Showing papers on "XANES published in 2005"

Investigation of the Charge Compensation Mechanism on the Electrochemically Li-Ion Deintercalated Li1-xCo1/3Ni1/3Mn1/3O2 Electrode System by Combination of Soft and Hard X-ray Absorption Spectroscopy

Abstract: In situ hard X-ray absorption spectroscopy (XAS) at metal K-edges and soft XAS at O K-edge and metal L-edges have been carried out during the first charging process for the layered Li1-xCo1/3Ni1/3Mn1/3O2 cathode material. The metal K-edge XANES results show that the major charge compensation at the metal site during Li-ion deintercalation is achieved by the oxidation of Ni2+ ions, while the manganese ions and the cobalt ions remain mostly unchanged in the Mn4+ and Co3+ state. These conclusions are in good agreement with the results of the metal K-edge EXAFS data. Metal L-edge XAS results at different charge states in both the FY and PEY modes show that, unlike Mn and Co ions, Ni ions at the surface are oxidized to Ni3+ during charge, whereas Ni ions in the bulk are further oxidized to Ni4+ during charge. From the observation of O K-edge XAS results, we can conclude that a large portion of the charge compensation during Li-ion deintercalation is achieved in the oxygen site. By comparison to our earlier res...

Near‐edge X‐ray absorption fine structure (NEXAFS) spectroscopy for mapping nano‐scale distribution of organic carbon forms in soil: Application to black carbon particles

Abstract: Received 17 December 2004; accepted 3 January 2005; published 16 February 2005. [1] Small-scale heterogeneity of organic carbon (C) forms in soils is poorly quantified since appropriate analytical techniques were not available up to now. Specifically, tools for the identification of functional groups on the surface of micrometer-sized black C particles were not available up to now. Scanning Transmission X-ray Microscopy (STXM) using synchrotron radiation was used in conjunction with Near-Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy to investigate nano-scale distribution (50-nm resolution) of C forms in black C particles and compared to synchrotron-based FTIR spectroscopy. A new embedding technique was developed that did not build on a C-based embedding medium and did not pose the risk of heat damage to the sample. Elemental sulfur (S) was melted to 220� C until it polymerized and quenched with liquid N2 to obtain a very viscous plastic S in which the black C could be embedded until it hardened to a noncrystalline state and was ultrasectioned. Principal component and cluster analysis followed by singular value decomposition was able to resolve distinct areas in a black carbon particle. The core of the studied biomass-derived black C particles was highly aromatic even after thousands of years of exposure in soil and resembled the spectral characteristics of fresh charcoal. Surrounding this core and on the surface of the black C particle, however, much larger proportions of carboxylic and phenolic C forms were identified that were spatially and structurally distinct from the core of the particle. Cluster analysis provided evidence for both oxidation of the black C particle itself as well as adsorption of non-black C. NEXAFS spectroscopy has great potential to allow new insight into black C properties with important implications for biogeochemical cycles such as mineralization of black C in soils and sediments, and adsorption of C, nutrients, and pollutants as well as transport in the geosphere, hydrosphere, and atmosphere.

Structural and oxidation state changes of the photosystem II manganese complex in four transitions of the water oxidation cycle (S0 --> S1, S1 --> S2, S2 --> S3, and S3,4 --> S0) characterized by X-ray absorption spectroscopy at 20 K and room temperature.

Abstract: Structural and electronic changes (oxidation states) of the Mn4Ca complex of photosystem II (PSII) in the water oxidation cycle are of prime interest. For all four transitions between semistable S-states (S0 → S1, S1 → S2, S2 → S3, and S3,4 → S0), oxidation state and structural changes of the Mn complex were investigated by X-ray absorption spectroscopy (XAS) not only at 20 K but also at room temperature (RT) where water oxidation is functional. Three distinct experimental approaches were used: (1) illumination-freeze approach (XAS at 20 K), (2) flash-and-rapid-scan approach (RT), and (3) a novel time scan/sampling-XAS method (RT) facilitating particularly direct monitoring of the spectral changes in the S-state cycle. The rate of X-ray photoreduction was quantitatively assessed, and it was thus verified that the Mn ions remained in their initial oxidation state throughout the data collection period (>90%, at 20 K and at RT, for all S-states). Analysis of the complete XANES and EXAFS data sets (20 K and ...

Ab initio and experimental pre-edge investigations of the Mn K -edge XANES in oxide-type materials

Abstract: Mn $K$ edge ab initio FEFF8.2 calculations of the pre-edge features of the x-ray-absorption near-edge structure (XANES) region were undertaken for a series of Mn-bearing oxide-type compounds. The aim of the study is to provide a reliable method for determining quantitative and accurate redox and symmetry information for manganese. In agreement with multiplet calculations by Glatzel and co-workers, FEFF8.2 predicts a doublet and a triplet for Mn(II) and Mn(III) in octahedral symmetry, respectively, in agreement with high-resolution XANES experiments. Site distortion increases notably the contribution from dipolar transitions and, consequently, the pre-edge feature integrated area. An even more intense pre-edge feature is calculated and measured for the ${T}_{d}$ symmetry (singletlike). For Mn(IV), a triplet is predicted and measured for the ${O}_{h}$ symmetry. However, additional transitions are found in Mn(IV)-rich compounds, that are related to metal-metal transitions. These transitions overlap strongly with the ``true pre-edge,'' making extraction of redox and symmetry information for Mn(IV) more challenging. However, a model of the pre-edge with pseudo-Voigt functions of fixed calculated width (based on core-hole lifetime and experimental resolution) helps to separate the contributions related to first-neighbor symmetry from those of the metal-metal pairs. Application to multivalent defective manganese oxide materials suggests that the pre-edge information varies linearly as a function of Mn redox state or symmetry but varies nonlinearly as a function of both parameters. Finally, the polymerization of the manganese networks can be estimated from the metal-metal transitions found in the pre-edge region.

Photocatalytic Hydrogen Production from Water over M-Doped La2Ti2O7 (M = Cr, Fe) under Visible Light Irradiation (λ > 420 nm)†

Abstract: In the search for efficient photocatalysts working under visible light, we have investigated the effect of cation substitution on a layered perovskite, La2Ti2O7. Among various metal dopants, only Cr and Fe induced intense absorption of visible light (λ > 400 nm), and only these catalysts produced H2 photocatalytically from water in the presence of methanol under visible light irradiation (λ > 420 nm). The polymerized complex method was found to be more efficient for fabrication of the present catalysts producing a more homogeneous structure than the solid-state reaction. The characterization by XRD, UV−vis DRS, XPS, and XANES revealed that doped Cr and Fe were present in the Cr3+ and Fe3+ states substituting for Ti sites in the La2Ti2O7 lattice. The theoretical calculation indicated that the most significant feature in the electronic band structure of the metal-doped La2Ti2O7 was the formation of a partially filled 3d band in the band gap of La2Ti2O7, while the contribution of these dopants on the valence...

In-Situ X-ray Absorption Spectroscopic Study on Variation of Electronic Transitions and Local Structure of LiNi1/3Co1/3Mn1/3O2 Cathode Material during Electrochemical Cycling

Abstract: In-situ X-ray absorption spectroscopic investigations have been carried out to examine the changes of the electronic transitions and local structure at the Mn, Co, and Ni K-edge for the LiNi1/3Co1/3Mn1/3O2 electrode during charging and discharging process in this study. It was found that only Ni atom in Li1-xNi1/3Co1/3Mn1/3O2 is electroactive from the evolution of the XANES spectra and the bond length variation of Ni−O. It was found that the redox pairs of Ni2+/Ni3+ and Ni3+/Ni4+ exist and the oxidation states of Mn and Co remain as MnIV and CoIII, respectively, in Li1-xNi1/3Co1/3Mn1/3O2 upon charge and discharge. The oxygen, rather than the transition-metal ions (Ni, Co, and Mn), functions as electron donor at the end of charge. In addition, the irreversible capacity at the first cycle derives mainly from the appearance of inactive Ni, which is evidenced by the energy shift E − E0 of the absorption edge for the Ni absorber and the bond length change of the Ni−O. A decrease/increase of Debye−Waller factor...

XANES Investigation of Phosphate Sorption in Single and Binary Systems of Iron and Aluminum Oxide Minerals

Abstract: Phosphate sorption on Fe- and Al-oxide minerals helps regulate the solubility and mobility of P in the environment. The objective of this study was to characterize phosphate adsorption and precipitation in single and binary systems of Fe- and Al-oxide minerals. Varying concentrations of phosphate were reacted for 42 h in aqueous suspensions containing goethite, ferrihydrite, boehmite, or noncrystalline (non-xl) Al-hydroxide, and in 1:1 (by mass) mixed-mineral suspensions of goethite/boehmite and ferrihydrite/non-xl Al-hydroxide at pH 6 and 22 °C. X-ray absorption near edge structure (XANES) spectroscopy was used to detect precipitated phosphate and distinguish PO4 associated with Fe(III) versus Al(III) in mixed-mineral systems. Changes in the full width at half-maximum height (fwhm) in the white-line peak in P K-XANES spectra provided evidence for precipitation in Al-oxide single-mineral systems, but not in goethite or ferrihydrite systems. Similarly, adsorption isotherms and XANES data showed evidence fo...

DFT and in situ EXAFS investigation of gold/ceria-zirconia low-temperature water gas shift catalysts: identification of the nature of the active form of gold.

Abstract: A combined experimental and theoretical investigation of the nature of the active form of gold in oxide-supported gold catalysts for the water gas shift reaction has been performed. In situ extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES) experiments have shown that in the fresh catalysts the gold is in the form of highly dispersed gold ions. However, under water gas shift reaction conditions, even at temperatures as low as 100 °C, the evidence from EXAFS and XANES is only consistent with rapid, and essentially complete, reduction of the gold to form metallic clusters containing about 50 atoms. The presence of Au−Ce distances in the EXAFS spectra, and the fact that about 15% of the gold atoms can be reoxidized after exposure to air at 150 °C, is indicative of a close interaction between a fraction (ca. 15%) of the gold atoms and the oxide support. Density functional theory (DFT) calculations are entirely consistent with this model and suggest that an import...

Determination of O and OH adsorption sites and coverage in situ on Pt electrodes from Pt L(2,3) X-ray absorption spectroscopy.

Abstract: The adsorption of atomic oxygen and hydroxide on a platinum electrode in 0.1 M HClO4 or H2SO4 electrolytes was studied in situ with Pt L(2,3) X-ray absorption spectroscopy (EXAFS and XANES). The Pt L3 edge absorption data, mu, were collected at room temperature in transmission mode on beamline X-11A at the National Synchrotron Light Source using a custom built cell. The Pt electrode was formed of highly dispersed 1.5-3 nm particles supported on carbon. A novel difference procedure (delta mu = mu(O[H]/Pt) - mu(Pt)) utilizing the L3 XANES spectra at different applied voltages was used to isolate the effects of O[H] (O or OH) adsorption in the XANES spectra. The Deltamu results are compared with results obtained from real-space full-multiple scattering calculations utilizing the FEFF8 code on model clusters. The experimental results, when compared with theoretical calculations, allow the adsorption site to be identified. At low coverages OH adsorbs primarily in 1-fold coordinated atop sites. As the coverage increases, O binds in the bridge/fcc sites, and at still higher coverages above 1.05 V RHE, O adsorbs into a higher coordinated n-fold or subsurface site, which is thought to be the result of Pt-O site exchange and oxide formation. These results are similar to those found in the gas phase. Direct specific adsorption of bisulfate anions in H2SO4 is spectroscopically seen in both the EXAFS and XANES data and is seen to impede oxygen adsorption consistent with previous reports.

Xanes spectroscopy of sulfur in earth materials

Abstract: X-ray absorption near-edge structure (XANES) spectroscopy is the ideal non-destructive technique for characterizing and quantifying S species in compositionally complex natural materials such as silicate glasses and minerals, coals, asphalts and asphaltenes, kerogens and humic substances. Sulfur absorption edges represent the transition of S 1 s and 2 p core electrons to unoccupied antibonding orbitals at the bottom of the conduction band. Shifts in the position of the absorption-edge feature of S K - and L -edge XANES spectra constitute a chemical ruler for oxidation state of both inorganic and organic species of S; the S K edge shifts from 2469.5 eV for chalcopyrite (2− oxidation state) to 2482 eV for gypsum (6+). However, chemical state of S in Earth materials is most readily assigned by comparing the overall XANES profile with spectra for reference compounds. Sulfur XANES spectra are reviewed for pyrite, troilite, pyrrhotite and NiAs-type Co 0.923 S and Ni 0.923 S, niningerite (MgS), oldhamite (CaS), alabandite (MnS) and cubic FeS, and sphalerite and related phases, as well as for selected solid-solutions of the monosulfides. Sulfur XANES spectra for FeS, CoS, NiS, MgS, CaS, MnS and ZnS have been simulated by multiple scattering calculations. The S K -edge XANES of transition-metal-bearing monosulfides generally show anomalous absorption consistent with hybridization of the final S 3 p σ* antibonding states with empty 3 d orbitals on the metal atoms. Various applications of S K - and L -edge XANES fingerprinting are discussed, including speciation of inorganic S in basaltic glasses, lazurite and hauyne, identification of organic functional groups of S in coals, kerogens and humic substances extracted from subtropical soils and marine sediments, and the association of sulfated sugars with calcification of coral aragonite skeletons.

Correlation of Water Activation, Surface Properties, and Oxygen Reduction Reactivity of Supported Pt–M/C Bimetallic Electrocatalysts Using XAS

Abstract: An analysis of X-ray absorption spectroscopy XAS data X-ray absorption near-edge structure XANES and extended X-ray absorption fine structure EXAFS at the Pt L3 edge for Pt‐M bimetallic materials M = Co, Cr, Ni, Fe and at the Co K edge for Pt‐Co is reported for Pt‐M/C electrodes in HClO4 at different potentials. The XANES data are analyzed using the method, which utilizes the spectrum at some potential V minus that at 0.54 V reversible hydrogen electrode RHE representing a reference spectrum. These data provide direct spectroscopic evidence for the inhibition of OH chemisorption on the cluster surface in the Pt‐M. This OH chemisorption, decreasing in the direction Pt Pt‐Ni Pt‐Co Pt‐Fe Pt‐Cr, is directly correlated with the previously reported fuel cell performance electrocatalytic activities of these bimetallics, confirming the role of OH poisoning of Pt sites in fuel cells. EXAFS analysis shows that the prepared clusters studied have different morphologies, the Pt‐Ni and Pt‐Co clusters were more homogeneous with M atoms at the surface, while the Pt‐Fe and Pt‐Cr clusters had a “Pt skin.” The cluster morphology determines which previously proposed OH inhibition mechanism dominates, the electronic mechanism in the pres

Direct Synthesis of Well Ordered and Unusually Reactive FeSBA-15 Mesoporous Molecular Sieves

Abstract: Large-pore hexagonal SBA-15 molecular sieves partially substituted with iron(III) have been synthesized for the first time in highly acidic media. The degree of iron(III) incorporation into SBA-15 can easily be controlled by a simple adjustment of the molar ratio of water and hydrochloric acid. All the materials have been characterized by XRD, N2 adsorption, UV−Vis DRS, ESR, and XANES spectroscopy. The characterization of the FeSBA-15 materials by UV−Vis DRS, ESR, and XANES spectroscopies suggests that the iron atoms are highly dispersed and mostly occupy isolated tetrahedral sites. XANES studies revealed that the proportion of tetrahedrally coordinated Fe atoms decreases with decreasing nSi/nFe ratios. Benzylation of benzene (or substituted benzenes) using benzyl chloride as the alkylating agent over FeSBA-15 with different nSi/nFe ratios was investigated as was the same reaction using AlSBA-15 as catalyst. The influence of parameters such as reaction temperature, reactant feed ratio, and the presence of...

Determination of Valence States of Chromium in Calcium Chromates by Using X‐ray Absorption Near‐Edge Structure (XANES) Spectroscopy

Abstract: Calcium chromates with the empirical formulas Ca 10 Cr 6 -O 25 , Ca 3 Cr 2 O 8 and Ca 5 Cr 3 O 12 , which form at temperatures >900°C at CaO:Cr 2 O 3 molar ratios of ≥3 in an oxidation atmosphere have been synthesized in the pure state. X-ray absorption near-edge structure (XANES) spectroscopy has been used to determine the average valence state of chromium in the samples. The presence of unusual chromium valence states, 4+ and 5+, which was proposed via X-ray diffractometry studies, is strongly supported.

XANES and ELNES in Ceramic Science

Abstract: Both X-ray absorption near edge structures (XANES) and electron energy loss near edge structure (ELNES) are important tools in ceramic science offering information on local environment of selected elements not only in crystals but also in amorphous materials. Recent technological progress enables measurements of XANES of ppm-level dopants using modern synchrotron facilities. Combined with transmission electron microscopy, ELNES can be used to analyze the local structures with subnanometer spatial resolution. First principles methods to reproduce and interpret the spectra have been established just recently. When a core-hole is adequately taken into account, most of K-edge spectra can be well reproduced using a modern band-structure method within one-electron approximation. The same is true for 1,2,3-edge spectra of non-transition metal compounds. However, multi-electron calculations are mandatory to reproduce 7-2,3-edge spectra of 3d transition-metal elements because of strong electronic correlations. In this paper, some recent results obtained in our group by the combination of XANES/ELNES experiments and theoretical calculations are reviewed.

Surface electronic states in three-dimensional SnO 2 nanostructures

Abstract: The electronic structure of three-dimensional SnO{sub 2} nanostructures (aerogels) is studied by soft x-ray absorption near-edge structure (XANES) spectroscopy. High-resolution O K-edge and Sn M{sub 3}- and M{sub 4,5}-edge XANES spectra of monolithic nanocrystalline rutile SnO{sub 2} aerogels with different surface areas (i.e., different surface-to-volume atom fractions) are compared with spectra of full-density rutile SnO{sub 2} and tetragonal SnO. Spectra are interpreted based on the electronic densities of states in SnO{sub 2} calculated with both cluster (self-consistent real-space multiple scattering) and band-structure (linear muffin-tin orbital) methods. Results show that, in contrast to the currently widely accepted picture, the presence of undercoordinated surface atoms not only affects the Fermi level position but also changes the structure of the conduction band by introducing additional Sn-related electronic states close to the conduction band minimum. These additional states are due to oxygen deficiency and are attributed to a surface reconstruction of SnO{sub 2} nanoparticles forming the aerogel skeleton. Results of this study are important for understanding the physical processes underlying the performance of gas sensors based on SnO{sub 2} nanostructures.

Release of N(2) from the carbon nanotubes via high-temperature annealing.

Abstract: Nitrogen (N)-doped carbon nanotubes (CNTs) were heated to 1000 °C under an ultrahigh vacuum. X-ray photoelectron spectroscopy (XPS) and X-ray absorption near-edge structure (XANES) reveal three dif...

pH dependence of the electronic structure of glycine.

Abstract: The carbon, nitrogen, and oxygen K-edge spectra were measured for aqueous solutions of glycine by total electron yield near-edge X-ray absorption fine structure (TEY NEXAFS) spectroscopy. The bulk solution pH was systematically varied while maintaining a constant amino acid concentration. Spectra were assigned through comparisons with both previous studies and ab initio computed spectra of isolated glycine molecules and hydrated glycine clusters. Nitrogen K-edge solution spectra recorded at low and moderate pH are nearly identical to those of solid glycine, whereas basic solution spectra strongly resemble those of the gas phase. The carbon 1s → π*CO transition exhibits a 0.2 eV red shift at high pH due to the deprotonation of the amine terminus. This deprotonation also effects a 1.4 eV red shift in the nitrogen K-edge at high pH. Two sharp preedge features at 401.3 and 402.5 eV are also observed at high pH. These resonances, previously observed in the vapor-phase ISEELS spectrum of glycine,1 have been rea...

A study of the manganites La0.5M0.5MnO3(M = Ca, Sr, Ba) prepared by hydrothermal synthesis

Abstract: We present results of the first systematic study of the hydrothermal synthesis of the manganites La0.5M0.5MnO3 (M = Ca, Sr, Ba), and a detailed characterisation of the products using a number of experimental methods. All three materials can be produced as phase-pure, polycrystalline powders in one step from solutions of metal salts in aqueous potassium hydroxide solution at temperatures of 240 °C for M = Sr or Ba and 270 °C for M = Ca. Iodometric titration and Mn K-edge X-ray absorption near edge spectroscopy (XANES) combined with elemental analysis for metals confirms the average manganese oxidation state as 3.5 in each of the materials. Scanning electron microscopy shows that the materials are made up of cube-shaped particles 1 µm in dimension for La0.5Ba0.5MnO3, 10 µm on average for La0.5Sr0.5MnO3 and 20 µm for La0.5Ca0.5MnO3. La0.5Ba0.5MnO3 is shown to adopt an A-site ordered perovskite structure (tetragonal, P4/mmm, a = 3.9160(1) A, c = 7.8054(2) A); this is determined using powder neutron diffraction, thermogravimetric analysis and magnetisation measurements. High-resolution powder neutron diffraction data from La0.5Sr0.5MnO3 show that the material adopts a distorted perovskite structure (tetragonal, I4/mcm, a = 5.44778(40) A, c = 7.7353(10) A) similar to that previously reported for materials of the same composition prepared by a solid-state route. Hydrothermal La0.5Ca0.5MnO3 adopts an orthorhombic perovskite structure (Pnma, a = 5.410(1) A, b = 7.604(1) A, c = 5.443(2) A). Finally, we examine poorly ordered precursors of La0.5Ba0.5MnO3, formed prior to the crystallisation of the perovskite, using Mn K-edge X-ray absorption fine structure spectroscopy. This shows that at the earliest stage of the reaction the manganese is found in an oxidation state of 3.5 in edge-shared octahedral sites, consistent with the formation of a K0.5MnO2·nH2O, birnessite-like phase before the formation of the perovskite.

Nanostructured Ti-W mixed-metal oxides: structural and electronic properties.

Abstract: In this article, the structural and electronic properties of Ti−W binary mixed oxide nanoparticles are investigated by using X-ray diffraction, Raman, X-ray absorption spectroscopies (XAS; near edge XANES and extended EXAFS), UV−vis spectroscopy, and density functional calculations. A series of Ti−W binary oxide samples having W content below 20 atom % and with particle size between 8 and 13 nm were prepared by a microemulsion method. The atoms in these nanoparticles adopted the anatase-type structure with a/b lattice constants rather similar to those of the single TiO2 reference and with a c cell parameter showing a noticeable expansion upon doping. Within the anatase structure, W occupies substitutional positions, while Ti atoms only suffer minor structural perturbations. A change in the W local order at first neighboring distance is observed when comparing samples having a W content below and above 15 atom %. Charge neutrality is mostly achieved by formation of cation vacancies located at the first cat...

Quantitative determination of platinum oxidation state by XANES analysis

Abstract: Quantitative determination of platinum oxidation state on supported platinum catalyst was examined by using Pt LIII-edge XANES spectra. The nano-sized platinum particles on the support were partially oxidized under oxidative atmosphere. The atomic ratio of oxygen to platinum (O/Pt) for these particles could be determined by the amount of desorbed oxygen in He carrier (O2-TPD) during elevated temperature. On the other hand, the XANES spectrum of the platinum catalysts oxidized under each corresponding condition was recorded under the same condition, then the area intensity of the white line of these spectra was evaluated by a simple simulation using a set of Gaussian and arctangent functions. A good linear relationship was confirmed between the white line intensity and the atomic ratio of oxygen to platinum, meaning that the oxidation state of the platinum could be quantitatively determined by a simple XANES analysis.