EXAFS structural parameters from

Table 9.3 Structural parameters from EXAFS of a carbon-supported Co-Mo-S phase [67,76].

Refined structural parameters from Cr K-edge EXAFS data for the 70Sn 10Cr 20Cu catalyst material calcined at 300°C (R = 61). ... 

Distances are usually the most reliably determined structural parameters from EXAFS. However, the range of data that can be collected, often times because of practical reasons like the presence of the K-edge of another metal, limits the resolution of distance determinations to between... 

EXAFS data closely resemble the crystal structure parameters from oxo-bridged Mn(iil) and (IV) complexes. Examination of a series of multinuclear Mn complexes reveals that the Mn-Mn distances range between 2.6 - 2.8 A in complexes where two Mn atoms are bridged by at least... 

This discussion of EXAFS on ruthenium-copper clusters has emphasized qualitative aspects of the data analysis. A quantitative data analysis, yielding information on the various structural parameters of interest, has also been made and published (8). Of particular Interest was the finding that the average compo tion of the first coordination shell of ruthenium and copper atoms about a ruthenium atom was about 90% ruthenium, while that about a copper atom was about 50% ruthenium. Details of the methods Involved in the quantitative analysis of EXAFS data on bimetallic clusters can be obtained from our original papers (8.12-15). 

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. 

Table 1 Structural Parameter as Derived from the Mo K-Edge EXAFS for Mo and Co-Mo Sulfide Catalysts Encaged in a NaY Zeolite...

The IR spectra in Fig.7 indicate the preferential adsorption of NO on the Co sites. It may be conjectured that the Mo sulfide species are physically covered by the Co sulfide species or that Co-Mo mixed sulfide species are formed and the chemical natures of the Co and Mo sulfides are mutually modified. The Mo K-edge EXAFS spectra were measured to examine the formation of mixed sulfide species between Co and Mo sulfides. The Fourier transforms are presented in Fig.8 for MoSx/NaY and CoSx-MoSx/NaY. The structural parameters derived from EXAFS analysis are summarized in Table 1. The structure and dispersion of the Mo sulfides in MoSx/NaY are discussed above. With the Co-Mo binary sulfide catalyst, the Mo-Co bondings are clearly observed at 0.283 nm in addition to the Mo-S and Mo-Mo bondings. The Mo-Co distance is close to that reported by Bouwens et al. [7] for a CoMoS phase supported on activated carbon. Detailed analysis of the EXAFS results for CoSx-MoSx/NaY will be presented elsewhere. It is concluded that the Co-Mo mixed sulfides possessing Co-S-Mo chemical bondings are formed in CoSx-MoSx/NaY. 

Table 2. Structural Parameters Obtained from Best Least-Squares Fits of the EXAFS for the 20 wt % Pt/C Catalyst Electrode for Various Potentials,...

Table 1. Structural parameters deduced from EXAFS for a number of metallothioneins . R denotes the distance of scattering atoms from the absorbing atom is a Debye-Waller factor and...

A back Fourier transform on individual peak can be then applied to isolate individual contributions. The so-obtained filtered EXAFS signal is analyzed in order to determine the related structural parameters N, R and a. A common method consists to build a theoretical model from the relation (2) using known electronic parameters for X(k), So2, Aj(ir,k) and Oy (k), and, by treating N, R, and a as free parameters, to minimize the difference between the theoretical and experimental curves. The electronic parameters can be evaluated from EXAFS signals recorded for references of known structures or from theoretical calculations using the efficient ab initio FeFF codes [7, 8],... 

The accuracy of structural parameters extracted from EXAFS depends on many factors such as the disorder, the extension of the experimental spectra and quality of these data. Typical accuracies for the determination of parameters are 1% for interatomic distances, 15% for the coordination numbers and 20% for Debye-Waller factors [6], But, in a situation more complex where several shells must be fitted simultaneously, one must be satisfied with much less accurate results. 

Comparison of Structural Parameters Determined by Density Functional Theory and by EXAFS Spectroscopy Characterizing Samples Made from Chemisorption of [Rh(CO)2(acac)] on Dealuminated Y Zeolite (Goellner, Gates et al., 2000) ... 

The reflectivity spectra R(E) and the reflectivity-EXAFS Xr(E) = R(E) — Rq(E)]/R()(E) are similar, but not identical, to the absorption spectra and x(E) obtained in transmission mode. R(E) is related to the complex refraction index n(E) = 1 — 8(E) — ifl(E) and P(E) to the absorption coefficient /i(E) by ji fil/An. P and 8 are related to each other by a Kramers-Kronig transformation, p and 8 may be also separated in an oscillatory (A/ , AS) and non-oscillatory part (P0,80) and may be used to calculate Xr- This is, briefly, how the reflectivity EXAFS may be calculated from n(E). which itself can be obtained by experimental transmission EXAFS of standards, or by calculation with the help of commercial programs such as FEFF [109] with the parameters Rj, Nj and a, which characterize the near range order. The fit of the simulated to measured reflectivity yields then a set of appropriate structure parameters. This method of data evaluation has been developed and has been applied to a few oxide covered metal electrodes [110, 111], Fig. 48 depicts a condensed scheme of the necessary procedures for data evaluation. 

Fig. 48. Scheme for the calculation of the X-ray reflectivity from experimental transmission EXAFS of standards or calculation with a first set of structure parameters (Rj, Nj, [Pg.347]

Table 6.2. Structural parameters for Zr—O bonding in Cc Zri. Oj solid solutions as detected from EXAFS measurements.

Structural Parameters Deduced from EXAFS OF Metallothioneins"... 

Table FV. Structure parameters obtained from EXAFS analysis of [Ni (CgO)4dpg 2] at different temperatures. ( ) means the atoms of neighboring stacking molecules.

Table 14.2 Structural parameters of 0.33ThF4 0.6HfF4 0.07LaF3 glass from EXAFS analysis of Lm edge.

The approach adopted amounts to a trial and error procedure in which a series of values is chosen for OsCu and CuOs subject to the constraint of Eq. 4.12. For each set of trial phase shift functions, Eqs. 4.10 and 4.11 for the function Xi(XT, incorporating expressions of the form of Eq. 4.9 for the various x/MO terms, are fit to the corresponding functions derived from the osmium and copper EXAFS data on the osmium-copper catalyst. The fitting exercise yields values of various structural parameters, including the distance between an osmium atom and a copper atom (nearest neighbor atoms). For a given set of phase shift functions for OsCu and CuOs, limited only by the constraint of Eq. 4.12, this distance as derived from the osmium EXAFS will not in general be equal to the distance derived from the copper EXAFS. 

In Figures 4.16 and 4.17 the uppermost fields (labeled a) illustrate the quality of fit of values of the function KnX](K), represented by the points, to the corresponding function (solid line) derived from the EXAFS data (32). The points were calculated for values of structural parameters corresponding to Af o = —4 eV in Figure 4.15. For the osmium EXAFS in Figure 4.16 the function fitted was K2x K), while for the copper EXAFS in Figure 4.17 it was K3x U0- The fits are excellent except at very low K values. The fits can be improved at the very low K values by modification of the details of the phase shift functions, but there is very little effect of such a modification on the values of the structural parameters obtained. 

Table 5. MnPSg and intercalated derivatives structural parameters extracted from EXAFS spectra. The numbers of neighbours are fixed...

Table 9. Structural parameters extracted from EXAFS spectra. In Mn,.,Ni PS3 at nickel edge, two different compositions of the second shell were tested 3 (P—P) and 3 Mn (a), 3 (P—P) and 3 Ni (b). Fit (b) represents the case of a mechnical MnPSs—NiPSj mixture. Since the fit yields a similar residue, the actual environment of Ni in MHj Ni PSj cannot be solved by EXAFS alone, but the mechanical mixture possibility has been ruled out by XRD.

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