Dispersive X-ray Absorption Fine Structure Research Articles

Development of Simultaneous Measurement System for X-ray Absorption Spectra at Two Absorption Edges.

Dispersive X-ray absorption fine structure (DXAFS) spectroscopy is a versatile measurement technique for analyzing chemical reactions in real time. We have developed a novel time-resolved DXAFS instrument based on two polychromators and a wide-range position-sensitive detector that permits direct observation of the synergistic effects of two elements. This system enables simultaneous acquisition of the X-ray absorption spectra for two different elements without any mechanical movement of the X-ray optics. The developed system was successfully applied to monitor both the synthesis of a Ni-Cu bimetallic catalyst, which revealed that the reduction of Ni occurred at a higher temperature than that of Cu, and the charge-discharge processes of a LixNi0.5Mn1.5O4-based lithium-ion battery, which demonstrated that the redox reactions of Ni and Mn occurred sequentially at specific electrode potentials.

In Situ Time-Resolved XAFS Studies on Laser-Induced Particle Formation of Palladium Metal in an Aqueous/EtOH Solution

The reaction kinetics of laser-induced particle formation in an aqueous/EtOH solution of PdCl42– without a photoactivator was investigated by transmission electron microscopy (TEM) and dispersive X-ray absorption fine structure (DXAFS) measurements. Pd particles were generated by the irradiation of a nanosecond pulsed 266 nm laser with a fluence of 19.9–59.7 mJ/cm2. The TEM observation showed the dependence of the particle size on the laser fluence and the promotion of particle growth by the irradiation of a high-fluence laser. The DXAFS data were analyzed by three methods: deconvolution of the X-ray absorption near-edge structure (XANES) spectrum by a linear combination fitting, model fitting of the extended XAFS (EXAFS) oscillation of the PdCl42– ion, and fitting of the spectrum edge of XANES using an error function. These methods give the ratio of Pd2+, the coordination number of Pd–Cl bond, and the edge width of XANES, which are related to the Pd2+ concentration. Temporal changes of the Pd2+ concentra...

Dispersive soft x-ray absorption fine-structure spectroscopy in graphite with an attosecond pulse

Phase transitions of solids and structural transformations of molecules are canonical examples of important photo-induced processes, whose underlying mechanisms largely elude our comprehension due to our inability to correlate electronic excitation with atomic position in real time. Here, we present a decisive step towards such new methodology based on water-window-covering (284 eV to 543 eV) attosecond soft X-ray pulses that can simultaneously access electronic and lattice parameters via dispersive X-ray absorption fine-structure (XAFS) spectroscopy. We validate attoXAFS with an identification of the {\sigma}* and {\pi}* orbital contributions to the density of states in graphite simultaneously with its lattice's four characteristic bonding distances. This work demonstrates the concept of attoXAFS as a powerful real-time investigative tool which is equally applicable to gas-, liquid- and condensed phase.

In-Situ Analysis on BaTiO3-LiCoO2 Composite Cathode for Lithium Ion Batteries

The relatively low diffusivity of Li ions in bulk compounds has been limiting the achievable power densities for the lithium ion batteries (LIBs). Decorating of oxides such as Al2O3 [1], onto the active materials effectively improve the high rate capability. The thin-film oxide coating shortens the Li-ion diffusion distance at the SEI and prevents the dissolution of active material into the liquid electrolyte under a large current density. This phenomenon indicates the oxide thin films play a role as an artificial solid electrolyte interface (SEI). In our previous studies[2,3], the active material, LiCoO2 (LC) powder, was decorated with ferroelectric barium titanate, BaTiO3 (BT), as an artificial SEI, via a simple sol-gel route. The BT-loaded composite cathode exhibited a significant improvement in rate capability and an exceptionally high charge–discharge rate. However, the contribution of the ferroelectric BT SEI to such significant improvement in the cell performance hasn’t been clarified yet. The in-situ analysis, namely, electrochemical impedance spectroscopy (EIS) and X-ray absorption fine structure (XAFS) analysis, were performed in this study. BT-LC composite cathodes were prepared using a simple sol-gel synthesis. Various amount of BT (0.5, 1, 5, and 15 mol%) were loaded into the active materials LC(Cell Seed; Nippon Chemical Industrial Co., Ltd., Tokyo, Japan).Prior to impedance measurement, the cells were cycled in a voltage window of 3.3–4.5 V. During impedance measurements, cells were charged and discharged under constant current–constant voltage (CC–CV) with a voltage window of 3.3–4.5 V. During charging and discharging, the impedance was measured 12 times at 30-min intervals under open-circuit conditions. The impedance measurement was repeated for three cycles while charging and discharging at 0.1 C. As for the time-resolved dispersive XAFS (DXAFS) measurement, the loaded amounts of the oxides, BT and Al2O3, were fixed as 1 mol% in the LC matrix. Prior to DXAFS measurement, the cells were cycled by same way as impedance measurement. During DXAFS measurements, specimens were charged and discharged under a constant current (CC) with a cut-off voltage of 3.3 and 4.5 V for two cycles. The applied current density was fixed at a high rate (10 C). The acquired EIS data indicated the charge transfer resistance (R ct) of the BT 1mol% loaded specimen was 10.2Ω, corresponding to less than 1/12 the value for the bare LC (R ct = 127Ω).[4] This significant reduction in R ct may, therefore, be interpreted as the promotion of Li diffusivity at the electrode–electrolyte interface, driven by polarization due to the ferroelectricity of the BT layer. Meanwhile, the capacity retention ratio dropped rapidly, accompanied by a slight increase in R ct for larger BT loads of 5 and 15 mol %, attributed to the thicker BT layer and the existence of the impurity phase, BaCO3. These results imply that the ferroelectric BT SEI affected the kinetics of Li ion diffusion at the cathode–electrolyte interface, giving rise to a significant enhancement in the high charge–discharge rate capability, as well as restricting cobalt dissolution into the electrolyte. The DXAFS results also indicated, the BT 1mol%-loaded sample displayed the largest magnitude in Co valence variation during the cell driving at a high rate, while the energy shift for the Al2O3-coated sample was slightly larger than that of bare LC.[5] The increase in the X-ray absorption energy during the charging was intensified at higher applied potential (approaching 4.5 V) for the BT-decorated composite. The strength of the polarization increased with the applied potential during charging. The larger density of Li ions deintercalating from the LC, leading to higher capability, was attributed to the enhanced polarization at the composite–electrolyte interface. The heavier Co oxidation during charging, exhibited by the ferroelectric BT-SEI compared with the paraelectric Al2O3-SEI, was interpreted as evidence of the strengthened polarization due to the larger permittivity of BT. [1] Y. S. Jung, A. S. Cavanagh, A. C. Dillon, M. D. Groner, S. M. George, and S.-H. Lee, J. Electrochem. Soc., 157, A75-A81 (2010). [2] T. Teranishi, Y. Yoshikawa, R. Sakuma, H. Hashimoto, H. Hayashi, A. Kishimoto and T. Fujii, Appl. Phys. Lett., 105, 143904-1-3 (2014). [3] T. Teranishi, Y. Yoshikawa, R. Sakuma, H. Okamura, H. Hashimoto, H. Hayashi, T. Fujii, A. Kishimoto, and Y. Takeda, ECS Electrochem. Lett., 4, A137-A140 (2015). [4] T. Teranishi, Y. Yoshikawa, R. Sakuma, H. Okamura, H. Hayashi, A. Kishimoto, and Y. Takeda, Jpn. J. Appl. Phys., 54, 10NB02-1-5 (2015). [5] T. Teranishi, Y. Yoshikawa, R. Miyahara, , H. Hayashi, A. Kishimoto, M. Katayama, and Yasuhiro Inada, J. Ceram. Soc. Jpn., in press (2016).

Ferroelectric SEI for Lithium Ion Batteries with Ultrahigh Rate Capability

Lithium ion batteries with drastically enhanced power densities will enable excellent acceleration and better fuel economy, making them promising candidates as the next-generation battery for hybrid and electric vehicles. Higher power density; i.e., a capacity with faster charging/discharging times, is generally limited by the diffusion of Li+ in bulk compounds. A thin layer such as Al2O3 [1] decorated on the active materials acts as an artificial solid electrolyte interface (SEI), and improves the rate performance.We claim that another breakthrough involves using the “ferroelectricity” of an artificial SEI to assist the Li ion diffusion. Dipole moments due to the polarization are generated in the ferroelectric SEI layer under a potential difference between two electrodes. Since the dipole moments are negatively charged at the active material–SEI interface, these negative charges might attract the positively charged Li+ as the battery charges and discharges. Such a “c arrier inducing effect of ferroelectricity”, would assist the smooth intercalation and deintercalation of Li ions, leading to an excellent power density. LC particles were coated with ferroelectric BT layers via a simple liquid phase reaction, the sol–gel route. Various amounts of BT (0.1 to 15mol%) were loaded on the LC powder as a ferroelectric artificial SEI. The size of decorated BT nanoparticles was ca. 50 nm. A 2032-type coin cell was employed for the evaluation. The cells were cycled in a voltage window of 3.3–4.5 V. The cells were first charged and discharged at 0.1C (1C = 160 mA/g) for five cycles, and then the charge–discharge rate was increased stepwise to 10C (6 min. for 100% cha- and discharge)/100C (36 sec. for 100% cha- and discharge) for five cycles at each rate.The initial capacity of the bare LC was ~187 mAh/g and the capacity was reduced significantly at above 1C, to 62 mAh/g at 35 cycles, 10C. This capacity corresponded to only 33% of the initial value. The composites with smaller BT loadings from 0.1–1 mol% exhibited relatively good retention of the discharge capacity at a high rate (10C); in particular, the 1 mol% BT had an excellent discharge capacity after 10C for five cycles (total 35 cycles) of 146 mAh/g, which is 78% of the initial capacity, versus as high as 238% of the capacity over the same cycles for the bare LC [2, 3]. To understand the origin of the enhancement in the high-rate capability, the impedances of the cells while charging and discharging were measured. The BT loading significantly reduced the resistance at lower frequencies; i.e., the resistance due to the cathode reaction, R ct. For the BT 1 mol%, R ct = 10.2 Ω, which was 1/12 that for the bare LC, R ct = 127 Ω [4]. This significant reduction in R ct could imply the promotion of the Li diffusivity at the electrode–electrolyte interface; i.e., at the BT SEI, attributed to the polarization due to the ferroelectric BT layer. In-situ time-resolved dispersive X-ray absorption fine structure (DXAFS) analysis of the composites was also done to characterize the cobalt ion valence shift between oxidized and reduced states. Two types of artificial SEIs, ferroelectric BT and paraelectric Al2O3, were compared. The magnitude of the shift in the X-ray absorption energy at the peak of the white line, E 1, during charging and discharging at a 10 C rate, increased in the order of bare LC (0.264 eV) < Al2O3 1 mol% (0.497 eV) < BT 1 mol% (1.15 eV); the corresponding discharge capacities of the cells at 10 C [5]. All these results indicated the ferroelectric SEI effectively contributed to the enhancement in the high rate capabilities of the cell.The ultrahigh rate cell performances of the LCs coated with various dielectric/ferroelectric SEIs will be also presented in the talk.[1] I. D. Scott, Y. S. Jung, A. S. Cavanagh, Y. Yan, A. C. Dillon, S. M. George, S. H. Lee, Nano Lett., 11, 414 (2011). [2] T. Teranishi, Y. Yoshikawa, R. Sakuma, H. Hashimoto, H. Hayashi, A. Kishimoto, T. Fujii, Appl. Phys. Lett., 105, 143904 (2014). [3] T. Teranishi, Y. Yoshikawa, R. Sakuma, H. Okamura, H. Hashimoto, H. Hayashi, T. Fujii, A. Kishimoto, and Y. Takeda, ECS Electrochem. Lett., 4, A137 (2015). [4] T. Teranishi, Y. Yoshikawa, R. Sakuma, H. Okamura, H. Hayashi, A. Kishimoto, and Y. Takeda, Jpn. J. Appl. Phys. 54, 10NB02 (2015).in press. [5] T. Teranishi, Y. Yoshikawa, R. Miyahara, , H. Hayashi, A. Kishimoto, M. Katayama, and Yasuhiro Inada, J. Ceram. Soc. Jpn., in press (2016).

&lt;i&gt;In Situ&lt;/i&gt; and Simultaneous Observation of Palladium Redox and Oxygen Storage/Release in Pd/Sr&amp;ndash;Fe&amp;ndash;O Perovskite Catalysts Using Dispersive XAFS

We have succeeded in in situ and simultaneously observing the redox reaction of palladium and the oxygen storage/release process in a newly developed palladium-promoted Sr­Fe­O (Pd/Sr­Fe­O) catalyst during redox-gas cycles using dispersive X-ray absorption fine structure (XAFS) analysis at 673K with a time resolution of less than 20ms. The Pd/Sr­Fe­O catalyst, which exhibits high performance for automotive emission control, has a unique “multi-phase-domain” structure, where a single grain is composed of nano-sized domains of three phases: SrFeO3¹¤ ,S r4Fe6O13¹¤ and SrFe12O19¹¤. In situ observation has shown a strong correlation between the redox of the palladium and the oxygen storage/release in the Pd/Sr­Fe­O catalyst, and the correlation factors differ between the reduction and oxidation reactions. The oxide phases exhibit a type of “oxygen buffer” effect in the oxidation cycle and delay the formation of Pd­O, resulting in the high performance of the catalyst.

Real‐time XAFS analysis of Rh/alumina catalyst

AbstractReduction rates of rhodium supported on alumina were investigated with in situ time‐resolved X‐ray absorption fine structure (XAFS). The catalytic sample was prepared with 0.5‐wt% rhodium on the alumina powder by a usual impregnation method. Two types of XAFS techniques were applied for the experiments: energy‐dispersive XAFS and usual step‐scanning XAFS. The reduction rates of the rhodium supported on alumina after oxidizing treatment at 500 °C were measured with dispersive XAFS. The reducing condition was with 3% H2 in He at various temperatures from 100 to 250oC. The spectra of Rh K‐edge X‐ray absorption near edge structure (XANES) were acquired every 0.2 s. The oxidation state of the rhodium was evaluated by the energy shift of Rh K‐edge. The reduction rate of the rhodium was on a line in an Arrhenius plot. Reduction of the rhodium on the alumina in 3% H2 in He at 900 °C after high temperature oxidization at 900 and 1000 °C were measured with step‐scan XAFS. Though a portion of the rhodium was reduced in a few minutes, the residual rhodium was hardly reduced. The hardly reducible rhodium was considered to be reacted with the alumina support and lost catalytic activities. The ratio of the deactivated rhodium has been estimated as a function of the oxidization temperature and period. Copyright © 2008 John Wiley &amp; Sons, Ltd.

Wavelength Dispersive X-ray Absorption Fine Structure Imaging by Parametric X-ray Radiation

The parametric X-ray radiation (PXR) generator system at Laboratory for Electron Beam Research and Application (LEBRA) in Nihon University is a monochromatic and coherent X-ray source with horizontal wavelength dispersion. The energy definition of the X-rays, which depends on the horizontal size of the incident electron beam on the generator target crystal, has been investigated experimentally by measuring the X-ray absorption near edge structure (XANES) spectra on Cu and CuO associated with conventional X-ray absorption imaging technique. The result demonstrated the controllability of the spectrum resolution of XANES by adjusting of the horizontal electron beam size on the target crystal. The XANES spectra were obtained with energy resolution of several eV at the narrowest case, which is in qualitative agreement with the energy definition of the PXR X-rays evaluated from geometrical consideration. The result also suggested that the wavelength dispersive X-ray absorption fine structure measurement associated with imaging technique is one of the promising applications of PXR.

Observing copper chloride during dioxin formation using dispersive XAFS

AbstractThe incineration of municipal solid waste generates dioxins. To control dioxins, it is necessary to determine the mechanism of their formation. Because copper chloride (CuCl2) is one of the strongest catalysts of dioxin formation in fly ash, it is important to study the chemical changes of this compound. To determine the chemical changes of copper, Cu‐K x‐ray absorption near edge structure (XANES) spectra were examined during the heating of model fly ash using a time‐resolved dispersive x‐ray absorption fine structure (DXAFS) technique. The change in the absorption edge of the Cu‐K XANES spectrum revealed a change in the valence of copper and the temperature dependence of the reduction and oxidation of CuCl2. From room temperature to 270 °C, CuCl2 was reduced, and from &gt; 270 to 300 °C, copper was reoxidized in the presence of 10% oxygen. At a constant temperature of 210 or 300 °C, copper was dynamically reduced and oxidized. In experiments using different gas streams, HCl gas had no effect on the reaction of copper, while oxygen gas was essential for the reoxidation of copper. Our results suggest that dioxin formation in fly ash occurs via the chlorination of carbon with the reduction of CuCl2 and carbon gasification catalyzed by copper oxides. Copyright © 2008 John Wiley &amp; Sons, Ltd.

In situ Characterization of Supported Metal Catalysts and Model Surfaces by Time‐Resolved and Three‐Dimensional XAFS Techniques

AbstractFor Abstract see ChemInform Abstract in Full Text.

In situ characterization of supported metal catalysts and model surfaces by time-resolved and three-dimensional XAFS techniques

This paper emphasizes novel issues in in situ characterization techniques for catalysts and model surfaces, namely, energy dispersive X-ray absorption fine structure (DXAFS) for time-resolved structure analysis and polarization-dependent total-reflection fluorescence X-ray absorption fine structure (PTRF-XAFS) for 3D structure analysis, which have been developed in the past 5–10 years. DXAFS is a powerful technique for elucidating dynamic structural changes at active metal sites on catalyst surfaces that involve a sequence of formation and breaking of metal–metal, metal–adsorbate, and metal–support bonds; it opens a new area of structure kinetics, revealing kinetic parameters during catalytic reactions, catalyst preparations, and catalyst deactivation and regeneration. PTRF-XAFS is another powerful technique for elucidating 3D structures of active metal sites at single crystal model surfaces, determining structural parameters in three different directions independently with high surface sensitivity, which opens up a new area of catalytic structural chemistry, presenting information on asymmetric and anisotropic bond arrangements and transformations. The issues addressed in this paper are relevant to important key issues in understanding new catalytic structures and reaction mechanisms and to developing new concepts and strategies for novel catalytic materials, which can hardly be addressed by other techniques.

Studies on Surface and Interface with X-ray Absorption Fine Structure (XAFS). Time Resolved XAFS Studies for Catalysts Preparation.

Time resolved dispersive XAFS (DXAFS) is a useful method to elucidate changing structures and is applicable to investigate the structures of short living species during chemical reactions. We applied this method to the chemical study of catalysts preparation. Recent results of our DXAFS study on the reduction process of Cu-ZSM−5 and on the decarbonylation of Mo(CO)6 in the cages of Y-type zeolite are describe in this article as well as a brief description of DXAFS measurement and DXAFS data analysis.

Time-resolved energy-dispersive XAFS study on the reduction process of Cu-ZSM-5 catalysts.

The time-resolved reduction process of copper cations in ZSM-5 during temperature-programmed reduction (300-700 K) was studied by energy dispersive X-ray absorption fine structure (DXAFS). The Cu K-edge DXAFS spectra for isolated Cu2+ species in the channels of ZSM-5 were recorded at an interval of 1 s during the reduction. The curve fitting analysis of the EXAFS data and the XANES analysis revealed that the isolated Cu2+ species in the channels were reduced stepwise. They were reduced to isolated Cu+ species at 400-450 K and the Cu+ species to Cu0 metallic clusters at 550-650 K. Small clusters like Cu4 were initially formed, followed by particle growth. A small part of them went out to the outer surfaces of ZSM-5 during the reduction.

Real Time Structural Electrochemistry of Platinum Clusters using Dispersive XAFS

ABSTRACTChemical reference tables state that the standard potential for the reaction of Pt with water, Pt + 2H2O → Pt(OH)2+ 2H++ 2e−, is 0.98 V, and electrochemical studies propose that this reaction may occur at potentials as low as 0.8 V [1–5]. Using dispersive x-ray absorption finestructure (XAFS) spectroscopy [6], we have directly probed the structural evolution of a Pt catalyst operating in-situ in a polymer electrolyte [7] fuel cell during cyclic voltammetry. The changes in the number of Pt and O nearest-neighbors and the Pt charge demonstrate a close correspondence with features in the voltammogram. Because dispersive XAFS is very sensitive to detecting structural changes, we have been able to detect the presence of chemisorbed oxygen at potentials of 0.6–0.9 V in the anodic sweep. Since double-layer charging is regarded as the only process in this region for bulk Pt [3–5], these results may reflect a limitation of previous (indirect) studies on Pt electrochemistry, or they may indicate that these clusters are different from their bulk metal counterparts. Exploiting the timeresolving capability of dispersive XAFS, we also monitored changes in the Pt charge and the number of O and Pt nearest-neighbors during the electrochemical oxidation and reduction of the Pt clusters in real-time. The results are inconsistent with those expected from the placeexchange mechanism [8–11] for the formation of the surface oxide on bulk Pt electrodes in aqueous solution;Our current model for understanding these behaviors is that, relative to bulk Pt, unusual types of surface sites play a major role in determining the reactivity of these clusters.