Fourier transformation of EXAFS spectra

Figure 1 shows Fourier transforms of EXAFS spectra of a few samples prepared. The radial distribution functions of these samples are different from that of nickel oxide or cobalt oxide [7]. All the Fourier transforms showed two peaks at similar distances (phase uncorrected) the peak between 1 and 2 A is ascribed to the M-0 bond (M divalent cation) and the peak between 2 and 3 A is ascribed to the M-O-M and M-O-Si bonds. The similar radial distribution functions in Figure 1 indicate that the local structures of X-ray absorbing atoms (Ni, Co, and Zn) are similar. No other bonds derived from metal oxides (nickel, cobalt and zinc oxides) were observed in the EXAFS Fourier transforms of the samples calcined at 873 K, which suggests that the divalent cations are incorporated in the octahedral lattice. 

The radial distribution functions in Figure 7 represent the Fourier transform of EXAFS spectra. They display several peaks according to the nearest atomic shells surrounding the central Mn atoms. The first peak in the RDF allows the calculation of the Mn-O distance and the coordination number of the first shell. The second peak corresponds to the Mn-Mn distances. 

Figure 3. The radial distribution function (RDF) obtained from the Fourier transformation of EXAFS spectra for the underexchanged Cu-ZSM-5-59 (a) sample has been exposed to ambient air after ion exchange and calcination (b) sample was oxidized in dry air at 773 K and was cooled in dry air to room temperature (c) sample was auto-reduced in ultra-high purity He at 773 K and was cooled to room temperature in He.

Figure 19.4 XAFS (XANES and Fourier transforms of EXAFS) spectra of Ti/Si binary oxide thin films. Ti02 content (%) (a. A) 6.6, (b, B) 9.5, and (c, C) 50.1...

In Figure 1 (a,b) the Fourier-Transformed (FT) EXAFS spectra of the Co/C and the Co-Mo/C catalyst together with the Co Sg reference compound are plotted. The absolute FT spectrum of Co Sg (Fig. 1 (a)) exhibits two peaks. The first peak is attributed to combined Co-S and Co-Co coordinations, the second one only to a Co-Co coordination (denoted Co-Co(2), to differentiate it from the Co-Co(1) coordination in the first peak). In bulk Co Sg 8/9 of the cobalt atoms are tetrahedrally coordinated and 1/9 are octahedrally coordinated by sulfurs. The Co-S coordination distances are in the range 2.13-2.39 A. The Co-Co coordination distance of Co-Co(1) is 2.50 A, for Co-Co(2) the distance is 3.51 A. From Figure 1 (a) it is apparent that in the catalyst spectra the first peak is shifted to lower r-values compared to that in Co Sg. This shift is larger for Co-Mo/C than for Co/C. It is furthermore clear that a Co-Co(2) coordination is also present in the Co/C catalyst, but not in the promoted catalyst. On the other hand, the latter catalyst shows an additional peak which is not present in COgSg and Co/C and, consequently, might be ascribed to Mo backscatterers. 

At the present time, of all EXAFS-like methods of analysis of local atomic structure, the SEES method is the least used. The reason is that the theory of the SEES process is not sufficiently developed. However the standard EXAES procedure of the Fourier transformation has been applied also to SEES spectra. The Fourier transforms of MW SEES spectra of a number of pure 3d metals have been compared with the corresponding Fourier transforms of EELFS and EX-AFS spectra. Besides the EXAFS-like nature of SEES oscillations shown by this comparison, parameters of the local atomic structure of studied surfaces (the interatomic distances and the mean squared atomic deviations from the equilibrium positions [12, 13, 15-17, 21, 23, 24]) have been obtained from an analysis of Fourier transforms of SEES spectra. The results obtained have, at best, a semi-quantitative character, since the Fourier transforms of SEES spectra differ qualitatively from both the bulk crystallographic atomic pair correlation functions and the relevant Fourier transforms of EXAFS and EELFS spectra. 

In order to obtain more structural information about the molybdenum species in Mo/NaY, EXAFS measurements of the cluster 1 and Mo/NaY were carried out. The Fourier transforms of the EXAFS data are shown in Figure 2. Structural parameters (Table 3) showed no change of the Mo-0, Mo-S and Mo-Mo distances, suggesting that there is no significant structural difference between the cluster 1 and the molybdenum compound in the Mo/NaY. From these EXAFS parameters and the UV-visible spectra, it is considered the structure of cluster 1 remained vinually intact after ion exchange. 

Figure 7. FTIR spectra of NO adsorption on Figure 8. Fourier transforms of k -weighted CoSx/NaY (2. ICo/SC) and CoSx-MoSx/NaY EXAFS modulations of the Mo K-edge for (2.1MO-I- 2.IC0/SC). MoSx/NaY and CoSx-MoSx/NaY.

Figure 6.10 shows Fourier transforms of Co K-edge EXAFS spectra of these calcined catalysts and polycrystalline Co oxides. Three peaks are clearly observed in EXAFS spectra of calcined Co(X)/Si02 catalysts at almost the same... 

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 6.16 Fourier transforms of Ru and Cu K-edge EXAFS spectra of Ru/Si02, Cu/Si02 and Ru-Cu/Si02 catalysts before and after exposure to oxygen at room temperature. The data show that almost all Cu in the bimetallic Ru-Cu catalyst is oxidized, while Ru is hardly affected. The monometallic Ru and Cu catalysts are oxidized to a limited extent only (from Sinfelt etal. [39]).

Figure 9.9 EXAFS spectra and Fourier transforms of a highly dispersed Rh/AFCb catalyst after reduction (left) and after exposure to CO at room temperature (right , Courtesy of H.F.J. van t Blik, Eindhoven).

Fig. 6a-c. One monolayer of cobalt on copper (111), grazing incidence, T = 77 K and T = 300 K relevant steps of the EXAFS analysis, a Experimental absorption spectra b fourier transform of the EXAFS oscillations c inverse Fourier transform of the first neighbour peak as a function of photoelectron kinetic energy E... 

Figure 2.15 Nd L, EXAFS of NdFj-BeFj glass (top) and crystalline NdFj (bottom), (a) Normalized EXAFS spectra (b) Fourier transform of (a) and (c) inverse transform (line) and simulated EXAFS (points) in the region 0.2 to 3.2 A. (After Rao, K.J. et al, 1983.)...

Fig. 15. (a) Fourier transforms of the in situ EXAFS spectra above the Cu A -edge for a... 

Fig. 19. Combined QEXAFS and temperature-programmed sulfiding results of a Mo/ A1203 catalyst during sulfiding in a H2S/Ar gas mixture (a) Fourier transforms of the in situ EXAFS spectra above the Mo X-edge (b) variation in the H2S concentration in the gas outlet from the in situ EXAFS cell as simultaneously recorded by a mass spectrometer (61).

Figure 1. The Fourier transforms of -weighted K-edge EXAFS spectra for Ni of Ni-481, Co of Co-380, and Zn of Zn-153.

Fig. 9.5 EXAFS of Rh/Al203 catalysts after reduction at (left) 200 °C and (right) 400 °C. Top spectra the magnitude of the Fourier transform of the measured EXAFS signal. Bottom spectra back-transformed EXAFS corresponding to distances between 0.8 and 3.2 from Rh atoms. The lower Fourier...

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