XANES region

Fig. 5 XANES region, -weighted Fourier transformed of the raw EXAFS functions and the corresponding first shell filtered, Fourier back transform (a, b and c, respectively) of TS-1 activated at 400 °C (full lines), after interaction with water (wet sample, dashed lines) and after interaction with NH3 (Pnh3 = 50 Torr, dotted lines). Adapted from [64] with permission. Copyright (2002) by the ACS...

A comparison of the Ni K-edge X-ray absorption spectra for redox-poised samples of hydrogenases from different bacteria in the edge and XANES regions... 

Figure 7.18 A comparison of the Ni K-edge X-ray absorption spectra for redox-poised samples of hydrogenases from different bacteria in the edge and XANES regions.Spectra are separated by redox level, with line types indicating the different bacterial sources (bold line, t roseopersicina light line, D. gigas dotted line, A vinosum dashed line, D. desulfuricans ATCC27774 dashed-dot line, coli). Reprinted with permission from Gu, et a/. (1996) and the American Chemical Society.

The XANES region of the Pt Lm and Ln absorption edges can be used to determine the fractional d-electron occupancy of the Pt atoms in the catalyst sample by a so-called white line analysis. Figure 2 shows the XAS spectrum collected at both Pt Lm and Lii absorption edges of Na2Pt(OH)e. The sharp features at the absorption edges are called white lines after the white line observed in early photographic film based XAS measurements. Mansour and coworkers have shown that comparison of the white line intensities of a sample with those of a reference metal foil provides a measure of the fractional d-electron vacancy, f, of the absorber atoms in the sample. is defined as follows ... 

The effect of the applied potential on the XANES region of the XAS spectra for Pt/C catalysts has been briefly introduced above and is related to both the adsorption of H at negative potentials and the formation of the oxide at more positive potentials. The adsorption of H and the formation of oxides are also apparent in the EXAFS and corresponding Fourier transforms, as seen in the work by Herron et al. shown in Figure 15. As the potential is increased from 0.1 to 1.2 V vs SCE, the amplitude of the peak in the Fourier transform at 2.8 A decreases and that at 1.8 A increases. The effect on the EXAFS, (A), data is less easily observed the amplitude of the oscillations at A > 8 A decreases as the potential is increased, with the greatest change seen between 0.8 and 1.0 V. The results of fitting these data are shown in Table 2. Note that a value for the inner potential... 

XAS has been successfully employed in the characterization of a number of catalysts used in low temperature fuel cells. Analysis of the XANES region has enabled determination of the oxidation state of metal atoms in the catalyst or, in the case of Pt, the d band vacancy per atom, while analysis of the EXAFS has proved to be a valuable structural tool. However, the principal advantage of XAS is that it can be used in situ, in a flooded half-cell or true fuel cell environment. While the number of publications has been limited thus far, the increased availability of synchrotron radiation sources, improvements in beam lines brought about by the development of third generation sources, and the development of more readily used analysis software should increase the accessibility of the method. It is hoped that this review will enable the nonexpert to understand both the power and limitations of XAS in characterizing fuel cell electrocatalysts. 

In Fig. 4.6, the relative absorption is plotted against energy, showing a relative variation of the absorption coefficient of about 30% in the XANES region, compared to the EXAFS modulation of less than 4%. Three regions can be identified in the X-ray absorption spectrum ... 

The low-energy XANES region of about 8 eV, called "edge or threshold region". The absorption edge is the absorption with a few eV determined by the core excitations ... 

Figure 8 Difference spectra of the XANES region for the L3 edges for Pt/Cl-Al203 (A) A linc(T) and (B) A iadS(T), and Pt/K-Al203 (C) A ijnC(T) and (D) A iads(T).

XAFS spectroscopy has been used frequently to determine the oxidation state of an element as a catalyst is subjected to treatments such as reduction, oxidation, or exposure to some reactant. It is the near edge of the X-ray absorption spectrum, the XANES region that is usually used for these measurements. As mentioned in the introduction, the first to recognize the value of XANES for this type of investigation was van Nord-strand (1960), and his report appeared well before the advent of synchrotron radiation sources. 

There are many other such examples, illustrated by the results of Figures 8 (Bare, S.R., unpublished data) and 9 (Ressler et al., 2002). The latter example is interesting in that instead of the edge position as a monitor of the average molybdenum oxidation state, the absolute energy position of a feature in the XANES region was used (indicated by an arrow in Figure 9). 

Fig. 13. Pictorial view of the final-state radial wave functions relevant for core transitions in a molecule. The core transitions take place in an effective molecular potential seen by the excited photoelectron. The final states in the continuum XANES region are quasi-bound multiplescattering resonances (MSR), also called shape resonances. Below the continuum threshold E0 transitions to unoccupied valence states appear. 0 is the energy of the core ionization potential (from ESCA). Ec is the energy where the wavelength of initially excited photoelectrons conforms to the interatomic distance. For E < E0, discrete transitions to unoccupied valence states. E0 < E < Ec, continuum XANES. For < Ec, the EXAFS theory breaks down. The dotted curves show the wave functions of the initially excited photoelectron. From Bianconi (30).

Currently the feeling is that more structural information, for example, locations and site symmetries of the atoms in a particular shell, is contained in the XANES region of an edge absorption spectrum than in EXAFS. However, the large cluster size ( 102 atoms) required for XANES calculations is a considerable drawback. 

XANES Spectroscopy. In the XANES region the photoelectron has, by definition, low kinetic energy. Because low-energy electrons have a long mean free-path and are strongly scattered by the surrounding... 

Supplementary information on the cobalt coordination can be derived from the XANES region. The near-edge structure of the samples is shown in Figure 2 The spectra all show a weak absorption peak near threshold which has been identified as a 1s —> 3d transition (17,18). This transition has been observed to be more intense in te-... 

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