Dispersive X-ray Absorption Spectroscopy Research Articles

What drives spontaneous oscillations during CO oxidation using O2 over supported Rh/Al2O3 catalysts?

Spontaneous oscillations during CO oxidation by O2 over Rh/Al2O3 catalysts have been investigated for stoichiometric (2CO:1O2) and net oxidizing (1CO:6O2) cases using parallel application of time-resolved Rh K edge energy dispersive X-ray absorption spectroscopy (XAFS), diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and online mass spectrometry (MS). Whilst oscillatory chemistry is clearly visible over a range of temperatures and feedstock compositions, no evidence can be found for the participation of any IR active surface carbonyls to this chemistry. Equally, Rh K edge XAFS yields no evidence for reduction/oxidation cycles in the supported Rh as underlying the oscillatory behavior at the gas inlet (surface) of the sample bed or at varying axial positions below this point. In the stoichiometric case, variations in sample temperature are observed by both in DRIFTS and in a thermocouple inserted at the top of the catalyst bed. Oscillations occurring under net oxidizing conditions (378K), however, yield no such detectable thermal variations during CO oxidation. These observations are discussed in terms of previously suggested mechanisms to explain this spontaneous behavior and in terms of the sensitivity of the applied probes to various aspects of the reactive system.

Accelerated ageing of shales of palaeontological interest: Impact of temperature conditions

The palaeontological collections of the Muséum national d’Histoire naturelle (MNHN, Paris, France) and the Muséum d’Histoire naturelle d’Autun (MHNA, Autun, France) include many fossil specimens originating from the argillaceous shales of the Autun basin (Saône-et-Loire, France). These fossils are preserved within sedimentary rocks containing unstable sulphide compounds, such as pyrite, which may deteriorate in contact with water and oxygen. This alteration provokes crystalline efflorescence and cracks, thus compromising the preservations of the fossils. This work constitutes the first step of a project that aims to understand the mechanisms of alteration of these materials in order to define conservation guidelines for palaeontological collections. For this purpose, eight damaged specimens originating from the Permian Autun basin (Saône-et-Loire, France) were selected and analyzed by X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy coupled to energy dispersive X-ray spectrometry (SEM/EDS) and X-ray absorption spectroscopy at the threshold of the sulphur Kα-edge (XANES). This methodology enabled the characterization of the matrices composition and the chemical nature of the alterations. Subsequently, we have sought to reproduce by artificial ageing the alteration phenomena encountered in the collections. New shale samples were collected on seven outcrops of the same Autun basin. They were analyzed and subjected to artificial ageing at 50% relative humidity (RH) and at temperatures ranging between 40°C and 90°C. Our work shows that damaged specimens and newly collected shale have a similar mineralogical composition. Yet the crystalline efflorescence material formed on the surface of damaged specimens belongs to the iron sulphate group whereas gypsum predominates on artificially aged shale samples. Reproducing the alterations observed on specimens by artificial ageing remains therefore problematic. Additionally, it appears that the temperature of ageing controls the nature of the damage: at 40°C, many samples are mechanically damaged whereas no or minor crystalline efflorescence occurs. At 90°C, it is the opposite tendency that is observed. Finally, mechanical damages do not seem to be correlated with the development of the efflorescence: samples with efflorescent crystals generally do not show clearly visible cracks; those that seem most fragmented do not show any visible efflorescence.

A reaction cell for time-resolvedin situXAS studies during wet chemical synthesis: the Cu2(OH)3Cl case

The use of in situ time-resolved dispersive X-ray absorption spectroscopy (DXAS) to monitor the formation of Cu2(OH)3Cl particles in an aqueous solution is reported. The measurements were performed using a dedicated reaction cell, which enabled the evolution of the Cu K-edge X-ray absorption near-edge spectroscopy to be followed during mild chemical synthesis. The formed Cu2(OH)3Cl particles were also characterized by synchrotron-radiation-excited X-ray photoelectron spectroscopy, X-ray diffraction and scanning electron microscopy. The influence of polyvinylpyrrolidone (PVP) on the electronic and structural properties of the formed particles was investigated. The results indicate clearly the formation of Cu2(OH)3Cl, with or without the use of PVP, which presents very similar crystalline structures in the long-range order. However, depending on the reaction, dramatic differences were observed by insitu DXAS in the vicinities of the Cu atoms.

Femtosecond x-ray absorption spectroscopy with hard x-ray free electron laser

We have developed a method of dispersive x-ray absorption spectroscopy with a hard x-ray free electron laser (XFEL), generated by a self-amplified spontaneous emission (SASE) mechanism. A transmission grating was utilized for splitting SASE-XFEL light, which has a relatively large bandwidth (ΔE/E ∼ 5 × 10−3), into several branches. Two primary split beams were introduced into a dispersive spectrometer for measuring signal and reference spectra simultaneously. After normalization, we obtained a Zn K-edge absorption spectrum with a photon-energy range of 210 eV, which is in excellent agreement with that measured by a conventional wavelength-scanning method. From the analysis of the difference spectra, the noise ratio was evaluated to be ∼3 × 10−3, which is sufficiently small to trace minute changes in transient spectra induced by an ultrafast optical laser. This scheme enables us to perform single-shot, high-accuracy x-ray absorption spectroscopy with femtosecond time resolution.

Mechanisms of radioluminescence of rare earths doped SrAl2O4 and Ca12Al14O33 excited by X-ray

Pure and rare earth doped SrAl2O4 and Ca12Al14O33 have been synthesized by a new sol–gel proteic methodology. Photoluminescence (PL) and radioluminescence (RL) spectra show different behavior when excited with either UV light or X-ray radiation. Such behavior may be connected to the X-ray absorption/reduction of Eu ions process in samples. X-ray absorption spectroscopy (XAS) was carried out simultaneously with X-ray excited optical luminescence (XEOL) and the analysis indicates that are depends on the energy of excitation and the kind of excited species. Dispersive X-ray absorption spectroscopy (DXAS) was carried out to follow the kinetic of reduction process in the samples during the irradiation. The reduction process does not produce stable Eu2+ species. An overall analysis of the results makes possible to build the mechanisms of radioluminescence of rare earths doped SrAl2O4 and Ca12Al14O33 systems.

Energy Dispersive XAFS: Characterization of Electronically Excited States of Copper(I) Complexes

Energy dispersive X-ray absorption spectroscopy (ED-XAS), in which the whole XAS spectrum is acquired simultaneously, has been applied to reduce the real-time for acquisition of spectra of photoinduced excited states by using a germanium microstrip detector gated around one X-ray bunch of the ESRF (100 ps). Cu K-edge XAS was used to investigate the MLCT states of [Cu(dmp)2]+ (dmp =2,9-dimethyl-1,10-phenanthroline) and [Cu(dbtmp)2]+ (dbtmp =2,9-di-n-butyl-3,4,7,8-tetramethyl-1,10-phenanthroline) with the excited states created by excitation at 450 nm (10 Hz). The decay of the longer lived complex with bulky ligands, was monitored for up to 100 ns. DFT calculations of the longer lived MLCT excited state of [Cu(dbp)2]+ (dbp =2,9-di-n-butyl-1,10-phenanthroline) with the bulkier diimine ligands, indicated that the excited state behaves as a Jahn–Teller distorted Cu(II) site, with the interligand dihedral angle changing from 83 to 60° as the tetrahedral coordination geometry flattens and a reduction in the Cu–N distance of 0.03 Å.

Luminescence properties of Er3+ and Tm3+ doped BaY2F8

This paper reports the characterization of BaY2F8 doped with different concentrations of Er3+ and Tm3+ ions and their potential use as scintillators in radiation detection. Two types of samples were studied, polycrystalline samples, obtained via solid state reaction, and single crystals obtained via the zone melting method under a HF flow. The radioluminescence when excited with X-ray, showed emission peaks characteristics of the 4f–4f transitions of rare earths. Thermoluminescence measurements were done to study the traps that can inhibit the scintillating process. The radiation damage was evaluated and it was showed that the formation of the absorption bands can be connected to color centers generated by radiation in the matrix. Measurements of Dispersive X-ray Absorption Spectroscopy revealed that there is no change in the absorption edge of the dopant during irradiation indicating that the color centers are not connected to changes in the valence of the dopants but are direct connected to the defects due to the BaYF2 matrix.

Introducing a Magnetic Guest to a Tetrel-Free Clathrate: Synthesis, Structure, and Properties of EuxBa8–xCu16P30 (0 ≤ x ≤ 1.5)

The europium-containing clathrate-I Eu(x)Ba(8-x)Cu(16)P(30) was synthesized from the elements. Powder X-ray diffraction in combination with energy dispersive X-ray absorption spectroscopy (EDXS) and metallographic studies showed the homogeneity range with x ≤ 1.5. Determination of the crystal structure confirmed the presence of an orthorhombic superstructure of clathrate-I and revealed that Eu atoms exclusively resided in small pentagonal-dodecahedral cages. Magnetic measurements together with X-ray absorption spectroscopy are consistent with a 4f(7) (Eu(2+)) ground state for Eu(x)Ba(8-x)Cu(16)P(30). Below 3 K the Eu moments order antiferromagnetically. Resistivity measurements revealed metallic behavior of the investigated clathrate, in line with the composition deviating from the Zintl counting scheme. Local vibrations of the guest atoms inside the cages are analyzed with the help of specific heat investigations.

Preparation of Tin Chalcogenide Nanoparticles Using Tribenzyltin(IV) Semi- and Thiosemicarbazone Precursors

The reactions of tribenzyltin(IV)chloride with one mole equivalent of sodium salts of semi- and thiosemicarbazone ligands gave complexes of the type Bz3SnL (L = semicarbazone or thiosemicarbazone moiety). These compounds have been characterized by elemental analysis, IR, 1H, 13C{1H}, and 119Sn (in few cases), NMR spectroscopy, and thermogravimetric analysis. Thermal decomposition of complexes was carried out. The materials obtained were characterized by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), energy dispersive X-ray analysis, absorption, and fluorescence spectroscopy. The XRD showed formation of SnS and SnO from thiosemicarbazone and semicarbazone complexes, respectively. The TEM images showed presence of nearly spherical particles.

Preparation and characterization of nanostructured copper bismuth diselenide thin films from a chemical route

Thin films of copper bismuth diselenide were prepared by chemical bath deposition technique ontoglass substrate below 60°C. The deposition parameters such as time, temperature of deposition and pH of the solution, were optimized. The set of films having different elemental compositions was prepared by varying Cu/Bi ratio from 0·13–1·74. Studies on structure, composition, morphology, optical absorption and electrical conductivity of the films were carried out and discussed. Characterization includes X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), energy dispersive X-ray analysis (EDAX), absorption spectroscopy, and electrical conductivity. The results are discussed and interpreted.

Monitoring of Pt Nanoparticle Formation by H2Reduction of PtO2: An in Situ Dispersive X-ray Absorption Spectroscopy Study

In situ dispersive X-ray absorption spectroscopy (DXAS) measurements were used to monitor the Pt nanoparticle formation by hydrogen reduction of PtO2 at 150 °C. The kinetics of the reduction process, as well its dependence on the percentage of H2 in the reductive gas mixture, was investigated. The results reveal that a complete reduction of the PtO2 was obtained using a gas mixture with 55% of H2 in the time of 20 min, resulting in nanoparticles with an estimated mean diameter of 2.8 nm. In addition, the in situ DXAS investigation of the H2S sulfidation of the formed Pt nanoparticles at 150 °C revealed a low degree of sulfidation, in opposition to the Pt−Pd bimetallic systems.

In Situ Spectroscopic Investigation of Low-Temperature Oxidation of Methane over Alumina-Supported Platinum during Periodic Operation

Methane oxidation over Pt/Al2O3 at transient inlet-gas conditions was studied in situ using synchronous energy dispersive X-ray absorption spectroscopy, Fourier transform infrared spectroscopy, and mass spectrometry. The employed combination of experimental techniques allows for simultaneous analysis of the electronic state of platinum, surface coverage of reaction intermediates/products, and catalytic activity/selectivity, respectively. By cycling of the feed gas composition between net-oxidizing and net-reducing conditions, the activity for methane oxidation can be increased as compared to continuous net-oxidizing conditions. Using the white-line area of time-resolved X-ray absorption near-edge structure spectra, a quantitative estimation of the surface O/Pt ratio indicates the formation of an inhomogeneous surface oxide on the platinum crystallites during reaction. The obtained temporary high activity can be explained through Langmuir-Hinshelwood kinetics and may result either from the formation of a partially oxidized platinum surface that is more effective for methane dissociation or, more likely, from a period with more reactive chemisorbed oxygen prior to oxide formation.

Degradation study of Pt-based alloy catalysts for the oxygen reduction reaction in proton exchange membrane fuel cells

Investigations of the durability of Pt/C, PtCo/C and PtFe/C catalysts for applications on the cathode of proton exchange membrane fuel cells (PEMFC) are carried out using an accelerated degradation procedure (ADP). Electronic and structural properties of the catalysts before and after the ADP are obtained by energy dispersive X-ray spectrometry, X-ray diffraction and X-ray absorption spectroscopy. The electrochemically active Pt surface area of the electrodes is estimated by cyclic voltammetry, while the electrochemical performance is evaluated via polarization measurements at a test station, for single cells fed with H 2/air at 70 °C, prior to and in the course of the ADP. Results show a decrease in the fuel cell performance in the course of the ADP. A decrease in the kinetics of the oxygen reduction reaction is in agreement with a loss of electrochemical active Pt surface area on the catalysts, as verified by cyclic voltammetry and X-ray diffraction.

Temperature dependence of morphology, structural and optical properties of ZnS nanostructures synthesized by wet chemical route

ZnS nanostructures have been synthesized by simple wet chemical route and annealed at two different temperatures of 50 °C and 180 °C. From the measurements of transmission electron microscopy and contact-mode atomic force microscopy, it is found that annealed temperature changes the morphology from nanoparticles to nanorods. The optical properties of the synthesized ZnS nanomaterial have been characterized by UV–visible absorption spectroscopy and photoluminescence spectroscopy. The structural and elemental analyses were carried out by powder X-ray diffraction pattern and energy dispersive X-ray absorption spectroscopy, respectively. Absorption edge of the nanoparticles (295 nm) and nanorods (326 nm) was shifted towards shorter wavelength compared to bulk ZnS (337 nm) due to the quantum confinement effect.

Dependence on composition of electronic properties and stability of Pt–Fe/C catalysts for oxygen reduction

This work presents studies of electronic characteristics and alloy stability carried out for Pt–Fe/C catalysts of different compositions (10–50% Fe, in atoms) and same particle size. The electronic properties are characterized by in situ dispersive X-ray absorption spectroscopy (DXAS). The results show a steady decrease in Pt d-band occupancy as the amount of Fe in the alloy increases. The alloy stability is evaluated by prolonged potential cycling up to 1.0 V. Catalysts with Fe content up to 30% show good stabilities and keep their activities for oxygen reduction after prolonged cycling. In contrast, catalysts with Fe content above 30% suffer pronounced Fe leaching.

The preparation of CuInSe 2 films by solvothermal route and non-vacuum spin-coating process

A low-cost non-vacuum process for fabrication of CuInSe 2 (CIS) films by a solvothermal route and spin-coating method is described. First, the CIS precursor powders are synthesized by solvothermal technology. Second, the CIS films are deposited via spin-coating from precursor slurry consisting of CIS particles and selenization process. Through X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and absorption spectroscopy measurement, the CIS nanoparticle precursors consisted mainly of plate-like particles showing the chalcopyrite structure with preferred (1 1 2) orientation. Typical near stoichiometric single phase CIS films with a micron-sized dense grains are prepared after selenization process. An energy band gap about 1.02 eV and an absorption coefficient exceeding 10 5 cm −1 are also obtained in our work.

The preparation of CuInSe 2 films by combustion method and non-vacuum spin-coating process

A low-cost non-vacuum process for fabrication of CuInSe 2 (CIS) films by combustion method and spin-coating process is described. First, the Cu, In oxides precursors are synthesized by combustion method, which is simpler and cheaper than the other preparation methods and is also the first applied in the CIS solar cells. Second, the CIS films are deposited via spin-coating from precursor slurry consisting of Cu, In oxides components and selenization process. Through X-ray diffraction (XRD), scanning electron microscope (SEM), energy dispersive x-ray spectroscopy (EDS) and absorption spectroscopy measurement, it is found that Cu, In oxides precursors show the dominant CuO, In 2O 3 and Cu 2In 2O 5 phases and have a uniform distribution of the particle size. The CIS films show the dominant chalcopyrite structure, In 2O 3 and Cu 2Se phases and have a micron-sized dense gain. A typical near stoichiometric little Cu-rich CIS films is obtained. An energy band gap about 0.9 eV and an absorption coefficient exceeding 10 5 cm −1 are also obtained in our work.

Pt-Cu/C and Pd Modified Pt-Cu/C electrocatalysts for the oxygen reduction reaction in direct methanol fuel cells

In this work, different carbon supported Pt-Cu catalysts and Pd modified Pt-Cu catalysts were prepared by chemical reduction of Pt, Cu and Pd precursors with sodium borohydride, and their electrocatalytic activity for the oxygen reduction reaction was studied. X-ray diffraction analysis indicated alloy formation. Transmission electron microscopy (TEM) showed that the metal nanoparticles are well-distributed on the carbon support. Dispersive X-ray absorption spectroscopy (DXAS) was used to achieve a better understanding of the intrinsic properties of the materials. The electrocatalysts were characterized electrochemically by cyclic voltammetry in 0.5 mol L-1 H2SO4 and their electrochemical activity was studied with steady-state experiments in a direct methanol fuel cell (DMFC). The introduction of Pd led to an improvement of the catalytic activity of the Pt-Cu catalysts. The best performance in a DMFC was obtained using, as cathode, a Pt50Pd40Cu10/C alloy catalyst thermally treated at 500 ºC in an H2/Ar atmosphere.

Advances in high brilliance energy dispersive X-ray absorption spectroscopy

Almost 30 years have elapsed since the design and evaluation of the first energy dispersive X-ray absorption spectrometer to measure Extended X-Ray Absorption Fine Structure (EXAFS) spectra rapidly using synchrotron radiation. Since then, applications of this method have greatly evolved, and technical solutions were found to adapt these spectrometers to even more challenging experiments. From the initial time-resolved studies to track rapid changes in the local and electronic structure of absorber atoms in disordered systems, to the investigation of matter at high pressure, new applications are now emerging such as studies at extreme pulsed magnetic fields and two dimensional mapping of heterogeneous samples with micron resolution, where each pixel contains full XAS information. In view of the construction of new synchrotron sources and beamlines, and of the upcoming X-Ray Free Electron Lasers, we give a few examples of the scientific impact of Energy Dispersive X-Ray Absorption Spectroscopy (EDXAS) in different scientific domains. We then give a brief overview of recent technical advances, new applications and future developments in the field of high brilliance EDXAS.

Alloys and oxides on carbon-supported Pt–Sn electrocatalysts for ethanol oxidation

This work reports studies of ethanol oxidation on Pt–Sn/C catalysts with nearly the same particle size and identical overall composition having different amounts of oxide and alloyed phases. Results of characterization of physical properties by transmission electron microscopy (TEM), X-ray diffraction (XRD), differential scanning calorimetry (DSC), X-ray photoelectron spectroscopy (XPS), and in situ dispersive X-ray absorption spectroscopy (DXAS) are presented. The variation in the amount of oxide and alloyed phases, promoted by heat treatments in mild temperature conditions, does not produce any significant particle growth. Cyclic voltammetry and oxidation of adsorbed CO in acid medium are used to probe the surface conditions. Data on the electrocatalytic activity towards ethanol oxidation, obtained by potential sweeps and chronoamperometry, are discussed and correlate well with the physical properties. This study, carried out in the absence of composition and particle size effects, shows that the electrocatalytic activity towards ethanol oxidation is strongly influenced by the changes in the amounts of Sn in alloyed and oxidized forms, and that the increase in the amount of alloy at expense of the oxides improves the catalytic activity.