Nickel EXAFS

Cd, and others. For example, nickel EXAFS of carbon monoxide dehydrogenase (CODH) from Clostridium thermoaceticum strain DSM 521 were collected and the Fourier filtered data was best fit to sulfur at 0.216 nm. A reasonable fit to oxygen/nitrogen could only be obtained by setting the zIEq to 40 and the Debye-Waller term to 5xl0 nm. A reasonable Ni-S fit was also obtained by assuming two Ni-S bond lengths at 0.222 and 0.211 nm, respectively. Addition of a Ni-M (M=Fe, Ni, Zn) term at 0.325 nm, also improved the... 

It is known that thin (-20 A) passive films form on iron, nickel, chromium, and other metals. In s ressive environments, these films provide excellent corrosion protection to the underlying metal. The structure and composition of passive films on iron have been investigated through iron K-edge EXAFS obtained under a variety of conditions, yet there is still some controversy about the exact nature of... 

Results from an array of methods, including X-ray absorption, EXAFS, esr and magnetic circular dichroism, suggest that in all ureases the active sites are a pair of Ni" atoms. In at least one urease,these are 350 pm apart and are bridged by a carboxylate group. One nickel is attached to 2 N atoms with a fourth site probably used for binding to urea. The second nickel has a trigonal bipyramidal coordination sphere. 

The discovery of a new heterodinuclear active site in [NiFe] hydro-genases opens the way for the proposal of catalytic cycles based on the available spectroscopic data on the different active site redox states, namely EXAFS studies that reveal that the Ni-edge energy upon reduction of the enzyme supports an increase in the charge density of the nickel (191). 

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. 

Ni3(2,2,2-tet)3(/u-N3)3](PF6)3 and its perchlorate analogue have been suggested as trinuclear complexes with an irregular triangle structure.2117 While a crystal structure is lacking, EXAFS and XANES spectra support the occurrence of azido-bridged trinuclear nickel(II) compounds with Ni- -Ni separations of 5.16 A and 5.12 A, respectively. Two J parameters had to be... 

Froment and co-workers have employed reflexafs77 (reflection EXAFS) for studying passive films on iron78 and nickel.79 80 The experiment consists of measuring the ratio of the reflected and incident intensities as a function of energy. Although an EXAFS spectrum can be obtained from such a measurement, the process is somewhat involved since the reflectivity is a complex function... 

A part of Figure 3 in Ref. 207, reproduced on the right, reports radial EXAFS data around the S Is absorption edge for sulfur adsorbed on the (100) plane of a g nickel single-crystal surface. The top trace corresponds to the deposition of atomic S sulfur by dehydrogenation of H2S, while g, the bottom data were obtained by adsorb- M ing thiophene on the clean surface at 100 K. Based on these data, what can be learned about the adsorption geometry of thiophene Propose a local structure for the sulfur atoms in reference to the neighboring nickel surface. 

Semiconducting black polymers characterized as 44 have been prepared via slow heating of triple-decker (allyl)nickel diborolyl triple-decker complexes.44 EXAFS studies reveal an Ni-Ni distance of 3.35 A and a powder conductivity of ca. 10-2 S cm-1. When doped with iodine or oxygen, the conductivity of these polymers... 

Figure 13 shows the calibration curve of phase-lag for the nickel foils of various thickness. This result suggests the possibility of estimating the thickness of plated materials on the metal. Figure 14 shows the PAXAS spectrum of nickel-plated copper. In the photoacoustic amplitude spectrum, EXAFS of copper at the subsurface of sample was still detected clearly. This means that PAXAS method can be applied... 

As noted earlier, metal ions in polar solvents will form complexes with the solvent molecules. X-Ray diffraction, EXAFS, and visible absorption spectroscopy show that nickel(II) ion in dilute aqueous solution is present as the green hexaaqua complex Ni(H20)62+, just as in solids such as NiS04-7H20, which is actually [Ni(H20)e]S04-H20. In the crystal, the extra water molecule is loosely associated with the sulfate ion independently of the nickel-aqua complex it is sometimes referred to as lattice water, as distinct from complexed water. 

EXAFS has provided detailed information about the local environment of the active Co and Ni sites and the Mo atoms to which they are attached in terms of the types of atoms within two atomic shells away from the atom being characterized. Cobalt and nickel were shown to be definitely bonded to the surfaces of small MoS2 crystallites. Representative structures for the environments of Mo and Co are illustrated in the following diagram. In such structures, Mo has a coordination number (CN) of 6, with six nearneighbor sulfur atoms, three nearby Mo atoms, and one nearby Co or Ni atom. Co-S configurations were either CN = 5 (square pyramidal) or CN = 6 (octahedral), with either one nearby Mo atom (low HDS activity) or two nearby Mo atoms (high HDS activity) (62). 

Figure 1 shows Fourier transforms of EXAFS spectra of a few samples prepared. The radial distribution functions of these samples are different from that of nickel oxide or cobalt oxide [7]. All the Fourier transforms showed two peaks at similar distances (phase uncorrected) the peak between 1 and 2 A is ascribed to the M-0 bond (M divalent cation) and the peak between 2 and 3 A is ascribed to the M-O-M and M-O-Si bonds. The similar radial distribution functions in Figure 1 indicate that the local structures of X-ray absorbing atoms (Ni, Co, and Zn) are similar. No other bonds derived from metal oxides (nickel, cobalt and zinc oxides) were observed in the EXAFS Fourier transforms of the samples calcined at 873 K, which suggests that the divalent cations are incorporated in the octahedral lattice. 

Table 1 shows the properties of smectite-type materials prepared. Smectite materials prepared at lower pH had fewer sodium ions, higher surface areas, and larger pore volumes for a series of samples containing the same divalent cation species (nickel and cobalt) in the octahedral sheet. The adsorption of methylene blue on all the synthetic smectites shows that the smectite fragments are negatively charged. The Si M ratios of synthetic smectites were about 8 6, indicating that most of divalent cations exist in octahedral layers and small amount of divalent cations would exist as hydroxide or oxide cluster in smectite materials. However, the amounts of the hydroxide or oxide cluster were small, because only smectite structures were observed in XRD patterns and EXAFS Fourier transforms of synthetic smectites calcined at 873 K. 

Fig. 2. Ni K-edge EXAFS spectrum of urease. The curve (+) is calculated for a single type of nickel site, and the minimization of parameters was based on those for the model complexes Ni(l-n-propyl-2-hydroxybenzylbenzimidazole)3(Cl04) and Ni(2-hydroxy-methylbenzimidazole)3Br2. Atoms (with distances in nanometers given in parentheses) in the simulation were N (0.204), O (0.206), O (0.225), C (0.294), C (0.312), N (0.392), and C (0.394). Reproduced, with permission, from Ref. 34.

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