Showing papers on "XANES published in 2000"

Reaction of NO2 with Zn and ZnO: Photoemission, XANES, and Density Functional Studies on the Formation of NO3

Abstract: Synchrotron-based high-resolution photoemission and X-ray absorption near-edge spectroscopy (XANES) have been used to study the interaction of NO2 with polycrystalline surfaces of metallic zinc and zinc oxide. NO2 exhibits a complex chemistry on metallic zinc. After adsorbing nitrogen dioxide, N, O, NO, NO2, and NO3 are present on the surface of the metal. At room temperature the NO2 molecule mainly dissociates into O adatoms and gaseous NO, whereas at low temperatures (<250 K) chemisorbed NO2 and NO3 dominate on the surface. NO2 is a very good oxidizing agent for preparing ZnO from metallic zinc. Zn reacts more vigorously with NO2 than metals, such as Rh, Pd, or Pt which are typical catalysts for the removal of NOx molecules (DeNOx process). At 300 K, the main product of the reaction of NO2 with polycrystalline ZnO is adsorbed NO3 with little NO2 or NO present on the surface of the oxide. No evidence was found for the full decomposition of the NO2 molecule (i.e., no NO2 → N + 2O). The results of density ...

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 ...

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 ...

The electronic structure and surface chemistry of glycine adsorbed on Cu(110)

Abstract: We present a combined density functional theory and x-ray emission spectroscopy study of the bonding and chemistry of glycine (NH2CH2COOH) chemisorbed on Cu(110). The amino acid deprotonates upon adsorption. The adsorbate exhibits a rich surface chemistry leading to several intermediate adsorption structures. The most stable geometry is found to involve both the carboxylic and amino functional end groups in the bond. This structure appears only after annealing to 400 K, which in the present work is attributed to a removal of surface or subsurface hydrogen from the metal. Comparison with experimental x-ray emission and near edge x-ray absorption fine structure (NEXAFS) spectra provide a detailed picture of the electronic structure for the most stable structure. This allows conclusions to be drawn regarding the covalent interaction of the adsorbate system. When combined with theoretical calculations addressing, e.g., the electrostatic adsorbate–substrate interaction, a complete picture of the surface chemic...

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...

The use of X‐ray absorption spectroscopy for monitoring the thickness of antiwear films from ZDDP

Abstract: X‐ray absorption near edge structure (XANES) spectroscopy at the P K‐edge was used to monitor ZDDP antiwear film thickness with rubbing time. Thermal immersion films of varying thickness were generated from the ZDDP and analysed using XANES spectroscopy and the particle induced X‐ray emission (PIXE) technique. P K‐edge XANES edge jumps and (1s → np) peak heights of the spectra were plotted against PIXE mass thickness values in order to establish calibration curves. Antiwear films were analysed using XANES spectroscopy, and average mass thicknesses were extrapolated from the calibration curves. A set of antiwear films formed in the presence of ZDDP and then further rubbed in base oil (no ZDDP) showed no significant decrease in film thickness. A set of antiwear films rubbed in the presence of ZDDP for various lengths of time showed an increase in film thickness, followed by thinning of the film. The decrease in film thickness is believed to be due to wear caused by the ZDDP solution decomposition products acting as an abrasive in the contact region.

Phase transformations and electronic properties in mixed-metal oxides: Experimental and theoretical studies on the behavior of NiMoO4 and MgMoO4

Abstract: Mixed-metal oxides play a relevant role in many areas of chemistry, physics, and materials science. We have examined the structural and electronic properties of NiMoO4 and MgMoO4 by means of synchrotron-based time-resolved x-ray diffraction (XRD), x-ray absorption near-edge spectroscopy (XANES), and first-principles density functional theory (DFT) calculations. Nickel molybdate can exist in two phases (α and β). Mo is in a near tetrahedral environment in the β-phase, whereas in the α-phase the metal exhibits a pseudo-octahedral coordination with two very long Mo–O distances (2.3–2.4 A). The results of DFT calculations indicate that the α-phase of NiMoO4 is ∼9 kcal/mol more stable than the β-phase. On the other hand, in the case of magnesium molybdate, an α-NiMoO4-type phase is ∼13 kcal/mol less stable than β-MgMoO4. These trends in stability probably result from variations in the metal–metal repulsion within the α-phases of the compounds. For the α→β transition in NiMoO4, the DFT calculations predict an e...

Characterization of Fine Particulate Matter Produced by Combustion of Residual Fuel Oil

Abstract: Combustion experiments were carried out on four different residual fuel oils in a 732-kW boiler. PM emission samples were separated aerodynamically by a cyclone into fractions that were nominally less than and greater than 2.5 |j.m in diameter. However, examination of several of the samples by computer-controlled scanning electron microscopy (CCSEM) revealed that part of the PM2.5 fraction consists of carbonaceous cenospheres and vesicular particles that range up to 10 |j.m in diameter. X-ray absorption fine structure (XAFS) spectroscopy data were obtained at the S, V, Ni, Fe, Cu, Zn, and As K-edges and at the Pb L-edge. Deconvolution of the X-ray absorption near edge structure (XANES) region of the S spectra established that the dominant molecular forms of S present were sulfate (26-84% of total S) and thiophene (13-39% of total S). Sulfate was greater in the PM2.5 samples than in the PM25+ samples. Inorganic sulfides and elemental sulfur were present in lower percentages. The Ni XANES spectra f...

Chloride Ingress into Aluminum Prior to Pitting Corrosion An Investigation by XANES and XPS

Abstract: Two distinct chloride (Cl - ) species were detected on and/or in the passive oxides of polycrystalline Al samples, which were anodically polarized below the stable pitting potential in Cl - -containing solutions. Chloride was found to be present as an adsorbed specie at the surface of the Al oxide, as well as an incorporated specie within the passive oxide. The two species of Cl - were recorded by X-ray absorption near edge structure (XANES), using both an electron yield detector and an X-ray fluorescence detector, and by X-ray photoelectron spectroscopy (XPS). Electron yield XANES and XPS results indicate that adsorbed Cl - migrates from the solution/Al oxide interface into the passive Al oxide film, prior to stable pit initiation. Cl - migration occurs once a critical anodic potential or critical adsorbed Cl - concentration is reached. The migration of Cl - is followed by a loss of oxidized Al from the passivating film, as determined by XPS, and can be attributed to (i) metastable pitting events or (ii) oxide dissolution. The ingress of Cl - into the oxide appears to be a key factor for the onset of metastable pitting or passive film dissolution.

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.

Evolution of Local Structure around Manganese in Layered LiMnO2 upon Chemical and Electrochemical Delithiation/Relithiation

Abstract: Mn K-edge X-ray absorption spectroscopic (XAS) analyses have been performed to probe the evolution of electronic and crystal structures of layered LiMnO2 upon chemical and electrochemical delithiation/relithiation. According to the X-ray absorption near-edge structure studies, it becomes clear that the trivalent manganese ion in LiMnO2 is significantly oxidized by acid treatment and is not fully recovered by subsequent lithiation reaction with n-BuLi. The extended X-ray absorption fine structure results presented here demonstrate that the local structure around manganese in LiMnO2 is changed from a layered α-NaFeO2-type structure to a spinel-like one upon chemical delithiation reaction. It is also found from the XAS analyses for the cycled LiMnO2 that the electrochemical charge−discharge process gives rise not only to the partial oxidation of manganese ion but also to the migration of Mn into the interlayer lithium site, resulting in the coexistence of the layered structure and the spinel one. Such result...

Boron K-edge XANES of borate and borosilicate minerals

Abstract: Synchrotron radiation B K- edge XANES spectra are reported for 19 borate and borosilicate minerals. The spectra are characterized by three prominent features; a sharp peak (A) at ~194 eV (the edge feature of trigonal B; IIIB), a broader peak (B) at 197–199 eV (the edge feature of tetrahedral B, IVB), and a broad peak (C) at 200–201 eV for tetrahedral B and 203–204 eV for trigonal B. The area of peak A is very sensitive to content of IIIB and its position yields information on B-O bond length and linkage of the BO3 group. The area of peak B is proportional to content of IVB but quantification is limited by overlap with peak C. The width of peak B increases with increasing divergence of B-O bond lengths, responding to splitting of σ*(t2) antibonding orbitals. The tetrahedral component of peak C appears to be a σ-shape resonance. For trigonal B minerals, the relative intensity of peak C and its satellite peaks increases with increase in mean size and/or atomic number of next-nearest-neighbor cations, C being most intense in vonsenite (Fe2Fe3+BO5). Priceite (Ca4B10O19·7H2O), of unknown structure, has 80% IVB, and an isolated BX3 group with IIIB-O = 1.373(5) A. Comparison of B K- edge XANES spectra collected using total electron yield (sampling depth <60 A) and fluorescence yield (sampling depth <1100 A) shows that borates and borosilicates readily reconstruct in surface and near-surface environments. More profound structural damage involving conversion of IVB to IIIB occurs in minerals with high contents of IVB, and hydrous Mg borates with interstitial H2O are unstable in the high vacuum of the spectrometer.

Interaction of SO2 with MgO(100) and Cu/MgO(100): Decomposition Reactions and the Formation of SO3 and SO4

Abstract: Synchrotron-based high-resolution photoemission and first-principles density functional calculations (DFT-GGA) were used to study the interaction of SO2 with clean and modified (OH, Oδ-, O vacancies, or Cu adatoms present) MgO(100) surfaces. The reaction of the molecule with pure and hydroxylated powders of MgO was investigated using X-ray absorption near-edge spectroscopy (XANES). At 100 K, the main product of the adsorption of sulfur dioxide on MgO(100) is sulfite (SO2,gas + Olattice → SO3,ads). No evidence is found for bonding of SO2 to Mg sites of the surface or decomposition of the molecule. DFT calculations show that a η3-S,O,O adsorption configuration leads to a SO3-like species, and this is much more stable than configurations which involve bonding to only Mg sites or formation of SO4. On a flat MgO(100) substrate, the formation of SO4 is not energetically viable. A SO3 → SO4 transformation is observed at temperatures between 150 and 450 K with a substantial reconstruction of the oxide surface. Fr...

Atomic environment of high-field strength Nd and Al cations as dopants and major components in silicate glasses: a Nd LIII-edge and Al K-edge X-ray absorption spectroscopic study

Abstract: The local atomic structure around Nd and Al ions present as dopants in SiO 2 glasses and as major constituents in other silicate glasses have been studied using Nd L III -edge EXAFS and Al K-edge XANES and EXAFS spectroscopy. The Nd EXAFS results provide the first direct evidence of clustering of Nd ions through the formation of Nd–O–Nd bonds in a SiO 2 glass doped with 2400 ppm Nd 2 O 3 . The Nd–O and Nd–Nd distances in this glass are found to be 2.35 and 3.80 A, respectively. Codoping of this glass with 7300 ppm Al 2 O 3 is shown to result in a homogeneous dispersion of the Nd atoms. The Nd–O coordination polyhedra in this glass relax to a larger volume with respect to that in its Al-free counterpart with a significant increase in the Nd–O distance to 2.62 A. Moreover, Nd–O–Si/Al bonds are formed at the expense of Nd–O–Nd bonds on addition of Al, indicating incorporation of Nd–O polyhedra into the silicate network; the corresponding Nd–Si distance is found to be 3.51 A. A similar atomic environment for Nd is also observed in Nd 2 Al 2 Si 3 O 12 glass where Nd is present as a major component rather than as a dopant. However, no such Nd–O–Si bonds are observed in a K–Nd-silicate glass indicating the fact that incorporation of Nd–O polyhedra into the silicate network without Al creates significant radial disorder in the next-nearest neighbor Si shell around Nd. The Al XANES and EXAFS spectra indicate that Al ions in all these glasses exist in four-fold coordination with Al–O distances ranging between 1.76 and 1.77 A. A second shell of Si is observed in all compositions at distances ranging between 2.92 and 3.00 A. Al–O–Nd bonds are also observed in the Nd 2 Al 2 Si 3 O 12 glass. It is suggested that Nd, when incorporated into the Si, Al–O network, plays the role of charge-balancing cation for [AlO 4 ] −1 units in aluminosilicate glasses.

Studies on the Behavior of Mixed-Metal Oxides and Desulfurization: Reaction of H2S and SO2 with Cr2O3(0001), MgO(100), and CrxMg1-xO(100)

Abstract: In environmental catalysis the removal or destruction of S-containing molecules is a very important issue nowadays. The interaction of H2S and SO2 with Cr2O3(0001), MgO(100) and CrxMg1-xO(100) surfaces (x = 0.05−0.08) was studied using synchrotron-based high-resolution photoemission. X-ray absorption near-edge spectroscopy (XANES) was used to examine the chemistry of the molecules on powders of Cr2O3, MgO and CrxMg1-xO catalysts. The activity toward the breaking of S−H and S−O bonds was found to increase in the sequence: MgO < Cr2O3 < CrxMg1-xO. The mixed-metal oxide displays a unique ability for breaking S−O bonds that makes it the best catalyst for the Claus process (2H2S + SO2 → 2H2O + 3Ssolid) and the reduction of SO2 by CO (SO2 + 2CO → 2CO2 + Ssolid). First-principles density-functional (DF-GGA) calculations revealed that the superior catalytic properties of the mixed-metal oxide are due to the special electronic properties of Cr cations contained in a matrix of MgO. These Cr atoms have a lower oxid...

Spectroscopy of π bonding in hard graphitic carbon nitride films: Superstructure of basal planes and hardening mechanisms

Abstract: X-ray-absorption near-edge spectroscopy (XANES or NEXAFS) has been used to obtain information on the orientation, corrugation, and cross-linking of graphitic carbon nitride planes, structural parameters that determine the mechanical properties of the material. The contribution of p electrons from carbon and nitrogen atoms to \ensuremath{\pi} bonding in graphitic carbon nitride has been studied with elemental and angular sensitivity by XANES. The density of ${\ensuremath{\pi}}^{*}$ states from nitrogen is composition dependent and presents angular anisotropy, while the density of ${\ensuremath{\pi}}^{*}$ states from carbon is isotropic and independent of composition. Both observations are consistent with a model of the superstructure of basal planes.

Quantitative Analysis of Ti−O−Si and Ti−O−Ti Bonds in Ti−Si Binary Oxides by the Linear Combination of XANES

Abstract: X-ray absorption near edge structure (XANES) of Ti K-edge of Ti−Si mixed oxides and titania supported on silica with various Ti contents was studied to investigate the fractions of Ti−O−Si and Ti−O−Ti bonds quantitatively by fitting the preedge of Ti K-edge with the linear combination of two reference XANES spectra. In mixed oxides, the fraction of Ti−O−Ti was increased up to 0.55 when Ti/Si was varied from 0.04 to 0.5. The greatest change of each fraction occurred around 0.15−0.2 of Ti/Si, which was coincident with the formation of anatase titania as observed by XRD. For titania supported on silica with a surface area of 300 m2 g-1, the preedge fitting results combined with XRD and XPS indicated that monolayer coverage was reached around 7−10 wt % Ti loading where the amount of Ti in Ti−O−Si was saturated to 0.56 mmol-Ti/g-material. This work demonstrated the possibility of the quantification of Ti−O−Si and Ti−O−Ti bonds in Ti−Si binary oxides by using the linear combination of reference XANES spectra. K...