Showing papers on "Extended X-ray absorption fine structure published in 2000"

Theoretical approaches to x-ray absorption fine structure

Abstract: Dramatic advances in the understanding of x-ray absorption fine structure (XAFS) have been made over the past few decades, which have led ultimately to a highly quantitative theory. This review covers these developments from a unified multiple-scattering viewpoint. The authors focus on extended x-ray absorption fine structure (EXAFS) well above an x-ray edge, and, to a lesser extent, on x-ray absorption near-edge structure (XANES) closer to an edge. The discussion includes both formal considerations, derived from a many-electron formulation, and practical computational methods based on independent-electron models, with many-body effects lumped into various inelastic losses and energy shifts. The main conceptual issues in XAFS theory are identified and their relative importance is assessed; these include the convergence of the multiple-scattering expansion, curved-wave effects, the scattering potential, inelastic losses, self-energy shifts, and vibrations and structural disorder. The advantages and limitations of current computational approaches are addressed, with particular regard to quantitative experimental comparisons.

Quantitative speciation of Mn-bearing particulates emitted from Autos burning (methylcyclopentadienyl)manganese tricarbonyl-added gasolines using XANES spectroscopy

Abstract: The chemical nature of Mn-containing particulates emitted from (methylcyclopentadienyl)manganese tricarbonyl-added gasoline engines has been elucidated using Mn K-edge X-ray absorption fine structure (XAFS) spectroscopy. Edge shift data from the X-ray absorption near-edge structure (XANES) spectra showed that the average Mn valence in these particulates is ∼2.2. Using a principal component analysis (PCA) algorithm, the number and type of probable species contained in these particulates were determined to be three, consisting of Mn3O4, MnSO4·H2O, and a divalent manganese phosphate, Mn5(PO4)[PO3(OH)]2·4H2O. The proportions of these Mn phases in each particulate sample were evaluated quantitatively using least-squares fitting (LSF) of the experimental XANES spectra with linear combinations of these principal component (model compound) spectra. Two groups of Mn-bearing particulates may be distinguished: group I having 4−9 wt % of Mn3O4 and exhibiting a single intense first major absorption maximum at the Mn ...

Oxidation States of Copper Ions in ZSM-5 Zeolites. A Multitechnique Investigation

Abstract: The redox behavior of Cu/ZSM-5 zeolites prepared by ion exchange from Cu2+ aqueous solutions has been followed by a variety of spectroscopic techniques to provide a thorough picture of the so-called “self-reduction” of cupric ions, which occurs upon dehydration of the hydrated system at various temperatures, and of the reverse process of reoxidation as well. Conflicting hypotheses on both these processes are, in fact, present in the literature. The experimental techniques employed in this work are electron paramagnetic resonance (EPR), IR, and optical spectroscopies, extended X-ray absorption fine structure, and X-ray absorption near-edge structure. The early stages of dehydration (from room temperature to about 470 K) involve cupric ion migration and formation of EPR silent moieties but no reduction to Cu+. The onset of this latter phenomenon starts at 470 K and, in the range 470−670 K, involves the majority of copper ions present in the system. Rehydration of Cu+-containing samples does not cause direct...

Binding of mercury(II) to reduced sulfur in soil organic matter along upland-peat soil transects

Abstract: The binding of Hg{sup 2+} in organic matter of soils and waters controls the transport and transformations of Hg in terrestrial and aquatic ecosystems. The authors developed a competitive complexation method using the strong complexation of Hg{sup 2+} by Br{sup {minus}} for determining the Hg{sup 2+} binding strength in organic soils at native and elevated Hg concentrations. The distribution coefficients determined in KBr suspensions for sorption of native HG{sup 2+} to soil organic carbon (SOC) (K{sub soc}) are in the range of 10{sup 22} to 10{sup 23}. The K{sub soc} significantly decreased with increased additions of Hg{sup 2+} and with decreasing pH. Using data for reduced organic S concentrations determined by x-ray absorption near-edge structure spectroscopy (XANES), the authors calculated surface complex formation constants on the order of 10{sup 32} for a model site having acidity constants of mercaptoacetic acid. This value is in fair agreement with the tabulated value of 10{sup 345} for Hg{sup 2+} binding in mercaptoacetic acid. At native Hg concentrations, formation constants and K{sub soc} values were similar for different types of soil organic matter along transects from uplands into wetlands, despite varying concentration of Hg and reduced organic S. Their adsorption data are consistent withmore » the conclusions from their previous extended x-ray absorption fine structure spectroscopy (EXAFS) study that in a humic acid and soil, Hg{sup 2+} ions bond in two-fold coordination involving one reduced S and one O or N.« less

Nanosized powders of NiZn ferrite: Synthesis, structure, and magnetism

Abstract: The structure and magnetic behavior of nanostructured powders of stoichiometric NiZn ferrite, Ni0.5Zn0.5Fe2O4, synthesized by coprecipitation, are investigated by extended x-ray-absorption fine structure spectroscopy (EXAFS), x-ray diffraction, Mossbauer spectroscopy, and vibrating sample magnetometry. Samples of high purity and high homogeneity were obtained by annealing at relatively low temperatures (300–800 °C) resulting in nanoparticles with average diameter between 9 and 90 nm, as determined by x-ray diffraction. EXAFS was applied to follow Ni, Zn, and Fe cations distribution and the evolution of the short range order of the samples with increasing annealing temperature. Our results show ferrimagnetic NiZn ferrite nanosized powders with high purity, 1:1 Ni to Zn stoichiometric ratio and superparamagnetic behavior. Moreover, the samples exhibit good structural ordering already after heat treatment at 400 °C. Analysis by vibrating sample magnetometry indicated a critical particle diameter for the tran...

Oxidation-reduction mechanism of iron in dioctahedral smectites: I. Crystal chemistry of oxidized reference nontronites

Abstract: The crystal chemistry of Fe in four nontronites (Garfield, Panamint Valley, SWa-1, and NG-1) was investigated by chemical analysis, X-ray goniometry, X-ray absorption pre-edge spectroscopy, powder and polarized extended X-ray absorption fine structure (EXAFS, P-EXAFS) spectroscopy, and X-ray diffraction. The four reference nontronites have Fe/(Fe + Al + Mg) ratios ranging from 0.58 to 0.78, and are therefore representative of the different chemical compositions of dioctahedral ferruginous smectites. Pre-edge and powder EXAFS spectroscopy indicate that NG-1 contains 14 to 20% of tetrahedrally coordinated Fe 3+ , whereas the other three samples have no detectable IV Fe 3+ . The partitioning of VI Fe 3+ between cis (M2) and trans (M1) sites within the octahedral sheet was determined from the simulation of X-ray diffraction patterns for turbostratic nontronite crystallites by varying the site occupancy of Fe. Based on this analysis, the four nontronite samples are shown to be trans-vacant within the detection limit of 5% of total iron. The in-plane and out-of-plane local structure around Fe atoms was probed by angular P-EXAFS measurements performed on highly oriented, self-supporting films of each nontronite. The degree of parallel orientation of the clay layers in these films was determined by texture goniometry, in which the half width at half maxi- mum of the deviation of the c* axis of individual crystallites from the film plane normal, was found to be 9.9° for Garfield and 19° for SWa-1. These narrow distributions of orientation allowed us to treat the self-supporting films as single crystals during the quantitative analysis of polarized EXAFS spectra. The results from P-EXAFS, and from infrared spectroscopy (Madejova et al. 1994), were used to build a two-dimensional model for the distribution of Fe, and (Al,Mg) in sample SWa-l. In this nontronite, Fe, Al, and Mg atoms are statistically distributed within the octahedral sheet, but they exhibit some tendency toward local ordering. Fe-Fe and (Al, Mg)-(Al,Mg) pairs are preferen- tially aligned along the (010) direction and Fe-(Al,Mg) pairs along the (31 - 0), and (3 - 1 0) directions. This distribution is compatible with the existence of small Fe domains separated by (Al,Mg), and empty octahedra, which segregation may account for the lack of magnetic ordering observed for this sample at low temperature (5 K) (Lear and Stucki 1990).

Photoinduced Ferrimagnetic Systems in Prussian Blue Analogues CIxCo4[Fe(CN)6]y (CI = Alkali Cation). 2. X-ray Absorption Spectroscopy of the Metastable State

Abstract: A CoFe Prussian blue analogue Rb1.8Co4[Fe(CN)6]3.3·13H2O was synthesized, which presents an important photomagnetic effect. The electronic structure and the local structure of the ground and of the excited states have been investigated. X-ray absorption spectroscopy measurements at the Co and Fe L2,3 edges and cobalt K-edge (XANES and EXAFS) evidence the local electronic transfer and the spin change of the cobalt ions induced by irradiation. We observed a 0.19 A increase of the Co−N bond length, associated with the transformation of CoIII low spin to CoII high spin. The CoII/CoIII ratio has been evaluated as a function of the irradiation time and revealed as an important parameter to understanding the bulk magnetic properties. The combined role of the diamagnetic FeII−CoIII pairs and hexacyanoferrate(III) vacancies is locally evidenced. This work is a new step in the understanding of the photoinduced electron transfer.

Near edge X-ray absorption fine structure measurements (XANES) and extended x-ray absorption fine structure measurements (EXAFS) of the valence state and coordination of antimony in doped nanocrystalline SnO2

Abstract: Colloids of nanocrystalline tin dioxide containing 9.1 at. % and 16.7 at. % antimony have been prepared by the coprecipitation method. High-resolution transmission electron microscopy (TEM) images show crystalline particles in the 2–6 nm size regime. X-ray powder diffraction patterns of nanocrystalline powders obtained by drying the colloids and heating to 100 °C indicate the same rutile lattice structure known from bulk SnO2. On heating to 500 °C in air, the nanocrystalline powder shows a slight increase in particle size but especially a change in color from yellowish to bluish which is accompanied by the development of n-type conductivity. The coordination of antimony in the SnO2 nanocrystallites has been investigated by extended x-ray absorption fine structure measurements (EXAFS) at the Sb K-edge at 5 K while its valence state was determined by near edge x-ray absorption fine structure measurements (XANES) at the Sb L1 edge. The Sb higher neighbor shell distances in the doped material differ from the ...

X-ray Absorption Fine Structure Study of the Formation of the Highly Dispersed PdO over ZSM-5 and the Structural Change of Pd Induced by Adsorption of NO

Abstract: Extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES) were used to investigate the local structure of Pd supported on ZSM-5, which was affected by the acid sites of support and the adsorption of nitrogen oxide. After the thermal treatment of the initially ion-exchanged Pd amine complex, the formation of metal Pd particles was found, whose size was larger than zeolite pore. The subsequent oxidation led to the disruption of Pd particles and the formation of the dispersed PdO, where the degree of dispersion was dependent on the acid amount of ZSM-5. It was also found on Pd/HZSM-5 (Si/Al2 = 24) that the formation of metal Pd and dispersed PdO was reversible upon reduction and oxidation treatments. These facts prove the presence of strong interaction between acid sites and PdO. The role of acid sites of zeolite was considered to keep the dispersed state of PdO. The adsorption of NO on highly dispersed PdO induced a significant change in the local structure of Pd at r...

Large zinc cation occupancy of octahedral sites in mechanically activated zinc ferrite powders

Abstract: The cation site occupancy of a mechanically activated nanocrystalline zinc ferrite powder was determined as (Zn0.552+Fe0.183+)tet[Zr0.452+Fe1.823+]octO4 through analysis of extended x-ray absorption fine structure measurements, showing a large redistribution of cations between sites compared to normal zinc ferrite samples. The overpopulation of cations in the octahedral sites was attributed to the ascendance in importance of the ionic radii over the crystal energy and bonding coordination in determining which interstitial sites are occupied in this structurally disordered powder. Slight changes are observed in the local atomic environment about the zinc cations, but not the iron cations, with respect to the spinel structure. The presence of Fe3+ on both sites is consistent with the measured room temperature magnetic properties.

Identification of active sites and adsorption complexes in Fe/MFI catalysts for NOx reduction

Abstract: FTIR, extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge spectroscopy (XANES) and ESR were used to identify the active sites and the adsorption complexes present in a variety of Fe/MFI samples prepared in different ways and displaying vastly different activities and selectivities in the reduction of NOx to N2 with hydrocarbons. Iron oxide particles, charged ferric oxide nano-clusters, isolated iron ions and oxygen-bridged binuclear iron ions have been identified with various degrees of reliability. In contact with appropriate gases, NO+ ions, mono- and dinitrosyl groups, nitro groups, nitrate ions and superoxide ions have been identified. Peroxide ions, though not detectable with the methods used here, were postulated by other authors on the basis of density functional calculations. The binuclear iron oxo-ions are more abundant in the best catalysts with high Fe/Al ratio that were prepared by sublimation. They are the most probable sites for NO oxidation to NO2 and its further conversion to adsorbed nitro groups and nitrate ions, steps that are crucial for NOx reduction. Superoxide and/or peroxide ions are likely involved in the NO oxidation to NO2. This process is fast when Fe/MFI is exposed to a mixture of NO+O2, but much slower if NO is chemisorbed first, before exposure to O2.

Cationic Order and Structure of [Zn−Cr−Cl] and [Cu−Cr−Cl] Layered Double Hydroxides: A XRD and EXAFS Study

Abstract: X-ray diffraction (XRD) and extended X-ray absorption fine structure (EXAFS) were used to evidence a cationic order within sheets of the layered double hydroxides (LDH) [Zn−Cr−Cl] and [Cu−Cr−Cl]. T...

In Situ X‐Ray Absorption Studies of a High‐Rate LiNi0.85Co0.15 O 2 Cathode Material

Abstract: We have performed an in situ X-ray absorption spectroscopy (XAS) study to investigate the evolution of the local electronic and atomic structure of a high-surface-area Li 1-x Ni 0.85 Co 0.15 O 2 (0 ≤ x ≤ 1) cathode material during electrochemical delithiation. We have measured the changes in the oxidation state, bond distance, coordination number, and local disorder of Ni and Co absorbers as a function of the state of charge of the material. The X-ray absorption near edge spectra shows that delithiation of Li 1-x Ni 0.85 Co 0.15 O 2 leads to the oxidation of Ni 3+ to Ni 4+ . Ni atoms oxidize during the initial stages of charge and attain a maximum oxidation state of Ni 4+ well before the end of charge (x 0.85). On the other hand, Co atoms do not oxidize during the initial stages of charge but oxidize close to the end of charge. Analysis of the extended X-ray absorption fine structure (EXAFS) shows that the oxidation of Ni 3+ to Ni 4+ leads to the expected reduction in the Jahn-Teller effect. Also, to within the accuracy of the EXAFS technique, Co absorbers occupy Ni-type sites in the NiO 2 slabs. Furthermore, Co doping has a strong effect on the overall structural evolution and leads to a slight expansion of the a and b axes close to the end of charge.

Structure and Bonding of a Site-Isolated Transition Metal Complex: Rhodium Dicarbonyl in Highly Dealuminated Zeolite Y

Abstract: The structure and bonding of rhodium dicarbonyl bonded to highly dealuminated zeolite Y has been determined by the combined application of extended X-ray absorption fine structure (EXAFS) and infrared spectroscopies and quantum chemical calculations based on density functional theory. The EXAFS and infrared spectra indicate the existence of nearly unique rhodium dicarbonyl species bonded at structurally equivalent positions in the zeolite pores. However, even this anchored structure, one of the simplest known, is not determined fully by the experimental results, and quantum chemical calculations were needed to eliminate the ambiguity. Taken together, the experimental and theoretical results indicate Rh+(CO)2 located at a four-ring of the faujasite framework; the rhodium center is bonded to two oxygen centers of the framework near an aluminum center with a Rh−O distance of 2.15−2.20 A. The results show how spectroscopy and theory used in combination can determine the structure and location of a metal compl...

The architecture of catalytically active centers in titanosilicate (TS-1) and related selective-oxidation catalysts

Abstract: Detailed X-ray spectroscopic studies yield the three-dimensional structure of Ti(IV) centered active sites in a series of TS-1 catalysts, in which titanium ions substitutionally replace a small fraction of the silicon sites in the pentasil structure, Silicalite-1. Advantage has been taken of the atomic structure of Ti(OSiPh3)4 , (determined separately by X-ray diffraction). A detailed EXAFS analysis of the X-ray absorption spectra of Ti(OSiPh3)4 which takes advantage of the full cluster multiple scattering methodology leads to the precise nature of the coordination of the Ti ions in a series of TS-1 samples. It proved possible to distinguish between the tetrapodal [i.e., Ti(OSi)4] and tripodal structure [i.e., Ti(OSi)3OH] shown (see T. Maschmeyer, F. Rey, G. Sankar and J. M. Thomas, Nature, 1995, 378, 159; ref. 1) to be present in high performance anchored titanosilicate epoxidation catalysts.

Metal Ion Coordination at the Water–Manganite (γ-MnOOH) Interface: II. An EXAFS Study of Zinc(II)

Abstract: The local structure of Cd(II) surface complexes adsorbed onto manganite (γ-MnOOH) has been investigated by extended X-ray absorption fine structure (EXAFS) spectroscopy. Adsorption experiments were carried out within the pH range 7.4–9.8 and with surface coverage from 2.2 to 8.9 μmol/m2. Quantitative analysis of the EXAFS spectra shows little difference in the local coordination environment of Cd regardless of the coverage. Analysis of the first shell required a third cumulant in an asymmetric distribution model (i.e., non-Gaussian distribution), indicating distorted CdO6 octahedra with an average Cd–O distance of 2.31 A. A single second shell of Mn neighbors at a constant distance of 3.33 A was detected. This comparatively short distance shows that inner-sphere complexes are formed. No Cd neighbor was found which means that cadmium hydroxide precipitation or cluster formation does not occur at the surface at the experimental conditions probed in this study. We assign the Cd–Mn distance to edge sharing between CdO6 and MnO6 octahedra, with a geometry similar to that in the mixed solid Cd2Mn3O8. This adsorption mode is possible on the {110} and {010} cleavage planes of manganite.

The metal–support interaction in Pt/Y zeolite: evidence for a shift in energy of metal d-valence orbitals by Pt–H shape resonance and atomic XAFS spectroscopy

Abstract: The turnover frequency (TOF) for conversion of neo-pentane was determined for Pt in Y zeolite with different numbers of protons and La C3 ions, different Si/Al ratios and with non-framework Al being present. Comparing Pt/NaY to Pt/H-NaY and Pt/K-USY with Pt/H-USY, respectively, shows an increase in the ln (TOF) which is proportional to the number of protons. Compared to NaY, the TOF of Pt in non-acidic NaLaY zeolite is about 25 times higher, which indicates also a strong influence on the charge of the cations on the TOF of Pt. The 20 times increase in the Pt TOF for K-USY compared to NaY is attributed to the effect of a higher Si/Al ratio and non-framework Al in the K-USY. EXAFS data collected on Pt/NaY and Pt/H-USY showed platinum particles consisting of 14‐20 atoms on an average. These results were confirmed by HRTEM, which also showed that the Pt particles were dispersed inside the zeolite. The EXAFS data indicate that the metal particles are in contact with the oxygen ions of the support. The peak in the Fourier transform of the atomic XAFS (AXAFS) spectrum of the Pt/H-USY is larger in intensity than the corresponding peak of the Pt/Na-Y data. A detailed analysis of the L2 and the L3 X-ray absorption near edge structure revealed a shape resonance due to the Pt‐H anti-bonding state (AS) induced by chemisorption of hydrogen on the surface of the platinum metal particles. The difference in energy (Eres) between the AS and the Fermi-level (EF) is 4.7 eV larger for Pt/H-USY than for Pt/NaY. Both the AXAFS spectra and the shape resonances of the Pt-NaY and the Pt/H-USY catalysts provide direct experimental evidence of how the support properties determine the electronic structure of the platinum metal particles. Previous AXAFS and shape resonance work lead to a model in which the position in energy of the Pt valence orbitals is directly influenced by changes in the potential (i.e. electron charge) of the oxygen ions of the support and how the proton density affects this oxygen charge. This work shows that the potential of the oxygen ions is also a function of the Si/Al ratio of the support and the polarisation power of the charge compensating cations (H C ,N a C ,L a 3C and extra-framework Al); the metal particles experience an interaction which is determined by several properties of the support. The data further reveal how the change in the Pt electronic structure directly influences the catalytic properties of the catalyst. While the TOF is dependent on the metal‐support interaction, the hydrogenolysis selectivity is determined by the Pt particle size, and increases linearly with increasing dispersion, or decreasing particle size. ©2000 Elsevier Science B.V. All rights reserved.

Structure and luminescence of annealed nanoparticles of ZnS:Mn

Abstract: Structural and light-emitting properties of nanoparticles of ZnS:Mn annealed in vacuum at temperatures up to 525 °C are presented. Annealing the 3.5 nm particles at temperatures up to 350 °C caused growth of some particles without substantial change in the luminescence or ZnS lattice. After annealing at 400–525 °C, the high-temperature wurtzite phase of ZnS appeared, accompanied by an increase of the average particle diameter to approximately 100 nm and a rearrangement of the Mn ions. Dramatic increase in cathodoluminescence emission was also observed and is compared to the structural information obtained from electron microscopy, x-ray diffraction, x-ray absorption fine structure, and electron paramagnetic resonance measurements.

XANES study of Cu2+-binding sites in aquatic humic substances.

Abstract: The structure and composition of the inner shell of copper (II) complexes with aquatic humic substances (HS) were studied by X-ray absorption near-edge structure (XANES) spectroscopy. The interpretation of the XANES data was based on FEFF8 code real-space multiple-scattering calculations for the prototype [Cu(H2O)6]2+ complex. These calculations reproduced the tetragonal distortion of the CuO6 octahedron representing the inner shell. They also demonstrated the polarization nature of this effect's manifestations (“splitting” of the derivative) in XANES. The extension of this interpretation to the Cu−HS system confirmed suppression of the tetragonal distortion of the inner shell in these species established by independent EXAFS experiments. XANES measurements for Cu/C ratios varying from 0.0005 to 0.03 demonstrated the nonuniform nature of copper binding sites. Experiments with Cu−ethylenediamine and Cu−glycine model systems and FEFF8 calculations indicated that for low copper concentrations (Cu/C < 0.005) ...

X-ray photoelectron spectroscopy and x-ray absorption near edge structure study of copper sites hosted at the internal surface of ZSM-5 zeolite: A comparison with quantitative and energetic data on the CO and NH3 adsorption

Abstract: The oxidation state of Cu species dispersed in a Cu-ZSM-5 zeolite obtained by a nonconventional gas-phase CuCl exchange, and nominally containing only Cu(I) species, was studied by x-ray photoelectron spectroscopy (XPS) and x-ray absorption near edge structure (XANES) analyses. The oxidation of Cu(I) species to Cu(II) by simple exposure to the atmosphere and subsequent reduction by thermal activation in vacuo was monitored. The quantitative and energetic aspects of the formation of carbonyl-like and amino-complexes at the metallic sites was studied by means of adsorption microcalorimetry. CO and NH3 were used as probe molecules in order to assess the coordinative unsaturation of the Cu(I) cations. Adsorption heats comprised in the 130–40 kJ mol−1 interval were obtained for the formation of both type of complexes. The perturbation induced on the Cu centers and/or on the zeolite matrix by the adsorption of the probe molecules was monitored by parallel experiments of XPS, IR, and XANES. A significant fractio...

An X-ray absorption spectroscopy study of the Fe K edge in nanosized maghemite and in Fe2O3–SiO2 nanocomposites

Abstract: The structural evolution of Fe2O3–SiO2 nanocomposite samples prepared using the sol–gel method have been studied by EXAFS (extended X-ray absorption fine structure) and XANES (X-ray absorption near edge structure) techniques in comparison with pure nanosized and microcrystalline maghemite samples. EXAFS data show that the difference between nanosized and microcrystalline maghemite is due to an increase of disorder of the superficial sites while no decrease of coordination numbers was detected. EXAFS and XANES data of the nanocomposite thermally treated at 900°C indicate that the sample is constituted by maghemite nanoparticles embedded into amorphous silica while the sample thermally treated at 300°C contains an amorphous precursor which is very likely to be ferrihydrite. No interactions between the metal oxide nanoparticles and the silica network are present since the EXAFS spectra of the nanocomposite samples can be interpreted as only having Fe–O and Fe–Fe interactions.