XANES structures

Figure 8. Empirical correlations of Fe porphyrins XANES structure with spin state. A) The "Ligand Field Indicator Region" identified by Chance et al. (Reproduced with permission from reference 44. Copyright 1986, Journal of Biological Chemistry) B) Spin state sensitive bands identified by Oyanagi et al. Redrawn from data in Reference 45.

The characteristics of reforming catalysts make them one of the most frequent cases where EXAFS is used. The very small metallic particles cannot be detected in transmission electron microscopy the long range order required for XRD analysis is absent, and the low metal contents make XPS analysis difficult. The observation of XANES structures at Pi and Re thresholds can be used to determine the electronic state. Some examples of reference compounds are shown in Figure 11.10. In particular a sharp peak (called a white line for historical reasons) is visible at the edge with an intensity related to the oxidisation state of the element. This peak in the absorption coefficient is a consequence of the existence of the empty electron states close to zero binding energy. 

An example of the use of ab initio XANES calculations to determine nanoparticle structure is the Zn/ferrihydrite sorption system examined by Waychunas et al. (2001). In the case of sorption complexes the XAI S spectrum of the sorbed species will contain information about the local structure of the substrate, and thus the structural nature of the full sorption complex. In the Zn/ferrihydrite system it was observed via EXAFS that the number of Fe next nearest neighbors about the sorbed Zn ion decreased as the Zn sorption density increased. Direct calculation of the XANES structure identified MS paths that changed in number as a function of cluster size (and thus number of neighbor Fe atoms), and gave rise to XANES features that changed in intensity (Fig. 32). These changes agreed well with the structural interpretation of the EXAFS and the crystal chemistry of Zn-Fe hydroxides. 

When the measurements were made with an efficient cell, similar inverse appearance was again observed for a thick cell (Fig. 12) Since the thickness of the cell approaches the 1 — e = 1 limit, no XANES is seen in the transmission spectrum except an edge jump. The current yield again shows sharp but inversed XANES structures characteristic of the spectrum obtained with low cell efficiency. 

Fig. 3.35 An XAS spectrum of ZnSe under a pressure of 10 GPa at the Zn K-edge obtained with an energy-dispersive set-up (X) the XANES structure, and (E) the EXAFS structure.

The fine structures at X-ray absorption edges contain a manifold of information about the geometrical (EXAFS and XANES) and electronic (XANES) structure of matter. The main advantages of this method are ... 

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. 

Figure 2 Molybdenum K-edge X-ray absorption spectrum, ln(i /i ) versus X-ray energy (eV), for molybdenum metal foil (25- jjn thick), obtained by transmission at 77 K with synchrotron radiation. The energy-dependent constructive and destructive interference of outgoing and backscattered photoelectrons at molybdenum produces the EXAFS peaks and valleys, respectively. The preedge and edge structures marked here are known together as X-ray absorption near edge structure, XANES and EXAFS are provided in a new compilation of literature entitled X-rsy Absorption Fine Structure (S.S. Hasain, ed.) Ellis Norwood, New York, 1991.

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... 

Some structural data obtained by these methods are also discussed in the following sections. The XANES spectra of organotin(lV) are usually not so informative. The advantages and disadvantages of EXAFS as a structural probe are discussed... 

X-ray absorption near edge structure (XANES) spectroscopy is a non-destructive and sensitive probe of the coordination number and geometry as well as of the effective charge of a chosen atom within a molecule and therefore also of the formal oxidation number. Recently, there have been a number of XANES studies at the sulfur K-edge demonstrating the sensitivity of... 

From an analysis of the intensity of the ls-3c pre-edge transition in the XANES spectra, Tsang et al. 72) have suggested that the histidine-coordinated Fe" of the reduced cluster may be 5-coordi-nated this is not supported by the X-ray structures. 

Time-resolved X-ray absorption is a very different class of experiments [5-7]. Chemical reactions are triggered by an ultrafast laser pulse, but the laser-induced change in geometry is observed by absorption rather than diffraction. This technique permits one to monitor local rather than global changes in the system. What one measures in practice is the extended X-ray absorption fine structure (EXAFS), and the X-ray extended nearedge strucmre (XANES). 

The extended fine structure (EXAFS) was used to determine bond distances, coordination number and disorder. The near edge (XANES) was used as an Indication of electronic state. Significant results Include, 1) a reversible change of shape of clean supported metal clusters as a function of temperature, 2) supported Pt clusters have more disorder or strain compared to the bulk metal, and 3) a clear determination of the bonds between the catalytic metal atoms and the oxygen atoms of the support. 

X-ray absorption spectroscopy combining x-ray absorption near edge fine structure (XANES) and extended x-ray absorption fine structure (EXAFS) was used to extensively characterize Pt on Cabosll catalysts. XANES Is the result of electron transitions to bound states of the absorbing atom and thereby maps the symmetry - selected empty manifold of electron states. It Is sensitive to the electronic configuration of the absorbing atom. When the photoelectron has sufficient kinetic energy to be ejected from the atom It can be backscattered by neighboring atoms. The quantum Interference of the Initial... 

In the following, structural data are obtained for Ft atoms and their near neighbors on active catalysts under controlled conditions. XANES Is used to Indicate the direction and amount of d-electron flow between the Ft catalyst and Its ligands, EXAFS to measure near neighbor structural parameters. We find EXAFS/XANES to be a sensitive and subtle Indicator of small changes In the environment of catalytic atoms. 

Evidence for the first reason was demonstrated in X-ray photoelectron spectroscopy (XPS) or X-Ray Absorption Near Edge Structure (XANES) studies, as well as by many voltammetric studies. ... 

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. 

If a vacant site is occupied by another Nb atom, such that it is a dimer, new catalysts may be designed. The Nb dimer catalyst(2) was prepared by reaction of [Nb(ri -C5H5)H-p-(T, Ti -CsH4)]2 with a Si02 at 313 K, followed by treatment with 02 at 773 K. A proposed structure(2) was characterized by EXAFS, x-ray absorption near-edge structure(XANES), FT-IR, UV-vis, and XPS, which shows Nb-Nb (coordination number 0.9) and Nb-Si(2.3)... 

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