Molybdenum EXAFS

Both assimilatory and dissimilatory nitrate reductases are molybdoenzymes, which bind nitrate at the molybdenum. EXAFS studies1050 have shown that there are structural differences between the assimilatory nitrate reductase from Chlorella vulgaris and the dissimilatory enzyme from E. coli. The Chlorella enzyme strongly resembles sulfite oxidase1050,1053 and shuttles between mon-and di-oxo forms, suggesting an oxo-transfer mechanism for reduction of nitrate. This does not appear to be the case for the E. coli enzyme, for which an oxo-transfer mechanism seems to be unlikely. The E. coli enzyme probably involves an electron transfer and protonation mechanism for the reduction of nitrate.1056 It is noteworthy that the EXAFS study on the E. coli nitrate reductase showed a long-distance interaction with what could be an electron-transfer subunit. 

Figure 2 Molybdenum K-edge X-ray absorption spectrum, ln(i /i ) versus X-ray energy (eV), for molybdenum metal foil (25- jjn thick), obtained by transmission at 77 K with synchrotron radiation. The energy-dependent constructive and destructive interference of outgoing and backscattered photoelectrons at molybdenum produces the EXAFS peaks and valleys, respectively. The preedge and edge structures marked here are known together as X-ray absorption near edge structure, XANES and EXAFS are provided in a new compilation of literature entitled X-rsy Absorption Fine Structure (S.S. Hasain, ed.) Ellis Norwood, New York, 1991.

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.

The Fourier transform of the EXAFS of Figure 5 is shown in Figure 6 as the solid curve It has two large peaks at 2.38 and 2.78 A as well as two small ones at 4.04 and 4.77 A. In this example, each peak is due to Mo—Mo backscattering. The peak positions are in excellent correspondence with the crystallographically determined radial distribution for molybdenum metal foil (bcc)— with Mo—Mo interatomic distances of2.725, 3.147, 4.450, and 5.218 A, respectively. The Fourier transform peaks are phase shifted by -0.39 A from the true distances. 

Molybdenum enzymes a survey of structural information from EXAFS and EPR spectroscopy. S. P. Cramer, Adv. Inorg. Bioinorg. Mech., 1983, 2, 260 (137). 

MgO-supported model Mo—Pd catalysts have been prepared from the bimetallic cluster [Mo2Pd2 /z3-CO)2(/r-CO)4(PPh3)2() -C2H )2 (Fig. 70) and monometallic precursors. Each supported sample was treated in H2 at various temperatures to form metallic palladium, and characterized by chemisorption of H2, CO, and O2, transmission electron microscopy, TPD of adsorbed CO, and EXAFS. The data showed that the presence of molybdenum in the bimetallic precursor helped to maintain the palladium in a highly dispersed form. In contrast, the sample prepared from the monometallie precursors was characterized by larger palladium particles and by weaker Mo—Pd interactions. ... 

A cationic molybdenum sulfide cluster [Mo3S4(H20)9] " with incomplete cubane-type structure and a cationic nickel-molybdenum mixed sulfide cluster [Mo3NiS4Cl(H20)9p " with complete cubane-type structure were introduced into zeolites NaY, HUSY and KL by ion exchange. Stoichiometry of the ion exchange was well established by elemental analyses. The UV-visible spectra and EXAFS analysis data exhibited that the structure of the molybdenum cluster remained virtually intact after ion exchange. MoNi/NaY catalyst prepared using the molybdenum-nickel sulfide cluster was found to be active and selective for benzothiophene hydrodesulfurization. 

This paper describes the successful incorporation of molybdenum and molybdenum-nickel clusters into zeolites with 12-membered ring by aqueous ion exchange and application of the resulting materials to HDS reaction of benzothiophene. Stoichiometry of the ion exchange was examined by elemental analysis. UV-visible spectroscopy and EXAFS measurements were carried out to investigate the structure of molybdenum species loaded on zeolites. 

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. 

In order to draw a crystallographic picture of the Co-Mo-S phase, we need precise data on the location of the cobalt atom with respect to the molybdenum and the sulfur atoms. For this, EXAFS is the indicated technique. 

Several groups [64-67,76] have reported EXAFS studies on sulfided cobalt-molybdenum catalysts. Figure 9.22 shows the Fourier transforms of M0S2 and of sulfided molybdenum and cobalt-molybdenum catalysts supported on carbon,... 

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]).

The Co/Mo = 0.125 catalyst has all the cobalt atoms present as Co-Mo-S and, therefore, the EXAFS studies of this catalyst can give information about the molybdenum atoms in the Co-Mo-S structure. The Fourier transform (Figure 2c) of the Mo EXAFS of the above catalyst shows the presence of two distinct backscatterer peaks. A fit of the Fourier filtered EXAFS data using the phase and amplitude functions obtained for well-crystallized MoS2 shows (Table II) that the Mo-S and Mo-Mo bond lengths in the catalyst are identical (within 0.01 A) to those present in MoS2 (R =... 

These examples would seem to indicate that the molybdenum atom, that for a long time was considered to be the specific site of dinitrogen coordination, is of little importance. It should be borne in mind, however, that the X-ray structure of the protein was obtained in the resting state. As noted, under such conditions, the Mo atom is assigned oxidation state IV and has a saturated coordination, hence not able to further coordinate nitrogen. EXAFS studies on the active protein indicate a Mo coordination different to that determined by X-ray diffraction. One hypothesis considers the dissociation of the homocitrate, induced by addition of electrons, that would leave vacant coordination sites which could then be saturated by the nitrogen molecule. 

Helz GR, Miller CV, Chamock JM, Mosselmans JEW, Pattrick RAD, Gamer CD, Vaughn DJ (1996) Mechanism of molybdenum removal from the sea and its concentration in black shales EXAFS evidence. Geochim Cosmochim Acta 60 3631-3642 Hille R (1996) The mononuclear molybdenum enzymes. Chem Rev 96 2757-2816... 

Detailed studies were conducted by infrared, TPD, XPS but also by more sophisticated techniques such as CP-MAS solid state NMR or EXAFS, on the various steps by which molybdenum can be deposited on alumina supports starting from [Mo(CO)e]. Indeed, thin films of molybdenum or of its oxides have wide application as gas sensors or solar cell catalysts. 

In a similar vein, it was shown that molybdenum amido derivatives of the isolobal terminal phosphide and carbide, " and related chalcogenide atoms,could be obtained. Detailed mechanistic studies of the Mo N(R)Ar 3/N2 system involving X-ray, EXAFS, magnetic, Raman, and isotopicaUy labelled NMR spectroscopy showed that the reaction proceeded through an intermediate involving an end-on bound N2 bridging the molybdenum centres (Scheme... 

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