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At least 487 records · Page 27Linked to original sources
PubMed2001

K- and L-edge XAFS determination of the local structure of aqueous Nd(III) and Eu(III).

The short-range coordination of Nd(III) and Eu(III) ions in aqueous solutions has been investigated by means of K-edge x-ray absorption spectroscopy. Experimental data are presented which demonstrate that an accurate determination of the Nd(III) and Eu(III) hydration structural parameters can be obtained from the analysis of the K-edge spectra. It is shown that the large widths of the core hole states do not appreciably reduce the potential structural information of the K-edge EXAFS data. In addition the available k-range of the K-edge spectra is much wider compared to that of the L3-edge which is affected by the presence of double-electron excitations in a more severe fashion.

Journal Article↗
PubMed2001

XAFS studies on the local structures of Ti-HMS mesoporous molecular sieves and their photocatalytic properties.

Various Ti-containing HMS mesoporous molecular sieves having high surface areas and pore volumes have been prepared at ambient temperature. The Ti-HMS samples exhibited a well-defined XRD pattern of typical hexagonal lattice. UV-vis and XAFS spectral for the calcined forms of Ti-HMS indicated that the Ti ions are tetrahedrally coordinated in the framework of HMS for the samples of Ti-HMS(1) and Ti-HMS(2), while at least a fraction of Ti-oxides in Ti-HMS(10) is in octahedral coordination. UV irradiation of the Ti-HMS catalysts in the presence of NO was found to lead to the decomposition of NO to form N2, O2, and N2O at 273 K with different yields and product distributions. The photocatalytic performance of Ti-oxides appears to be completely modified by their incorporation structure and reaction environment. High efficiency and selectivity for the formation of N2 and O2 in the photocatalytic decomposition of NO was achieved with the Ti-HMS(1) having highly dispersed isolated tetrahedral titanium oxide species, while the reactivity decreased with an increase in the Ti content.

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PubMed2001

Local structure of Pb(II) ion catalysts anchored within zeolite cavities and their photocatalytic reactivity for the elimination of N2O.

The Pb2+/ZSM-5 catalyst was prepared by an ion-exchange method and its photocatalytic activity for the decomposition of N2O under UV irradiation was investigated. In-situ UV-Vis absorption spectroscopy and XAFS (XANES and FT-EXAFS) investigations revealed that the Pb2+ ions exist in a highly dispersed state within the pores of the zeolites. UV irradiation of the catalysts in the presence of N2O led to the photocatalytic decomposition of N2O into N2 at temperatures as low as 298K. The effective wavelength of the irradiated UV light indicated that the excited state of the Pb2+ ions included within the zeolite cavities plays a significant role in the photocatalytic decomposition of N2O molecules.

Journal Article↗
PubMed2001

Investigations on the local structure of Ag+/ZSM-5 catalysts and their photocatalytic reactivities for the decomposition of N2O at 298 K.

Ag+/ZSM-5 catalysts were prepared by an ion-exchange method and UV-irradiation of the catalysts in the presence of N2O led to the photocatalytic decomposition of N2O into N2 and O2 at 298 K. Investigations on the effective wavelength of irradiated UV-light for the reaction as well as the in-situ characterization of the catalysts by means of XAFS, UV-Vis, photoluminescence and FT-IR spectroscopies revealed that the photoexcitation of the Ag+-N2O complexes formed between gaseous N2O and the isolated Ag+ ions play a significant role in this reaction.

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PubMed2001

Study on local structure around Ce and Gd atoms in CeO2-Gd2O3 binary system.

X-ray absorption spectra near Ce and Gd K-edges of binary oxide of Ce(x)Gd(1-x)O(3+x)/2 (0 < or = x < or = 0.5) in C-type structure solid solution were measured by use of the beamline BL01B1 at SPring-8. Interatomic distances between rare-earth and oxygen atoms. R(Gd-O) and R(Ce-O), were evaluated with the extended x-ray absorption fine structure analysis and found to increase with x. R(Gd-O) was larger than R(Ce-O) by about 0.1 A. Metal-oxygen distances estimated as their linear combinations, xR(Ce-O) + (1-x)R(Gd-O), well agreed with those determined by X-ray diffraction.

Journal Article↗
PubMed2001

Local structure of NiTi nanocrystals studied by EXAFS and XRD.

A series of NiTi nanocrystals with different annealing temperatures, prepared by sputtering method, were investigated by extended x-ray absorption fine structure (EXAFS) and x-ray diffraction. It was found that the structure of nano-phase powder is different from bulk NiTi alloy with bcc structure as target materials. When increasing the annealing temperature, a small fraction of the (Ni,Ti) type nanocrystal with the hexagonal structure was presented except target materials and Ni, and it is atomic occupation in random. Finally there were four Ti and two Ni atoms around central Ni atoms, and the bond length of Ni-Ti and Ni-Ni were 0.2462nm and 0.2585nm at 800 degrees C annealed.

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PubMed2002

Local structure in molecular complexes probed by multiple-scattering XAS.

The GNXAS (n-body distribution function X-ray absorption spectroscopy) method for multiple-scattering (MS) data analysis of EXAFS (extended X-ray absorption fine structure) data and the results recently obtained on molecular complexes relevant to biological matter are briefly reviewed and discussed. Practical MS calculations for important molecular fragments like Fe-O-O and Fe-C-N-Cu are presented in detail showing the potential of the techniques for measuring bond-angle distributions. The optimal conditions for obtaining accurate structural refinements using EXAFS measurements and modern data-analysis schemes are discussed as well as the current perspectives in the exploitation of the technique.

Absorptiometry, Photon↗
PubMed2004

XAFS imaging of Tsukuba gabbroic rocks: area analysis of chemical composition and local structure.

Gabbroic rocks were collected at Mount Tsukuba in Japan, and their XAFS images were studied using a projection-type X-ray fluorescence (XRF) microscope, which is a powerful new tool recently developed for extremely rapid imaging. The instrument employs a grazing-incidence arrangement in order that primary X-rays illuminate the whole sample surface, as well as parallel-beam optics and an extremely close geometry in order to detect XRF by a high-performance X-ray CCD system with 1024 x 1024 pixels. The XRF image indicated that black amphibole and white feldspar, both of which are typical mineral textures of the rock, contain iron. The origin has been suggested to be several small yellowish-brown minerals contained there. The XAFS imaging has been carried out by repeating the exposure of XRF images during the energy scan of the primary X-rays. It has been found that the structure is qualitatively close to that of olivine, and the main differences found in both areas can be explained as a difference in iron and magnesium concentration, i.e. the mixed ratio of forsterite (Mg(2)SiO(4)) and fayalite (Fe(2)SiO(4)). The feasibility of the present XAFS imaging method has been demonstrated for realistic inhomogeneous minerals.

Altitude↗
PubMed2005

Local structure fluctuations as a signature of an inhomogeneous ground state in high-Tc superconductors.

In-plane polarized Cu K-edge XAFS on La2CuO4.1 is presented, which indicates a radial in-plane Cu-O distribution function that is not a single Gaussian. Fits to the isolated Cu-O XAFS signal show the presence of a two-site radial distribution function, similar to that found in other La-based cuprate superconductors at temperatures below the temperature associated with the pseudogap appearance, T*. The appearance of the two-site distribution is interpreted as evidence of a non-homogeneous ground state, preceding the superconducting transition. Similar results found in other copper-oxide superconductors indicate that this non-homogeneous ground state is a general feature of these materials.

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