XAFS Spectra Research Articles

Synthesis–structure–performance correlation for polyaniline–Me–C non-precious metal cathode catalysts for oxygen reduction in fuel cells

In this report, we present the systematic preparation of active and durable non-precious metal catalysts (NPMCs) for the oxygen reduction reaction in polymer electrolyte fuel cells (PEFCs) based on the heat treatment of polyaniline/metal/carbon precursors. Variation of the synthesis steps, heat-treatment temperature, metal loading, and the metal type in the synthesis leads to markedly different catalyst activity, speciation, and morphology. Microscopy studies demonstrate notable differences in the carbon structure as a function of these variables. Balancing the need to increase the catalyst’s degree of graphitization through heat treatment versus the excessive loss of surface area that occurs at higher temperatures is a key to preparing an active catalyst. XPS and XAFS spectra are consistent with the presence of Me–Nx structures in both the Co and Fe versions of the catalyst, which are often proposed to be active sites. The average speciation and coordination environment of nitrogen and metal, however, depends greatly on the choice of Co or Fe. Taken together, the data indicate that better control of the metal-catalyzed transformations of the polymer into new graphitized carbon forms in the heat-treatment step will allow for even further improvement of this class of catalysts.

Study on analysis of crystal structure in CeO 2 doped with Er 2O 3 or Gd 2O 3

To simulate the effects of burnable poison doping in nuclear fuel UO 2, Er 2O 3 (or Gd 2O 3)-doped CeO 2 pellets were prepared. Changes in lattice constant and atomic disordering for CeO 2 due to the Er 2O 3 and Gd 2O 3 doping were measured by means of XRD and XAFS. By the Er 2O 3 doping, the lattice constant decreased, and a disordering of lattice structure was induced in the samples. The doping with Er 2O 3 also induced the disordering of atomic arrangement around Er atoms, which was observed through the change in XAFS spectra. In contrast, the effect of Gd 2O 3 doping was smaller than that of Er 2O 3 doping. The result was discussed in terms of ionic size of dopants in CeO 2 crystal.

A confocal set-up for micro-XRF and XAFS experiments using diamond-anvil cells

A confocal set-up is presented that improves micro-XRF and XAFS experiments with high-pressure diamond-anvil cells (DACs). In this experiment a probing volume is defined by the focus of the incoming synchrotron radiation beam and that of a polycapillary X-ray half-lens with a very long working distance, which is placed in front of the fluorescence detector. This set-up enhances the quality of the fluorescence and XAFS spectra, and thus the sensitivity for detecting elements at low concentrations. It efficiently suppresses signal from outside the sample chamber, which stems from elastic and inelastic scattering of the incoming beam by the diamond anvils as well as from excitation of fluorescence from the body of the DAC.

Improvement in XAFS beamline BL01B1 at SPring-8

Beamline BL01B1 was constructed for conventional XAFS measurements using a bending magnet light source at SPring-8 in 1997 [1]. BL01B1 covers a wide energy range from 3.8 to 113 keV using an adjustable inclined double-crystal monochromator and a double mirror system. Here, we describe a newly developed apparatus and the current status of the BL01B1. A time-efficient QEXAFS method was developed to measure high quality spectra up to high-k regions in the shortest possible time. The method involves sweeping the monochromator continuously at a variable angular speed depending on the k-region using a VME stepping motor controller. During scanning, a pair of crystals of the monochromator rotates without translation motion like a channel-cut crystal. This QEXAFS method was applied to the fluorescence mode using a 19-element Ge detector combined with a digital X-ray processor system. A new conversion electron yield detector having a rotational sample stage was developed for single crystalline samples. Angular oscillations of 4 degrees in the sample rotation stage around the axis normal to the sample surface successfully remove diffraction noise in the XAFS spectra.

2 Dimensional position sensitive XAFS by using in-house X-ray spectrometer

Position sensitive XAFS measurements by using the in-house X-ray spectrometer and the position sensitive X-ray detector were attempted. The in-house spectrometer produces monochromized divergent X-ray beam. Therefore, an extended direct-beam image projected on the detector can be taken, and its extension rate is depending on the arrangement of the sample and the detector. A position sensitive XAFS measurement was demonstrated by using Ni metal foil and NiO powder as a model sample. Ge(220) or Si(400) Johansson-type bent single-crystal was used as monochromator, and Mo and LaB6 were used as the target and filament, respectively. Tube voltage and current were operated at 16 kV and 100 mA (1.6 kW). XAFS spectra were measured by transmission-mode with sample set/reset method and required time of each experiment is about 6 hours in total. It was confirmed that metal and oxide spectra with enough quality for structural analysis were clearly separated by each position in the sample.

XAFS study of BaCe 1−xTi xO 3 and Ba 1−yCe 1−xY xO 3 protonic solid electrolytes

The paper reports on the doping-influenced evolution of the local structure in a new class of solid electrolytes, BaCe 1− x Ti x O 3 and Ba 1− y Ce 1− x Y x O 3 perovskites, to be used in intermediate temperature fuel cells. The local environments of the Ti, Y and Ce cations were studied by means of X-ray absorption spectroscopy. The Ti XAFS spectra reveal a decrease of the local symmetry of titanium site with increasing doping. The Ce and Y EXAFS functions show a larger amount of oxygen vacancies created by Y doping than that caused by barium understoichiometry.

A XAFS study on the mechanism of isotopic fractionation of molybdenum during its adsorption on ferromanganese oxides

Due to its potential as a paleoredox proxy, there have been many studies on Mo isotopic fractionation during adsorption onto ferromanganese oxides in seawater. However, the mechanisms of both adsorption and isotopic fractionation are still under debate due to the lack of structural information on the adsorbed species. In this study, XAFS analyses were performed to reveal the mechanism, based on structural information at the molecular level, of Mo isotopic fractionation during adsorption onto ferromanganese oxides. Molybdenum L3-edge XANES and K-edge EXAFS revealed that Mo species adsorbed on the surface of ferrihydrite was a tetrahedrally coordinated outer-sphere complex, while that on δ-MnO2 was an octahedrally coordinated inner-sphere complex. Additionally, it was also revealed that δ-MnO2 was the dominant host phase of Mo in the hydrogenetic ferromanganese nodules from the comparison of their XAFS spectra. Previous studies reported that lighter isotopes of Mo were preferentially incorporated into ferromanganese oxides from seawater. This fractionation can be explained based on the structural difference between tetrahedral MoO42- (=a major species in seawater) and the octahedral species adsorbed on the Mn oxide phase in ferromanganese nodules. In contrast, little change in Mo local structures during its adsorption onto ferrihydrite also suggested the little or no fractionation of Mo isotopes in the presence of Fe hydroxides without Mn oxides. These facts imply that the Mo isotopic composition in ancient marine sediments can distinguish redox boundaries of Fe2+/Fe(OH)3 and of the more oxic Mn2+/MnO2.

STUDY OF THIN FILMS AND AQUEOUS CHLORINE SOLUTION USING X-RAY ABSORPTION FINE STRUCTURE SPECTROSCOPY

The capability of thin film measurement and performance at low energy using XAFS at the X-ray demonstration and development beamline (XDD) at SSLS has been explored. XDD is supplied with synchrotron radiation from one of the two superconducting dipoles of the Helios 2 storage ring. The photon energy is tunable from 2.5 keV to 10 keV using a channel-cut Si(111) monochromator and the photon flux is of the order of 109 photons/s at energies of several keV. The energy resolution, detector, and control system required by XAFS were optimized. As a demonstration, the XAFS spectra of Mn-doped ZnO thin films at Mn K-edge and chlorine aqueous solution at Cl K-edge were measured and discussed.

Improved Fe/Mg-Al hydrotalcite catalyst for Baeyer–Villiger oxidation of ketones with molecular oxygen and benzaldehyde

Metal (Me: Fe, Co, Ni or Cu) containing Mg-Al hydrotalcite-type anionic clay catalysts have been prepared by adopting the “memory effect” of hydrotalcite and tested for Baeyer–Villiger oxidation of ketones. Mg-Al hydrotalcite was calcined to the mixed oxide and dipped in an aqueous solution of metal (Me) salt; metal species was incorporated by “memory effect” to form Me/Mg-Al hydrotalcite as the active phase on the surface of the Mg-Al mixed oxide. The activity and the structure of these catalysts have been compared with those of metal supported catalysts prepared by coprecipitation and impregnation. Among the metals tested, iron was the most effective and use of Fe(NH 4) 2(SO 4) 2·6H 2O as the metal salt resulted in the highest activity. The activity of the catalyst was higher than those prepared by coprecipitation from the nitrates of Mg, Fe and Al. Judging from Mössbauer and Fe K-edge XAFS spectra, Fe species possess the Fe 3+ valence state, are mainly octahedrally coordinated and form Fe O Fe cluster-type structure on the Mg-Al mixed oxides. This clearly indicates that the Fe 3+ O Fe 3+ cluster-type compounds are more active for the Baeyer–Villiger oxidation than well dispersed Fe 3+ species formed on the hydrotalcite prepared by coprecipitation. Six-membered cyclic ketones, such as cyclohexanone, 2-norbornone and 2-adamantanone, were effectively oxidized to the corresponding lactones by using O 2/benzaldehyde as an oxidizing agent.

Soft x-ray XAFS experiments of SiO2-P2O5 sol-gel film

Structural evolutions in a heat-treatment process have been studied using Si and P K-edge XAFS in SiO2-P2O5 sol-gel layers on silicon substrates. The Si K-edge EXAFS results indicate that structural changes occur between 1173 K and 1273 K. In the case of heat-treated samples at 1273 K, the Si-O interatomic distance is 1.63–1.64 Å. On the other hand, the distances are 1.66–1.68 below 1273 K. Furthermore, structural properties around Si and P in the sol-gel process have been studied. In the XAFS spectra, the ratio of P to Si shows 84% decrease after heat-treatment above 1073 K. For the Si-O and P-O distances, wet-gel has shorter interatomic distance than sol-gel raw solution, TEOS and TEP, respectively.

XAFS spectra and photoluminescence properties of Eu-doped Al2O3-SiO2

XAFS spectra and photoluminescence properties of Eu-doped Al2O3-SiO2

XAFS study on the trace amounts of ytterbium ions incorporated in calcium carbonate crystal

We measured XAFS spectra of trace amount of ytterbium (Yb) incorporated in calcite, a stable phase of calcium carbonate. Calcium carbonate was precipitated from a mixed solution of CaCl2 aq and NaHCO3 aq with a given amount of YbCl3. Concentrations of Yb were 5 μmol kg−1 in the starting solutions and Yb/Ca molar ratio in the precipitated calcium carbonate was 1.2 × 10−3. Ytterbium LIII-edge XAFS spectra were collected in the fluorescence mode at the beamline BL-12C of the KEK-PF. Analysis of the XAFS results indicated, (1) Approximately 15% Yb ion existed as divalent ions in the calcite. (2) Yb3+, major Yb species in the calcite, is located at the Ca2+ site in the calcite. The local structure around Yb3+ is significantly different from that of Ca2+. The coordination number of Ca2+ in the crystal structure of calcite is 6 and Ca–O distance is 2.36 Å. In contrast, the nearest Yb–O coordination for Yb-doped calcite is split into two shells, the nearest 4 oxygen atoms and the second nearest 2 oxygen atoms, and their Yb–O distances are 2.24 Å and 2.77 Å, respectively. It was shown that localized structural relaxation occurred around Yb3+ which has smaller ionic radius than Ca2+.

Combining selective sequential extractions, X-ray absorption spectroscopy, and principal component analysis for quantitative zinc speciation in soil.

Selective sequential extractions (SSE) and, more recently, X-ray absorption fine-structure IXAFS) spectroscopy have been used to characterize the speciation of metal contaminants in soils and sediments. However, both methods have specific limitations when multiple metal species coexist in soils and sediments. In this study, we tested a combined approach, in which XAFS spectra were collected after each of 6 SSE steps, and then analyzed by multishell fitting, principal component analysis (PCA) and linear combination fits (LCF), to determine the Zn speciation in a smelter-contaminated, strongly acidic soil. In the topsoil, Zn was predominately found in the smelter-emitted minerals franklinite (60%) and sphalerite (30%) and as aqueous or outer-sphere Zn2+ (10%). In the subsoil, aqueous or outer-sphere Zn2+ prevailed (55%), but 45% of Zn was incorporated by hydroxy-Al interlayers of phyllosilicates. Formation of such Zn-bearing hydroxy-interlayers, which has been observed here for the first time, may be an important mechanism to reduce the solubility of Zn in those soils, which are too acidic to retain Zn by formation of inner-sphere sorption complexes, layered double hydroxides or phyllosilicates. The stepwise removal of Zn fractions by SSE significantly improved the identification of species by XAFS and PCA and their subsequent quantification by LCF. While SSE alone provided excellent estimates of the amount of mobile Zn species, it failed to identify and quantify Zn associated with mineral phases because of nonspecific dissolution and the precipitation of Zn oxalate. The systematic combination of chemical extraction, spectroscopy, and advanced statistical analysis allowed us to identify and quantify both mobile and recalcitrant species with high reliability and precision.

Structural Investigation of the Aqueous Eu2+ Ion: Comparison with Sr2+ Using the XAFS Technique

Structural parameters of the Sr2+ and, for the first time, of the Eu2+ ions in aqueous solution were determined by the XAFS method. For the Sr2+, the use of an improved theoretical approach led to a first shell coordination number of 8.0 (3), a Sr−O distance of 2.600 (3) Å and a Debye−Waller factor of σ2 = 0.0126 (5) Å2. These results were confirmed by an analysis performed with experimental phase and amplitude, extracted from the solid reference compound [Sr(H2O)8](OH)2. The same theoretical approach was used for the analysis of the Eu2+ XAFS spectra in aqueous solution. This gives a first coordination shell of Eu2+ formed by 7.2 (3) water molecules, an Eu−O distance of 2.584 (5) Å, and a high Debye−Waller factor of σ2 = 0.0138 (5) Å2. Whereas Eu3+ occurs as an equilibrium between the [Eu(H2O)8]3+ and the [Eu(H2O)9]3+ ions, Eu2+ occurs in aqueous solution as an equilibrium between a predominant [Eu(H2O)7]2+ ion and a minor [Eu(H2O)8]2+ species.

XAFS study of gadolinium and samarium bisporphyrinate complexes.

The comparative X-ray absorption spectroscopy study of gadolinium and samarium bisporphyrinate complexes represented by the formulas Gd(III)H(oep)(tpp), Gd(III)(oep)(2), Gd(III)H(tpp)(2) and Sm(III)H(oep)(tpp), Sm(III)(oep)(2), Sm(III)H(tpp)(2) is reported. The XAFS spectra are recorded on the LURE-DCI storage ring (Orsay, France) in transmission mode on the microcrystalline samples at the Gd and Sm L(3) edges. The local environment for Ln(3+) ions has been reconstructed applying one-shell and two-shell XAFS analysis procedures. The protonated and nonprotonated bisporphyrinate complexes present different XAFS features. After our analysis on the title derivatives, the gadolinium ion (at 80 K) is found to be bonded: (i) to eight nitrogen atoms at R(Gd-N) 2.50 A, for Gd(III)(oep)(2) [Debye-Waller (DW) factor 0.004 A(2)]; (ii) to seven nitrogen atoms at R(Gd-N) 2.49 A, for Gd(III)H(oep)(tpp) [DW factor 0.005 A(2)] and one nitrogen at long distance; and (iii) to six nitrogen atoms at R(Gd-N) 2.50 A [DW factor 0.006 A(2)] and two nitrogen atoms at long distance for Gd(III)H(tpp)(2). A similar coordination sphere has been detected for the corresponding Sm derivatives. So, the samarium ion (at room temperature) is bonded: (i) to eight nitrogen atoms at R(Sm-N) 2.53 A, for Sm(III)(oep)(2) [DW factor 0.006 A(2)]; (ii) to seven nitrogen atoms at R(Sm-N) 2.53 A, for Sm(III)H(oep)(tpp) [DW factor 0.006 A(2)] and one nitrogen at long distance; and (iii) to six nitrogen atoms at R(Sm-N) 2.50 A, for Sm(III)H(tpp)(2) [DW factor 0.006 A(2)] and two nitrogen atoms at long distance. As far as concerns Ln(III)(oep)(2) complexes, the increase of Ln-N distance in the series Gd(3+) < Eu(3+) < Sm(3+) reflects an increase in the ionic radii, which are in good agreement with previously published XRD data on Eu(III)(oep)(2). Moreover, the protonated Ln(III)H(oep)(tpp) and Ln(III)H(tpp)(2) complexes possess systematically shorter distances of about 0.02 A between the XAFS and XRD data. This difference is attributed to the asymmetry of the distribution concerning Ln-N distances.

XAS investigation of Al2O3-coated nano-composite ZrO2

In order to complement the structural characterisation by high resolution electron microscopy and perturbed angular correlation spectroscopy (PAC) of Al2O3 coated nano-composite ZrO2, XAFS spectra have been acquired and analysed. The electron micrographs showed fringes of a well ordered lattice and well defined crystal faces of the as-produced powder whereas the PAC spectra indicated a strongly distorted short range order. On the basis of the XAFS data, a structural model for the ZrO2 core of the nano-composite ZrO2/Al2O3 with a well ordered Zr lattice and a sevenfold, strongly distorted nearest neighbour oxygen shell is proposed. A smooth temperature dependence without an indication for a phase transformation up to a temperature of 700K has been revealed.

In situ QEXAFS investigation at Co K-edge of the sulfidation of a CoMo/Al2O3 hydrotreating catalyst

In situ sulfidation of a commercial alumina-supported CoMo hydrotreating catalyst (3 wt% Co, 12.3 wt% Mo) has been studied by QEXAFS at Co K-edge. Sulfidation was performed by heating progressively the oxide precursor from room temperature (RT) up to 673 K in a H2/10% H2S flow (ramp 4 K/min). XAFS spectra were recorded each 10 K with an acquisition time of only 30 s. The obtained XANES and EXAFS data were compared with those of a Co/Al2O3 (2.36 wt% Co) used as reference sample. It is evidenced that sulfidation of Co atoms in a CoMo/Al2O3 catalyst starts at room temperature while the sulfidation of a Co/Al2O3 catalyst begins at 473 K.

Theory on Extended X-Ray Emission Spectra

A simple theoretical approach is given to study XAFS-like spectra (extended X-ray emission fine structure, EXEFS) observed in X-ray fluorescence spectra based on one-step quantum mechanical formula derived by Almbladh and Hedin. Electronic relaxation and shake-off processes are crucial to explain the spectral features. Here the “shake-off” processes are different from those found in the typical XPS and XAFS spectra; the “shake-off” electrons are ejected during the fluorescence emission processes. Formal EXEFS formulas derived here are the same as the usually used XAFS formula. However, we should carefully use the phase shifts by taking account of the relaxation effects due to additional core hole production; they should be different from those used in the XAFS analyses. The radial dipole integral should be replaced by the corresponding monopole radial integral in the EXEFS analyses.

XAFS spectra in the high-energy region measured at SPring-8.

XAFS spectra in the high-energy region measured at SPring-8.

Probing Depth Study of Conversion Electron/He Ion Yield XAFS Spectroscopy on Strontium Titanate Thin Films

The probing depth of conversion electron/He ion yield XAFS methods has been studied in order to apply these methods to estimate the local distortion of strontium titanate thin films on thick substrates. Several strontium titanate thin film samples with different thicknesses were prepared and the edge-jump amplitudes at both the Sr and Ti K-edges were obtained. The probing depths were estimated to be 300 nm for the Sr K-edge and 30 nm for the Ti K-edge by the conventional exponential fit. The XAFS spectra and the probing depths measured by conversion electron and He ion yield methods is much the same, though the background curves are different.