Showing papers on "Extended X-ray absorption fine structure published in 2018"

Electrochemical trapping of metastable Mn3+ ions for activation of MnO2 oxygen evolution catalysts.

Abstract: Electrodeposited manganese oxide films are promising catalysts for promoting the oxygen evolution reaction (OER), especially in acidic solutions. The activity of these catalysts is known to be enhanced by the introduction of Mn3+ We present in situ electrochemical and X-ray absorption spectroscopic studies, which reveal that Mn3+ may be introduced into MnO2 by an electrochemically induced comproportionation reaction with Mn2+ and that Mn3+ persists in OER active films. Extended X-ray absorption fine structure (EXAFS) spectra of the Mn3+-activated films indicate a decrease in the Mn-O coordination number, and Raman microspectroscopy reveals the presence of distorted Mn-O environments. Computational studies show that Mn3+ is kinetically trapped in tetrahedral sites and in a fully oxidized structure, consistent with the reduction of coordination number observed in EXAFS. Although in a reduced state, computation shows that Mn3+ states are stabilized relative to those of oxygen and that the highest occupied molecular orbital (HOMO) is thus dominated by oxygen states. Furthermore, the Mn3+(Td) induces local strain on the oxide sublattice as observed in Raman spectra and results in a reduced gap between the HOMO and the lowest unoccupied molecular orbital (LUMO). The confluence of a reduced HOMO-LUMO gap and oxygen-based HOMO results in the facilitation of OER on the application of anodic potentials to the δ-MnO2 polymorph incorporating Mn3+ ions.

Enhanced Adsorption of p-Arsanilic Acid from Water by Amine-Modified UiO-67 as Examined Using Extended X-ray Absorption Fine Structure, X-ray Photoelectron Spectroscopy, and Density Functional Theory Calculations.

Abstract: p-Arsanilic acid (p-ASA) is an emerging organoarsenic pollutant comprising both inorganic and organic moieties For the efficient removal of p-ASA, adsorbents with high adsorption affinity are urgently needed Herein, amine-modified UiO-67 (UiO-67-NH2) metal–organic frameworks (MOFs) were synthesized, and their adsorption affinities toward p-ASA were 2 times higher than that of the pristine UiO-67 Extended X-ray absorption fine structure (EXAFS), X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) calculation results revealed adsorption through a combination of As–O–Zr coordination, hydrogen bonding, and π–π stacking, among which As–O–Zr coordination was the dominant force Amine groups played a significant role in enhancing the adsorption affinity through strengthening the As–O–Zr coordination and π–π stacking, as well as forming new adsorption sites via hydrogen bonding UiO-67-NH2s could remove p-ASA at low concentrations (<5 mg L–1) in simulated natural and wastewaters to an a

Metal-organic layers stabilize earth-abundant metal-terpyridine diradical complexes for catalytic C-H activation.

Abstract: We report the synthesis of a terpyridine-based metal-organic layer (TPY-MOL) and its metalation with CoCl2 and FeBr2 to afford CoCl2·TPY-MOL and FeBr2·TPY-MOL, respectively. Upon activation with NaEt3BH, CoCl2·TPY-MOL catalyzed benzylic C-H borylation of methylarenes whereas FeBr2·TPY-MOL catalyzed intramolecular Csp3 -H amination of alkyl azides to afford pyrrolidines and piperidines. X-ray absorption near edge structure (XANES), extended X-ray absorption fine structure (EXAFS), X-ray photoelectron spectroscopy, UV-Vis-NIR spectroscopy, and electron paramagnetic spectroscopy (EPR) measurements as well as density functional theory (DFT) calculations identified M(THF)2·TPY-MOL (M = Co or Fe) as the active catalyst with a MII-(TPY˙˙)2- electronic structure featuring divalent metals and TPY diradical dianions. We believe that site isolation stabilizes novel MII-(TPY˙˙)2- (M = Co or Fe) species in the MOLs to endow them with unique and enhanced catalytic activities for Csp3 -H borylation and intramolecular amination over their homogeneous counterparts. The MOL catalysts are also superior to their metal-organic framework analogs owing to the removal of diffusion barriers. Our work highlights the potential of MOLs as a novel 2D molecular material platform for designing single-site solid catalysts without diffusional constraints.

Selective CO Methanation on Highly Active Ru/TiO2 Catalysts: Identifying the Physical Origin of the Observed Activation/Deactivation and Loss in Selectivity

Abstract: Ru/TiO2 catalysts are highly active and selective in the selective methanation of CO in the presence of large amounts of CO2 but suffer from a considerable deactivation and loss of selectivity during time on stream. Aiming at a fundamental understanding of these processes, we have systematically investigated the physical reasons responsible for these effects, using catalysts with different surface areas and combining time-resolved kinetic and in situ/operando spectroscopy measurements as well as ex situ catalyst characterization. This allowed us to identify and disentangle contributions from different effects such as structural effects, adlayer effects, such as site blocking effects, and changes in the chemical (surface) composition of the catalysts. Operando X-ray absorption near edge spectroscopy (XANES)/extended X-ray absorption fine structure analysis (EXAFS) measurements revealed that an initial activation phase is largely due to the reduction of oxidized Ru species, together with a distinct change i...

High-Resolution Extended X-ray Absorption Fine Structure Analysis Provides Evidence for a Longer Fe···Fe Distance in the Q Intermediate of Methane Monooxygenase

Abstract: Despite decades of intense research, the core structure of the methane C–H bond breaking diiron(IV) intermediate, Q, of soluble methane monooxygenase remains controversial, with conflicting reports supporting either a “diamond” diiron core structure or an open core structure. Early extended X-ray absorption fine structure (EXAFS) data assigned a short 2.46 A Fe–Fe distance to Q (Shu et al. Science 1997, 275, 515) that is inconsistent with several theoretical studies and in conflict with our recent high-resolution Fe K-edge X-ray absorption spectroscopy (XAS) studies (Castillo et al. J. Am. Chem. Soc. 2017, 139, 18024). Herein, we revisit the EXAFS of Q using high-energy resolution fluorescence-detected extended X-ray absorption fine structure (HERFD-EXAFS) studies. The present data show no evidence for a short Fe–Fe distance, but rather a long 3.4 A diiron distance, as observed in open core synthetic model complexes. The previously reported 2.46 A feature plausibly arises from a background metallic iron c...

One-Pot Pyrolysis Method to Fabricate Carbon Nanotube Supported Ni Single-Atom Catalysts with Ultrahigh Loading

Abstract: The practical application of single atom catalysts (SACs) is constrained by the low achievable loading of single metal atoms Here, nickel SACs stabilized on a nitrogen-doped carbon nanotube structure (NiSA-N-CNT) with ultrahigh Ni atomic loading up to 203 wt % have been successfully synthesized using a new one-pot pyrolysis method employing Ni acetylacetonate (Ni(acac)2) and dicyandiamide (DCD) as precursors The yield and formation of NiSA-N-CNT depends strongly on the Ni(acac)2/DCD ratio and annealing temperature Pyrolysis at 350 and 650 °C led to the formation of Ni single atom dispersed melem and graphitic carbon nitride (Ni-melem and Ni-g-C3N4) Transition from a stacked and layered Ni-g-C3N4 structure to a bamboo-shaped tubular NiSA-N-CNT structure most likely occurs via a solid-to-solid curling or rolling-up mechanism, thermally activated at temperatures of 700–900 °C Extended X-ray absorption fine structure (EXAFS) experiments and simulations show that Ni single atoms are stabilized in the N-C

Optical absorption of carbon-gold core-shell nanoparticles

Abstract: In order to enhance the solar thermal energy conversion efficiency, we propose to use carbon-gold core-shell nanoparticles dispersed in liquid water. This work demonstrates theoretically that an absorbing carbon (C) core enclosed in a plasmonic gold (Au) nanoshell can enhance the absorption peak while broadening the absorption band; giving rise to a much higher solar absorption than most previously studied core-shell combinations. The exact Mie solution is used to evaluate the absorption efficiency factor of spherical nanoparticles in the wavelength region from 300 nm to 1100 nm as well as the electric field and power dissipation profiles inside the nanoparticles at specified wavelengths (mostly at the localized surface plasmon resonance wavelength). The field enhancement by the localized plasmons at the gold surfaces boosts the absorption of the carbon particle, resulting in a redshift of the absorption peak with increased peak height and bandwidth. In addition to spherical nanoparticles, we use the finite-difference time-domain method to calculate the absorption of cubic core-shell nanoparticles. Even stronger enhancement can be achieved with cubic C-Au core-shell structures due to the localized plasmonic resonances at the sharp edges of the Au shell. The solar absorption efficiency factor can exceed 1.5 in the spherical case and reach 2.3 in the cubic case with a shell thickness of 10 nm. Such broadband absorption enhancement is in great demand for solar thermal applications including steam generation.

Exact Stoichiometry of CexZr6–x Cornerstones in Mixed-Metal UiO-66 Metal–Organic Frameworks Revealed by Extended X-ray Absorption Fine Structure Spectroscopy

Abstract: Bimetallic Ce/Zr-UiO-66 metal–organic frameworks (MOFs) proved to be promising materials for various catalytic redox applications, representing, together with other bimetallic MOFs, a new generation of porous structures. However, no direct proof for the presence of both metals in a single cornerstone of UiO-type MOFs was reported so far. Employing element-selective X-ray absorption spectroscopy techniques herein, we demonstrate, for the first time, that our synthesis route allows obtaining Ce/Zr-UiO-66 MOFs with desired Ce content and bimetallic CeZr5 cornerstones. Performing multiple-edge extended X-ray absorption fine structure analysis, we determine the exact stoichiometry of the cornerstones, which explains the dependence of thermal and chemical stability of the materials on Ce content.

Fe-Containing magnesium aluminate support for stability and carbon control during methane reforming

Abstract: We report a MgFexAl2–xO4 synthetic spinel, where x varies from 0 to 0.26, as support for Ni-based catalysts, offering stability and carbon control under various conditions of methane reforming. By incorporation of Fe into a magnesium aluminate spinel, a support is created with redox functionality and high thermal stability, as concluded from temporal analysis of products (TAP) experiments and redox cycling, respectively. A diffusion coefficient of 3 × 10–17 m2 s–1 was estimated for lattice oxygen at 993 K from TAP experiments. X-ray diffraction (XRD) and extended X-ray absorption fine structure (EXAFS) modeling identified that the incorporation of iron occurs as Fe3+ in the octahedral sites of the spinel lattice, replacing aluminum. Simulation of the X-ray absorption near edge structure (XANES) spectrum of the reduced support showed that 60 ± 10% of iron was reduced from 3+ to 2+ at 1073 K, while there was no formation of metallic iron. A series of Ni/MgFexAl2–xO4 catalysts, where x varies from 0 to 0.26,...

Palladium Carbide and Hydride Formation in the Bulk and at the Surface of Palladium Nanoparticles

Abstract: The presence of a core/shell behavior in Pd nanoparticles (NPs) during the formation of the metal-hydride phase has recently been highlighted combining X-ray absorption and scattering experiments [J. Phys. Chem. C 2017, 121, 18202]. In this work, we focus on the formation of the carbide phase in the bulk region and on the surface of supported palladium NPs because it affects the catalytic activity and selectivity in hydrogenation reactions. We present in situ X-ray absorption spectroscopy study of carbide formation and decomposition in 2.6 nm palladium nanoparticles supported on carbon during exposure to acetylene, hydrogen, and their mixtures at 100 °C, taken as a representative temperature for hydrogenation reactions. Fourier analysis of extended X-ray absorption fine structure (EXAFS) spectra was used to determine the average Pd–Pd bond distance in the NPs, reflecting the formation of bulk palladium carbide, while theoretical calculation of X-ray absorption near-edge structure (XANES) using the finite ...

Structural properties and catalytic activity of a novel ternary CuO/gC3N4/Bi2O3 photocatalyst

Abstract: In the present study, CuO/gC3N4/Bi2O3 composite is constructed as a ternary visible light active photocatalyst. Since CuO plays a critical role in enhancing the photocatalytic activity of the formed composite, its structural properties are particularly studied using synchrotron X-ray absorption spectroscopy (XAS), including X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS). XANES confirms the presence of Cu species with +2 oxidation state in the form of CuO. EXAFS furthermore confirms that each Cu cation is coordinated to four O anions in an approximately square planar configuration. The length of the Cu-O coordination is estimated to be 1.92 A, slightly shorter than that of bulk CuO (1.95 A). The CuO/gC3N4/Bi2O3 composite exhibits highly enhanced photocatalytic activity in the 2,4-dichlorophenol decomposition under visible light. The enhanced photocatalytic activity is due to the increased population of electrons and the successful consumption of the photoproduced electrons by the dissolved oxygen through the one-electron transfer reaction.

Mechanistic insights into sequestration of U(VI) toward magnetic biochar: Batch, XPS and EXAFS techniques.

Abstract: The magnetic iron oxide (Fe3O4) nanoparticles stabilized on the biochar were synthesized by fast pyrolysis of Fe(II)-loaded hydrophyte biomass under N2 conditions. The batch experiments showed that magnetic biochar presented a large removal capacity (54.35mg/g) at pH3.0 and 293K. The reductive co-precipitation of U(VI) to U(IV) by magnetic biochar was demonstrated according to X-ray diffraction, X-ray photoelectron spectroscopy and X-ray absorption near edge structure analysis. According to extended X-ray absorption fine structure analysis, the occurrence of U-Fe and U-U shells indicated that high effective removal of uranium was primarily inner-sphere coordination and then reductive co-precipitation at low pH. These observations provided the further understanding of uranium removal by magnetic materials in environmental remediation.

Unraveling the Chemical State of Cobalt in Co-Based Catalysts during Ethanol Steam Reforming: an in Situ Study by Near Ambient Pressure XPS and XANES

Abstract: The steam reforming of ethanol (ESR) has been studied by near ambient pressure XPS (NAP-XPS), extended X-ray absorption fine structure (EXAFS), and X-ray absorption near edge structure (XANES) unde

Molecular identification of Cr(VI) removal mechanism on vivianite surface

Abstract: Experimental and theoretical studies were conducted to identify the molecular-scale reaction mechanism for Cr(VI) removal by a ferrous phosphate mineral, vivianite. The surface-normalized rate constant for Cr(VI) removal in a vivianite suspension at pH 7 was higher than those obtained for other Fe(II)-containing minerals (i.e., magnetite and pyrite). The highest rate constant was obtained at pH 5, which was 35- and 264-times higher than those obtained at pH 7 and 9, respectively, indicating the dramatic acceleration of removal kinetics with decreasing pH of suspension. The X-ray photoelectron spectroscopy (XPS) and X-ray absorption near-edge structure (XANES) spectroscopy revealed that Cr(VI) removal involved reduction of Cr(VI) to Cr(III) coupled with oxidation of Fe(II) to Fe(III) on the vivianite surface. In addition, the density functional theory (DFT)-optimized structure of the Cr(VI)-vivianite complex was consistent with that obtained from extended X-ray absorption fine structure (EXAFS) spectroscop...

Operando study of palladium nanoparticles inside UiO-67 MOF for catalytic hydrogenation of hydrocarbons

Abstract: Functionalization of metal-organic frameworks with metal nanoparticles (NPs) is a promising way for producing advanced materials for catalytic applications. We present the synthesis and in situ characterization of palladium NPs encapsulated inside a functionalized UiO-67 metal-organic framework. The initial structure was synthesized with 10% of PdCl2bpydc moieties with grafted Pd ions replacing standard 4,4'-biphenyldicarboxylate linkers. This material exhibits the same high crystallinity and thermal stability of standard UiO-67. Formation of palladium NPs was initiated by sample activation in hydrogen and monitored by in situ X-ray powder diffraction and X-ray absorption spectroscopy (XAS). The reduction of PdII ions to Pd0 occurs above 200 °C in 6% H2/He flow. The formed palladium NPs have an average size of 2.1 nm as limited by the cavities of UiO-67 structure. The resulting material showed high activity towards ethylene hydrogenation. Under reaction conditions, palladium was found to form a carbide structure indicated by operando XAS, while formation of ethane was monitored by mass spectroscopy and infra-red spectroscopy.

Aggregation-Induced Emission (AIE) in Ag-Au Bimetallic Nanocluster.

Abstract: Herein we report the synthesis and structure determination of a non-fluorescent Au4 Ag5 (dppm)2 (SAdm)6 (BPh4 ) (dppm=bis(diphenylphosphino)methane and HSAdm=1-adamantane mercaptan) nanocluster in methanol with extremely strong AIE when aggregating to the solid state (i.e., film or crystal). This phenomenon was rarely reported in structural determined noble metal nanoclusters. The extended X-ray absorption fine structure (EXAFS) measurement ruled out the hypothesis that the luminescence originated from the structure change in different states. Besides, the crystal structure (determined by X-ray diffraction) revealed that the tightly combined left- and right-handed enantiomers induced the strong restriction of intramolecular motions (RIM), which may have an impact on aggregation-induced emission.

Time-resolved operando studies of carbon supported Pd nanoparticles under hydrogenation reactions by X-ray diffraction and absorption

Abstract: The formation of palladium hydride and carbide phases in palladium-based catalysts is a critical process that changes the catalytic performance and selectivity of the catalysts in important industrial reactions, such as the selective hydrogenation of alkynes or alkadienes. We present a comprehensive study of a 5 wt% carbon supported Pd nanoparticle (NP) catalyst in various environments by using in situ and operando X-ray absorption spectroscopy and diffraction, to determine the structure and evolution of palladium hydride and carbide phases, and their distribution throughout the NPs. We demonstrate how the simultaneous analysis of extended X-ray absorption fine structure (EXAFS) spectra and X-ray powder diffraction (XRPD) patterns allows discrimination between the inner “core” and outer “shell” regions of the NP during hydride phase formation at different temperatures and under different hydrogen pressures, indicating that the amount of hydrogen in the shell region of the NP is lower than that in the core. For palladium carbide, advanced analysis of X-ray absorption near-edge structure (XANES) spectra allows the detection of Pd–C bonds with carbon-containing molecules adsorbed at the surface of the NPs. In addition, H/Pd and C/Pd stoichiometries of PdHx and PdCy phases were obtained by using theoretical modelling and fitting of XANES spectra. Finally, the collection of operando time-resolved XRPD patterns (with a time resolution of 5 s) allowed the detection, during the ethylene hydrogenation reaction, of periodic oscillations in the NPs core lattice parameter, which were in phase with the MS signal of ethane (product) and in antiphase with the MS signal of H2 (reactant), highlighting an interesting direct structure–reactivity relationship. The presented studies show how a careful combination of X-ray absorption and diffraction can differentiate the structure of the core, shell and surface of the palladium NPs under working conditions and prove their relevant roles in catalysis.

Iron Vacancies Accommodate Uranyl Incorporation into Hematite.

Abstract: Radiotoxic uranium contamination in natural systems and nuclear waste containment can be sequestered by incorporation into naturally abundant iron (oxyhydr)oxides such as hematite (α-Fe2O3) during mineral growth. The stability and properties of the resulting uranium-doped material are impacted by the local coordination environment of incorporated uranium. While measurements of uranium coordination in hematite have been attempted using extended X-ray absorption fine structure (EXAFS) analysis, traditional shell-by-shell EXAFS fitting yields ambiguous results. We used hybrid functional ab initio molecular dynamics (AIMD) simulations for various defect configurations to generate synthetic EXAFS spectra which were combined with adsorbed uranyl spectra to fit experimental U L3-edge EXAFS for U6+-doped hematite. We discovered that the hematite crystal structure accommodates a trans-dioxo uranyl-like configuration for U6+ that substitutes for structural Fe3+, which requires two partially protonated Fe vacancies ...

Evidence of a cubic iron sub-lattice in t-CuFe2O4 demonstrated by X-ray Absorption Fine Structure.

Abstract: Copper ferrite, belonging to the wide and technologically relevant class of spinel ferrites, was grown in the form of t-CuFe2O4 nanocrystals within a porous matrix of silica in the form of either an aerogel or a xerogel, and compared to a bulk sample. Extended X-ray absorption fine structure (EXAFS) spectroscopy revealed the presence of two different sub-lattices within the crystal structure of t-CuFe2O4, one tetragonal and one cubic, defined by the Cu2+ and Fe3+ ions respectively. Our investigation provides evidence that the Jahn-Teller distortion, which occurs on the Cu2+ ions located in octahedral sites, does not affect the coordination geometry of the Fe3+ ions, regardless of their location in octahedral or tetrahedral sites.

Lanthanide-Based Porous Coordination Polymers: Syntheses, Slow Relaxation of Magnetization, and Magnetocaloric Effect

Abstract: Two lanthanide-containing structurally analogous porous coordination polymers (PCPs) have been isolated with the general molecular formula [Ln2(L1)2(H2O)4(ox)]n.4nH2O (where L1 = fumarate, ox = oxalate; Ln = Dy (1), Gd (2)). Thermogravimetric analysis (TGA) and TG-MS measurements performed on 1 and 2 suggest that not only the solvated water molecules in the crystal lattice but also the four coordinated water molecules on the respective lanthanides in 1 and 2 are removed upon activation. Due to the removal of the waters, 1 and 2 lost their crystallinity and became amorphous, as confirmed by powder X-ray diffraction (PXRD). We propose the molecular formula [Ln2(L1)2(ox)]n for the amorphous phase of 1 and 2 (where Ln = Dy (1′), Gd (2′)) on the basis of XANES, EXAFS, and other experimental investigations. Magnetization relaxation dynamics probed on 1 and 1′ reveal two different relaxation processes with effective energy barriers of 53.5 and 7.0 cm–1 for 1 and 45.1 and 6.4 cm–1 for 1′, which have been rational...

Fe5C2 nanoparticles as low-cost HER electrocatalyst: the importance of Co substitution

Abstract: Constructing and understanding the doping effect of secondary metal in transition metal carbide (TMC) catalysts is pivotal for the design of low-cost hydrogen evolution reaction (HER) electrocatalysts. In this work, we developed a wet-chemistry strategy for synthesizing Co-modified Fe5C2 nanoparticles ((Fe1−xCox)5C2 NPs) as highly active HER electrocatalysts in basic solution. The structure of (Fe1−xCox)5C2 NPs was characterized by X-ray diffraction (XRD), extended X-ray absorption fine structure spectra (EXAFS) and scanning/transmission electron microscopy (S/TEM), indicating that the isomorphous substitution of cobalt in the lattice of Fe5C2. (Fe0.75Co0.25)5C2 exhibited the best HER activity (174 mV for j = −10 mA/cm2). Computational calculation results indicate that Co provides the most active site for HER. X-ray adsorption spectra (XAS) studies further suggested that the electron transfer in Fe-C bonds are enhanced by the substitution of Co, which modulates the hydrogen adsorption on the adjacent electronic-enriched carbon, and therefore promotes HER activity. Our results affirm the design of low-cost bimetallic TMCs based HER catalysts.

Elucidating the Nature of the Excited State of a Heteroleptic Copper Photosensitizer by using Time‐Resolved X‐ray Absorption Spectroscopy

Abstract: We report the light-induced electronic and geometric changes taking place within a heteroleptic CuI photosensitizer, namely [(xant)Cu(Me2 phenPh2 )]PF6 (xant=xantphos, Me2 phenPh2 =bathocuproine), by time-resolved X-ray absorption spectroscopy in the ps-μs time regime. Time-resolved X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) analysis enabled the elucidation of the electronic and structural configuration of the copper center in the excited state as well as its decay dynamics in different solvent conditions with and without triethylamine acting as a sacrificial electron donor. A three-fold decrease in the decay lifetime of the excited state is observed in the presence of triethylamine, showing the feasibility of the reductive quenching pathway in the latter case. A prominent pre-edge feature is observed in the XANES spectrum of the excited state upon metal to charge ligand transfer transition, showing an increased hybridization of the 3d states with the ligand p orbitals in the tetrahedron around the Cu center. EXAFS and density functional theory illustrate a significant shortening of the Cu-N and an elongation of the Cu-P bonds together with a decrease in the torsional angle between the xantphos and bathocuproine ligand. This study provides mechanistic time-resolved understanding for the development of improved heteroleptic CuI photosensitizers, which can be used for the light-driven production of hydrogen from water.

Role of local structural distortion in driving ferroelectricity in GdCrO 3

Abstract: Temperature-dependent synchrotron x-ray diffraction and extended x-ray absorption fine structure (EXAFS) studies were performed to understand the role of structural characteristics in driving the magnetoelectric multiferroic properties of ${\mathrm{GdCrO}}_{3}$. The results suggest that the distortion in the structure appears to be associated with the off-center displacement of Gd atoms together with octahedral rotations via displacement of the oxygen ions in ${\mathrm{GdCrO}}_{3}$. Further, a comparative EXAFS study of ${\mathrm{GdCrO}}_{3}$ with a similar system, ${\mathrm{YCrO}}_{3}$, suggests that the oxygen environment of Gd in ${\mathrm{GdCrO}}_{3}$ is different from that of Y in ${\mathrm{YCrO}}_{3}$, which results in an orthorhombic $Pna{2}_{1}$ structure in ${\mathrm{GdCrO}}_{3}$ in contrast to the monoclinic $P{2}_{1}$ structure in ${\mathrm{YCrO}}_{3}$.

On the coordination of Zn2+ ion in Tf2N- based ionic liquids: structural and dynamic properties depending on the nature of the organic cation.

Abstract: A synergic approach combining molecular dynamics (MD) simulations and X-ray absorption spectroscopy has been used to investigate diluted solutions of zinc bis(trifluoromethanesulfonyl)imide (Zn(Tf2N)2) in Tf2N- based ionic liquids (ILs) having different organic cations, namely the 1-butyl-3-methylimidazolium ([C4(mim)]+), 1,8-bis(3-methylimidazolium-1-yl)octane ([C8(mim)2]2+), N,N,N-trimethyl-N-(2-hydroxyethyl)ammonium ([Choline]+) and butyltrimethylammonium ([BTMA]+) ions. All of the ILs tend to dissolve the Zn(Tf2N)2 species giving rise to a different structural arrangement around the Zn2+ as compared to that of the salt crystallographic structure. A quantitative analysis of the Zn K-edge extended X-ray absorption fine structure (EXAFS) spectra of the solutions has been carried out based on the microscopic description of the systems derived from the MD simulations. A very good agreement between theoretical and experimental EXAFS signals has been obtained, allowing us to assess the reliability of the MD structural results for all the investigated solutions. The Zn2+ ion has been shown to be coordinated by six oxygen atoms of the Tf2N- anions arranged in an octahedral geometry in all the Tf2N- based ILs, regardless of the organic cation of the IL solvent. However, the nature of the organic cation has a small influence on the overall spatial arrangement of the Tf2N- anions in the Zn2+ first solvation shell: two different Zn-Tf2N complexes are found in solution, a 5-fold one, with one bidentate and four monodentate Tf2N- anions, and a 6-fold one with only monodentate ligands, with the ratio between the two species being slightly dependent on the IL cation. The IL ion three-dimensional arrangements in the different IL solutions were also investigated by carrying out a thorough analysis of the MD simulations, highlighting similarities and differences between imidazolium and ammonium based IL systems.