Showing papers on "Extended X-ray absorption fine structure published in 2020"

Enhanced CO2 Electroreduction on Neighboring Zn/Co Monomers by Electronic Effect

Abstract: It is of great significance to reveal the detailed mechanism of neighboring effects between monomers, as they could not only affect the intermediate bonding but also change the reaction pathway. This paper describes the electronic effect between neighboring Zn/Co monomers effectively promoting CO2 electroreduction to CO. Zn and Co atoms coordinated on N doped carbon (ZnCoNC) show a CO faradaic efficiency of 93.2 % at -0.5 V versus RHE during a 30-hours test. Extended X-ray absorption fine structure measurements (EXAFS) indicated no direct metal-metal bonding and X-ray absorption near-edge structure (XANES) showed the electronic effect between Zn/Co monomers. In situ attenuated total reflection-infrared spectroscopy (ATR-IR) and density functional theory (DFT) calculations further revealed that the electronic effect between Zn/Co enhanced the *COOH intermediate bonding on Zn sites and thus promoted CO production. This work could act as a promising way to reveal the mechanism of neighboring monomers and to influence catalysis.

Properties of Iron-Modified-by-Silver Supported on Mordenite as Catalysts for NOx Reduction

Abstract: A series of mono and bimetallic catalysts based on a Fe-Ag mixture deposited on mordenite was prepared by ion-exchange and evaluated in the catalytic activity test of the de-NOx reaction in the presence of CO/C3H6. The activity results showed that the most active samples were the Fe-containing ones, and at high temperatures, a co-promoter effect of Ag on the activity of Fe catalysts was also observed. The influence of the order of cation deposition on catalysts formation and their physicochemical properties was studied by FTIR (Fourier Transform Infrared Spectroscopy) of adsorbed NO, XANES (X-ray Absorption Near-Edge Structure), and EXAFS (Extended X-ray Absorption Fine Structure) and discussed in terms of the state of iron. Results of Fe K-edge XANES oscillations showed that, in FeMOR catalysts, iron was present in a disordered state as Fe3+ and Fe2+. In FeAgMOR, the prevailing species was Fe3+, while in the AgFeMOR catalyst, the state of iron was intermediate or mixed between FeMOR and FeAgMOR. The Fe K-edge EXAFS results were characteristic of a disordered phase, the first coordination sphere being asymmetric with two different Fe-O distances. In FeAgMOR and AgFeMOR, coordination of Fe-O was similar to Fe2O3 with a few amount of Fe2+ species. We may conclude that, in the bimetallic FeAgMOR and AgFeMOR samples, a certain amount of tetrahedral Al3+ ions in the mordenite framework is replaced by Fe3+ ions, confirming the previous reports that these species are active sites for the de-NOx reaction. Based on the thermodynamic analysis and experimental data, also, it was confirmed that the order of deposition of the components influenced the mechanism of active sites’ formation during the two steps ion-exchange synthesis.

Quantitative temporally and spatially resolved X-ray fluorescence microprobe characterization of the manganese dissolution-deposition mechanism in aqueous Zn/α-MnO2 batteries

Abstract: Rechargeable aqueous Zn/α-MnO2 batteries are a possible alternative to lithium ion batteries for scalable stationary energy storage applications due to their low cost, safety and environmentally benign components. A critical need for advancement of this battery system is a full understanding of the electrochemical reaction mechanisms, which remain unclear. In this report, operando, spatiotemporal resolved synchrotron X-ray fluorescence mapping measurements on a custom aqueous Zn/α-MnO2 cell provided direct evidence of a Mn dissolution-deposition faradaic mechanism that governs the electrochemistry. Simultaneous visualization and quantification of the Mn distribution in the electrolyte revealed the formation of aqueous Mn species during discharge and depletion on charge. The findings are supported by ex situ transmission electron microscopy (TEM), X-ray diffraction, Mn K-edge X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) measurements. The elucidated mechanism is fundamentally different from the previously proposed Zn2+ insertion or conversion reactions. These findings provide a foundation for developing dissolution- deposition chemistries suitable for scalable stationary energy storage with aqueous electrolyte.

Controllable P Doping of the LaCoO3 Catalyst for Efficient Propane Oxidation: Optimized Surface Co Distribution and Enhanced Oxygen Vacancies.

Abstract: The properties of LaCoO3 are modified by a controllable P doping strategy via a simple sol-gel route. It is demonstrated that appropriate P doping is beneficial for forming a relatively pure perovskite phase and hinders the growth of perovskite nanoparticles. The combined results of density functional theory (DFT), extended X-ray absorption fine structure (EXAFS), X-ray absorption near-edge structure (XANES), temperature-programmed reduction of hydrogen (H2-TPR), X-ray photoelectron spectroscopy (XPS), and temperature-programmed desorption of ammonia (NH3-TPD) reveal that appropriate P doping gives rise to more oxygen vacancies, optimized distribution of Co ions, and improved surface acidity, which are beneficial for the adsorption of active oxygen species and the activation of propane molecules, resulting in an excellent catalytic oxidation performance. Especially, LaCo0.97P0.03O3 exhibits more surface-active oxygen species, higher bulk Co3+ proportion, increased surface Co2+ species, and increased acidity, resulting in its superior propane oxidation performance, which is dominated by the Langmuir-Hinshelwood mechanism. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) confirms that the presence of P will accelerate oxygen mobility, which in turn promotes the oxidation rate. Moreover, the obtained LaCo0.97P0.03O3 catalyst displays excellent thermal stability during the 60 h durability test at 400 °C and strong resistance against 5 vol % H2O and/or 5 vol % CO2 for prolonged 150 h.

Observation of Non-FCC Copper in Alkynyl-Protected Cu53 Nanoclusters.

Abstract: The only feasible access to non-face-centered cubic (FCC) copper was by physical vapor deposition under high vacuum. Now, non-FCC copper is observed in a series of alkynyl-protected Cu53 nanoclusters (NCs) obtained from solution-phase synthesis. Determined by single-crystal X-ray crystallography, the structures of Cu53 (C≡CPhPh)9 (dppp)6 Cl3 (NO3 )9 and its two derivatives reveal an ABABC stacking sequence involving 41 Cu atoms. It can be regarded as a mixed FCC and HCP structure, which gives strong evidence that Cu can be arranged in non-FCC lattice at ambient conditions when proper ligands are provided. Characterization methods including X-ray absorption fine structure, XPS, ESI-MS, UV/Vis, Auger spectroscopy, and DFT calculations were carried out. CuII was shown to successively coordinate with introduced ligands and changed to CuI after bonding with phosphine. The following addition of NaBH4 and the aging step further reduced it to the Cu53 NC.

Cation Distribution in Monodispersed MFe2O4 (M = Mn, Fe, Co, Ni, and Zn) Nanoparticles Investigated by X-ray Absorption Fine Structure Spectroscopy: Implications for Magnetic Data Storage, Catalysts, Sensors, and Ferrofluids

Abstract: Spinel ferrite MFe2O4 (M = Mn, Fe, Co, Ni, and Zn) nanoparticles, coordinated with oleylamine, oleic acid, and trioctylphosphine oxide, were effectively synthesized by the microwave-assisted rapid ...

Neural network assisted analysis of bimetallic nanocatalysts using X-ray absorption near edge structure spectroscopy

Abstract: X-ray absorption spectroscopy is a common method for probing the local structure of nanocatalysts. One portion of the X-ray absorption spectrum, the X-ray absorption near edge structure (XANES) is a useful alternative to the commonly used extended X-ray absorption fine structure (EXAFS) for probing three-dimensional geometry around each type of atomic species, especially in those cases when the EXAFS data quality is limited by harsh reaction conditions and low metal loading. A methodology for quantitative determination of bimetallic architectures from their XANES spectra is currently lacking. We have developed a method, based on the artificial neural network, trained on ab initio site-specific XANES calculations, that enables accurate and rapid reconstruction of the structural descriptors (partial coordination numbers) from the experimental XANES data. We demonstrate the utility of this method on the example of a series of PdAu bimetallic nanoalloys. By validating the neural network-yielded metal-metal coordination numbers based on the XANES analysis by previous EXAFS characterization, we obtained new results for in situ restructuring of dilute (2.6 at% Pd in Au) PdAu nanoparticles, driven by their gas and temperature treatments.

Experimental and theoretical evidence for hydrogen doping in polymer solution-processed indium gallium oxide

Abstract: The field-effect electron mobility of aqueous solution-processed indium gallium oxide (IGO) thin-film transistors (TFTs) is significantly enhanced by polyvinyl alcohol (PVA) addition to the precursor solution, a >70-fold increase to 7.9 cm2/Vs. To understand the origin of this remarkable phenomenon, microstructure, electronic structure, and charge transport of IGO:PVA film are investigated by a battery of experimental and theoretical techniques, including In K-edge and Ga K-edge extended X-ray absorption fine structure (EXAFS); resonant soft X-ray scattering (R-SoXS); ultraviolet photoelectron spectroscopy (UPS); Fourier transform-infrared (FT-IR) spectroscopy; time-of-flight secondary-ion mass spectrometry (ToF-SIMS); composition-/processing-dependent TFT properties; high-resolution solid-state 1H, 71Ga, and 115In NMR spectroscopy; and discrete Fourier transform (DFT) analysis with ab initio molecular dynamics (MD) liquid-quench simulations. The 71Ga{1H} rotational-echo double-resonance (REDOR) NMR and other data indicate that PVA achieves optimal H doping with a Ga···H distance of ∼3.4 A and conversion from six- to four-coordinate Ga, which together suppress deep trap defect localization. This reduces metal-oxide polyhedral distortion, thereby increasing the electron mobility. Hydroxyl polymer doping thus offers a pathway for efficient H doping in green solvent-processed metal oxide films and the promise of high-performance, ultra-stable metal oxide semiconductor electronics with simple binary compositions.

Structural and CO2 Capture Properties of Ethylenediamine-Modified HKUST-1 Metal-Organic Framework

Abstract: The high structural and compositional flexibility of metal–organic frameworks (MOFs) shows their great potential for CO2 capture and utilization in accordance with the environmental guidelines of low-carbon technology developments. HKUST-1 as one of the most intensively studied representatives of MOFs for such purposes excels because of its simplicity of production and high ability to tune its intrinsic properties by various functionalization processes. In the present work, ethylenediamine functionalization was performed for the first time in order to thoroughly investigate the amine sorption sites’ impact on the CO2 capture performance of HKUST-1. The placement of ethylenediamine moieties on Cu2+ free-metal sites has been examined in detail and confirmed by using various spectroscopic techniques such as Fourier transform infrared spectroscopy, electron paramagnetic resonance, Raman, and Cu K-edge extended X-ray absorption fine structure/X-ray absorption near edge structure. N2 and CO2 sorption tests have proven that the functionalization reduces both the specific surface area and the CO2 sorption capacity, but on the other hand, it increases the binding energy by 85% (from −20.3 kJ/mol to −36.8 kJ/mol) and CO2/N2 selectivity at 0.15/0.85 bar by 100% and notably improves the kinetics of adsorption in comparison to the pristine HKUST-1 material.

The missing pieces of the PuO2 nanoparticle puzzle

Abstract: The nanoscience field often produces results more mystifying than any other discipline. It has been argued that changes in the plutonium dioxide (PuO2) particle size from bulk to nano can have a drastic effect on PuO2 properties. Here we report a full characterization of PuO2 nanoparticles (NPs) at the atomic level and probe their local and electronic structures by a variety of methods available at the synchrotron, including extended X-ray absorption fine structure (EXAFS) at the Pu L3 edge, X-ray absorption near edge structure (XANES) in high energy resolution fluorescence detection (HERFD) mode at the Pu L3 and M4 edges, high energy X-ray scattering (HEXS) and X-ray diffraction (XRD). The particles were synthesized from precursors with different oxidation states of plutonium (III, IV, and V) under various environmentally and waste storage relevant conditions (pH 8 and pH > 10). Our experimental results analyzed with state-of-the-art theoretical approaches demonstrate that well dispersed, crystalline NPs with a size of ∼2.5 nm in diameter are always formed in spite of diverse chemical conditions. Identical crystal structures and the presence of only the Pu(IV) oxidation state in all NPs, reported here for the first time, indicate that the structure of PuO2 NPs is very similar to that of the bulk PuO2. All methods give complementary information and show that investigated fundamental properties of PuO2 NPs, rather than being exotic, are very similar to those of the bulk PuO2.

Adsorption of Rare Earth Elements onto DNA-Functionalized Mesoporous Carbon.

Abstract: The recovery and separation of rare earth elements (REEs) are of national importance owing to the specific usages, high demand, and low supply of these elements. In this research, we have investigated the adsorption of rare earth elements onto DNA-functionalized mesoporous carbons with a BET surface area of 605 m2/g and a median mesopore width of 48 A. Three types of single-stranded DNA, one with 100 base units of thymine, another with 20 units of thymine, and the third, a 2000 unit long DNA from salmon milt were grafted on the carboxylated mesoporous carbon surface. All of the DNA-functionalized mesoporous carbons demonstrated higher adsorption of REEs compared to pristine mesoporous carbon and DNA grafted with 100 units of thymine demonstrated slightly higher adsorbed amounts compared to others. Pure neodymium (Nd(III)) adsorption in the aqueous phase demonstrated an adsorbed amount of 110.4 mg/g with respect to the initial concentration of 500 mg/g. A pH variation study with pure Nd(III) demonstrated that the adsorbed amount is higher at elevated pH compared to that at lower pH, thereby suggesting possible recovery at lower pH. Adsorption of a mixture of 16 REEs, including Sc, Lu, Tm, Yb, Er, Ho, Tb, Dy, Y, Eu, Gd, Sm, Ce, Nd, Pr, and La revealed that the adsorbed amount increased with an increase in the atomic weight and metallic radii of elements within the lanthanides. The calculation of the distribution coefficients for all of the equilibrium adsorption amounts suggested that adsorption is more effective in the lower concentration region. The Nd L3-edge X-ray absorption near edge structure (XANES) confirmed a 3+ oxidation state of Nd in the adsorbed phase. The extended X-ray absorption fine structure (EXAFS) confirmed the local atomic structure relaxation of Nd complexes in the adsorbed phase and shortening of the Nd-O bond distance by about 0.03-0.04 A, which may be associated with their local complexation at the carbon surface.

Solution XANES and EXAFS analysis of active species of titanium, vanadium complex catalysts in ethylene polymerisation/dimerisation and syndiospecific styrene polymerisation

Abstract: Mechanistic studies in homogeneous catalysis through the solution transition metal K Edge XANES (X-ray absorption near-edge structure) and EXAFS (Extended X-ray absorption fine structure) analysis for vanadium and titanium complex catalysts for ethylene polymerisation/dimerization, and syndiospecific styrene polymerisation, including interpretation of the XANES spectra, have been introduced. The core excitation spectra of the complexes based on the time-dependent density functional theory (TD-DFT) can be used to interpret the Ti and V K-edge features and to extract information on the electronic structure from the XANES spectra. Theoretical calculations and experimental XAS analysis should have great potential for analysing the active species.

Elusive Coordination of the Ag+ Ion in Aqueous Solution: Evidence for a Linear Structure.

Abstract: X-ray absorption spectroscopy (XAS) has been employed to study the coordination of the Ag+ ion in aqueous solution. The conjunction of extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES) data analysis provided results suggesting the preference for a first shell linear coordination with a mean Ag-O bond distance of 2.34(2) A, different from the first generally accepted tetrahedral model with a longer mean Ag-O bond distance. Ab initio molecular dynamics simulations with the Car-Parrinello approach (CPMD) were also performed and were able to describe the coordination of the hydrated Ag+ ion in aqueous solution in very good agreement with the experimental data. The high sensitivity for the closest environment of the photoabsorber of the EXAFS and XANES techniques, together with the long-range information provided by CPMD and large-angle X-ray scattering (LAXS), allowed us to reconstruct the three-dimensional model of the coordination geometry around the Ag+ ion in aqueous solution. The obtained results from experiments and theoretical simulations provided a complex picture with a certain amount of water molecules with high configurational disorder at distances comprised between the first and second hydration spheres. This evidence may have caused the proliferation of the coordination numbers that have been proposed so far for Ag+ in water. Altogether these data show how the description of the hydration of the Ag+ ion in aqueous solution can be complex, differently from other metal species where hydration structures can be described by clusters with well-defined geometries. This diffuse hydration shell causes the Ag-O bond distance in the linear [Ag(H2O)2]+ ion to be ca. 0.2 A longer than in isolated ions in solid state.

Transition metal atom–doped monolayer MoS2 in a proton-exchange membrane electrolyzer

Abstract: There has been a substantial research effort worldwide to develop non-noble metal catalysts for H2 production from water splitting using renewable energy sources, but most data were evaluated by voltammetry in laboratories. Here, exposed basal planes of MoS2 monolayer nanosheets with metal dopants across the first transition metal (TM) series in the periodic table (Fe, Co, Ni, Cu) are used as cathode catalysts for the proton-exchange membrane (PEM) water splitting in an electrolyzer under typical conditions of strong acidity with more negative applied voltage. Extended X-ray absorption fine structure spectroscopy (EXAFS) analysis and high-angle annular dark-field scanning transmission electron microscopy (HAADF–STEM) images show a direct proof on the single TM atoms residing at the surface basal sites, which subtly modify the electrocatalytic activity of the monolayer MoS2, depending on their electronic and metal-hydrogen binding ability. We report that Co-sMoS2 yields the highest current density in an electrolyzer with the hydrogen evolution reaction (HER) activity comparable with that of the commercial 20 wt% Pt/C under industrial applicable conditions. A general trend for the other TMs has also been established as evidenced by the change in TM effective nuclear charge across the periodic table, which perturbed the TM-Mo interaction and hence affects the HER activity.

Correlating the size and cation inversion factor in context of magnetic and optical behavior of CoFe2O4 nanoparticles

Abstract: Herein, the size dependent behavior of cobalt ferrite nanoparticles was investigated using synchrotron radiation based techniques. Scanning electron micrographs revealed the enhancement of particle/crystallite size with increase of annealing temperature. Moreover, the shape of these particles also changed with increase of crystallite size. Saturation magnetization increased with increase of crystallite size. The higher saturation magnetization for larger crystallite size nanoparticles was attributed to a cation distribution similar to that of bulk CoFe2O4. The optical band-gap of these nanoparticles decreased from 1.9 eV to 1.7 eV with increase of crystallite size. The enhancement of the optical band-gap for smaller crystallites was due to phenomena of optical confinement occurring in the nanoparticles. Fe L Co L-edge near edge extended X-ray absorption fine structure (NEXAFS) measurements showed that Fe and Co ions remain in the 3+ and 2+ state in these nanoparticles. The results obtained from Fe & Co K-edge X-ray absorption near edge structure (XANES)-imaging experiments further revealed that this oxidation state was possessed by even the crystallites. Extended X-ray absorption fine structure (EXAFS) measurements revealed distribution of Fe and Co ions among tetrahedral (A) and octahedral (B) sites of the spinel structure which corroborates the results obtained from Rietveld refinement of X-ray diffraction patterns (XRD). X-ray magnetic circular di-chroism (XMCD) measurements revealed negative exchange interaction among the ions situated in tetrahedral (A) and octahedral (B) sites. Theoretical and experimental calculated magnetic moments revealed the dominancy of size effects rather than the cation redistribution in the spinel lattice of CoFe2O4 nanoparticles.

Calcium-Uranyl-Carbonato Species Kinetically Limit U(VI) Reduction by Fe(II) and Lead to U(V)-Bearing Ferrihydrite.

Abstract: Reaction conditions and mechanisms promoting or inhibiting U reduction exert a central control on U solubility and, therefore, U transport and its associated risks. Here, we vary and track common aqueous uranium species to show that a kinetic restriction inhibits homogeneous reduction of the calcium-uranyl-carbonato species (CaUO2(CO3)32- and Ca2UO2(CO3)3) by Fe(II)(aq), while ferrihydrite surface-catalyzed reduction of all aqueous uranyl by Fe(II) proceeds. Using U L3 high energy resolution fluorescence detection (HERFD) X-ray absorption near edge structure (XANES) spectroscopy, U L3 extended X-ray absorption fine structure (EXAFS) spectroscopy, and transmission electron microscopy (TEM), we also show that U(V) is generated and incorporated into ferrihydrite formed from homogeneous U(VI) reduction by Fe(II)(aq). Through elucidation of the mechanisms that inhibit reduction of the calcium-uranyl-carbonato species and promote stabilization of U(V), we advance our understanding of the controls on U solubility and thus improve prediction of U transport in surface and subsurface systems.

Structural dynamics of an iron molybdate catalyst under redox cycling conditions studied with in situ multi edge XAS and XRD

Abstract: The structural dynamics and phase transformations of an iron molybdate catalyst with excess molybdenum trioxide (Mo/Fe = 2.0) were studied during redox cycling of the catalyst using in situ multi-edge X-ray absorption spectroscopy (XAS) at the Mo K-edge (transmission mode) and Fe K-edge (fluorescence mode) in combination with X-ray diffraction (XRD). X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) analysis showed that heating under reducing conditions with methanol up to 400 °C produced MoO2 and FeMoO4. Linear combination fitting (LCF) analysis showed that iron was reduced completely, while molybdenum remained partly oxidized (60% as Mo(VI)). Complementary in situ XRD also supported the phase transformation due to reduction of Fe2(MoO4)3 and MoO3 to FeMoO4 and MoO2. Subsequent heating under oxidative conditions from 200 to 500 °C transformed the catalyst into its initial state via Fe2O3 and extra MoO3 as intermediate phases. This underlines the segregation and iron enrichment during redox cycling. MoO3 volatilization, observed under industrial reaction conditions of a methanol and oxygen containing atmosphere, causes this segregation to be permanent. Complete regeneration could only be achieved at 500 °C, which is significantly higher than industrial reaction temperatures. Overall, multi edge in situ XAS along with complementary XRD was found to be an ideal tool for tracing the different amorphous and crystalline phases present during redox cycling of the catalyst.