EXAFS Study Research Articles

Mechanistic study of simultaneous arsenic and fluoride removal using granular TiO2-La adsorbent

Naturally co-occurring arsenic (As) and fluoride (F) in groundwater have caused increasing public concerns, and their simultaneous removal is still a challenge due to the lack of efficient adsorptive materials and their competition for adsorption sites. This study fabricated a novel TiO2-La granular composite adsorbent with LaCO3OH showing orientated growth on TiO2 {100} facet. The material exhibited high As(III) (114mg/g) and F (78.4mg/g) adsorption capacities and a wide application pH, ranging 3–9, achieving a high percentage (>90%) of As(III) and F adsorption. Coadsorption experiment results demonstrated that high F concentrations inhibit As(III) adsorption, whereas the coexisting As(III) has no significant effect on F removal. The molecular-level mechanisms with EXAFS and DFT studies demonstrated that As(III) adsorption is favorable only on Ti sites as evidenced by an As-Ti distance of 3.36Å when pH<7, while La adsorption sites can also be occupied with an As-La distance at 3.42Å when pH>10. The F adsorption is pH-dependent and mainly occurred on La sites. The granular TiO2-La with high As(III) and F adsorption capacities can be used to simultaneously remove As(III) and F. The insights gained from this study shed a new light on the interaction mechanism of As(III) and F with the TiO2-La composite.

Co (II) Complexes with Schiff Base Ligands: Synthesis and EXAFS Study

Thesynthesis of transition metal Schiff base complexes of Co(II) are prepared by chemical root method. Obtained by the condensation of O-phenylenediamine, salicylaldehyde and isatin / 2-hydroxy- 1 Naphthaldehyde is presented. The complexes were characterized by Co- K- edge EXAFS measurements using the dispersive beam line at 2.5GeV energy of Indus-2 synchrotron radiation source RRCAT Indore. The recorded EXAFS data were analyzed using the computer software Athena for determine the nearest neighboring distances (bond lengths) of these complexes with conventional methods and were compared with Fourier transform (FT) analysis.

Micro-structural origin of elongation in swift heavy ion irradiated Ni nanoparticles: A combined EXAFS and DFT study

Aiming for perpendicular magnetic storage usage, shape anisotropy is introduced in Ni nanoparticles (NPs) embedded inside a thin SiO2 matrix using swift heavy ion irradiation (SHI). Systematic increase in NPs' aspect ratio along the direction of incident SHI beam is observed up to 5 × 1013 ions/cm2 (5e13) fluence from grazing incidence small angle X-ray scattering measurements. Strikingly, at higher fluences the major dimension (along SHI beam) got reduced. This observation is totally intriguing as usually particle elongation increases with applied fluence. To understand this anomaly, as a first step, we have performed a combined near and far edge X-ray absorption spectroscopy (XANES and EXAFS) analysis. This shows irradiated Ni NPs sustain their metallic phase even with increased structural disorder and reduced atomic co-ordination. However an atypical reduction in local structural anisotropy beyond 5e13 fluence is observed from angle dependent EXAFS: following the same trend as NP elongation. To have a better insight, the role of the electronic spin of individual atoms in controlling particle shape is investigated. Using the lattice temperature profiles derived from thermal spike model, we have carried out ab initio molecular dynamics (MD) simulations to understand the structural distortion at higher temperatures. The experimentally observed structural and shape anisotropy in the irradiated NPs is only realized when spin polarization contributions were considered in our calculations. From the pair correlation function and atom-wise spin density plots, we conclude that presence of spin affects the second co-ordination shell and controls the atomic arrangements in such a manner that elongated structures are preferred in the irradiated system at intermediate fluence (i.e. 5e13). Further increase in MD temperature to 6000 K (corresponding to 1 × 1014 ions/cm2 fluence) results in disordered spin alignment and melting of moderately weak spin polarized Ni NPs: thus violating conventional trend of increasing particle elongation w.r.t SHI fluence.

The cytochrome b Zn binding amino acid residue histidine 291 is essential for ubihydroquinone oxidation at the Qo site of bacterial cytochrome bc1

The ubiquinol:cytochrome (cyt) c oxidoreductase (or cyt bc1) is an important membrane protein complex in photosynthetic and respiratory energy transduction. In bacteria such as Rhodobacter capsulatus it is constituted of three subunits: the iron-sulfur protein, cyt b and cyt c1, which form two catalytic domains, the Qo (hydroquinone (QH2) oxidation) and Qi (quinone (Q) reduction) sites. At the Qo site, the pathways of bifurcated electron transfers emanating from QH2 oxidation are known, but the associated proton release routes are not well defined. In energy transducing complexes, Zn2+ binding amino acid residues often correlate with proton uptake or release pathways. Earlier, using combined EXAFS and structural studies, we identified Zn coordinating residues of mitochondrial and bacterial cyt bc1. In this work, using the genetically tractable bacterial cyt bc1, we substituted each of the proposed Zn binding residues with non-protonatable side chains. Among these mutants, only the His291Leu substitution destroyed almost completely the Qo site catalysis without perturbing significantly the redox properties of the cofactors or the assembly of the complex. In this mutant, which is unable to support photosynthetic growth, the bifurcated electron transfer reactions that result from QH2 oxidation at the Qo site, as well as the associated proton(s) release, were dramatically impaired. Based on these findings, on the putative role of His291 in liganding Zn, and on its solvent exposed and highly conserved position, we propose that His291 of cyt b is critical for proton release associated to QH2 oxidation at the Qo site of cyt bc1.

Toward a Unified Identification of Ti Location in the MFI Framework of High-Ti-Loaded TS-1: Combined EXAFS, XANES, and DFT Study

Titanium silicalite-1 (TS-1) has been shown to be a heterogeneous catalyst with remarkable efficiency and selectivity; however, the nature of the active Ti site in the MFI framework remains elusive. Here we report combined experimental and theoretical research on Ti distribution in the 12 crystallographically distinct T sites of the MFI framework in high-Ti-loaded TS-1 (2.7 wt % in TiO2). Using a multishell fit to extended X-ray absorption fine structure, we show that T4 is the most populated site, in marked contrast to the preferential substitution sites and the definitely excluded sites assumed hitherto by diffraction studies. The identification is supported by a good agreement between calculated and experimental X-ray absorption near-edge structure studies and by full periodic density functional theory (DFT) computation. In spite of having the identical most favored site, the preference order for the remaining sites predicted by DFT does not fully match the experimental results. This suggests that Ti d...

Rotation of X-ray polarization in the glitches of a silicon crystal monochromator.

EXAFS studies on dilute samples are usually carried out by collecting the fluorescence yield using a large-area multi-element detector. This method is susceptible to the 'glitches' produced by all single-crystal monochromators. Glitches are sharp dips or spikes in the diffracted intensity at specific crystal orientations. If incorrectly compensated, they degrade the spectroscopic data. Normalization of the fluorescence signal by the incident flux alone is sometimes insufficient to compensate for the glitches. Measurements performed at the state-of-the-art wiggler beamline I20-scanning at Diamond Light Source have shown that the glitches alter the spatial distribution of the sample's quasi-elastic X-ray scattering. Because glitches result from additional Bragg reflections, multiple-beam dynamical diffraction theory is necessary to understand their effects. Here, the glitches of the Si(111) four-bounce monochromator of I20-scanning just above the Ni K edge are associated with their Bragg reflections. A fitting procedure that treats coherent and Compton scattering is developed and applied to a sample of an extremely dilute (100 micromolal) aqueous solution of Ni(NO3)2. The depolarization of the wiggler X-ray beam out of the electron orbit is modeled. The fits achieve good agreement with the sample's quasi-elastic scattering with just a few parameters. The X-ray polarization is rotated up to ±4.3° within the glitches, as predicted by dynamical diffraction. These results will help users normalize EXAFS data at glitches.

Highly selective palladium-copper bimetallic electrocatalysts for the electrochemical reduction of CO2 to CO

Selective and efficient conversion of carbon dioxide (CO2) to a reusable form of carbon via the electrochemical reduction of CO2 has attracted much attention recently, as it is a promising approach for the storage of renewable energy. Herein, we synthesize palladium-copper bimetallic nanoparticles with different compositions, which serve as a well-defined platform to understand their fundamental catalytic activity in CO2 reduction. Among PdCu/C and Pd/C catalysts tested, Pd85Cu15/C catalyst shows the highest CO Faradaic efficiency of 86%, CO current density of 6.9mAcm−2 and mass activity for CO production of 24.5Ag−1 at −0.89V vs. RHE in CO2-saturated 0.1M KHCO3 solution, which is about 5 times, 8 times and 2.2 times higher than Pd/C catalyst, respectively. It was suggested from EXAFS and CO TPD-MS studies that the highly selective CO production on Pd85Cu15/C catalyst is due to the presence of an optimum ratio of the copper element and low-coordination sites over monometallic Pd active for H2 evolution with low overpotential. We believe that controllable size and composition for the bimetallic nanoparticles are critical to the CO2 reduction activity enhancement and high CO Faradaic efficiency. The insights gained through this work may shed light in a foundation for designing efficient catalysts for electrochemical reduction of CO2.

Structural and magnetic properties of nanoclusters formed in III-V semiconductors

Studies of X-ray magnetic circular dichroism (XMCD) were performed for a set of GaMnAs films with different Mn concentrations priory and after high temperature annealing (500 and 600 oC). After thermal treatment, GaMnAs samples with zinc blende structure and MnAs hexagonal nano-clusters were formed. In most of the samples, both types of clusters were detected by EXAFS studies. Dependence of the orbital and the spin moments on magnetic field were calculated from XMCD data by applying the sum rule. It was shown that both moments were much larger for MnAs nano-clusters. When these inclusions are formed even in a small amount, they dominate the XMCD signal. Interestingly, in some of samples the zinc blende GaMnAs nano-clusters were observed at a surface while in the bulk of hexagonal MnAs. Therefore, the location of magnetic ions in the host matrix is crucial for their magnetic properties. This unique information can be provided by XAS and XMCD.

Structural properties of sodium-rich carbonate-silicate melts: An in-situ high-pressure EXAFS study on Y and Sr

In-situ EXAFS combined with a Paris-Edinburgh press (PEP) is an outstanding tool to investigate the local environment of trace elements in melts at high pressure and temperature. A novel design of the pressure assembly ensures a highly stable experimental setup (reaching temperatures of up to 2000 K at 2.5 GPa) while permitting the necessary level of X- ray transmission. This study focuses on the structural incorporation of the geochemically important trace elements Y and Sr in sodium-rich silicate-carbonate melts. Y and Sr K edge EXAFS were collected in transmission mode of the melt (at ∼2.5 GPa, 1600 K) and its respective quench products. Distinct changes in the XANES region suggest a change in site symmetry during the cooling process.

EXAFS study of the structural properties of In and In + C implanted Ge

The structural configurations of In implanted Ge have been studied via x-ray absorption spectroscopy with and without the codoping of C. In the case of In singly implanted Ge, while the In atoms occupy an substitutional site in Ge (InGe4) at low In concentration (≤ 2 at. %), they precipitate into a metallic phase (In metal) and form complexes composed of one vacancy and three Ge atoms (InVGe3) at concentration ≥ 0.6 at. %. This behaviour can be suppressed by the addition of C leading to In-C pairing to form InCGe3 complexes. This cluster enables In atoms to recover a four-fold coordinated structure and has the potential to improve the electrical activation of In atoms in Ge.

Polyoxometalate Cluster-Incorporated Metal-Organic Framework Hierarchical Nanotubes.

A simple method to prepare metal-organic framework (MOF) nanotubes is developed by employing polyoxometalates (POMs) as modulators. The local structure of the MOF nanotubes is investigated combining XANES and EXAFS studies. These nanotubes show both an excellent catalytic performance in the detoxification of sulfur compounds in O2 atmosphere and a remarkable cycling stability as the anode material for lithium-ion batteries.

Synchrotron EXAFS and XANES spectroscopy studies of transition aluminas doped with La and Cr for catalytic applications

Transition aluminas doped with Cr find widespread application in the dehydrogenation catalysis industry, while La-stabilized transition aluminas are used extensively for high-temperature application as catalytic supports (Wefers and Misra in Oxides and hydroxides of aluminum, Alcoa Laboratories, Pittsburgh, 1987). In this work, detailed synchrotron XAFS spectroscopy studies were conducted to shed light upon the atomic mechanisms of surface and subsurface reconstructions and/or catalytic support stabilization of doped aluminas. It was demonstrated that in four transition aluminas doped with Cr, it is the atoms which are mostly in the state of oxidation Cr3+ and enter nanoparticles of Cr-bearing phases (Cr2O3 in the case of gamma- and chi-alumina). In the transition series aluminas: “gamma- chi- theta- eta-alumina,” the change of properties (in particular, the dramatic increase in dehydrogenation catalytic activity and catalyst longevity and the coloration of samples) takes place because of the reduction in the average size of Cr clusters and their appearance on the Al2O3 surface, probably responsible for change in catalytic activity. It was demonstrated that in the samples of gamma-alumina doped with La any substantial change in the local coordination of the La atoms takes place only upon heating up to 1400 °C. This makes the La-stabilized gamma-alumina a perfect catalytic support for the numerous applications, e.g., catalytic three-way conversion of automobile exhaust gases. This change manifested itself in the form of increased La–O bond lengths and the La coordination number (from 8 to 12). Furthermore, it was demonstrated that the local environment of La in this new La-bearing phase cannot be explained in terms of the LaAlO3 formation. The absence of the La atoms in the second coordination sphere favors monoatomic distribution of La atoms on grain boundaries, proving that only very small amount of this rare earth material is required to achieve full stabilization. It is inferred that the tendency of La atoms to get surrounded by oxygen atoms, and also the impossibility of going into the alumina bulk, could be a major reason of the increased thermal stability of gamma-alumina doped with lanthanum.

Robust and Porous β-Diketiminate-Functionalized Metal-Organic Frameworks for Earth-Abundant-Metal-Catalyzed C-H Amination and Hydrogenation.

We have designed a strategy for postsynthesis installation of the β-diketiminate (NacNac) functionality in a metal-organic framework (MOF) of UiO-topology. Metalation of the NacNac-MOF (I) with earth-abundant metal salts afforded the desired MOF-supported NacNac-M complexes (M = Fe, Cu, and Co) with coordination environments established by detailed EXAFS studies. The NacNac-Fe-MOF catalyst, I•Fe(Me), efficiently catalyzed the challenging intramolecular sp(3) C-H amination of a series of alkyl azides to afford α-substituted pyrrolidines. The NacNac-Cu-MOF catalyst, I•Cu(THF), was effective in promoting the intermolecular sp(3) C-H amination of cyclohexene using unprotected anilines to provide access to secondary amines in excellent selectivity. Finally, the NacNac-Co-MOF catalyst, I•Co(H), was used to catalyze alkene hydrogenation with turnover numbers (TONs) as high as 700,000. All of the NacNac-M-MOF catalysts were more effective than their analogous homogeneous catalysts and could be recycled and reused without a noticeable decrease in yield. The NacNac-MOFs thus provide a novel platform for engineering recyclable earth-abundant-element-based single-site solid catalysts for many important organic transformations.

Effect of aging on the CO oxidation properties of copper manganese oxides prepared by hydrolysis–coprecipitation using tetramethyl ammonium hydroxide

Copper manganese mixed oxides were prepared by a hydrolysis-coprecipitation method using tetramethyl ammonium hydroxide and calcined at 573–873 K. The effect of aging treatment after the coprecipitation process on the catalytic properties of the calcined mixed oxides for CO oxidation was investigated. XRD and EXAFS studies showed that spinel phases were mainly formed in the aged catalysts and the degree of disorder in the Cu–Mn spinel oxide phases increased. The aging treatment suppressed the formation of crystallites of mixed oxides, but the treatment promoted the crystallization of impurity CuO phase. The aged catalyst exhibited higher activity for CO oxidation than the unaged catalysts. The optimum calcination temperature for obtaining the highest activity was changed by aging treatment. Refluxing during the aging treatment led to the detrimental effect on the CO oxidation activities.

Investigation of Fe doped ZnO thin films by X-ray absorption spectroscopy

XANES and EXAFS studies have been carried out on Fe doped ZnO thin films having different Fe doping concentration ranging from 1% to 10% and the observed ferromagnetism in the samples is explained in the light of XANES and EXAFS observations.

Investigations on local structures in new Bi2-2xLa2xUO6 (x = 0-0.05) solid solutions: a combined XRD, EXAFS, PL and EPR study.

La doped Bi2UO6 solid solutions of the general formula Bi2-2xLa2xUO6 (x = 0-0.05) were prepared by the solid state reaction of Bi2O3, La(OH)3 and U3O8 in a stoichiometric ratio. These solid solutions were characterized by X-ray diffraction, Extended X-ray Absorption Fine Structure (EXAFS) and X-ray Absorption Near Edge Spectroscopy (XANES) studies. It was found that La goes to the Bi sites of the Bi2UO6 lattice during the formation of these solid solutions. For further confirmation, a photoluminescence (PL) study on an iso-structural Bi1.96Eu0.04UO6 was performed, and this supports the above observation. The solid solutions were also probed by EPR studies. The PL and EPR studies suggest that there are doping induced oxygen vacancies in these solid solutions.

Restructuring of supported Pd by green solvents: an operando quick EXAFS (QEXAFS) study and implications for the derivation of structure–function relationships in Pd catalysis

Aqueous ethanolic solvents rearrange Pd catalysts facilely toward a common state irrespective of starting Pd dispersion or support material.

EXAFS Study of a Model Flotation System - Xanthate/Zinc Sulphate: Structural Determination of Interaction Forms Existing in Aqueous Solution and Deposition

This work presents first results of an EXAFS study of a model flotation system – the xanthate/zinc sulphate in aqueous solution, initial solutions of zinc sulphate and reference bulk samples. All EXAFS spectra (Zn-K edge) of the samples studied (both in solution and deposition) were recorded at EXAFS station of Siberian Synchrotron Terahertz Radiation Center (SSTRC, Novosibirsk) based on VEPP-3 storage ring. Some differences of local structure of initial solution of zinc sulfate (∼10-2 mol/l) compared to the bulk reference sample were found. The presence of different Zn-O forms were proposed. For the model flotation system - xanthate/zinc sulphate in aqueous solution, the Zn-S forms were predominantly found. The structural determination of interaction forms existing in aqueous solution and deposition were carried out in comparison with the initial bulk and reference samples. All possible structural models were discussed. A preliminary structural analysis has done. Perspectives of the used approach are shown.

A combined theoretical and experimental EXAFS study of the structure and dynamics of Au147 nanoparticles

We demonstrated the capability of combined EXAFS and DFT calculations for characterizing the structural and thermal properties of Au147 clusters.

Structural and optical properties of sol–gel derived Cr-doped ZnO diluted magnetic semiconductor nanocrystals: an EXAFS study to relate the local structure

Structural, local structural and optical properties of sol–gel derived Zn1−xCrxO (0 ≤ x ≤ 0.06) nanoparticles have been thoroughly studied by several complementary techniques.