Applications of EXAFS

It is interesting to note that oxygen XANES exhibits pre-edges if bound to a transition metal (e.g. Co), as well. 

XANES probes the transition of core electrons into unoccupied electronic states. 

The final states may be mixed with other orbitals and can be used to determine the coordination environment of an element in a compound, its electron density and oxidation state. The intensity can be used to accurately determine relative oxidation state ratios. Furthermore, XANES can be used to determine relative amounts of species by linear combination of individual compounds. In recent years, the codes for XANES calculation (especially John Rehr s FEFF code), have significantly improved and it can be expected that theoretical models of compounds will be accurately determined by XANES measurement and calculation in the future. 

XANES and EXAFS complement each other very well and are ideal to determine the site structure of even mixtures of catalysts or oxides. 

XAS is in general less accurate than XRD techniques. The accuracy depends on the edge energy, the resolution of the monochromator and, last but not least, the quality of model building. As a general mle distances can be determined up to 5 X 10 A and coordination numbers at maximum to 10%. This is clearly inferior to XRD. 

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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. 

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). 

The last decade has seen enormous developments in SS NMR, and techniques that were used only in soluhon have become available also for solid samples. Nowadays, even conformers can be identified by solid-state NMR [22]. Therefore, this section outlines these improvements as well as their application to the characterization of selected surface organometallic species. An application of EXAFS to the characterization of surface organometallic tantalum species is also shown. Structural data obtained through EXAFS (such as that in Table 11.1) are often essential for comparisons with data obtained by DFT calculations. 

J. Stohr in X-ray Absorption, Principles, Techniques, Applications of EXAFS, SEXAFS and XANES, ed. R. Prins and D.C. Koeningsberger (Wiley, New York, 1988) 443. 

The energy of X-rays is large enough to promote Is core electrons to unoccupied orbitals, e.g. 3d, 4s, 4p. The energy at which this absorbance takes place is specific for the element concerned. Therefore iron centres can be detected in the presence of other metals. However, as the technique will detect all iron centres in a protein, interpretation of a spectrum from a protein with more than one iron site (or in the presence of contaminants) is complicated. The technique requires an intense and tuneable supply of X-rays and therefore is limited to synchrotron sources. High protein concentrations ( lmM iron) are also required. Recent reviews of the application of EXAFS to biological systems include those by Penner-Hahn and Hodgson [110] and Scott [111],... 

One of the earliest successful applications of EXAFS to probe a me-talloenzyme was the study of the molybdenum site of nitrogenase. Studies were made on both the C. pasteurianum and A. vinelandii MoFe-proteins and on isolated FeMoco (116). These studies showed definitively that molybdenum is present as part of a polynuclear cluster containing sulfur and iron, with Mo—S and Mo—Fe distances of —2.36 and —2.72 A, respectively. This work inspired the successful development of many chemical systems containing Mo—Fe—S clusters, and XAS studies of these systems strengthened the basis for the interpretation of corresponding data for the natural system. The most accurate picture of the molybdenum site of FeMoco currently available involves a coordination of about three oxygen (or nitrogen), sulfur, and iron atoms at —2.10, —2.37, and —2.70 A, respectively (117). 

Figure 11.18 shows an application of EXAFS on sulphided catalysts as used in hydrotreating processes [39]. The results in the corresponding table have been obtained by computer fitting. The great disadvantage of EXAFS is that data analysis is complicated and time consuming considerable expertise is required... 

A iKitable exception is the recent, application of EXAFS to plasticized Zn sulfonated polystyrene ionomers (73-711. Thi.s technique was used to investigate the effect of various diluents on the local struct.ure of the Zn cation. Plasticizers termed, "non-coordinatirig" (because they do not complex metal cations) - in particular, toluene, acetonitrile, and DOP - were shown to have a minimal influence on the cation s local structure. At most, these cause a slight increase in disorder, but they do not. alter the coordination structure around the cation. These diluents clearly partition themselves into the nonpolar regions of the ionomer. 

While the phenomenon of EXAFS has been known for a long time (15-17), its possibilities for investigating the structures of noncrystalline materials have only recently been appreciated. The application of EXAFS in catalyst studies is of particular interest, since for a number of very highly dispersed catalysts of technological importance it appears to be the only method capable of yielding detailed structural information. 

All these features explain why the applications of EXAFS to the study of very dispersed materials have increased so much in the last few years. Figure 2 shows the tremendous growth of the number of papers on catalysits characterization by EXAFS coupled to the use of synchrotron light. This figure comes from the very detailed and exhaustive review Bart and Vlaic recently published on the subject... 

Hasnain, S. S. (1988) Application of EXAFS to biochemical systems in Topics in Current Chemistry 147, 73-93, edited by E. Mandelkow. 

The other cofactor often neglected in discussions of the OEC is chloride. Again, EXAFS data have been obtained to address whether or not chloride is bound to the manganese cluster (33, 358, 359). These experiments are technically very difficult. There is the possibility that chloride (or bromide in bromide-substituted samples) may be observed in the EXAFS spectrum. However, we believe that it is unlikely that one can resolve this issue, because the signal-to-noise ratio may not be sufficient (156) to observe only one chloride per four manganese ions. Very recently, chloride EXAFS of manganese model complexes has been reported (360). This new application of EXAFS may be able to resolve this issue. 

Sinfelt JH, Via GH, Lytle FW (1984) Application of EXAFS in catalysis. Structure of bimetallic cluster catalysts. Catal Rev - Sci Eng 26 81-140... 

Stem EA, Heald SM (1983) Basic principles and applications of EXAFS. In Handbook on Synchrotron Radiation, Vol. lb, Koch EE (ed) North Holland, New York, p. 955-1014 Stem EA, Heald SM (1979) X-ray filter assembly for fluorescence measurements of X-ray absorption fine structure. Rev Sci Inst 50 1579-1582... 

The applications of EXAFS spectroscopy to metal carbonyl cluster chemistry can be subdivided into a number of broad and overlapping areas ... 

Application of EXAFS would advance our understanding of the nature of solid-bound Cd species of soils as the methods develop further and can operate at the trace levels of Cd present in soils. However, sampling techniques required for these methods often annihilate or change irreversibly the surface species of interest. Molecular-level information about the mechanisms, orientation or dynamics of surface species inferred from data obtained by these methods may bear little resemblance to the chemical mechanisms operating in a natural... 

To demonstrate the application of EXAFS to this problem, we compare the FTs of the MoK edge of four different samples and a reference compoimd (Fig. 8) ... 

EXAFS may now be a vaunted acronym, but it represents only part of the information available in X-ray absorption spectroscopy, albeit the most understood part. Since this is the first article in this Specialist Periodical Review series on the technique, the first part of the chapter will be devoted to an introduction into the basis, methodology, information content of "mainstream" X-ray absorption spectroscopy. Then examples will be presented to illustrate its application to research in catalysis. Finally, some relatively new techniques will be described which seem to offer very substantial promise for catalysts investigations. Earlier articles by Joyner, Cox and Pettifer on the application of EXAFS to catalyst characterisation have been published in the monograph of Thomas and Lambert. ... 

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