Mo K-edge EXAFS

Figures Background-subtracted, normalized, and ili -weighted Mo K-edge EXAFS, versus k (A ), for molybdenum metal foil obtained from the primary experimental data of Figure 2 with Eq = 20,025 eV.

Figures Fourier transform (soiid curve), Osir ) versus r (A, without phase-shift correction), of the Mo K-edge EXAFS of Figure 5 for moiybdenum metal foii. The Fourier filtering window (dashed curve) is applied over the region -1.5-4.0 A to isolate the two nearest Mo-Mo peaks.

The Mo K-edge EXAFS spectra for the catalysts and reference compounds (MoSj and NajMoOJ were measured on the BL-lOB instruments of the Photon Factory at the National Laboratory for High Energy Physics by using a synchrotron radiation. The EXAFS spectra were obtained at room temperature without exposing the sample to air by using an in situ EXAFS cell with Kapton windows [12]. Data analysis was earned out assuming a plane wave approximation. 

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. 

Figure 9.22 Left Mo K-edge EXAFS Fourier transforms of MoS2 and sulfided, carbon supported Mo and Co-Mo catalysts, showing the reduced S and Mo coordination in the first shells around molybdenum in the catalyst (from Bouwens et at. [68]). Right Co K-edge Fourier transforms of the same catalysts and of a Co9S8 reference. Note the presence of a contribution from Mo neighbors in the Fourier transform of the Co-Mo-S phase (from Bouwens et at. [76]).

Extended X-ray absorption fine structure (EXAFS) studies on the Fe/Mo/S aggregate in nitrogenase have made available structural data that are essential in the design of synthetic analog clusters. Analyses of the Mo K-edge EXAFS of both the Fe-Mo protein and the FeMoco (9) have shown as major features 3-4 sulfur atoms in the first coordination sphere at 2.35 A and 2-3 iron atoms further out from the Mo atom at 2.7 A. The Fe EXAFS of the FeMoco (10,11) shows the average iron environment to consist of 3.4 1.6 S(C1) atoms at 2.25(2) A, 2.3 +0.9 Fe atoms at 2.66(3) A, 0.4 0.1 Mo atoms at 2.76(3) A and 1.2 1.0 0(N) atoms at 1.81(7) A. In the most recent Fe EXAFS study of the FeMoco (11) a second shell of Fe atoms was observed at a distance of 3.75 A. 

Fig. 10.18. Magnitude of the Fourier Transform of the Mo K edge EXAFS spectrum of carbon-supported, sulphided Co-Mo and Mo catalysts, along with EXAFS parameters (from...

This chemistry may be relevant to the nature and function of the molybdenum center of E. coli formate dehydrogenase. The Mo K-edge EXAFS of both the oxidized and reduced form of this enzyme were found to be very similar, each inolving a des-oxo-molybdenum site with four Mo S bonds at 2.35 A, (probably) one Mo O bond at 2.1 A, and one Mo—Se interaction at 2.62 A. The Se K-edge EXAFS showed clear evidence for a S Se contact of 2.19 A, presumably indicative of a novel seleno-sulfide ligand to the molybdenum (121). 

Early Mo K-edge EXAFS data for sulfite oxidase showed that the oxidized resting state contains a [Mo 02] unit, whereas the fully reduced enzyme possesses a [Mo O] unit (8). Both units are ligated by two or three sulfur atoms and possibly additional N or O ligands. Two of the coordinated sulfur atoms are presumably provided by molyb-dopterin, as shown in 2. However, similar EXAFS results would be expected if the molybdenum atom were bound to thiolate groups of the protein itself... 

Recently, Mo K-edge EXAFS data have been obtained at low pH/high Cl and high pH/low Cl for each of the three oxidation states of the molybdenum center of sulfite oxidase (69) by poising the potential with redox dyes (78). This latter EXAFS study provides strong evidence that one chloride ion binds to the Mo(IV) and Mo(V) states of the enzyme at low pH/high Cl", but that Cl" does not appear to bind to the Mo(VI) state of the enzyme. Combination of the EXAFS and EPR data for sulfite oxidase yields the minimal structures for the molybdenum center shown in Fig. 6. 

Further support for an Mo=0 group in the reduced states of xanthine oxidase was provided by Mo K-edge EXAFS studies of the Mo(V) state of xanthine oxidase inhibited with pyridine-3-carboxaldehyde and of the Mo(IV) state of alloxanthine-inhibited enzyme (102). Both of these inhibited species showed clear evidence for an Mo=0 group (1.70 A), but neither exhibited an Mo=S distance of 2.15 A, as is observed for the oxidized state of functional xanthine oxidase (8,68,100-102). Comparison of the EPR parameters for model compounds containing the [Mo =0] and [Mo =S] groups also favors the presence of an Mo=0 group in the Mo(V) states of xanthine oxidase (Section IV.C.2) (110). Section VI presents a molecular mechanism for xanthine oxidase that combines the wealth of EPR and EXAFS data available for the various forms of the enzyme (17,64,107-109) with recent developments in model molybdenum chemistry (107-109, 111-113). 

Figure 2 Fourier Transforms of Fe K-edge Figure 3 Fourier Transform of As, Cr, Se EXAFS for Fe/en-MCM-41 and anion and Mo K-edge EXAFS for anion coordinated Fe/en-MCM-41. coordinated Fe/en-MCM-41.

Table 3 Curve-fitting Results of As, Cr, Se and Mo K-edge EXAFS ...

Figure 10. Fourier transforms (k X(k)) of the Mo K-edge EXAFS for MoSx/NaY and MoS2/NaY.

Fig. 2. (a) Mo K-edge EXAFS of AlMoe and CoMoa- (b) Corresponding Fourier transforms. 

One of the early triumphs of biological x-ray absorption spectroscopy was the deduction that the nitrogenase M center is an Mo-Fe-S cluster. (It is also worth noting that nitrogenase was the first enzyme to be studied by x-ray absorption spectroscopy.) Early work on lyophilized protein samples indicated the presence of two major contributions to the Mo K-edge EXAFS, which were attributed to Mo-S ligands, plus a more distant Mo-Fe contribution. Subsequently, these conclusions have been confirmed and extended, using samples in solution and with much more sensitive detection systems. 

Largely on the basis of the Mo K-edge EXAFS results and model studies discussed below, several proposals for the structure of the M center have been put forward. These are illustrated in Figure 7.31. 

Mo K-edge EXAFS spectrum (left panel) and EXAFS Fourier transform (right panel) of Klebsiella pneumoniae nitrogenase MoFe protein. The solid line is the processed experimental spectrum and the dashed line a calculated one. ... 

It has been shown through a Raman, and Mo K-edge EXAFS spectroscopic study, that upon impregnation of alumina with AHM solution a decomposition occurs with extraction of aluminium atoms of the support... 

Fig. 5. (A) Fourier transform of Mo K-edge EXAFS spectrum of the Mo-Fe protein from nitrogenase [Adapted from S. P. Cramer, in X-Ray Absorption Principles, Applications, Techniques of EXAFS, SEXAFS and XANES (D. C. Koningsberger and R. Prins, eds.), p. 257. Wiley, New York, 1988]. The Fourier transform shows two peaks indicating that at least two major components, Mo-S (and Mo-O/N) and Mo-Fe, are required to explain the Mo EXAFS spectrum from nitrogenase. (B) Model for the FeMo-co proposed based on the Mo EXAFS data and comparison with data from model compounds [T. E. Wolff, J. M. Berg, C. Warrick, K. O. Hodgson, R. H. Holm, and R. B. Frankel, J. Am. Chem. Soc. 100, 4630 (1978)]. (C) Structure of the FeMo-co based on a more recent X-ray crystal structure [J Kim and D. C. Rees, Science 257, 1677 (1992)]. The similarities between the structures in (B) and (C) are remarkable.

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