Fourier-transform EXAFS spectra

Table 10.2 Curve-fitting results of Fourier transformed EXAFS spectra (16 K) at V K-edge for a V precursor (L-leucine), its fresh supported V complex (3.4% V) and that treated with 2-naphthol the coordination number of V=0 was fixed as unity.

Laperche Traina (1998) studied Pb uptake on hydroxyapatite at low initial solution concentration of Pb (103 mg/L). For this, EXAFS was used to characterize the local coordination environment of Pb on the apatite. The baseline corrected, Fourier-transformed EXAFS spectra revealed fc-values at >3 A, suggesting that Pb was not randomly sorbed. Radial structure functions (RSF) showed three intense peaks, characteristic of pyromorphite. 

Fig. 9.1.11 Fourier-transformed EXAFS spectra at Pd K-edge of PVP-stabilized Pd/Pt bimetallic nanoparticles at Pd/Pt ratio = I/O, 10/1, 4/1, and 1/1. (From Ref. 25a.)...

Detailed information on the structure of the supported cobalt sulfide phase could be obtained by studying the imaginary Fourier Transformed EXAFS spectra as well as the cobalt 1s —> 3d transition in the XANES spectra. It is shown that the cobalt atoms in the Co-Mo-S (II) phase have an octahedral-like sulfur coordination while the sulfided Co/C catalyst has a larger fraction of octahedral cobalt than Co Sg. On the basis of these results, the high HDS activity of a sulfided Co/C catalyst can be understood since it appears that the structure of the cobalt sulfide phase in Co/C is in agreement with that in Co-Mo/C. In the Co-Mo-S (II) phase approximately one cobalt atom is in contact with one molybdenum atom at a distance of 2.85 A. 

Figure 7. A log-log plot of a sorption isotherm, with an inflection indicating the transition from adsorption to surface precipitation processes. On the right, illustrative Fourier transformed EXAFS spectra for the adsorbate (solid curve) are compared with that for a precipitate (dotted curve) to show the adsorption — precipitation transition. Data are from Charlet and Manceau (48) for Cr(III) sorbed on hydrous ferric oxide.

Figure 6. Analysis of Pt/Co bimetallics, a) TPR patterns of CPA/SiOi, C03O4, CPA/C03O4, and CPA/(Co304+Si0, b) Fourier Transformed EXAFS spectra ofPt foil md Pt/(Co304+SiOj).

Fig. 6 Fourier transformed EXAFS spectra of sPEEK-ZrP membranes prepared by contact of ion-exchange membranes with H3PO4 (IM, 80 C) for 5 min (V) and 15h ( ), compared with that of well-crystallised a-ZrP prepared using the HE method ( )...

Fourier transformed EXAFS spectra of e purple enzyme reveal three major peaks, assigned as follows first shell, Fe-0(N) second shell, Fe— Fe and Fe-P(C) third shell, Fe—N(C) (imidazole). Due to interference by Fe-0 (tyrosine) bonds at 1.8-1.9 A, an Fe-p-oxo bond could not be detected. The Fe— Fe distance of 3.0 A lies within the range expected for a p-oxo bridged structure. The spectrum of the purple, phosphate bound form also provides direct evidence for phosphate coordination to one of the iron atoms, with an Fe—P distance of 3.0 A. 

Figure 4. (a). Raw XAS data for the Fe k-edge (7112 eV) of an Fe foil incorperating the XANES and EXAFS regions with the cubic spline funtion (dotted line) representing the background to be subtracted, (b). Normalized EXAFS data that has been transformed to k-space using Eq. 9. And (c). Fourier Transformed EXAFS spectrum in distance space. 

The As-HAO system presents special difficulties for IR and XAFS spectroscopic analysis. In an XAFS spectrum, the magnitude of peaks in the Fourier transformed EXAFS spectrum is a function of several variables, two of which are atomic number (z) and distance from the central As atom. With only half as many electrons as Fe, the scattering power of Al is weak, therefore peaks representing As-Al scattering in the Fourier-transformed EXAFS are smaller and more difficult to interpret. IR and Raman spectra of As(V) sorbed on gibbsite are difficult to interpret for an entirely different reason substantial overlap of peaks representing Al(V)-0/Al-OH vibrations and As(V)-0/As(V)-OH vibrations (Myneni et al, 1998). 

Top XAS spectrum of the vanadium(IV) complex shown in the inset. Bottom Fourier transform EXAFS spectrum of the same complex. R is the distance in A. The spectrum shows three peaks, assigned as indicated. The V Br distance has not been detected. 

Fig. 1. Fourier transformed EXAFS spectrum of and MPPNP bound to...

Figure 12.6 shows the Fourier-transformed EXAFS spectrum of potassium tetrachloroaurate reacted with wheat biomass and amine resin, the Au(0) foil and a potassium tetrachloroaurate model compound. Similar to results obtained for the reduction of Cr(Vl) on agave biomass, the reduction of potassium tetrachloroaurate on wheat biomass (Figure 12.6a) or on an amine resin (Figure 12.6b) at pH... 

Figure 9. Data reduction and data analysis in EXAFS spectroscopy. (A) EXAFS spectrum x(k) versus k after background removal. (B) The solid curve is the weighted EXAFS spectrum k3x(k) versus k (after multiplying (k) by k3). The dashed curve represents an attempt to fit the data with a two-distance model by the curve-fitting (CF) technique. (C) Fourier transformation (FT) of the weighted EXAFS spectrum in momentum (k) space into the radial distribution function p3(r ) versus r in distance space. The dashed curve is the window function used to filter the major peak in Fourier filtering (FF). (D) Fourier-filtered EXAFS spectrum k3x (k) versus k (solid curve) of the major peak in (C) after back-transforming into k space. The dashed curve attempts to fit the filtered data with a single-distance model. (From Ref. 25, with permission.)...

The essence of analyzing an EXAFS spectrum is to recognize all sine contributions in x(k)- The obvious mathematical tool with which to achieve this is Fourier analysis. The argument of each sine contribution in Eq. (8) depends on k (which is known), on r (to be determined), and on the phase shift

characteristic property of the scattering atom in a certain environment, and is best derived from the EXAFS spectrum of a reference compound for which all distances are known. The EXAFS information becomes accessible, if we convert it into a radial distribution function, 0 (r), by means of Fourier transformation ... 

Fig. 10 a Co K-edge XAS spectrum for CoP collected in transmission mode, showing the approximate regions where XANES and EXAFS features are observed and the assignment of dipolar and quadrupolar transitions, b EXAFS (x) vs. k curve, c Fourier transform of EXAFS... 

Fig. 16. Effect of soaking TS-1 with water on the XANES (a) and UV-Raman (b) spectra dried TS-1 (solid line) soaked TS-1 (dotted line). The inset in part (a) reports the U-weighted, phase-uncorrected Fourier transforms of the corresponding EXAFS spectrum [Reprinted from Ricchiardi et al. (41) with permission. Copyright (2001) American Chemical Society].

Figure 6.14 EXAFS and Fourier transform of rhodium metal, showing a) the measured EXAFS spectrum, b) the uncorrected Fourier Transform according to equation (6-10), c) the first Rh-Rh shell contribution being the inverse of the main peak in the Fourier Transform, and d) the phase- and amplitude-corrected Fourier Transform according to (6-11). The Fourier transform is a complex function, and hence the transforms give the magnitude of the transform (the positive and the negative curve are equivalent) as well as the imaginary part, which oscillates between the magnitude curves (from Martens (361).

Figure 2. a) X-ray absorption spectrum near the Mo K-edge of the Co/Mo = 0.125 unsupported Co-Mo catalyst recorded in situ at room temperature b) normalized Mo EXAFS spectrum c) absolute magnitude of the Fourier transform d) fit of the first shell e) fit of the second shell. The solid line in d) and e) is the filtered EXAFS, and the dashed line is the least squares fit. 

The right panel of Figure 1.3 displays the radial function obtained by Fourier transformation of the -weighed background-subtracted EXAFS data from the solid heated to 420°C [31], This spectrum shows two major peaks, one at about 1.5 A associated with backscattering from O neighbors, and a second at 3 A related to the Nb-Mo pairs. The measured distances are consistent with a combination of niobium oxo species and heteropolymolybdate fragments, presumably the catalytically active phase. 

The EXAFS function is obtained from the X-ray absorption spectrum by subtracting the absorption due to the free atom. A Fourier transform of the EXAFS data gives a radial distribution function which shows the distribution of the neighbouring atoms as a function of internuclear distance from the absorbing atom. Shells of neighbours, known as coordination shells, surround the absorbing atom. Finally, the radial distribution function is fitted to a series of trial structural models until a structure which best fits the... 

Figure 6.17 EXAFS data of a reduced Pt/AEO catalyst. Full lines are measured data dotted lines represent fits. Left magnitude of a -weighted Fourier transform of the range 1,9

Figure 7. Fourier transform of the background-subtracted EXAFS spectrum for Cr3 53-montmorillonite.

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