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Showing papers on "XANES published in 1994"


Journal ArticleDOI
TL;DR: In this article, the effect of the presence or absence of ligands such as H 2 O and NH 3 on the coordination sphere of a pure Ti-silicalite was investigated.
Abstract: X-ray absorption at the Ti K edge (both XANES and EXAFS) of a very pure Ti-silicalite containing a small fraction of Ti(IV) substituting for Si(IV) has been performed in order to study the effect of the presence or absence of ligands such as H 2 O and NH 3 on the Ti(IV) coordination sphere. In particular, the effect of an outgassing treatment at 400 K and of the interaction with NH 3 has been studied and described in detail. It has been found that the Ti(IV) is fourfold coordinated in the samples outgassed at 400 K and expands its coordination sphere number with action of adsorbates. The process is partially reversible at room temperature

362 citations


Journal ArticleDOI
TL;DR: In this paper, a series of high surface area, microporous, Ti-Si mixed oxides having a wide range of elemental compositions was prepared by a sol-gel method, and X-ray absorption spectroscopy at the Ti K edge was used to probe the first-shell coordination environment around Ti.
Abstract: A series of high surface area, microporous, Ti-Si mixed oxides having a wide range of elemental compositions was prepared by a sol-gel method. Since the materials were noncrystalline, X-ray absorption spectroscopy at the Ti K edge was used to probe the first-shell coordination environment around Ti. Incorporation of Si into the mixed oxide shortened the average Ti-O bond distance derived from EXAFS and disrupted the normal octahedral coordination of pure titania. The shortest Ti-O bond distance was 1.82 [angstrom] and was attributed to Ti primarily residing in tetrahedral sites, which was also indicated by a significant pre-edge peak present in Ti K edge XANES. Shifts in the UV absorption edges for the mixed oxides were attributed to titania domain sizes decreasing with increasing Si content in accord with the well-known quantum size effect. The smallest domain size was estimated as less than 1 nm for a sample with a Ti:Si atomic ratio of 1:8. Lattice vibrations in the materials were probed with FT-Raman and FT-IR absorption spectroscopies, and the results were consistent with a shortening of the Ti-O bond and the formation of Ti-O-Si linkages in the mixed oxides. In situ experiments showed that adsorbed water profoundly affects the localmore » environment around surface Ti atoms. 44 refs., 11 figs., 3 tabs.« less

254 citations


Journal ArticleDOI
TL;DR: In this paper, the energy shift of a pre-edge peak in X-ray absorption near-edge structure (XANES) spectra obtained using the synchrotron Xray fluorescence microprobe was used for determining Fe 3+ √Fe ratios in Fe-bearing minerals.

201 citations



Journal ArticleDOI
TL;DR: By comparing the ordered and disordered species it is found that the charge transfer depends primarily on the local environment (coordination number of like and unlike atoms) and not on atomic separation.
Abstract: A series of Au-Cu alloys of various stoichiometries and order have been studied using x-ray photoemission spectroscopy (XPS) and x-ray absorption spectroscopy (XAS) Significant electronic changes are associated with alloying and with changes in the local environment The Au 5d electron charge depletion has been determined independently from XPS core-level and M\"ossbauer isomer shifts and from x-ray absorption near-edge structure (XANES) measurements Strong correlations were found using both methods in the parameters investigated here, especially in the elucidation of the charge-transfer mechanism It is found that at the Au site there is a loss of d charge, upon alloying, which increases as Au becomes more dilute in Cu This along with XANES evidence of d charge gain at the Cu site, conduction (primarily 6s) charge gain at the Au site, and overall charge gain at the Au site verifies the charge compensation model in which Au loses d charge but is overcompensated by a gain of conduction charge This observation is in line with electronegativity (Au is more electronegative than Cu) and electroneutrality (the overall gain of charge is small) arguments It is also found that there is a strong linear correlation between the 5d hole count at the Au site and (1) the M\"ossbauer isomer shift, (2) alloy d-band width, (3) ``Au'' apparent spin-orbit splitting in the alloy d band By comparing the ordered and disordered species (which have different local environments) it is found that the charge transfer depends primarily on the local environment (coordination number of like and unlike atoms) and not on atomic separation The charge transfer (d-band depletion) was larger in the ordered species and can be explained by the number of Cu and Au nearest neighbors as compared to the disordered phases

127 citations


Journal ArticleDOI
TL;DR: In this paper, the effect of the presence or absence of ligands such as H2O and NH3 on the coordination sphere of pure Ti-silicalite has been investigated.
Abstract: X-ray absorption at the Ti K edge (both XANES and EXAFS) of a very pure Ti-silicalite containing a small fraction of Ti(IV) substituting Si(IV), has been performed in order to study the effect of the presence or absence of ligands such as H2O and NH3 on the Ti(IV) coordination sphere. In particular, the effect of an outgassing treatment at 400 K and of the interaction with NH3 has been studied and described in detail. It has been found that the Ti(FV) is fourfold coordinated in the samples outgassed at 400 K and expands its coordination sphere number under the action of adsorbates.

93 citations


Journal ArticleDOI
TL;DR: In this article, the results of 27Al nuclear magnetic resonance spectroscopy (NMR) and Al-K-edge X-ray Absorption Near Edge Structure (XANES) of natural imogolite and allophanes and some crystalline reference minerals were compared.
Abstract: This paper compares the results of 27Al nuclear magnetic resonance spectroscopy (NMR) and Al-K-edge X-ray Absorption Near Edge Structure (XANES) of natural imogolite and allophanes and some crystalline reference minerals. All soil allophanes studied contain 4-coordinated Al (AlIV). The highest relative proportion of AlIV, 21% of the total Al, was found in Si-rich allophane. This value is close to that found in spring allophanes, which were previously considered to be different from soil allophanes. For a quantitative determination of the AlIV/Altotal ratio, NMR is more reliable than XANES, because of the sensitivity of the chemical shift to low AlIV concentrations, but XANES may be used even if paramagnetic impurities (mostly Fe) are present. Al-K XANES also yields more information than NMR on the local environment of AlVI and especially site multiplicity. AlVI XANES of imogolite and allophanes are similar regardless of the Al/Si ratio. They yield two well-resolved resonances with maxima near 1568 and 1570 eV, which indicates the presence of a unique AlVI site by comparison with crystalline references. The presence of only one AlVI site indicates that imogolite and allophanes have an octahedral sheet with a structure similar to 2/1 dioctahedral phyllosilicates but different from gibbsite or kaolinite, previously considered as structural analogues. The 27AlIV MAS NMR peak maxima of allophanes are between 58.6 and 59.8 ppm, in the range observed for crystalline and amorphous framework alumino-silicates, and less positive than those of sheet silicates, which are typically in the range 65–75 ppm. 27Al-H1 CPMAS NMR spectra suggest that both AlIV and AlVI have Al-O-H linkages.

93 citations


Journal ArticleDOI
TL;DR: In this article, the structure and local electronic environment of PdAu bimetallic clusters supported on silica were investigated. But the results were limited to four samples with total metal loadings of approximately 100 [mu]mol g[sup [minus]1] and varying Pd Au compositions.
Abstract: The extended X-ray absorption fine structure (EXAFS) above the Pd K and Au L[sub III] edges and the X-ray absorption near edge structure (XANES) associated with the Au L[sub III] edge were used to study the structure and local electronic environment of PdAu bimetallic clusters supported on silica. Four samples with total metal loadings of approximately 100 [mu]mol g[sup [minus]1] and varying PdAu compositions were investigated. Transmission electron microscopy performed on a bimetallic sample (Pd:Au mole ratio equal to 60:40) exposed to air failed to detect any metal particles, indicating that their average diameter was less than 1 nm, which corresponds to [approximately] 100% metal exposed. The EXAFS for bimetallic clusters (Pd:Au = 60:40) in vacuum at 77 K shows that Pd atoms decorate a core of Au atoms. The Pd-Pd interatomic distance in the bimetallic sample is the same as in bulk Pd, whereas the Au-Au interatomic distance is 3% shorter than that in bulk Au. The XANES associated with the Au L[sub III] edge indicates that Au has an atomic-like electronic structure when alloyed with Pd, which is due to Au atoms having fewer like nearest neighbors in the clusters compared to bulk Au. 38 refs., 8 figs.,more » 3 tabs.« less

77 citations


Journal ArticleDOI
TL;DR: In this article, the authors applied the near-edge X-ray absorption fine structure (NEXAFS) technique to investigate the electronic and structural properties of thin vanadium carbide films on a vanadium (110) singlecrystal surface.

74 citations



Journal ArticleDOI
TL;DR: In this article, the authors investigated the mechanism of the selective nitric oxide reduction by hydrocarbons on Cu-ZSM-5 catalysts in an oxygen-rich gas mixture using X-ray Absorption Near Edge Structure (XANES) method.
Abstract: Reported here is our recent investigation of the mechanism of the selective nitric oxide reduction by hydrocarbons on Cu-ZSM-5 catalysts in an oxygen-rich gas mixture. We studied the copper oxidation state change during the catalytic reaction using the X-ray Absorption Near Edge Structure (XANES) method. We observe that even under strongly net oxidizing conditions, a significant fraction of the copper ions in ZSM-5 is reduced to Cu I at elevated temperature, when propene is present in the reactant stream. XANES spectra show that the Cu I 1s → 4p transition intensity, which is proportional to cuprous ion concentration, changes with the reaction temperture in a pattern similar to the NO conversion activity. For comparison purposes, we also studied the Cu I concentration change using a gas mixture in which propene was replaced by a stoichiometrically equivalent concentration of methane. Unlike propene, methane provides no NO selective reduction pathway over Cu-ZSM-5. No window of enhanced Cu I concentration was observed using methane as the reductant. Our study indicates that, even in a strongly oxidizing environment, cupric ion can be partially reduced by propene to form Cu I , possibly by way of allylic intermediate, which may be a crucial step for effective NO conversion through a redox mechanism.

Journal ArticleDOI
TL;DR: In this article, the first Cu precipitates are found to be of BCC structure, i.e. coherent with the matrix, at the longest ageing times, they have become of FCC structure.
Abstract: Samples of FeCu, FeCuMn, FeCuNi, and FeCuCr alloys containing 1.5 wt% of each solute have been electron irradiated around 290 degrees C or thermally aged at 500 degrees C for various times. It is known that such treatments induce Cu precipitation; the Cu depletion of the matrix is measured by the resistivity decrease of the samples. The crystallographic environment of the solute atoms in the irradiated or aged samples has been studied by XAS (X-ray absorption spectroscopy). The data show that Cr, Mn, and Ni atoms mainly remain in BCC solid solution during the Cu precipitation. The first Cu precipitates are found to be of BCC structure, i.e. coherent with the matrix. At the longest ageing times, they have become of FCC structure. In the electron-irradiated samples, the data show that, up to a fluence of 5 C cm-2, most of the Cu precipitates (>80%) are still of BCC structure. The fraction of BCC precipitates has been estimated from both the X-ray-absorption near-edge structure (XANES) and extended X-ray-absorption fine-structure (EXAFS) data. It appears that, for both FeCu and FeCuMn samples, similarly aged, the FCC fraction is larger in cold-rolled samples than in pre-annealed and quenched ones. The results obtained are consistent, in the FeCu case, with a simple linear correspondence between fluence and time, 1 C cm-2 at 300 degrees C being equivalent to approximately 10 h at 500 degrees C. The Cu precipitation is found to be accelerated, at least in its first stages, by the presence of Mn in the alloy.

Journal ArticleDOI
TL;DR: In this paper, the authors used the scanning transmission X-ray microscope on the X1A undulator beam line, at the National Synchrotron Light Source, has been used for imaging various biological as well as polymer samples at 55 nm Rayleigh resolution.
Abstract: The scanning transmission X-ray microscope on the X1A undulator beam line, at the National Synchrotron Light Source, has been used for imaging various biological as well as polymer samples at 55 nm Rayleigh resolution. The microscope is operated mainly in direct imaging mode, where X-rays of fixed energy are diffractively focussed to a microprobe and the sample is scanned in two dimensions. However, by varying the X-ray energy while keeping the beam focussed to one spot on the sample we can also determine the localized chemical composition of the sample from the carbon X-ray absorption near edge spectra in an area smaller than 0.2 μm by 0.2 μm. The spatial distribution of the chemical constituents can be obtained by imaging at absorption maxima of specific chemical bonds, while retaining the 0.05 μm spatial resolution of the microscope. The chemical sensitivity of X-ray-absorption near-edge spectroscopy provides a powerful contrast mechanism for imaging organic systems. The well-known differences in energy among π resonances of different chemical bonds have been used to distinguish different polymer phases and have been applied to biological systems.

Journal ArticleDOI
TL;DR: First resonant $K-shell absorption spectra of atomic clusters covering the size range from the atom to the solid are reported and the character of a transition changes from a Rydberg orbital in a small cluster into a bulk excitation in larger systems are observed.
Abstract: First resonant $K$-shell absorption spectra of atomic clusters covering the size range from the atom to the solid are reported. Discrete absorption into Rydberg states as well as the near edge ionization continuum (XANES/EXAFS) of neon clusters ($N\ensuremath{\approx}15\ensuremath{-}4000$) is investigated with excellent spectral resolution. Shifts and splittings of the absorption features ($1s\ensuremath{\rightarrow}3p,4p$) provide information on local properties of the different atomic sites. Besides identifying surface and bulk contributions we observe how the character of a transition changes from a Rydberg orbital in a small cluster into a bulk excitation in larger systems.

Journal ArticleDOI
TL;DR: In this article, an analysis of the X-ray absorption near the edge structure (XANES) and extended Xray absorption fine structure (EXAFS) for the Co and Fe K-edges of these composite hydrous oxides revealed that, regardless of the oxidation state of nickel sites in the films, the guest metal ions are present as Co[sup 3+] and Fe[sup 4+].
Abstract: Aspects of the structural and electronic properties of hydrous oxide films of composite (9:1) Ni/Co and (9:1) Ni/Fe, prepared by electrodeposition, have been examined in alkaline electrolytes using in situ X-ray absorption fine structure (XAFS). An analysis of the X-ray absorption near the edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) for the Co and Fe K-edges of these composite hydrous oxides revealed that, regardless of the oxidation state of nickel sites in the films, the guest metal ions are present as Co[sup 3+] and Fe[sup 3+] and that the cobalt-oxygen distance d(Co-O) = 1.9 [+-] 0.02 [angstrom] and d(Fe-O) = 1.92 [+-] 0.02 [angstrom]. The latter values are in excellent agreement with d(Me-O) (Me = Co or Fe) in CoOOH and [beta]- and [gamma]-FeOOH, respectively, determined by conventional X-ray diffraction. Two clearly defined Me-Ni first coordination shells could be observed in the Fourier transforms (FT) of the K-edge EXAFS of the guest metal recorded at a potential at which both Ni[sup 2+] and Ni[sup 3+] sites are expected to be present. 28 refs., 10 figs., 3 tabs.

Journal ArticleDOI
TL;DR: In this paper, the excitation spectrum of the L 3 -M 5 X-ray emission of Dy compounds in the pre-edge region of Dy L 3 Xray absorption near edge structure (L 3 -XANES) is theoretically investigated based upon the coherent second order optical formula with multiplet coupling effects.
Abstract: The excitation spectrum of the L 3 -M 5 X-ray emission of Dy compounds in the pre-edge region of Dy L 3 X-ray absorption near edge structure (L 3 -XANES) is theoretically investigated based upon the coherent second order optical formula with multiplet coupling effects. The spectral broadening of the excitation spectrum is determined by the M 5 core hole lifetime, being free from the L 3 core hole lifetime. The fine pre-edge structure of the L 3 edge due to the 2 p →4 f quadrupole transition can be seen in the excitation spectrum, while this structure is invisible in the conventional XANES, in agreement with the recent experimental results. We clarify the conditions for the excitation spectrum to be regarded as the absorption spectrum with a smaller width. The resonant X-ray emission spectra for various incident photon energies around the L 3 edge are also calculated.

Journal ArticleDOI
TL;DR: In this paper, the Xanes spectra of nickel K-edge and X-ray diffraction patterns of several nickel oxycompounds were measured and the shift in energy of the Ni K-absorption edge toward higher values correlates well with the increase in the oxidation state of nickel from +2 to +4 in the compounds studied.
Abstract: The structure of the higher oxide forms of nickel (where Ni has a valency greater than +2) is interesting from the standpoint of developing advanced nickel batteries for consumer applications and for electric vehicle propulsion. Xanes spectra of nickel K-edge and X-ray diffraction patterns of several nickel oxycompounds were measured. The shift in energy of the Ni K-absorption edge toward higher values correlates well with the increase in the oxidation state of nickel from +2 to +4 in the compounds studied. The inflection point energy of the Ni pre-edge structure shifts approximately 1 eV per unit change in valency. The results provide evidence which support the assignment that the valency of Ni in [beta]-NiOOH is +3; Ni[sub 3]O[sub 2](OH)[sub 4] and commercial nickel peroxide (NiO[sub 2]) likewise contain trivalent nickel. The edge shift for quadrivalent nickel in KNilO[sub 6] is reported for the first time.

Journal ArticleDOI
TL;DR: In this paper, a Pt-KH-MOR was used for n-hexane isomerization and hydrocracking via bifunctional catalysis at 240-300°C and 1 atm.

Journal ArticleDOI
TL;DR: In this paper, the densities of unoccupied states at the conduction bands of sphalerite, chalcopyrite and stannite are determined using S K- and L-edge XANES features (up to 15 eV above the edge), combined with published metal K-edge x-ray absorption spectra (XANES).
Abstract: Sulfur K-edge x-ray absorption spectra (XANES and EXAFS) and L-edge XANES of sphalerite (ZnS), chalcopyrite (CuFeS2) and stannite (Cu2FeSnS4) have been recorded using synchrotron radiation. The K- and L-edge XANES features are interpreted using a qualitative MO/energy band structure model. The densities of unoccupied states at the conduction bands of sphalerite, chalcopyrite and stannite are determined using S K- and L-edge XANES features (up to 15 eV above the edge), combined with published metal K-edge XANES. The SK- and L-edge XANES also indicate that, for sphalerite, the Fe2+ 3d band at the fundamental gap has little or no bonding hybridization with S 3p and S 3s orbitals; for chalcopyrite, the Cu+ 3d and Fe3+ 3d bands have strong mixing with S 3p and S 3s states, while for stannite the Cu+ 3d band strongly hybridizes with S 3p and S 3s orbitals, but the Fe2+ 3d band does not. The post-edge XANES features (15–50 eV above the edge) of sphalerite, chalcopyrite and stannite are similar. These features are related to the tetrahedral coordination of sulfur in all these structures, and interpreted by a multiple scattering model. The resonance energies from both the K-edge and L-edge XANES for these minerals are well correlated with reciprocal interatomic distances and lattice spaces. Sulfur K-edge EXAFS analyses using Fourier transform and curve fitting procedures are presented. Comparison of the structural parameters from EXAFS with x-ray structure data shows that the first shell bond distances (BD) from EXAFS are usually accurate to ±0.02 A, and that coordination numbers (CN) are generally accurate to ±20 percent. For sphalerite, EXAFS analysis yields the structure parameters for the first three neighbour shells around a sulfur atom; the BD and CN even for the third shell are in close agreement with the x-ray structure, and the Debye-Waller term decreases from the first shell to the third shell. It is shown that sphalerite (ZnS) is a good model compound for EXAFS analysis of sulfur in chalcogenide glasses and metalloproteins.

Journal ArticleDOI
TL;DR: In this paper, the S K and L-edge x-ray absorption near-edge structures (XANES) of low bornite, cubanite, chalcocite, covellite, enargite and tetrahedrite have been measured with synchrotron radiation.
Abstract: The S K and L-edge x-ray absorption near-edge structures (XANES) of low bornite, cubanite, chalcocite, covellite, enargite and tetrahedrite have been measured with synchrotron radiation The near-edge features are interpreted based on comparison with the S K- and L-edge spectra of chalcopyrite and a MO/energy band structure model The XANES spectra of these sulfides reflect the DOS of unoccupied S s-, p- and d-like states near and above the Fermi level In tetrahedral Cu-Fe sulfides, the Fe3+ 3d crystal field band has much more significant DOS of unoccupied S 3p-and 3s-like states than the Cu+ 3d crystal field band For Cu sulfides, the Cu+ 3d crystal field band has the higher DOS of S 3p- and 3 s-like states in tetrahedral structure than in structures with the triangular CuS3 cluster The shifts in both S K- and L-edges correlate approximately linearly with the energy gap

Journal ArticleDOI
TL;DR: In this paper, the S K and L -edge X-ray absorption near-edge structure (XANES) spectra of sphalerite, wurtzite, greenockite, metacinnabar, and trigonal HgS have been obtained using synchrotron radiation.

Journal ArticleDOI
TL;DR: In this article, the effect of pressure on titanium coordination in glasses, with composition K2TiSi4O11, quenched isobarically from liquids equilibrated at high pressure (5, 10, 15, 20, 25, 30 kbar respectively) and T=1600° C has been investigated by X-ray absorption spectroscopy (XAS).
Abstract: The effect of pressure on titanium coordination in glasses, with composition K2TiSi4O11, quenched isobarically from liquids equilibrated at high pressure (5, 10, 15, 20, 25, 30 kbar respectively) and T=1600° C has been investigated by X-ray absorption spectroscopy (XAS). The XANES spectra collected at the Ti K-edge clearly show a variation with pressure that is related to changes in the geometrical environment around the Ti atoms. By comparison with spectra of standard materials, the XANES spectra of the glasses suggest a relatively low average coordination number (near 5) in samples quenched at low pressure and a higher coordination number (near 6) in samples quenched from the highest pressure. The combination of XANES data with density and compressibility measurements supports the idea that a mixture of 6- and lower coordinated (4- and/ or 5-coordinated) Ti geometries are present in the 1 bar glass, and an increasing proportion of 6-coordinated Ti occurs in the glasses synthesized at progressively higher pressures.

Journal ArticleDOI
TL;DR: These highly structured spectra of well-characterized ordered crystals provide a set of standard data which is used to test the multiple-scattering x-ray-absorption code FEFF V, and the dependence of the calculation on cluster size, order of scattering, and choice of self-energy is demonstrated.
Abstract: The x-ray-absorption near-edge structure (XANES) spectra of single-crystal alkali halide salts have been measured at low temperature (T\ensuremath{\approxeq}80 K) with high resolution in energy. By employing the electron partial-yield detection technique, spectra of NaF, NaCl, and NaBr were obtained near the sodium K edge and spectra of LiF, NaF, and KF were obtained near the fluorine K edge. All spectra showed sharp absorption features at the absorption threshold and broader features extending 50--80 eV above threshold. Some narrow features close to threshold are attributed to core-level excitons. Contributions from atomic multiple-electron excitations, estimated by a comparison to the K-edge photoabsorption spectrum of Ne in the gas phase, are found to be very small. Thus, the intense broader peaks observed further above threshold arise from single-electron scattering resonances. These highly structured spectra of well-characterized ordered crystals provide a set of standard data which is used to test the multiple-scattering x-ray-absorption code feff v. For some spectra, at photon energies more than \ensuremath{\approxeq}10 eV above threshold, the calculations achieve very good agreement with experiment. The dependence of the calculation on cluster size, order of scattering, and choice of self-energy is demonstrated.

Journal ArticleDOI
TL;DR: In this paper, the reductive deposition of metallic Cu on porous silicon (PS) surface from aqueous solution of Cu{sup 2+}(aq) ions is reported, and X-ray absorption near-edge structure (XANES) at the Cu L{sub 3,2} edge has been used to characterize these samples.
Abstract: The reductive deposition of metallic Cu on porous silicon (PS) surface from aqueous solution of Cu{sup 2+}(aq) ions is reported. X-ray absorption near-edge structure (XANES) at the Cu L{sub 3,2} edge has been used to characterize these samples. It is found that the reduction of Cu{sup 2+}(aq) to Cu occurs readily at room temperature and appears to be controlled by the availability of active sites as well as the concentration of Cu{sup 2+}(aq) and that only a very thin film of Cu is formed even with concentrated solutions. The surface of Cu/PS is oxidized when exposed to the ambient atmosphere. The spectroscopic features and the implication of the results are discussed. 50 refs., 9 figs., 1 tab.

Journal ArticleDOI
TL;DR: The coordination environment around Ni at minor concentration levels (2 wt. % NiO) in Na2Si2O5 glass and melt (293-1250K) has been investigated by Ni K-edge X-ray Absorption Fine Structure (XAFS) spectroscopy as discussed by the authors.
Abstract: The coordination environment around Ni at minor concentration levels (2 wt. % NiO) in Na2Si2O5 glass and melt (293–1250K) has been investigated by Ni K-edge X-ray Absorption Fine Structure (XAFS) spectroscopy. X-ray Absorption Near Edge Structure (XANES) spectra indicate that both IVNi and VNi occur in the glass, whereas IVNi is dominant in the melt. The average anharmonic Ni-O distances derived from XAFS spectra of the glass at 293 K and the melt above 1150 K are 2.00 (±0.02) and 1.97 (± 0.02) A, respectively. The Ni-O distance in the glass is consistent with a mixture of IVNi and VNi, whereas that in the melt is consistent with dominantly IVNi, suggesting a Ni-coordination change during glass melting. This major change in Ni coordination indicates significant reorganization of the melt during quenching and crystallization. These observations for a compositionally-simple melt/glass system may help explain the enrichment of Ni in early-formed igneous minerals. They also suggest the possibility of a pressure-induced coordination change around Ni in the mantle which may influence geochemical modeling of the deep Earth.

Journal ArticleDOI
01 Jan 1994-Polymer
TL;DR: In this article, a biaxially stretched polypropylene (PP) foils treated with oxygen plasma at low pressure were analyzed by core level and valence band region X-ray photoelectron spectroscopy (XPS), atomic force microscopy, and near edge Xray absorption fine structure (NEXAFS).

Journal ArticleDOI
TL;DR: UPS spectra of multilayers and monolayers of chlorinated ethylene molecules condensed or adsorbed on Pt(111) and Pt(110) surfaces at 95 K are in close agreement.


Journal ArticleDOI
TL;DR: In this paper, the authors applied near-edge X-ray absorption fine structure (NEXAFS) to characterize the surface and bulk properties of LixNi1−xO catalysts.
Abstract: We have applied near-edge X-ray absorption fine structure (NEXAFS) to characterize the surface and bulk properties of LixNi1−xO catalysts. In our experimental set-up, NEXAFS spectra of powder materials could be obtained by measuring the intensity of either electron-yield or fluorescence-yield. While the electron-yield method is sensitive only to the top few atomic layers, the fluorescence-yield method can detect species up to a few thousands angstroms deep into the bulk structure. The ability to distinguish surface and bulk compositions is demonstrated in studies of a number of Li0.5Ni0.5O samples, of which the surface compositions vary as a function of preparation procedures. In addition, NEXAFS investigations following the reaction of LixNi1−xO with CH4 have also been carried out and the results indicate that the initial surface reaction intermediates are Li2CO3.

Journal ArticleDOI
TL;DR: X-ray absorption spectroscopy results allow us to discuss the coordination number and geometry of the ferrous ion in the native enzyme and analysis of the outer shells performed using multiple scattering theory shows that there are histidines in the coordination sphere.
Abstract: X-ray absorption spectroscopy has been utilized to investigate the structure of the active site of iron(II) catechol 2,3-dioxygenase from Pseudomonas putida mt2 both in the native and the 2-chlorophenol inhibited forms. XANES (X-ray absorption near edge structure) and EXAFS (extended X-ray absorption fine structure) results allow us to discuss the coordination number and geometry of the ferrous ion in the native enzyme. The metal geometry is not significantly affected by the binding of the inhibitor. The EXAFS spectrum is consistent with an iron(II) bound to six N/O atoms at an average distance of 2.05 A or to five N/O at an average distance of 2.04 A. The stimulation of the experimental data is greatly enhanced by considering the iron ligands divided in two different shells. Analysis of the outer shells performed using multiple scattering theory shows that there are histidines in the coordination sphere. The best fitting is obtained assuming the presence of two of them. Similar results are obtained for the inhibited enzyme, which, however, are indicative of a slight shortening of the average metal-donor bond distances. The direct binding of inhibitors to the metal center is confirmed by 1H NMR data.