination of the X-ray crystal structures of spin transition compounds at two or more temperatures thus approaching the structures of the pure HS and LS electronic isomers. A recent survey [6] concentrates particularly on these studies. 

Formation of polynuclear lead species with parameters close to isolated lead bromophenoxides during DPC synthesis was found by EXAFS of frozen active reaction mixtures (Pb-0 = 2.34 A, Pb Pb = 3.83 A). Noteworthy, in samples of final reaction mixtures, where catalyst was inactive, short Pb Pb distances were absent. These polynuclear compounds have been tested as lead sources in large-scale runs (small scale reactions were inconclusive due to heterogeneity of reaction mixtures because these compounds are less soluble than PbO). It was found that the use of lead bromophenoxides instead of PbO increases both Pd TON (by 25-35%), and reaction selectivity (from 65 - 67 % to 75 - 84 %). Activity of different lead bromophenoxides was about the same (within experimental error) but the best selectivity was observed for complex Pb602(0Ph)6Br2. Therefore, the gain in selectivity vs. loss due to additional preparation step should be analyzed for practical application. 

X-Ray Absorption Principles,Applications,Techniques of EXAFS, SEXAFS, and XANES. Edited by D. C. Konnigsberger... 

In this chapter, I will try to present an introduction to these various techniques with emphasis on EXAFS and X-ray standing waves and their application to the study of electrochemical interfaces. Each technique will be treated from theoretical and experimental points of view, and selected examples from the literature will be employed to illustrate their application to the study of electrochemical interfaces. 

A variant of the typical X-ray tube described above is the rotating anode, which is capable of generating much higher intensities. Although rotating anode sources can and have been used in EXAFS experiments, the intensities are of such magnitude that data acquisition for extended periods of time is required and their application is furthermore limited to bulk samples. 

The use of surface EXAFS in the study of passive films represents a natural application of the technique and, in fact, the studies by Kruger and co-workers70 73 on the passive film on iron represent the first reported. 

The applications of polarized x-ray absorption spectroscopy (PXAS) for structure determination in inorganic and bioinorganic systems are discussed. PXAS studies of oriented samples add angular detail to the information obtained from x-ray absorption edges and from EXAFS. In some cases, PXAS can be used to determine molecular orientation. In other cases, PXAS can be used to infer the details of electronic structure or of chemical bonding. Some of the potential future applications of PXAS are discussed. 

XAS data comprises both absorption edge structure and extended x-ray absorption fine structure (EXAFS). The application of XAS to systems of chemical interest has been well reviewed (4 5). Briefly, the structure superimposed on the x-ray absorption edge results from the excitation of core-electrons into high-lying vacant orbitals (, ] ) and into continuum states (8 9). The shape and intensity of the edge structure can frequently be used to determine information about the symmetry of the absorbing site. For example, the ls+3d transition in first-row transition metals is dipole forbidden in a centrosymmetric environment. In a non-centrosymmetric environment the admixture of 3d and 4p orbitals can give intensity to this transition. This has been observed, for example, in a study of the iron-sulfur protein rubredoxin, where the iron is tetrahedrally coordinated to four sulfur atoms (6). 

In this chapter, we briefly discuss the theoretical background of polarized x-ray absorption spectroscopy (PXAS). Many of the recent applications of synchrotron radiation to polarized absorption edge structure and to EXAFS are discussed, with particular emphasis being given to the study of discrete molecular systems. We present here some indication of the potential applications of PXAS to systems of chemical and biological interest. 

The results confirm that the novel metal nitrate conversion method using nitric oxide in place of air advocated by Sietsma et al. in patent applications WO 2008029177 and WO 2007071899 leads to, after activation in H2, catalysts with smaller cobalt crystallites, as measured by EXAFS and hydrogen chemisorption/ pulse reoxidation. In spite of the lower extent of cobalt reduction for H2-activated nitric oxide calcined catalysts, which was recorded by TPR, XANES, EXAFS,... 

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