EXAFS analysis

Fig. 3. EXAFS analysis results for A) oxidic precursors before reduction, B) fresh samples after reduction, and C), D) bulk reference samples.

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. 

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. 

EXAFS analysis is a powerful spectroscopic method for structural analysis which has been extensively applied to the problem of structure determination in nanoparticles, and especially bimetallic nanoparticles [170-172]. The X-ray absorption spectrum of an element contains absorption edges corresponding to the excitation of electrons from various electronic states at energies characteristic of that element, i.e., K edges arise from the excitation of electrons from Is states, and LI, II, III edges from excitations from 2s, 2p 1/2, and 2p3/2 states. When the X-ray energy is increased above an edge, oscillations (fine... 

In order to clarify the mechanism responsible of the energy transfer, we investigated the aggregation state of Au atoms and the chemical environment of Er ions in our samples, by performing an EXAFS analysis on the Er + Au 600 °C sample. The experiment evidenced an Au-Au coordination proving the presence of Au metallic aggregates, with an estimated Au-Au distance i = 2.78+ 0.01 A, consistent with the presence of small... 

The high activity of the Rh/y-Al203 system even at such a low Rh loading (0.1% w/w) can be rationalized on the basis that, as indicated by IR studies of adsorbed CO and EXAFS analysis, all the Rh atoms are zerovalent coordination by the NR3 molecules protects them from oxidation by the solid support [24,35]. 

The introduction of redox activity through a Co11 center in place of redox-inactive Zn11 can be revealing. Carboxypeptidase B (another Zn enzyme) and its Co-substituted derivative were oxidized by the active-site-selective m-chloroperbenzoic acid.1209 In the Co-substituted oxidized (Co111) enzyme there was a decrease in both the peptidase and the esterase activities, whereas in the zinc enzyme only the peptidase activity decreased. Oxidation of the native enzyme resulted in modification of a methionine residue instead. These studies indicate that the two metal ions impose different structural and functional properties on the active site, leading to differing reactivities of specific amino acid residues. Replacement of zinc(II) in the methyltransferase enzyme MT2-A by cobalt(II) yields an enzyme with enhanced activity, where spectroscopy also indicates coordination by two thiolates and two histidines, supported by EXAFS analysis of the zinc coordination sphere.1210... 

Ravel, B. 2001. EXAFS analysis using FEFF and FEFFIT workshop, June 27. 

Thus, each coordination shell contributes a sine function multiplied by an amplitude, as illustrated in Fig. 6.12 for the simple case of a dimer. EXAFS analysis boils down... 

X-ray imaging, selenium in, 22 101. See also Extended X-ray absorption fine structure (EXAFS) analysis X-ray imaging techniques, 16 504 X-ray imaging tests, 26 440 X-ray-induced X-ray emission, 24 109 X-ray instruments, 26 411... 

Duff, D.G. et al., Structural characterization of colloidal platinum by high resolution electron microscopy and EXAFS analysis, Angew. Chem. 101, 610, 1989 Angew. Chem. Int. Ed. Engl., 28, 590,1989. 

Although hints of an Fe atom at a longer distance from the Ni site were suggested by some early EXAFS analysis, the Fe centre was not incorporated into EXAFS fits until it was revealed by crystallography. The reason for this is that for oxidized enzymes with an Ni-Fe distance of ca. 2.9 A, the scattering due to Fe is a very small component of the overall EXAFS and inclusion of the Fe in the fits does not improve the goodness of the fits. However, in most reduced enzymes the Ni-Fe distance is shortened to 2.5-2.6 A, and the inclusion of the Fe is necessary in order to get an optimal fit (Gu et al. 1996 Davidson et al. 2000). 

The EXAFS analysis is in good agreement with the proposed formula on MCM(5oo) of (=SiO)i 65Ti(Np)2.35, with a mean number of 1.7 oxygen atoms at 1.81 A... 

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