Showing papers on "XANES published in 2019"

In Situ X-ray Absorption Spectroscopy of a Synergistic Co-Mn Oxide Catalyst for the Oxygen Reduction Reaction.

Abstract: Identifying the catalytically active site(s) in the oxygen reduction reaction (ORR), under real-time electrochemical conditions, is critical to the development of fuel cells and other technologies. We have employed in situ synchrotron-based X-ray absorption spectroscopy (XAS) to investigate the synergistic interaction of a Co–Mn oxide catalyst which exhibits impressive ORR activity in alkaline fuel cells. X-ray absorption near edge structure (XANES) was used to track the dynamic structural changes of Co and Mn under both steady state (constant applied potential) and nonsteady state (potentiodynamic cyclic voltammetry, CV). Under steady state conditions, both Mn and Co valences decreased at lower potentials, indicating the conversion from Mn(III,IV) and Co(III) to Mn(II,III) and Co(II), respectively. Rapid X-ray data acquisition, combined with a slow sweep rate in CV, enabled a 3 mV resolution in the applied potential, approaching a nonsteady (potentiodynamic) state. Changes in the Co and Mn valence states...

An improved laboratory-based x-ray absorption fine structure and x-ray emission spectrometer for analytical applications in materials chemistry research

Abstract: X-ray absorption fine structure (XAFS) and x-ray emission spectroscopy (XES) are advanced x-ray spectroscopies that impact a wide range of disciplines. However, unlike the majority of other spectroscopic methods, XAFS and XES are accompanied by an unusual access model, wherein the dominant use of the technique is for premier research studies at world-class facilities, i.e., synchrotron x-ray light sources. In this paper, we report the design and performance of an improved XAFS and XES spectrometer based on the general conceptual design of Seidler et al. [Rev. Sci. Instrum. 85, 113906 (2014)]. New developments include reduced mechanical degrees of freedom, much-increased flux, and a wider Bragg angle range to enable extended x-ray absorption fine structure (EXAFS) measurement and analysis for the first time with this type of modern laboratory XAFS configuration. This instrument enables a new class of routine applications that are incompatible with the mission and access model of the synchrotron light sources. To illustrate this, we provide numerous examples of x-ray absorption near edge structure (XANES), EXAFS, and XES results for a variety of problems and energy ranges. Highlights include XAFS and XES measurements of battery electrode materials, EXAFS of Ni with full modeling of results to validate monochromator performance, valence-to-core XES for 3d transition metal compounds, and uranium XANES and XES for different oxidation states. Taken en masse, these results further support the growing perspective that modern laboratory-based XAFS and XES have the potential to develop a new branch of analytical chemistry.

Hydrodeoxygenation of phenol over Ni/Ce1-xNbxO2 catalysts

Abstract: This work investigated the effect of different niobium concentration on the performance of Ni/Ce1-xNbxO2 catalyst for hydrodeoxygenation of phenol in gas phase at 300 °C. Also, the structural modification promoted by Nb addition was studied by several techniques. In situ X-ray diffraction and Raman analyses indicated that high niobium concentration lead to Nb2O5 oxide with hexagonal structure, while in situ X-ray photoelectron spectroscopy (XPS) experiments indicated an enrichment of the surface with Nb. In general, the incorporation of niobium changed the lattice parameter of ceria and promoted the formation of oxygen vacancies as observed by XPS and X-ray absorption near edge structure (XANES) analyses, which favored the activity of the sample. Increasing Nb content increased the selectivity to deoxygenated products (benzene). The superior benzene selectivity is likely due to the higher interaction between the oxygen of phenol molecule and the oxophilic sites represented by the Nb5+ cations of the support that promotes the hydrogenation of carbonyl group.

Negative Charging of Au Nanoparticles during Methanol Synthesis from CO2 /H2 on a Au/ZnO Catalyst: Insights from Operando IR and Near-Ambient-Pressure XPS and XAS Measurements.

Abstract: The electronic and structural properties of Au/ZnO under industrial and idealized methanol synthesis conditions have been investigated. This was achieved by kinetic measurements in combination with time-resolved operando infrared (DRIFTS) as well as in situ near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) and X-ray absorption near-edge spectroscopy (XANES) measurements at the O K-edge together with high-resolution electron microscopy. The adsorption of CO during the reaction revealed the presence of negatively charged Au nanoparticles/Au sites during the initial phase of the reaction. Near-ambient-pressure XPS and XANES demonstrate the build-up of O vacancies during the reaction, which goes along with a substantial increase in the rate of methanol formation. The results are discussed in comparison with previous findings for Cu/ZnO and Au/ZnO catalysts.

Co-regulating the surface and bulk structure of Li-rich layered oxides by a phosphor doping strategy for high-energy Li-ion batteries

Abstract: Li-rich layered materials, despite their high specific capacity up to 250 mA h g−1, suffer from structural transformation either in the initial activation or after cycling, causing continuous voltage decay and capacity fading. Anion doping has been widely considered as a way to stabilize the intrinsic structure and improve the electrochemical performance of Li-rich materials, though with the pain of process complexity and limitation. Here, we report a simple co-precipitation method with a dual sedimentating agent to realize phosphor doping in both the surface and bulk. X-ray diffraction Rietveld refinement results indicate that the doped sample presents a larger lattice spacing than the normal sample and a Li3PO4 protective layer in situ forms on the surface. Synchrotron scanning transmission X-ray microscopy (STXM) reveals commendable homogeneity in the phase distribution between the surface and bulk in the doped sample. X-ray absorption near edge structure (XANES) shows a more homogeneous local chemical environment of the doped sample by investigating the Mn, Ni, and Co L-edges and O K-edge spectra. The doped sample displays a high discharge capacity of 295 mA h g−1 with an initial coulombic efficiency of 90.5% at 0.1C, showing a high rate performance of 247 mA h g−1 at 1C and a superior capacity retention of 73% after 500 cycles. Moreover, this doping strategy also inhibits the critical voltage decay of Li-rich materials during cycling. The prolonged structural evolution analysis demonstrates that phosphor doping can play a stabilizing role in Li-rich materials to restrain the transformation from layer to spinel.

Mapping XANES spectra on structural descriptors of copper oxide clusters using supervised machine learning

Abstract: Understanding the origins of enhanced reactivity of supported, subnanometer in size, metal oxide clusters is challenging due to the scarcity of methods capable to extract atomic-level information from the experimental data. Due to both the sensitivity of X-ray absorption near edge structure (XANES) spectroscopy to the local geometry around metal ions and reliability of theoretical spectroscopy codes for modeling XANES spectra, supervised machine learning approach has become a powerful tool for extracting structural information from the experimental spectra. Here, we present the application of this method to grazing incidence XANES spectra of size-selective Cu oxide clusters on flat support, measured in operando conditions of the methanation reaction. We demonstrate that the convolution neural network can be trained on theoretical spectra and utilized to "invert" experimental XANES data to obtain structural descriptors-the Cu-Cu coordination numbers. As a result, we were able to distinguish between different structural motifs (Cu2O-like and CuO-like) of Cu oxide clusters, transforming in reaction conditions, and reliably evaluate average cluster sizes, with important implications for the understanding of structure, composition, and function relationships in catalysis.

Thin Films of Fully Noble Metal-Free POM@MOF for Photocatalytic Water Oxidation.

Abstract: P2W18Co4@MOF-545, which contains the sandwich-type polyoxometalate (POM) [(PW9O34)2Co4(H2O)2]10- (P2W18Co4) immobilized in the porphyrinic metal-organic framework (MOF), MOF-545, is a "three-in-one" (porosity + light capture + catalysis) heterogeneous photosystem for the oxygen-evolution reaction (OER). Thin films of this composite were synthesized on transparent and conductive indium tin oxide (ITO) supports using electrophoretic (EP) or drop-casting (DC) methods, thus providing easy-to-use devices. Their electro- and photocatalytic activities for OER were investigated. Remarkably, both types of films exhibit higher turnover numbers (TONs) than the original bulk material previously studied as a suspension for the photocatalytic OER, with TONs after 2 h equal to 1600 and 403 for DC and EP films, respectively, compared to 70 for the suspension. This difference of catalytic activities is related to the proportion of efficiently illuminated crystallites, whereby a DC thin film offers the largest proportion of POM@MOF crystallites exposed to light due to its lower thickness when compared to an EP film or crystals in suspension. Such devices can be easily recycled by simply removing them from the reaction medium and washing them before reuse. The films were fully characterized with extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES) spectroscopies, Raman, scanning electron microscopy, and electrochemistry before and after catalysis. The combination of all of these techniques shows the stability of both the POM and the MOF within the composite upon water-oxidation reaction.

Dual Graphitic-N Doping in a Six-Membered C-Ring of Graphene-Analogous Particles Enables an Efficient Electrocatalyst for the Hydrogen Evolution Reaction.

Abstract: Graphene-based materials still exhibit poor electrocatalytic activities for the hydrogen evolution reaction (HER) although they are considered to be the most promising electrocatalysts. We fabricated a graphene-analogous material displaying exceptional activity towards the HER under acidic conditions with an overpotential (57 mV at 10 mA cm-2 ) and Tafel slope (44.6 mV dec-1 ) superior to previously reported graphene-based materials, and even comparable to the state-of-the art Pt/C catalyst. X-ray absorption near-edge structure (XANES) and solid-state NMR studies reveal that the distinct feature of its structure is dual graphitic-N doping in a six-membered carbon ring. Density functional theory (DFT) calculations show that the unique doped structure is beneficial for the activation of C-H bonds and to make the carbon atom bonded to two graphitic N atoms an active site for the HER.

Efficient simulation of near-edge x-ray absorption fine structure (NEXAFS) in density-functional theory: Comparison of core-level constraining approaches

Abstract: Widely employed Near-Edge X-Ray Absorption Fine Structure (NEXAFS) spectroscopy probes a system by excitation of core electrons to unoccupied states. A variety of different methodologies are available to simulate corresponding spectra from first-principles. Core-level occupation constraints within ground-state Density-Functional Theory represent a numerically most efficient means to this end that provides access to large systems, examples being surface adsorption, proteins, polymers, liquids, and buried, condensed phase interfaces (e.g., solid-liquid and solid-solid). Here, we systematically investigate the performance of different realizations of this approximate approach through the simulation of K-edge NEXAFS-spectra of a set of carbon and nitrogen-containing organic molecules. Variational collapse to the ground state and oscillatory convergence are the major complications of these approximate computational protocols. We present a modified version of the maximum-overlap method to achieve a self-consistent inclusion of electrons in virtual states for systems where convergence is hampered due to degeneracies. Our results demonstrate that reliable spectra allowing for a semi-quantitative analysis of experimental data are already obtained at the semi-local level of density functionals and with standard numeric atomic orbital basis sets.

Cr(III) Adsorption by Cluster Formation on Boehmite Nanoplates in Highly Alkaline Solution

Abstract: The development of advanced functional nanomaterials for selective adsorption in complex chemical environments requires partner studies of binding mechanisms. Motivated by observations of selective Cr(III) adsorption on boehmite nanoplates (γ-AlOOH) in highly caustic multicomponent solutions of nuclear tank waste, here we unravel the adsorption mechanism in molecular detail. We examined Cr(III) adsorption to synthetic boehmite nanoplates in sodium hydroxide solutions up to 3 M, using a combination of X-ray diffraction (XRD), Raman, X-ray photoelectron spectroscopy (XPS), scanning/transmission electron microscopy (S/TEM), electron energy loss spectroscopy (EELS), high-resolution atomic force microscopy (HR-AFM), time-of-fight secondary ion mass spectrometry (ToF-SIMS), Cr K-edge X-ray absorption near edge structure (XANES)/extended X-ray absorption fine structure (EXAFS), and electron paramagnetic resonance (EPR). Adsorption isotherms and kinetics were successfully fit to Langmuir and pseudo-second-order kinetic models, respectively, consistent with monotonic uptake of Cr(OH)4- monomers until saturation coverage of approximately half the aluminum surface site density. High resolution AFM revealed monolayer cluster self-assembly on the (010) basal surfaces with increasing Cr(III) loading, possessing a structural motif similar to guyanaite (β-CrOOH), stabilized by corner-sharing Cr-O-Cr bonds and attached to the surface with edge-sharing Cr-O-Al bonds. The selective uptake appears related to short-range surface templating effects, with bridging metal connections likely enabled by hydroxyl anion ligand exchange reactions at the surface. Such a cluster formation mechanism, which stops short of more laterally extensive heteroepitaxy, could be a metal uptake discrimination mechanism more prevalent than currently recognized.

Operando Structure Determination of Cu and Zn on Supported MgO/SiO2 Catalysts during Ethanol Conversion to 1,3-Butadiene

Abstract: The electronic structure and reactivity of Cu- and Zn-promoted wet-kneaded MgO/SiO2 catalysts was interrogated during ethanol reaction to 1,3-BD. A multimodal nature of characterization, including in situ or operando X-ray, electron, light spectroscopies, and steady state reactivity measurements demonstrated critical information on the temporal evolution of the catalyst active sites including key measurements performed in operando conditions using synchrotron techniques (EXAFS and XANES). In situ DRIFT spectroscopy allowed decoupling of the aldol condensation and dehydrogenation reactive steps due to the promotion with enhanced ability to carry out aldol condensation, as correlated with the steady state reactivity experiments. In situ UV–vis spectroscopy presented a complex picture of the adsorbates with π–π* electronic transitions due to the allylic cations, cyclic or aromatic species while also suggesting oligomeric CuO species were formed. Operando X-ray measurements combined with ab initio multiple sc...

Classification of local chemical environments from x-ray absorption spectra using supervised machine learning

Abstract: X-ray absorption near-edge structure (XANES) spectroscopy is a robust and element-specific tool for probing the atomic structure of materials. Traditional spectroscopy methods work in the forward direction by simulating XANES spectra from atomic models. Here, the authors present the opposite: a computational method predicting local structural geometry from XANES spectra in which the so-called inverse problem is solved using supervised machine learning. The robustness and fidelity of the method are demonstrated by an average of 86% classification accuracy in the K-edge XANES spectra of hundreds of materials across eight $3d$ transition metal families.

Tracking the Metal-Centered Triplet in Photoinduced Spin Crossover of Fe(phen)32+ with Tabletop Femtosecond M-Edge X-ray Absorption Near-Edge Structure Spectroscopy.

Abstract: Fe(II) coordination complexes are promising alternatives to Ru(II) and Ir(III) chromophores for photoredox chemistry and solar energy conversion, but rapid deactivation of the initial metal-to-ligand charge transfer (MLCT) state to low-lying (d,d) states limits their performance. Relaxation to a long-lived quintet state is postulated to occur via a metal-centered triplet state, but this mechanism remains controversial. We use femtosecond extreme ultraviolet (XUV) transient absorption spectroscopy to measure the excited-state relaxation of Fe(phen)32+ and conclusively identify a 3T intermediate that forms in 170 fs and decays to a vibrationally hot 5T2g state in 39 fs. A coherent vibrational wavepacket with a period of 249 fs and damping time of 0.63 ps is observed on the 5T2g surface, and the spectrum of this oscillation serves as a fingerprint for the Fe–N symmetric stretch. The results show that the shape of the M2,3-edge X-ray absorption near-edge structure (XANES) spectrum is sensitive to the electron...

Investigating the luminescence mechanism of Mn-doped CsPb(Br/Cl)3 nanocrystals

Abstract: Inorganic lead halide perovskite CsPbX3 (X = Cl, Br, or I) nanocrystals are promising candidate materials for light-emitting devices and optoelectronics. Mn-Doped CsPbX3 is of particular interest, as the Mn-doping introduces an additional emission band, making this material a promising white-light emitter. In this study, Mn-doped CsPb(Br/Cl)3 nanocrystals are prepared at room-temperature and ambient pressure. The chemical environment of Mn, and the luminescence of these nanocrystals are analyzed in detail using X-ray diffraction (XRD), extended X-ray absorption fine structure (EXAFS), X-ray absorption near-edge structure (XANES) and X-ray excited optical luminescence (XEOL). Although the introduction of Mn does not alter the long-range order of the CsPbX3 crystal, it leads to a local lattice contraction with the bond length of Mn–X much shorter than Pb–X. We also find excitation energy-dependence in both the intensity and wavelength of the perovskite excitonic emission band, while only in intensity of the Mn emission band. Detailed fitting of the XEOL reveals that the perovskite emission band is dual-channel, and it is the excitation energy-dependent intensity variation of these two channels that drives the observed red-shift of the combined emission band. Our findings also confirm that the Mn emission band is driven by exciton-Mn energy transfer and clarify the Mn chemical environment and the luminescence mechanism in Mn-doped CsPb(Br/Cl)3 nanocrystals.

In-situ X-Ray Absorption Near Edge Structure Spectroscopy of a Solid Catalyst using a Laboratory-Based Set-up

Abstract: An in-situ laboratory-based X-ray Absorption Near Edge Structure (XANES) Spectroscopy set-up is presented, which allows performing long-term experiments on a solid catalyst at relevant reaction conditions of temperature and pressure. Complementary to research performed at synchrotron radiation facilities the approach is showcased for a Co/TiO 2 Fischer-Tropsch Synthesis (FTS) catalyst. Supported cobalt metal nanoparticles next to a (very small) fraction of cobalt(II) titanate, which is an inactive phase for FTS, were detected, with no signs of re-oxidation of the supported cobalt metal nanoparticles during FTS at 523 K, 5 bar and 200 h, indicating that cobalt metal is maintained as the main active phase during FTS.

Nickel-Vanadium Layered Double Hydroxide under Water-Oxidation Reaction : New Findings and Challenges

Abstract: Nickel–vanadium layered double hydroxide has recently been considered as a highly active, low-cost electrocatalyst and as a benchmark non-noble metal-based electrocatalyst for water oxidation. The material showed a current density of 27 mA/cm2 at an overpotential of 350 mV, which is comparable to the best-performing nickel–iron-layered double hydroxides for water oxidation in alkaline media. The enhanced conductivity and facile electron transfer were suggested among important factors for the high activity of nickel–vanadium layered double hydroxide. In the present study, the stability of an Ni–V catalyst was investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), X-ray absorption near edge structure (XANES), extended X-ray absorption fine structure (EXAFS), and electrochemical characterization methods. These methods show that the initial Ni–V catalyst during water oxidation in alkaline conditions is co...

Semiconductor Nanocrystal Engineering by Applying Thiol- and Solvent-Coordinated Cation Exchange Kinetics.

Abstract: Thiol- and solvent-coordinated cation exchange kinetics have been applied to engineer the composition and crystallinity of novel nanocrystals. The detailed thermodynamics and kinetics of the reactions were explored by NMR spectroscopy, time-dependent photoluminescence (PL) characterizations and theoretical simulations. The fine structure of the colloidal semiconductor nanocrystals (CSNCs) was investigated by X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS). In this way, high-quality p-type Ag-doped ZnS quantum dots (QDs) and Au@ZnS hetero-nanocrystals with a cubic phase ZnS shell were synthesized successfully.The unprecedented dominant Ag+ -dopant-induced fluorescence and p-type conductivity in the zinc-blende ZnS are reported.

Efficient Simulation of Near-Edge X-ray Absorption Fine Structure (NEXAFS) in Density-Functional Theory: Comparison of Core-Level Constraining Approaches

Abstract: Widely employed Near-Edge X-Ray Absorption Fine Structure (NEXAFS) spectroscopy probes a system by excitation of core electrons to unoccupied states. A variety of different methodologies are available to simulate corresponding spectra from first-principles. Core-level occupation constraints within ground-state Density-Functional Theory (DFT) represent a numerically most efficient means to this end that provides access to large systems, examples being surface adsorption, proteins, polymers, liquids, and buried, condensed phase interfaces (e.q. solid-liquid and solid-solid). Here, we systematically investigate the performance of different realizations of this approximate approach through the simulation of K-edge NEXAFS-spectra of a set of carbon and nitrogen-containing organic molecules. Variational collapse to the ground state and oscillatory convergence are the major complications of these approximate computational protocols. We present a modified version of the maximum-overlap method to achieve a self-consistent inclusion of electrons in virtual states for systems where convergence is hampered due to degeneracies. Our results demonstrate that reliable spectra allowing for a semi-quantitative analysis of experimental data are already obtained at the semi-local level of density functionals and with standard numeric atomic orbital basis sets.

NEXAFS spectroscopy study of lithium interaction with nitrogen incorporated in porous graphitic material

Abstract: Nitrogen-doped carbon nanomaterials have greater capacity and better cycling stability for Li-ion batteries as compared to undoped carbon materials. In situ near-edge X-ray absorption fine structure (NEXAFS) spectroscopy in combination with quantum chemical modeling has been applied to determine the chemical states of the incorporated nitrogen after interaction with lithium. NEXAFS N K-edge spectra of nitrogen-doped porous carbon were measured before and after thermal deposition of Li vapors. The simulation and interpretation of NEXAFS data were carried out based on density functional theory calculations of initial and lithiated graphene fragments that contained different nitrogen species. The preferable interactions of Li with pyridinic and hydrogenated pyridinic nitrogen which are located at edges of atomic vacancies and graphene planes were revealed.