Showing papers on "XANES published in 2004"

X-ray absorption spectroscopy of low temperature fuel cell catalysts.

Abstract: 1.Introduction 2.X-ray Absorption Spectroscopy 2.1. XANES 2.2. EXAFS 3.Data Collection and In Situ Cells 4.XAS as a Characterization Method: Pt/C 4.1. Particle Size 4.2. Potential Dependence 4.3. Adsorbates 5.Pt Containing Alloy Catalysts 5.1. PtRu Alloys 5.1.1. Compositional Analysis 5.1.2. Potential Dependence 5.1.3. Adsorbates 5.2. Other Pt Containing Alloy Anode Catalysts 5.3. Pt Containing Alloy Cathode Catalysts 6.Non-Pt Catalysts 7.Conclusion 8.References

Investigation of the Structural and Electrochemical Properties of Size-Controlled SnO2 Nanoparticles

Abstract: Four types of size-controlled SnO2 nanoparticles were synthesized by a colloidal method to investigate the structural and electrochemical properties of various particle sizes. The samples were characterized by means of transmission electron microscopy (TEM), powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), near-edge X-ray absorption fine structure (NEXAFS), and electrochemical measurements. TEM and XRD analyses showed that the samples were well dispersed with an average particle size of ∼20 nm (sample A), ∼11 nm (sample B), ∼5.6 nm (sample C), and ∼2.3 nm (sample D). XPS analyses of four size-controlled SnO2 samples confirmed that the chemical state of the Sn was Sn4+, as evidenced by the presence of SnO2, not Sn2+ related to the Sn(OH)2. The Sn NEXAFS spectra of the four samples indicated that the samples had a tetragonal rutile structure and showed no noticeable change in the oxidation state of tin ions with decreased particle size, in good agreement with the XPS analysis. The ele...

X-ray absorption spectroscopic study of Fe reference compounds for the analysis of natural sediments

Abstract: Synchrotron X-ray absorption spectroscopy (XAS) is becoming an increasingly popular tool for the analysis of element speciation in complex natural mixtures such as soils and sediments. Identification of a particular mineral or amorphous solid in a heterogeneous mixture by XAS depends on the spectral uniqueness of the element in the bonding environment associated with a component, and on absorption effects from the components and the matrix. A suite of 27 common, Fe-bearing reference compounds, including sulfides, carbonates, phosphates, oxides, oxyhydroxides, and phyllosilicates, was analyzed to empirically assess the utility of X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) for identifying a particular Fe mineral (or class of minerals) in a soil or sediment mixture. We show that XANES spectral features are useful for distinguishing qualitatively among major mineral classes, but not necessarily for identifying minerals within classes. A practical detection limit (based on empirical mixtures) for most mineral classes is on the order of 5% of the total atomic Fe absorption, but detection limits vary depending on the spectral uniqueness of the components, the number of components, and the matrix. Calibration curves for Fe sulfide and non-sulfide (phyllosilicate ± oxide) component mixtures were made independently from the analyses of XANES and EXAFS fluorescence spectra of binary and ternary mineral mixtures (at 5% total Fe) in a quartz matrix to mimic natural sediments. Because of differences in sample and matrix absorption and fluorescence among sulfide and phyllosilicate minerals, apparent component fractions of pyrite derived from fits deviated significantly from linear binary mixtures. We show that corrections for non-linear fluorescence (as much as 20%) are particularly important for binary mineral mixtures with different densities and absorption characteristics (such as sulfides and phyllosilicates), and for mixtures with similar component abundances (i.e., far from one end-member). An application of the fluorescence calibration for XANES and EXAFS is shown for natural samples. This study points out the importance of a high-quality, experimentally consistent reference compound library, and the need for calibration of fluorescence spectra, in order to quantify accurately the component fractions of complex mixtures using XAS.

X-ray absorption study of Ti-activated sodium aluminum hydride

Abstract: Ti K-edge x-ray absorption near-edge spectroscopy was used to explore the Ti valence and coordination in Ti-activated sodium alanate. An empirical relationship was established between the Ti valence and the Ti K-edge onset based on a set of standards. This relationship was used to estimate oxidation states of the titanium catalyst in 2 and 4mol% Ti-doped NaAlH4. The results demonstrate that the formal titanium valence is zero in doped sodium alanate and nearly invariant during hydrogen cycling. A qualitative comparison of the edge fine structure suggests that the Ti is present on the surface in the form of amorphous TiAl3.

Time-resolved Studies for the Mechanism of Reduction of Copper Oxides with Carbon Monoxide: Complex Behavior of Lattice Oxygen and the Formation of Suboxides

Abstract: Temperature-programmed reduction (TPR), synchrotron-based time-resolved X-ray diffraction (TR-XRD), extended X-ray absorption fine-structure (EXAFS), and X-ray absorption near-edge structure (XANES) were employed in this work to systematically study the reaction of CuO and Cu2O with CO gas molecules. Both TPR and isothermal reduction results showed that CuO was easier to reduce than Cu2O under the same reaction conditions. In situ measurements of XRD and XANES showed a direct transformation pathway for CuO reduction (CuO → Cu) when there was a large supply of CO, while they showed a sequential step pathway involving one intermediate (CuO → Cu2O → Cu) with a limited supply of CO. An induction period for CuO reduction was seen and increased with decreasing temperature. O vacancies in CuO were observed during the induction period that could foster formation of a nonstoichiometric metastable copper-oxide species. The metastable species can either react rapidly with CO to form metallic copper during high CO fl...

The effect of redox state on the local structural environment of iron in silicate glasses: A combined XAFS spectroscopy, molecular dynamics, and bond valence study

Abstract: A series of 27 silicate glasses of various compositions containing 0.2–2 at.% iron were synthesized at various oxygen fugacity values. The glasses were examined using X-ray absorption fine structure (XANES) spectroscopy at the Fe K-edge in order to determine iron oxidation state and first-neighbor coordination number. Spectral information extracted from the pre-edge region and principal component analysis (PCA) of the XANES region, together with a spectral inversion, were used to derive the end-member spectral components for Fe(II) and Fe(III). Linear trends in the pre-edge features were observed for most compositional series of the glasses examined as a function of Fe(II)/Fe(III) content. These linear trends are believed to be due to the similarity of average coordination numbers for both Fe(II) and Fe(III) end-members in each series. This result is consistent with model simulations of the XANES region and molecular dynamics (MD) simulations for the two end-member compositions which also show that Fe(II) and Fe(III) have similar average coordination numbers. These simulations also suggest the presence of five-coordinated Fe(III) in the melt phase. Based on a bond valence analysis of these MD simulations, a simple model is proposed to help predict the speciation of iron in oxide and silicate glasses and melts.

Electronic structure of titania aerogels from soft x-ray absorption spectroscopy

Abstract: The electronic structure of ${\mathrm{TiO}}_{2}$ aerogels is studied by soft x-ray absorption near-edge structure (XANES) spectroscopy. High-resolution O K-edge and Ti ${L}_{2,3}$-edge XANES spectra of aerogels are compared with those of rutile, anatase, and unrelaxed amorphous phases of full-density ${\mathrm{TiO}}_{2}.$ Results show that all the main spectroscopic features of aerogels, reflecting the element-specific partial density of empty electronic states and correlation effects, can be attributed to the absence of long-range order in stoichiometric amorphous ${\mathrm{TiO}}_{2}.$ Based on these results, we discuss the effects of short- and long-range order on the electronic structure of ${\mathrm{TiO}}_{2}.$

X-ray absorption study of Ti-activated sodium aluminum hydride

Abstract: Ti K-edge x-ray absorption near edge spectroscopy (XANES) was used to explore the Ti valence and coordination in Ti-activated sodium alanate. An empirical relationship was established between the Ti valence and the Ti K-edge onset based on a set of standards. This relationship was used to estimate oxidation states of the titanium catalyst in 2 mol% and 4 mol% Ti-doped NaAlH4. These results demonstrate that the formal titanium valence is zero in doped sodium alanate and nearly invariant during hydrogen cycling. A qualitative comparison of the edge fine structure suggests that the Ti is present on the surface in the form of amorphous TiAl3.

A XANES determination of the oxidation state of chromium in silicate glasses

Abstract: Cr K -edge X-ray absorption near-edge structure (XANES) spectra were recorded for silicate glasses of various compositions in the system CaO + MgO + Al2O3 + SiO2 ± TiO2, quenched from melts equilibrated as a function of oxygen fugacity ( f O2) at 1400°C. The spectra vary systematically with f O2 (log f O2 ~0 to −16) indicating changes in the Cr oxidation state. The intensity of a shoulder on the absorption edge (attributed to the 1 s → 4 s transition) was quantified using the corresponding peak in the XANES derivative spectrum and used to determine Cr2+/∑Cr. The resulting Cr2+/∑Cr values are in agreement with the theoretical dependence on log f O2 suggesting that the 1s → 4s feature is diagnostic of Cr2+ in a silicate glass and σ(Cr2+/∑Cr) = ± 0.015. The Cr2+/∑Cr ratio for a given log f O2 changes with the glass composition which may reflect the ability of the melt to accommodate the Jahn-Teller distorted coordination geometry which stabilizes Cr2+. Cr2+/∑Cr varies between ~0.3 and 0.8 over the log f O2 range bounded by the Ni/NiO and Fe/FeO f O2 buffers, suggesting that Cr2+ may be important in natural melts even though this oxidation state has never been identified in a terrestrial material. The development of a methodology for determining Cr2+/∑Cr from XANES spectra of quenched glasses is an essential precursor to in situ experiments on Fe-bearing silicate melts at high temperature.

Structure and Composition of Cu-Doped CdSe Nanocrystals Using Soft X-ray Absorption Spectroscopy

Abstract: The local structure and composition of Cu ions dispersed in CdSe nanocrystals is examined using soft X-ray absorption near edge spectroscopy (XANES). Using Cu L-edge XANES and X-ray photoelectron measurements (XPS), we find that the Cu ions exist in the Cu(I) oxidation state. We also find that the observed Cu L-edge XANES signal is directly proportional to the molar percent of Cu present in our final material. Se L-edge XANES indicates changes in the Se density of states with Cu doping, due to a chemical bonding effect, and supports a statistical doping mechanism. Photoluminescence (PL) measurements indicate that Cu ions may act as deep electron traps. We show that XANES, XPS, and PL are a powerful combination of methods to study the electronic and chemical structure of dopants in nanostructured materials.

Calculation of near-edge x-ray-absorption fine structure at finite temperatures: Spectral signatures of hydrogen bond breaking in liquid water

Abstract: We calculate the near-edge x-ray-absorption fine structure of H(2)O in the gas, hexagonal ice, and liquid phases using heuristic density-functional based methods. We present a detailed comparison of our results with experiment. The differences between the ice and water spectra can be rationalized in terms of the breaking of hydrogen bonds around the absorbing molecule. In particular the increase in the pre-edge absorption feature from ice to water is shown to be due to the breaking of a donor hydrogen bond. We also find that in water approximately 19% of hydrogen bonds are broken.

Higher Order Speciation Effects on Plutonium L3 X-ray Absorption Near Edge Spectra

Abstract: Pu L3 X-ray near edge absorption spectra for Pu(0−VII) are reported for more than 60 chalcogenides, chlorides, hydrates, hydroxides, nitrates, carbonates, oxy-hydroxides, and other compounds both as solids and in solution, and substituted in zirconolite, perovskite, and borosilicate glass. This large database extends the known correlations between the energy and shape of these spectra from the usual association of the XANES with valence and site symmetry to higher order chemical effects. Because of the large number of compounds of these different types, a number of novel and unexpected behaviors are observed, such as effects resulting from the medium and disorder that can be as large as those from valence.

Incorporation of Uranium in Zirconolite (CaZrTi2O7)

Abstract: Up to 015 formula unit (fu) of U4+ incorporated in the Zr site of zirconolite by firing CaUxZr(1−x)Ti2O7 compositions in argon at 1400°C allows retention of the 2M polytype Further U4+ substitution for Zr, up to 04 fu, produces the 4M polytype (containing ∼04 fu of U) plus the 2M polytype containing 015 fu of U The pyrochlore structure (containing 06 fu of U) forms in conjunction with the 4M polytype at U contents of 04 fu up to 07 fu Higher U contents give the pyrochlore structure, but the solid-state reactivity of even alkoxide-based preparations becomes increasingly poor for x > ∼07 and hot pressing in graphite dies at ∼1250°C is necessary to achieve near single-phase pyrochlore structures for x= 1 When samples of CaUxZr1−xTi2O7 stoichiometry (x= 01 and 02) are oxidized at 1400°C in air, diffuse reflectance spectroscopy (DRS) shows evidence for U5+ formation at the expense of U4+ via enhanced absorption bands, ∼50 nm in half-width, near 970 and 1500 nm and correspondingly weakened absorption at 1150 and 1660 nm Weight gains consistent with complete oxidation of U4+ to U5+ are observed when finely powdered argon-fired samples with x= 01 and 04 are heated in air to 1200°C Evidence for U valence states higher than +4 in both argon- and air-heated materials containing charge compensators to encourage U5+ or U6+ formation was also derived from DRS (showing U5+ in particular), and XANES DRS shows weak absorption bands attributable to U4+ in zirconolites containing ∼02 fu of U incorporated in the Ca site via Mg or Al substitutions in the Ti sites, with the spectrum being closely but not exactly similar to that attributed to U4+ in the Zr site Zirconolite and pyrochlore compositions which are melted at 1500° or 1550°C in argon and furnace-cooled yield broadly similar phase assemblages to the corresponding sintered materials, but there is evidence of incongruent melting in all materials

Chemical State and Local Structure around Titanium Atoms in NaAlH4 Doped with TiCl3 Using X-ray Absorption Spectroscopy

Abstract: X-ray absorption spectroscopy is applied to investigate NaAlH4 doped with 5 mol % TiCl3 by ball milling. XANES analysis indicates that after the ball milling Ti species are reduced from Ti (+3) to Ti (0). Release or absorption of hydrogen does not affect the chemical state obtained after ball milling. EXAFS analysis shows that the Ti atoms are associated only with Ti as next neighbors in the ball-milled state as well as during subsequent desorption and absorption of hydrogen. Furthermore, an increase of the particle size and an ordering of the local structure is seen to evolve with the desorption and the absorption of hydrogen.

Experimental and theoretical studies on the structure of N-doped carbon nanotubes: Possibility of intercalated molecular N2

Abstract: The concentration distribution and electronic structure of N atoms doped in multiwalled banboo-like carbon nanotubes (CNTs) are examined by photon energy-dependent x-ray photoelectron spectroscopy and x-ray absorption near edge structure. The inner part of the nanotube wall has a higher N concentration and contains molecular N2 presumably intercalated between the graphite layers. These results are supported by the self-consistent charge-density-functional-based tight-binding calculation of double-walled CNTs, showing that the intercalation of N2 is energetically possible and the graphite-like N structure conformer becomes more stable when the inner wall is more heavily doped.

Electronic structure of ZnO nanorods studied by angle-dependent x-ray absorption spectroscopy and scanning photoelectron microscopy

Abstract: Angle-dependent x-ray absorption near-edge structure (XANES) and scanning photoelectron microscopy measurements were performed to differentiate local electronic structures at the tips and sidewalls of highly aligned ZnO nanorods. The overall intensity of the O K-edge XANES spectra is greatly enhanced for small photon incident angles. In contrast, the overall intensity of the Zn K-edge XANES is much less sensitive to the photon incident angle. Both valence-band photoemission and O K-edge XANES spectra show substantial enhancement of O 2p derived states near the valence band maximum and conduction band minimum, respectively. The spatially resolved Zn 3d core level spectra from tip and sidewall regions show the lack of chemical shift. All the results consistently suggest that the tip surfaces of the highly aligned ZnO nanorods are terminated by O ions and the nanorods are oriented in the [0001] direction.

Diameter dependence of the electronic structure of ZnO nanorods determined by x-ray absorption spectroscopy and scanning photoelectron microscopy

Abstract: OK-, ZnL3, and K-edges x-ray absorption near-edge structure (XANES) spectra and scanning photoelectron microscopy (SPEM) spectra were obtained for ZnO nanorods with various diameters. The analysis of the XANES spectra revealed increased numbers of O2p and Zn4p unoccupied states with the downsizing of the nanorods, which reflects the enhancement of surface states when the diameter is decreased. Valence-band photoemission spectra show a significant narrowing of the valence band for the 45nm diameter nanorod. The Zn3d intensities in the Zn3d SPEM spectra are drastically diminished for all nanorods as compared to the ZnO reference film, which can be interpreted as a reduction in density of itinerant final states or in transition probability.

Soft X-Ray Absorption Spectroscopic Study of a LiNi0.5Mn0.5 O 2 Cathode during Charge

Abstract: Soft X-ray (200 to 1000 eV) absorption spectroscopy at the O K-edge and the metal L II,III -edges, in both the fluorescence yield (FY) and the partial electron yield (PEY) mode, has been used to probe the electronic structure of electrochemically deintercalated Li 1-x Ni 05 Mn 05 O 2 FY and PEY spectra of the transition metal L II,III -edges, indicated that Mn ions remain mostly unchanged in the Mn 4+ state at all levels of charge However, the Ni FY L-edge spectra show a continuous shift to higher energy during charge, but remain mostly unchanged in the PEY data The results of the FY data show that the Ni ions in the bulk are oxidized form Ni 2+ to Ni 4+ during charge The difference between the surface-sensitive PEY data and the bulk-sensitive FY data indicates that the surface of Li 1-x Ni 05 Mn 05 O 2 has a different electronic structure than the bulk The shift in the O K-edge to lower energies and the development of a shoulder on the low energy side of the first pre-edge peak indicates that the holes compensating the lithium ion deintercalation are located in O 2p states as well as Ni 3d states These results show that soft X-ray absorption is a powerful technique for studying the electronic structure of new battery materials and it provides unique complementary information to that obtained from hard X-ray (above 1000 eV) absorption studies at the transition metal K-edges

Systematic empirical analysis of calcium?oxygen coordination environment by calcium K-edge XANES

Abstract: The X-ray absorption near edge structure (XANES) at the calcium K-edge is rich in information, but complex and difficult to interpret fully. We present here a systematic study of a range of calcium/oxygen containing compounds and minerals and show that the XANES may be used to obtain qualitative information on the calcium coordination environment.

Fischer-Tropsch synthesis: study of the promotion of Pt on the reduction property of Co/Al2O3 catalysts by in situ EXAFS of Co K and Pt LIII edges and XPS.

Abstract: The addition of platinum metal to cobalt/alumina-based Fischer-Tropsch synthesis (FTS) catalysts increases both the reduction rate and, consequently, the density of active cobalt sites. Platinum also lowers the temperature of the two-step conversion of cobalt oxide to cobalt metal observed in temperature programmed reduction (TPR) as Co3O4 to CoO and CoO to Co0. The interaction of the alumina support with cobalt oxide ultimately determines the active site density of the catalyst surface. This interaction can be controlled by varying the cobalt loading and dispersion, selecting supports with differing surface areas or pore sizes, or changing the noble metal promoter. However, the active site density is observed to depend primarily on the cluster size and extent of reduction, and there is a direct relationship between site density and FTS rate. In this work, in situ extended X-ray absorption fine structure (EXAFS) at the LIII edge of Pt was used to show that isolated Pt atoms interact with supported cobalt clusters without forming observable Pt--Pt bonds. K-edge EXAFS was also used to verify that the cobalt cluster size increases slightly for those systems with Pt promotion. X-ray absorption near-edge spectroscopy (XANES) was used to examine the remaining cobalt clusters after the first stage of TPR, and it revealed that the species were almost entirely cobalt (II) oxide. After the second stage of TPR to form cobalt metal, a residual oxide persists in the sample, and this oxide has been identified as cobalt (II) aluminate using X-ray photoelectron spectroscopy (XPS). Sequential in situ reduction of promoted and unpromoted systems was also monitored through XPS, and Pt was seen to increase the extent of cobalt reduction by a factor of two.

Determination of Sulfur Environments in Borosilicate Waste Glasses Using X-ray Absorption Near-Edge Spectroscopy

Abstract: Sulfur can be the waste-loading limiting constituent for vitrification of sulfur-bearing radioactive wastes due to low solubility in silicate melts. Methods to improve sulfur loading would benefit from improved understanding of the structural aspects of sulfur incorporation in borosilicate and other glasses. To this end, sulfur XANES spectra were collected for eight crystalline standards and twenty-four glasses, including borosilicate, phosphate, and borate compositions. Spectra for the standards show a systematic energy shift of the sulfur K-edge from 2469 to 2482 eV, as sulfur valence increases from 2− (in sulfides) to 6+ (in sulfates). Most crucible glasses investigated have simple edges near 2482 eV that indicate sulfur in the form of sulfate only. Other glasses, some synthesized under reducing conditions, have complicated edges, indicating sulfate, sulfite, and more reduced species that may include S, S–S doublets, or short polysulfide chains. Sulfide species (S2−) were not dominant in any of the samples over the range of redox conditions investigated. These results indicate that sulfur incorporation is considerably more complex than would be suggested by the conventional interpretation of the redox-dependence of sulfur solubility, which considers only sulfate and sulfide species. Raman data indicate that several of the glasses investigated are not homogeneous with regard to all sulfur species.

Kβ-detected XANES of framework-substituted FeZSM-5 zeolites

Abstract: The valence and local symmetry of iron in framework-substituted FeZSM-5 with a high Fe dilution (Si/Fe = 360) was studied by means of Kβ-detected X-ray absorption spectroscopy. This technique combines high-resolution (ΔE ∼1 eV) fluorescence detection of the 3p to 1s (Kβ) transition with the X-ray absorption near-edge structure (XANES) at the Fe K-edge. An absorption-like spectrum is recorded by detecting the Kβ fluorescence intensity as a function of the incident energy that is scanned through the K absorption edge. Kβ-detected XANES spectra allow for a more precise separation of the weak K pre-edge structure from the main edge as compared to conventional absorption spectroscopy. Subsequent analysis and interpretation of the pre-edge spectral features therefore is more accurate. The pre-edge is sensitive to changes in the local coordination and oxidation state of Fe. Using this technique we were able to quantitatively determine the degree of iron extraction out of a zeolite framework upon steaming. With t...

Full quantitative multiple-scattering analysis of X-ray absorption spectra: application to potassium hexacyanoferrat(II) and -(III) complexes.

Abstract: A recently developed method to the full quantitative analysis of the XAS spectra extending from the absorption edge to the high-energy region is presented. This method is based on the use of two independent approaches to the analysis of the EXAFS and XANES data, the well-known GNXAS and the newly developed MXAN procedures. Herein, we report the application of this technique to two iron complexes of known structure where multiple-scattering effects are prominent, the potassium hexacyanoferrat(II) and -(III) crystals and aqueous solutions. The structural parameters obtained from refinements using the two methods are equal and compare quite well with crystallographic values. Small discrepancies between the experimental and calculated XANES spectra have been observed, and their origin has been investigated in the framework of non-muffin-tin correction. The ligand dependence of the theoretical spectra has been also examined. Analysis of the whole energy range of the XAS spectra has been found to be useful in elucidating both the type of ligands and the geometry of iron sites. These results are of particular use in studying the geometrical environment of metallic sites in proteins and complexes of chemical interest.