EXAFS Experiments Research Articles

Определение параметров локальной атомной структуры методом совместного анализа EXAFS- и EXELFS-данных

A method has been developed for studying the local atomic structure of 3d-metal-light element systems based on a joint analysis of the EXAFS and EXELFS data. This method permits to form a mathematical connection for the joint processing of experimental data; it takes into account the theoretical formalism of both X-ray absorption and electron beam scattering for the analysis of compounds consisting of chemical elements, the scattering parameters of which vary greatly. The analysis of the simulated and experimental EXAFS and EXELFS spectra has shown that for a one- and two-component object under study, the problem contains a complete set of initial data, which ensures obtaining the desired functions with minimal errors. When studying objects containing three or more chemical elements, the problem of the lack of information arises, and the parameters of the local environment can only be obtained for those atoms for which the experimental data are analyzed. The obtained experimental results were compared with the model calculations. It is shown that the experimental parameters of the local atomic structure coincide with those obtained by the model calculations, which are within the error limits of the method. The comparison of the found PCFs (pair correlation functions) with the data of the model calculations makes it possible to unambiguously identify peaks that have no physical meaning.

Enhanced oxygen reduction activity of size-selected platinum subnanocluster catalysts: Ptn (n = 3–9)

Ptn subnanoclusters (n = 3–9) on a carbon substrate exhibit 1.6–2.2 times higher activity than the standard Pt/C catalysts. EXAFS experiments and DFT calculations show plausible structures and energetics for reaction intermediates in the processes.

Insights into the solution structure of the hydrated uranyl ion from neutron scattering and EXAFS experiments.

The solution structure of 1.0 M Uranyl Chloride has been determined by the EPSR modelling of a combination of neutron scattering and EXAFS data. The experimental data show an equilibrium in solution between [UO2(H2O)5]2+ and [UO2Cl(H2O)4]+ with a stability constant of 0.23 ± 0.03 mol-1 dm-3. A much smaller fraction of the neutral [UO2Cl2(H2O)3] ion is also observed. The data also show, for the first time in solution, that the uranyl ion is a very poor hydrogen bond acceptor, but the coordinated waters show enhanced hydrogen bond ability compared to the bulk water.

Valence control of charge and orbital frustrated system YbFe2O4 with electrochemical Li+ intercalation

We report an attempt valence control of the mixed valence iron triangular oxide YbFe2O4 to develop an effective technique controling the frustration of charges in strongly correlated electron systems. The electrochemical doping of Li + into YbFe2O4 was examined on a cell-type sample similar to the Li-ion secondary battery cell. Systematic changes in the lattice constant and Fe – Fe and Fe–Yb distance were observed with Li doping. Maximum value of the doping was over 300 mAh/g. An EXAFS experiment indicated that Li positioned between Yb octahedron layer (U-layer) and Fe-bipyramidal layer (W-layer). However, detailed change of iron valence state of YbFe2O4was not clearly observed because of the superimpose of the signal from iron metal nano particles in XANES observation. We discuss that the uncertainty might arise from the inhomogeneous distribution of the sample particle size, which might prevent the homogeneous doping of Li because the doping occurs on the surface of each nano-particles.

Investigation of ionic local structure in molten salt fast reactor LiF-ThF4-UF4 fuel by EXAFS experiments and molecular dynamics simulations

Abstract This article focuses on the speciation of molten fluoride mixtures based on ThF4 and UF4 actinides used in molten salt reactors. The local structure of molten AF-MF4 systems (A = Li+, Na+, K+; M = Th4+, U4+) was studied in situ by combining measurements by high temperature X-ray absorption spectroscopy and molecular dynamics simulations. In the molten state, 20 higher than the melting temperature, these mixtures are composed of free fluorine and anionic species [MF7]3−, [MF8]4− and [MF9]5− whose distribution varies with the amount of MF4 (M = Th4+, U4+). Regardless of the cation, these complexes consist of an average of 8 F− neighbors for MF4 content is lower than 35 mol%. This value decreases to 7 as the size of the alkaline ion A (A = Li+, Na+, K+) increases. The [MFx]4−x species are linked together by bridging fluorine ions to form long chains [MxFY]4x−y. The addition of a few percent UF4 to the eutectic LiF-ThF4 composition (77.5 mol% - 22.5 mol%) at leads to a slight increase in the population of free fluorine ions due to the breakdown of the Th-F links to form complexes [UFx]4−x isolated or connected to the (ThxFy)4x−y chains. This change in the structure of the liquid results in a slight decrease in viscosity when a few mol. % of UF4 is added.

Revisiting the cobalt(II) hydration from molecular dynamics and X-ray absorption spectroscopy

Solution chemistry of Co(II) is receiving a renewal attention due to the high interest for knowing the speciation in seawater of its Co radioactive isotope which appeared in the Japan sea as a consequence of the Fukushima-Daichii nuclear power plant accident. Experimental EXAFS and XANES spectra of a dilute Co(II) aqueous solution have been recorded and structural data derived from their analysis. Based on QM calculations, an ab-initio intermolecular potential has been generated for the Co(II)–HO interaction using the hydrated ion model that uses a polarisable and flexible solvent description through the MCDHO2 model. Classical molecular dynamics simulations of Co(II) in water have been performed and X-ray Absorption spectra have been simulated and compared with the experimental ones. Energetic, structural, dynamical and spectroscopical properties of the cobalt cation in solution have been computed and compared with previous experimental and theoretical data. These comparisons have assessed the good performance of the developed intermolecular potential.

Insights into the Composition and Function of a Bismuth-Based Catalyst for Reduction of CO2 to CO

The electrochemical conversion of CO2 to CO is a process important for generation of carbon-based fuels using energy produced from intermittent, renewable energy sources. Electrodeposited thin films of bismuth-based materials are promising platforms for this transformation when used for CO2 electrolysis in the presence of imidazolium-based ionic liquids including [BMIM]OTf and [EMIM]OTF. In this study, the composition and function of a bismuth-based carbon monoxide evolving catalyst (Bi-CMEC) has been probed. In particular, the composition of both the electrodeposited bismuth material and the catholyte has been scrutinized in an effort to understand the factors that drive efficient catalyst operation. A combination of XPS depth profiling, XANES, and EXAFS experiments were employed to identify the bulk composition of the bismuth catalyst, which is shown to be composed of both metallic and oxidized phases. The identity of the catholyte solvent has been shown to influence the nature and efficacy of CO2 reduc...

Lattice Strained Ni-Co alloy as a High-Performance Catalyst for Catalytic Dry Reforming of Methane

Dry reforming of methane (DRM) is an effective route to convert methane and carbon dioxide to syngas. Herein, we report an efficient nickel–cobalt bimetallic catalyst with an activity of 4.97 molCH4 molNi–1 s–1 at 800 °C. It is active at low temperature as well, and near-thermodynamic equilibrium conversion was achieved as low as 350 °C, with a high yield of H2 implying inhibition of side reactions: i.e., a reverse water-gas shift (RWGS) reaction. The formation of Ni-Co alloy during the reaction was observed, and its lattice contraction was revealed by HAADF-STEM and EXAFS experiments. The lattice-strained Ni-Co alloy has good CO2 dissociation ability, which is responsible for its superior catalytic performance. Its weak chemisorption with H2 results in inhibition of RWGS side reactions. This work is helpful in the design and development of other metal alloy materials with novel structures and/or electronic configurations for catalytic applications.

UO22+ structure in solvent extraction phases resolved at molecular and supramolecular scales: a combined molecular dynamics, EXAFS and SWAXS approach.

A new polarizable force field for describing the solvation of the uranyl (UO22+) cation in solvent extraction phases has been developed for molecular dynamics simulations. The validity of the polarizable force field has been established by comparison with EXAFS and SWAXS experiments. This new force field allows for describing both the UO22+ hydration and solvation properties in good agreement with the experiments. In aqueous phases we demonstrated that the UO22+ force field has been improved from the previous one we developed. Indeed, the UO22+ structural and dynamics properties, i.e., the dynamics of the water molecules in the vicinity of the uranyl cation, calculated from molecular dynamics are in better agreement with the EXAFS experiments. Furthermore, the transferability of the UO22+ force field proposed here has been validated on typical solvent extraction phases containing uranyl nitrate salts with extractant molecules, namely DMDOHEMA molecules, in n-heptane. The good agreements observed between the theoretical (MD simulations) and experimental UO22+ structures at the molecular (EXAFS) and supramolecular (SWAXS) scales prove the accuracy of the UO22+ force field developed and proposed in the present paper.

Relationship Between Structure and Coordination Strength of N and N,O-Hybrid Donor Ligands with Trivalent Lanthanides

ABSTRACTTo effectively separate lanthanides (Ln(III)) from actinides (An(III)), symmetrical 2,2′-bipyridyl (Bpy), 1,10-phenanthroline (Phen), and unsymmetrical N-methyl-N-tolyl-1,10-phenanthroline-2-carboxamide (MeTol-PTA) were investigated. According to the crystal structures and EXAFS experiments, the decreasing ionic radius from light to heavy Ln led to decreases in the Ln–N (Bpy) and Ln–N (Phen) distances, while log β simply increased due to the electrostatic interaction and the order of Ln–O (MeTol-PTA) < Ln–N (Bpy, Phen) < Ln–N (MeTol-PTA) was obtained. This indicated that the bulky phenanthroline moiety of MeTol-PTA may not allow N (MeTol-PTA) to come close to Ln. Consequently, the log β of MeTol-PTA exhibited a local maximum (around Nd).

Compositional partitioning during the spinodal decomposition in Cu–Ni–Sn alloy

Spinodal decomposition in Cu–9.4at%Ni–3.1at%Sn alloy was elucidated with the new insight from the experimental EXAFS analysis supported by ab initio total energy calculations suggesting the strong influence of the first near-neighbour atoms. Enthalpy of mixing was calculated for all crystallographically unique first near-neighbour configurations and finally an average positive enthalpy of mixing of 1604 J/mol was obtained. Combination of ab initio results, XRD and EXAFS analysis indicate that one of the daughter phase becomes rich in Ni and Sn than the other phase; in contrary to the earlier proposition that Cu/Ni ratio remains constant in both daughter phases. It is also shown that the present thermodynamic description requires further refinement to extend the miscibility gap towards lower Ni content in Cu–Ni–Sn system.

Revealing the Influence of Silver in Ni–Ag Catalysts on the Selectivity of Higher Olefin Synthesis from Stearic Acid

Results on the conversion of stearic acid to olefins over Ni–Ag/γ-Al2O3 catalysts are presented. XANES and EXAFS experiments in situ and DFT calculations were applied to reveal the structure of active sites therein. It is shown that the introduction of Ag to Ni catalysts leads to an increase in the olefin yield. After a reduction in hydrogen (350°C, 3 h) alumina-supported nanoparticles of nickel sulfides and metallic Ag are formed. The role of metal hydrides formed during the reaction is extensively discussed.

EXAFS of Nickel Oxide in the Ionic Liquid Emic/AlCl3

Introduction Earlier research has shown that nickel metal and nickel alloys can be deposited from nickel chloride dissolved in the IL AlCl3/EMIC but this depended on Lewis acidity of the IL which in turn affects the structure of the Ni ions that form in the solution (1, 2). The structures of several transition metal chlorides have been studied in situ in ILs using EXAFS (3-6). However, the oxides of these metals have not received as much attention. When working with the water and air sensitive ILs, there is always a concern about oxygen and moisture contamination and its effect on the properties of the IL. Many of the transition metal chlorides are studied in the anhydrous state and thus they are also sensitive to oxygen and moisture contamination in the IL. In this study, we examine the structure of the nickel ions that form in the AlCl3/EMIC IL when nickel(II) oxide is added to simulate the effect of oxygen contamination. The structure of NiO was studied with CVs and EXAFS in both Lewis acidic and Lewis basic AlCl3/EMIC ILs. Experimental Solutions were prepared in a Vacuum Atmosphere nitrogen filled drybox. Voltammetric experiments were carried out at ambient temperature. The ionic liquid solutions were sealed in 2 mil thick polyethylene bags and were examined with EXAFS. The electrochemical experiments were conducted with an EG&G PARC model 283 potentiostat controlled with the EG&G PARC PowerSuite software package. The EXAFS experiments were conducted on beamline X-11A at the National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory (BNL). All of the EXAFS data were measured at room temperature with the storage ring operating at 2.8 MeV and beam currents in the range of 150 to 260 mA. The EXAFS data were analyzed using the standard XDAP data analysis package. Results and Discussion Data were collected for NiO and anhydrous NiCl2 powder samples for reference standards to be used in the analysis of the IL samples. The Fourier transform for the NiO powder standard shows several shells. The first shell is the nickel - oxygen interaction and the second shell is the nickel - nickel interaction. The shells that are at greater distances are additional oxygen and nickel interactions, but they also include multiple scattering contributions. The Fourier transform of the NiCl2 powder standard shows two primary shells. The first shell is the nickel - chlorine interaction and the second shell is the nickel – nickel interaction. The Fourier transform for NiO dissolved in the N=0.43 basic IL shows a single peak, indicating a single shell around the central nickel atom. The Fourier transform was phase-corrected with both the chloride shell from the NiCl2 standard, and oxide shell from the NiO standard, in order to determine which element was contributing to the backscattering. When the FT is phase-corrected with the correct backscattering atom, the peak in the imaginary part corresponds to the maximum of the positive peak in the magnitude of the FT. The phase corrected FT shows that the backscattering atoms must be primarily from a Ni–Cl interaction where the two peaks are mostly in phase. Acknowledgements The authors would like to acknowledge the financial support of the Naval Research Laboratory, the American Society for Engineering Education and the U.S Department of Energy. This research was carried out at the National Synchrotron Light Source, Brookhaven National Laboratory, which is supported by the U.S. Department of Energy, Division of Materials Sciences and Division of Chemical Sciences, under Contract No. DE-AC02-98CH10886.

Triggering of spin-flipping-modulated exchange bias in FeCo nanoparticles by electronic excitation.

The exchange coupling between ferromagnetic (FM)-antiferromagnetic (AF) interfaces is a key element of modern spintronic devices. We here introduce a new way of triggering exchange bias (EB) in swift heavy ion (SHI) irradiated FeCo-SiO2 films, which is a manifestation of spin-flipping at high irradiation fluence. The elongation of FeCo nanoparticles (NPs) in SiO2 matrix gives rise to perpendicular magnetic anisotropy at intermediate fluence. However, a clear shift in hysteresis loop is evident at the highest fluence. This reveals the existence of an AF exchange pinning domain in the NPs, which is identified not to be oxide shell from XANES analysis. Thermal spike calculations along with first-principles based simulations under the framework of density functional theory (DFT) demonstrate that spin flipping of 3d valence electrons is responsible for formation of these AF domains inside the FM NPs. EXAFS experiments at Fe and Co K-edges further unravel that spin-flipping in highest fluence irradiated film results in reduced bond lengths. The results highlight the possibility of miniaturization of magnetic storage devices by using irradiated NPs instead of conventionally used FM-AF multilayers.

Experimental Challenges in Studying Hydrogen Absorption in Ultrasmall Metal Nanoparticles

Recent advances on synthesis, characterisation and hydrogen absorption properties of ultra-small metal nanoparticles (defined here as objects with average size ≤ 3 nm) are briefly reviewed in the first part of this work. The experimental challenges encountered in performing accurate measurements of hydrogen absorption in Mg- and noble metal-based ultra-small nanoparticles are addressed. The second part of this work reports original results obtained for ultra-small bulk immiscible Pd-Rh nanoparticles. Carbon supported Pd-Rh nanoalloys in the whole binary chemical composition range have been successfully prepared by liquid impregnation method followed by reduction at 300 °C. EXAFS investigations suggested that the local structure of these nanoalloys is partially segregated into Rh-rich core and Pd-rich surface coexisting within the same nanoparticles. Downsizing to ultra-small dimensions completely suppresses the hydride formation in Pd-rich nanoalloys at ambient conditions, contrary to bulk and larger nanosized (5-6 nm) counterparts. The ultra-small Pd90Rh10 nanoalloy can absorb hydrogen forming solid solutions under these conditions, as suggested by in situ XRD. Apart from this composition, common laboratory techniques such as, in situ XRD, DSC and PCI failed to clarify the hydrogen interaction mechanism : either adsorption on developed surfaces or both adsorption and absorption with formation of solid solutions. Concluding insights were brought by in situ EXAFS experiments at synchrotron: ultra-small Pd75Rh25 and Pd50Rh50 nanoalloys absorb hydrogen forming solid solutions at ambient conditions. Moreover, the hydrogen solubility in these solid solutions is higher with increasing Pd content and this trend can be understood in terms of hydrogen preferential occupation in the Pd-rich regions, as suggested by in situ EXAFS. The Rh-rich nanoalloys (Pd25Rh75 and Pd10Rh90) only adsorb hydrogen on the developed surface of ultra-small nanoparticles. In summary, in situ characterization techniques carried out at large scale facilities are unique and powerful tools for in-depth investigation of hydrogen interaction with ultra-small nanoparticles at local level.

Electronic Structure Changes As Source of Chemical and Interfacial Instability in LiCoPO4 As Positive Electrode for High Voltage Li-Ion Batteries

The energy storage capability of a battery scales with the potential difference between its electrodes. Yet operation of positive electrode materials at high potentials introduces challenges of stabilization of charged states. As of today, no positive electrode material has been demonstrated to durably and safely operate above 4.5 V vs. Li+/Li0. LiCoPO4-based electrodes theoretically offer high specific capacity and high potentials of operation, around 4.8V vs. Li+/Li0, but these electrodes are prone to failure during cycling. Failure occurs through chemical and structural degradation in the bulk of the active material or at its interfaces with cell components, especially the electrolyte. The development of Li-ion battery electrodes operating at high potential is indispensable to meet the specific energy target of 250 kWh/kg at the packaged cell level. Changes in the electronic structure and chemical stability of olivine-type LiCoPO4 and Fe-substituted LiCoPO4 were explored as both a function of ion substitution and oxidation state. Soft Ex situX-Ray absorption spectroscopy (XAS) made it possible to compare the changes in chemical bonding between electrode bulk and surface as a function of lithium content. This technique can probe the density of states at the transition metal and O levels. The evolution of these levels revealed changes in the metal-oxide covalence when lithium was deintercalated from the structure, and, thus, the material was oxidized. An increase in covalence can lead to the destabilization of the anions. If this process takes place in the bulk of the material, this destabilization can lead to thermal degradation via oxygen loss. At the surface, even small degrees of destabilization are sufficient to produce oxidizing species that attack the electron-rich solvent molecules in the electrolyte, leading to irreversible capacity loss. Increased metal-oxygen covalence was universally observed in the spectroscopy in the form of a rising pre-O K-edge peak at ~530 eV as a function of lithium deintercalation in both bulk and surface. However, accompanying changes in the Co spectroscopy were only observed in the Fe-substituted sample. These changes are also indicative of increased hybridization between Co 3d and O 2p orbitals. Co K-edge XANES and EXAFS experiments further corroborated these findings, in which virtually no changes in the Co K-edge were observed upon oxidation of unsubstituted LiCoPO4. Fe doping appears to play a substantial role in getting Co to participate in redox chemistry, and the mechanism by which it occurs is currently being explored using Density Functional Theory. Fe-substituted LiCoPO4 is an exciting new positive electrode material that may prove useful in advancing Li-ion battery technology.

Modeling the Structure of Complex Aluminosilicate Glasses: The Effect of Zinc Addition.

An empirical potential structure refinement of neutron and X-ray diffraction data combined with extended absorption fine structure evidence has been applied to the investigation of two distinct sets of complex aluminosilicate glasses containing different quantities of zinc. Data come from (i) neutron and X-ray total scattering experiments, which have been performed at the ISIS neutron spallation source (SANDALS beamline) and at the European Synchrotron Radiation Facility (ID11 beamline), and (ii) EXAFS experiments which have been performed at the European Synchrotron Radiation Facility (BM23 beamline). By careful examination of the modeled ensemble of atoms, a wide range of structural information has been extracted: coordination numbers, bond distances, cluster sizes, type of oxygen sharing, and the preference of large cations to adopt a charge-compensating role. The first series of glasses, which is characterized by a fixed network modifier element content (i.e., Na), shows how the introduction of Zn at the expense of Si and Al network forming elements does not significantly alter the polymerization degree, as a result of its dominant 4-fold coordination. In the case of the second series, which is characterized by fixed network forming element content (i.e., Si and Al), it is shown how the replacement of a network modifier element (i.e., Ca) with the introduction of Zn does not change the propensity of Zn to be mainly 4-fold coordinated by promoting the network. Where appropriate the experimental results have been compared with classical theoretical approaches such as stoichiometric models based on Zachariasen's rules and computational routines.

Application of soy hull biomass in removal of Cr(VI) from contaminated waters. Kinetic, thermodynamic and continuous sorption studies

Soy hull was evaluated as a new material for Cr(VI) removal from aqueous solutions. Cr(VI) removal was associated to a redox mechanism, in which Cr(VI) was reduced to Cr(III) by the biomass. The redox capacity of soy hull was 1.12mmolg−1. A kinetic model that considers the redox reaction between Cr(VI) and the biomass surface was proposed. The maximum sorption capacity was 7.286mgg−1 at 20°C and pH 1.5. Activation parameters and mean free energies suggest that the sorption process follows a mechanism of chemical sorption. Thermodynamic parameters show that Cr(VI) removal was spontaneous. The isosteric heat of sorption indicated that soy hull has an energetically homogeneous surface. XPS spectra showed that chromium bound on the biomass was Cr(III). These results were confirmed by XANES and EXAFS experiments. EPR spectra showed the presence of Cr(V)-soy hull at short contact time and only a signal corresponding to Cr(III)-soy hull at long contact times. Continuous sorption data were fitted to Thomas and modified dose–response models. The bed depth service time (BDST) model was used to scale-up the continuous sorption experiments.

X-Ray Absorption Fine Structure Investigation of Copper(II) Mixed Ligand Complexes with Pyridinedicarboxylic Acid as Primary Ligand

The X-ray absorption fine structure (XAFS) spectra at the K-edge of the copper complexes Cu(PDC)(Mim)3 H2O (1) and Cu(PDC)2(EA)2H2O (2) (where PDC – Pyridine-2,3-dicarboxylic acid, Mim – 2-methylimidazole, and EA – ethyl acetate) have been investigated. The experimental extended X-ray absorption fine structure data of complex 1 have been analyzed by fitting the theoretical model generated from its own crystallographic data. The crystallographic data for complex 2 are not available. It has been found by comparing the intensity of the pre-edge peaks and X-ray absorption near edge structure features of complexes 1 and 2 that both complexes possess square pyramidal geometry around the copper centers and thus complex 2 is analogous to complex 1. Hence, the theoretical model generated for complex 1 has been fitted to the experimental EXAFS data of complex 2 to determine the structural parameters of complex 2. The coordination geometry of both complexes has been depicted. Further, the chemical shifts have been used to determine the oxidation state as well as to estimate the effective nuclear charge on the copper atom.

Controlling Crystal Structure in Embedded Magnetic Nanoparticles

"Magnetic nanoparticles and nanocomposite materials have attracted much interest due to their novel magnetic behaviour, and their potential use in a range of applications. One of the main reasons for their novel magnetism, appreciated for some time, is the high proportion of under-coordinated atoms at the surface of nanoparticles. In the case of magnetic transition metals this leads to a narrowing of the 3d bands that are responsible for magnetism in these materials, leading to size-dependent nanoparticle properties. The atomic structure adopted by nanoparticles is also a key factor in determining their magnetism. Unlike in bulk materials atomic structure in nanoparticles can be changed more readily by, for example, embedding them in suitable matrix materials. Here we describe how a high level of control over crystal structure in nanoparticles can be achieved, using EXAFS to ""fingerprint"" their crystal structure, and show how this in turn leads to a high degree of control over nanoparticle magnetism. We describe a flexible co-deposition process based around a gas aggregation source, which enables a high degree of control over structure in transition metal nanoparticles embedded in various matrices. EXAFS experiments and analysis used to probe atomic structure in embedded Fe and Co nanoparticles are described [1]. Examples presented include the system of Fe nanoparticles embedded in a CuAu alloy matrix where we show that is not only the ability to change the atomic structure of embedded nanoparticles that is important but the ability to fine-tune their structure once changed [2]. In this case, this enables the atomic magnetic Fe moment to be fine-tuned to a value higher than in the bulk Fe structure, in agreement with theory. In some systems alloying at the particle/interface can be significant. We describe how this is the case for Fe nanoparticles in Pd [3], and how such alloying could be useful in forming magnetic nanocomposites with superior properties."