Data reduction, EXAFS

EXAFS data reduction and fitting were carried out using the WinXAS,18 Atoms,19 FEFF,20 and FEFFIT20 programs. The k- and r-ranges were chosen to be 3-15 A-1 and 1.0-3.0 A, respectively. XANES spectra were compared qualitatively after normalization. 

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.)...

Data reduction of EXAFS spectra was performed using WinXAS [14], The normalized spectra were analyzed over the Arrange of 2.5 to 10 A1. A square-weighted degree 7 spline was used to remove the background of the x(k) function. Finally, the data in -space were converted to R-space using a Bessel window to obtain the radial distribution function. 

Figure 9.5 shows EXAFS data reported by Koningsberger et al. [19] of a highly dispersed (H/M=1.2) Rh/Al203 catalyst under hydrogen after reduction at 200 and... 

In order to obtain data with reduced temperature smearing, experiments were also carried out at 77 K. However, such experiments could not be carried out in. situ and the catalysts were thus exposed to air before the measurements. EXAFS data of three catalysts with Co/Mo atomic ratios of 0.0., 0.25, and 0.50 were obtained. The results show many similarities with the data recorded in situ and were fitted in a similar fashion using phase and amplitude functions of the well-crystallized model compound M0S2 recorded at 77 K. The results, which are given in Table III, show that the bond lengths for the first and second coordination shell are the same for all the catalysts and identical to the values obtained for the catalyst recorded in situ (Table II). The coordination numbers for both shells appear, however, to be somewhat smaller. Although coordination numbers determined by EXAFS cannot be expected to be determined with an accuracy better than + 20, the observed reduction... 

Pt/Ru bimetallic nanoparticles were prepared by coreduction of the corresponding metal salts in the presence of glucose (26). EXAFS data indicated the existence of a Pt-Ru bond in this case, proving the formation of bimetallic nanoparticles. Pd/Rh bimetallic nanoparticles, stabilized by PVP, were also prepared by the alcohol reduction method (27). 

As with xanthine oxidase, the sulfido ligand of the active form of aldehyde oxidoreductase is readily replaced by an oxido ligand to yield a cofactor with a structure that resembles that of oxidized sulfite oxidase and assimilatoiy nitrate reductase. Both x-ray and EXAFS data are available for the bis(oxido) form, and, with the exception of the oxido replaced sulfido ligand, few changes are obvious in the overall structure of the oxidized form of the desulfo cofactor. Upon reduction of the enzyme the oxido ligand is presumably reduced to hydroxido, an observation that is supported by EPR data for the Mov state. 

In Figure 2a the raw EXAFS data is shown after reduction (RED400, solid line), after evacuation at 50°C (VAC50, dotted line) and after evacuation at 200°C (VAC200, dashed-dotted line). It is clearly shown that the evacuation treatment changed the nodes of the oscillations. The distance between the nodes increases in the order... 

In Figure 6A, the raw EXAFS data for the Pt/K-Al203 catalyst after reduction, evacuation at 323 K and 473 K are shown. The quality of the data is representative for all spectra. The evacuation treatment clearly changes the nodes of the oscillations. The distance between the nodes is smallest after reduction at 673 K and increases when the catalyst is evacuated at a... 

The Fourier transforms of the EXAFS data for the hydroquinone-reduced OEC derivative is also shown in Figure 10 (65). Hydroquinone reduction leads to dramatic changes in the EXAFS. The nearest neighbor (1.9 A Mn-O/N) peak decreases in amplitude, and a new shell of low-Z ligands appears at ca. 2.2 A. The peak at 2.2 A most likely represents the longer Mn-O bonds associated with the Mn(II). The peak due to 2.7-A Mn-Mn distances decreases to V2 to % of its original amplitude, and the 3.3-A feature is no longer discernible above the noise level. 

Cytochrome c Oxidase. This is a ubiquitous enzyme that catalyzes the terminal step in the four-electron reduction of 02 to 2H20. It contains both heme iron and copper and there are two kinds of copper sites, CuA and CuB, in each of which there is an association of copper with heme iron. The CuB site seems to have one copper atom linked to a heme unit, but details are still obscure.37 From modeling studies and EXAFS data, opinion presently favors some sort of Fe—X—Cu bridge... 

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