Three-Dimensional Visualization of Adsorption Distribution in a Single Crystalline Particle of a Metal-Organic Framework.

Many unique adsorption properties of metal-organic frameworks (MOFs) have been revealed by diffraction crystallography, visualizing their vacant and guest-loaded crystal structures at the molecular scale. However, it has been challenging to see the spatial distribution of the adsorption behaviors throughout a single MOF particle in a transient equilibrium state. Here, we report three-dimensional (3D) visualization of molecular adsorption behaviors in a single crystalline particle of a MOF by in situ X-ray absorption fine structure spectroscopy combined with computed tomography for the first time. The 3D maps of water-coordinated Co sites in a 100 µm-scale MOF-74-Co crystal were obtained with 1 µm spatial resolution under several water vapor pressures. Through the visualization of the water vapor adsorption process, 3D spectroimaging revealed the mechanism and spatial heterogeneity of guest adsorption inside a single particle of a crystalline MOF.

Application of transition-edge sensors for micro-X-ray fluorescence measurements and micro-X-ray absorption near edge structure spectroscopy: a case study of uranium speciation in biotite obtained from a uranium mine.

In this study, we successfully applied a transition-edge sensor (TES) spectrometer as a detector for microbeam X-ray measurements from a synchrotron X-ray light source in the hard X-ray region to determine uranium (U) distribution at the micro-scale and its chemical species in biotite obtained from a U mine. It is difficult to separate the fluorescent X-ray of the U Lα1 line at 13.615 keV from that of the Rb Kα line at 13.395 keV in the X-ray fluorescence spectrum with an energy resolution of approximately 220 eV using a conventional silicon drift detector (SDD). Meanwhile, the fluorescent X-rays of U Lα1 and Rb Kα were fully separated by a TES with 50 eV energy resolution at an energy of around 13 keV. The successful peak separation by the TES led to an accurate mapping analysis of trace U in micro-X-ray fluorescence measurements and a decrease in the signal-to-background ratio in micro-X-ray absorption near edge structure spectroscopy. Thus, it could be a powerful tool for studying the U distribution and speciation in various environmental samples.

In situ 3D X-ray imaging of water distribution in each layer of a membrane electrode assembly of a polymer electrolyte fuel cell.

In situ 3D computed tomography imaging with statistical analysis successfully revealed the water accumulation and drainage characteristics in the stacked gas diffusion layers (GDLs) and membrane electrode assembly (MEA) of a polymer electrolyte fuel cell. Efficient water drainage at the interface between the cathode GDL and MEA was confirmed upon supplying oxygen to the cathode.

Oxidation and phase transfer of individual Cr-doped dendritic FeOx particles visualized by full-field nano-XAFS spectroimaging.

Iron oxides with various compositions and polymorphs have been widely used as compounds that require reversible redox properties, such as catalysts. However, partial decomposition during phase transitions often causes irreversible degradation of the redox properties of iron oxides. Cr doping into the crystalline framework of iron oxide dendrites improves the stability of the structural transformation of iron oxides. We spatially visualized the FeOx-dendrite phase distribution during oxidation in crystalline dendritic FeOx and Cr-FeOx particles by full-field nano-X-ray absorption fine structure spectroimaging. The spectroimaging visualized propagation in the phase transitions in the individual FeOx particles and changes in the phase transition behaviors of the Cr-FeOx particles. The statistical analysis of the spectroimaging data revealed the phase transition trends in parts of the FeOx and Cr-FeOx particles in three Fe density zones (particle thicknesses) and the probability densities of the phase proportions in the dendrites.

Sulfur poisoning of Pt and PtCo anode and cathode catalysts in polymer electrolyte fuel cells studied by operando near ambient pressure hard X-ray photoelectron spectroscopy.

We have investigated the S adsorption behaviours on Pt (average particle diameter of ∼2.6 nm) and Pt3Co (∼3.0 nm) anode and cathode electrode catalysts in polymer electrolyte fuel cells (PEFCs) under working conditions for the fresh state just after the aging process and also the degraded state after accelerated degradation tests (ADT), by studying near ambient pressure hard X-ray photoelectron spectroscopy (HAXPES). S 1s HAXPES of both the anode and cathode electrodes shows not only the principal S species from the sulfonic acid group (-SO3H) in the Nafion electrolyte but also other characteristic S species such as zero-valent S (S0) adsorbed on the carbon support and anionic S (S2-) adsorbed on the Pt electrode. The S2- species on Pt should be ascribed to S contamination poisoning the Pt catalyst electrode. The S2- species on the cathode can be oxidatively removed by applying a high cathode-anode bias voltage (≥0.8 V) to form SO32-, while at the anode the S2- species cannot be eliminated because of reductive environment in hydrogen gas. The important finding is the difference in S adsorption behaviours between the Pt/C and Pt3Co/C electrodes after ADT. After ADT, the Pt/C anode electrode exhibits much larger S2- adsorption than the Pt3Co/C anode electrode. This indicates that the Pt3Co/C anode is more desirable than the Pt/C one from the viewpoint of S poisoning. The reason for more tolerance of the Pt3Co/C anode catalyst against S poisoning after ADT can be ascribed to the more negative charge of the surface Pt atoms in the Pt3Co/C catalyst than those in the Pt/C one, thus yielding a weaker interaction between the surface Pt and the anionic S species as S2-, SO32-, and SO42-. A similar behaviour was observed also in the cathode catalyst. The present findings will nevertheless provide important information to design novel Pt-based PEFC electrodes with higher performance and longer durability.

Surface Freezing of Cetyltrimethylammonium Chloride-Hexadecanol Mixed Adsorbed Film at Dodecane-Water Interface.

The surface freezing transition of a mixed adsorbed film containing cetyltrimethylammonium chloride (CTAC) and n-hexadecanol (C16OH) was utilized at the dodecane-water interface to control the stability of oil-in-water (O/W) emulsions. The corresponding surface frozen and surface liquid mixed adsorbed films were characterized using interfacial tensiometry and X-ray reflectometry. The emulsion samples prepared in the temperature range of the surface frozen and surface liquid phases showed a clear difference in their stability: the emulsion volume decreased continuously right after the emulsification in the surface liquid region, while it remained constant or decreased at a much slower rate in the surface frozen region. Compared to the previously examined CTAC-tetradecane mixed adsorbed film, the surface freezing temperature increased from 9.5 to 25.0 °C due to the better chain matching between CTAC and C16OH and higher surface activity of C16OH. This then renders such systems much more attractive for practical applications.

Feed gas exchange (startup/shutdown) effects on Pt/C cathode electrocatalysis and surface Pt-oxide behavior in polymer electrolyte fuel cells as revealed using in situ real-time XAFS and high-resolution STEM measurements.

The synchronizing measurements of both cyclic voltammograms (CVs) and real-time quick XAFSs (QXAFSs) for Pt/C cathode electrocatalysts in a membrane electrode assembly (MEA) of polymer electrolyte fuel cells (PEFCs) treated by anode-gas exchange (AGEX) and cathode-gas exchange (CGEX) cycles (startup/shutdown conditions of FC vehicles) were performed for the first time to understand the opposite effects of the AGEX and CGEX treatments on the Pt/C performance and durability and also the contradiction between the electrochemical active surface area (ECSA) decrease and the performance increase by CGEX treatment. While the AGEX treatment decreased both the ECSA and performance of MEA Pt/C due to carbon corrosion, it was found that the CGEX treatment decreased the ECSA but increased the Pt/C performance significantly due to high-index (331) facet formation (high-resolution STEM) and hence the suppression of strongly bound Pt-oxide formation at cathode Pt nanoparticle surfaces. Transient QXAFS time-profile analysis for the MEA Pt/C also revealed a direct relationship between the electrochemical performance or durability and transient kinetics of the Pt/C cathode.

Visualization and understanding of the degradation behaviors of a PEFC Pt/C cathode electrocatalyst using a multi-analysis system combining time-resolved quick XAFS, three-dimensional XAFS-CT, and same-view nano-XAFS/STEM-EDS techniques.

We developed a multi-analysis system that can measure in situ time-resolved quick XAFS (QXAFS) and in situ three-dimensional XAFS-CT spatial imaging in the same area of a cathode electrocatalyst layer in a membrane-electrode assembly (MEA) of a polymer electrolyte fuel cell (PEFC) at the BL36XU beamline of SPring-8. The multi-analysis system also achieves ex situ two-dimensional nano-XAFS/STEM-EDS same-view measurements of a sliced MEA fabricated from a given place in the XAFS-CT imaged area at high spatial resolutions under a water-vapor saturated N2 atmosphere using a same-view SiN membrane cell. In this study, we applied the combination method of time-resolved QXAFS/3D XAFS-CT/2D nano-XAFS/STEM-EDS for the first time for the visualization analysis of the anode-gas exchange (AGEX) (simulation of the start-up/shut-down of PEFC vehicles) degradation process of a PEFC MEA Pt/C cathode. The AGEX cycles bring about serious irreversible degradation of both Pt nanoparticles and carbon support due to a spike-like large voltage increase. We could visualize the three-dimensional distribution and two-dimensional depth map of the amount, oxidation state (valence), Pt2+ elution, detachment, and aggregation of Pt species and the formation of carbon voids, where the change and movement of the Pt species in the cathode catalyst layer during the AGEX cycles did not proceed exceeding the 1 µm region. It is very different from the case of an ADT (an accelerated durability test between 0.6-1.0 VRHE)-degraded MEA. We discuss the spatiotemporal behavior of the AGEX degradation process and the degradation mechanism.

Reversible structural transformation and redox properties of Cr-loaded iron oxide dendrites studied by in situ XANES spectroscopy.

Cr-Loaded iron oxide with a dendritic crystalline structure was synthesized and the reversible crystalline phase transition during redox cycling of the iron oxide was investigated. X-ray diffraction and transmission electron microscopy analyses revealed that Cr was well dispersed and loaded in the iron oxide dendrite crystals, whose lattice constant was dependent on the Cr loading. Temperature-programmed oxidation and reduction experiments revealed the reversible redox properties of the Cr-loaded iron oxide dendrites, whose redox temperature was found to be lower than that of Cr-free iron oxide dendrites. In situ Fe K-edge and Cr K-edge X-ray absorption near-edge structure (XANES) analysis indicated that Cr loading extended the redox reaction window for conversion between Fe3O4 and γ-Fe2O3 owing to compressive lattice strain in the iron oxide spinel structures.

Model building analysis - a novel method for statistical evaluation of Pt L3-edge EXAFS data to unravel the structure of Pt-alloy nanoparticles for the oxygen reduction reaction on highly oriented pyrolytic graphite.

Extended X-ray absorption fine structure (EXAFS) is a powerful tool to determine the local structure in Pt nanoparticles (NP) on carbon supports, active catalysts for fuel cells. Highly oriented pyrolytic graphite (HOPG) covered with Pt NP gives samples with flat surfaces that allow application of surface science techniques. However, the low concentration of Pt makes it difficult to obtain good quality EXAFS data. We have performed in situ highly sensitive BCLA-empowered Back Illuminated EXAFS (BCLA + BI-EXAFS) measurements on Pt alloy nanoparticles. We obtained high quality Pt L3-edge data. We have devised a novel analytical method (model building analysis) to determine the structure of multi-component nanoparticles from just a single absorption edge. The generation of large numbers of structural models and their comparison with EXAFS fits allows us to determine the structures of Pt-containing nanoparticles, catalysts for the oxygen reduction reaction. Our results show that PtCo, PtCoN and AuPtCoN form a Pt-shell during electrochemical dealloying and that the ORR activity is directly proportional to the Pt-Pt bond length.

An Organic-Inorganic Hybrid Exhibiting Electrical Conduction and Single-Ion Magnetism.

The first three-dimensional (3D) conductive single-ion magnet (SIM), (TTF)2 [Co(pdms)2 ] (TTF=tetrathiafulvalene and H2 pdms=1,2-bis(methanesulfonamido)benzene), was electrochemically synthesised and investigated structurally, physically, and theoretically. The similar oxidation potentials of neutral TTF and the molecular precursor [HNEt3 ]2 [M(pdms)2 ] (M=Co, Zn) allow for multiple charge transfers (CTs) between the SIM donor [M(pdms)2 ]n- and the TTF.+ acceptor, as well as an intradonor CT from the pdms ligand to Co ion upon electrocrystallisation. Usually TTF functions as a donor, whereas in our system TTF is both a donor and an accepter because of the similar oxidation potentials. Furthermore, the [M(pdms)2 ]n- donor and TTF.+ acceptor are not segregated but strongly interact with each other, contrary to reported layered donor-acceptor electrical conductors. The strong intermolecular and intramolecular interactions, combined with CT, allow for relatively high electrical conductivity even down to very low temperatures. Furthermore, SIM behaviour with slow magnetic relaxation and opening of hysteresis loops was observed. (TTF)2 [Co(pdms)2 ] (2-Co) is an excellent building block for preparing new conductive SIMs.

An Integrated Single-Electrode Method Reveals the Template Roles of Atomic Steps: Disturb Interfacial Water Networks and Thus Affect the Reactivity of Electrocatalysts.

A method enabling the accurate and precise correlation between structures and properties is critical to the development of efficient electrocatalysts. To this end, we developed an integrated single-electrode method (ISM) that intimately couples electrochemical rotating disk electrodes, in situ/operando X-ray absorption fine structures, and aberration-corrected transmission electron microscopy on identical electrodes. This all-in-one method allows for the one-to-one, in situ/operando, and atomic-scale correlation between structures of electrocatalysts with their electrochemical reactivities, distinct from common methods that adopt multisamples separately for electrochemical and physical characterizations. Because the atomic step is one of the most fundamentally structural elements in electrocatalysts, we demonstrated the feasibility of ISM by exploring the roles of atomic steps in the reactivity of electrocatalysts. In situ and atomic-scale evidence shows that low-coordinated atomic steps not only generate reactive species at low potentials and strengthen surface contraction but also act as templates to disturb interfacial water networks and thus affect the reactivity of electrocatalysts. This template role interprets the long-standing puzzle regarding why high-index facets are active for the oxygen reduction reaction in acidic media. The ISM as a fundamentally new method for workflows should aid the study of many other electrocatalysts regarding their nature of active sites and operative mechanisms.

Ambient Pressure Hard X-ray Photoelectron Spectroscopy for Functional Material Systems as Fuel Cells under Working Conditions.

Heterogeneous interfaces play important roles in a variety of functional material systems and technologies, such as catalysis, batteries, and devices. A fundamental understanding of efficient functions at interfaces under realistic conditions is crucial for sophisticated designs of useful material systems and novel devices. X-ray photoelectron spectroscopy is one of the most promising and common methods to investigate such material systems. Although X-ray photoelectron spectroscopy is usually conducted under high vacuum because of the requirement of electron detection with the precise measurement of kinetic energies, extensive efforts have been devoted to the measurements in gaseous environments. Very recently, we have succeeded in measuring X-ray photoelectron spectra under real ambient atmosphere (105 Pa), using synchrotron radiation hard X-rays with the photon energy of 8 keV and the windowless electron spectrometer system. In this Account, the novel useful technique of real ambient pressure hard X-ray photoelectron spectroscopy is reviewed. As examples of (near) ambient pressure hard X-ray photoelectron spectroscopy, hydrogen storage of Pd nanoparticles is at first investigated by recording Pd 3d and valence band spectra under hydrogen atmosphere. The Pd 3d and valence band spectra are found to change rather abruptly depending on the hydrogen pressure, demonstrating a behavior like phase transformation. Subsequently, as a main topic in this Account, we describe investigations of the electronic states of platinum nanoparticles on the cathode electrocatalyst in a polymer electrolyte fuel cell (PEFC) under the voltage operating conditions using the near ambient pressure hard X-ray photoelectron spectroscopic system. The Pt 4f and 3d X-ray photoelectron spectra of the cathode Pt/C catalysts clearly show that the oxidized Pt species is at most divalent and the tetravalent Pt species does not exist on the Pt nanoparticles even at the positive cathode-anode voltage of ∼1.4 V. Although the water oxidation reaction may take place at the potential, such a reaction does not lead to a buildup of detectable tetravalent Pt in the PEFC. The voltage-dependent Pt 3d X-ray photoelectron spectra show a clear hysteresis between the voltage increase and decrease processes. The fraction of oxidized Pt species matched the ratio of surface to total Pt atoms in the nanoparticles, which suggests that Pt oxidation occurs as a reaction event at only the first Pt layer of the Pt nanoparticles and the inner Pt atoms do not participate in the reaction practically. The developed technique is a valuable in situ tool for the investigation of the electronic states of PEFCs and other interesting functional material systems and devices under realistic working conditions.

SPring-8 BL36XU: Synchrotron Radiation X-Ray-Based Multi-Analytical Beamline for Polymer Electrolyte Fuel Cells under Operating Conditions.

We designed and constructed a beamline BL36XU at the 8 GeV synchrotron radiation facility SPring-8 to provide information required for the development of next-generation polymer electrolyte fuel cells (PEFCs) by clarifying the dynamic aspects of structures and electronic states of cathode catalysts under PEFC operating conditions and in the deterioration processes by accelerated durability test protcols. To investigate the mechanism and degradation process for the cathode electrocatalysis in practical PEFCs, we developed advanced time- and spatially-resolved in-situ/operando X-ray absorption fine structure measurement systems and complementary analytical systems (X-ray emission spectroscopy (XES), X-ray diffraction (XRD), X-ray computer tomography (CT) and hard X-ray photoelectron spectroscopy (HAXPES)) and combined them to develop multi-analytical systems at BL36XU. Multi-analytical systems are very powerful for observing spatial-temporal features of the transient processes occurring in complex systems such as PEFCs. This account describes the design, performance, and research results of the BL36XU and multi-analytical in-situ/operando systems.

Operando XAFS Imaging of Distribution of Pt Cathode Catalysts in PEFC MEA.

Three-dimensional imaging using X-ray as a probe is state-of-the-art for the characterization of heterogeneous materials. In addition to simple imaging of sample morphology, imaging of elemental distribution and chemical states provides advanced maps of key structural parameters of functional materials. The combination of X-ray absorption fine structure (XAFS) spectroscopy and three-dimensional imaging such as computed tomography (CT) can visualize the three-dimensional distribution of target elements, their valence states, and local structures in a non-destructive manner. In this personal account, our recent results on the three-dimensional XAFS imaging for Pt cathode catalysts in the membrane electrode assembly (MEA) of polymer electrolyte fuel cell (PEFC) are introduced. The distribution and chemical states of Pt cathode catalysts in MEAs remarkably change under PEFC operating conditions, and the 3D XAFS imaging revealed essential events in PEFC MEAs.

Development of Surface Fluorescence X-Ray Absorption Fine Structure Spectroscopy Using a Laue-Type Monochromator.

Surface fluorescence X-ray absorption fine structure (XAFS) spectroscopy using a Laue-type monochromator has been developed to acquire structural information about metals with a very low concentrate on a flat highly oriented pyrolytic graphite (HOPG) surface in the presence of electrolytes. Generally, surface fluorescence XAFS spectroscopy is hindered by strong scattering from the bulk, which often chokes the pulse counting detector. In this work, we show that a bent crystal Laue analyzer (BCLA) can efficiently remove the scattered X-rays from the bulk even in the presence of solution. We applied the technique to submonolayer (∼1014  atoms cm-2 ) Pt on HOPG and successfully obtained high signal/noise in situ XAFS data in combination with back-illuminated fluorescence XAFS (BI-FXAFS) spectroscopy. This technique allows in situ XAFS measurements of flat electrode surfaces to be performed in the presence of electrolytes.

Dynamic Behavior of Rh Species in Rh/Al2O3 Model Catalyst during Three-Way Catalytic Reaction: An Operando X-ray Absorption Spectroscopy Study.

The dynamic behavior of Rh species in 1 wt% Rh/Al2O3 catalyst during the three-way catalytic reaction was examined using a micro gas chromatograph, a NOx meter, a quadrupole mass spectrometer, and time-resolved quick X-ray absorption spectroscopy (XAS) measurements at a public beamline for XAS, BL01B1 at SPring-8, operando. The combined data suggest different surface rearrangement behavior, random reduction processes, and autocatalytic oxidation processes of Rh species when the gas is switched from a reductive to an oxidative atmosphere and vice versa. This study demonstrates an implementation of a powerful operando XAS system for heterogeneous catalytic reactions and its importance for understanding the dynamic behavior of active metal species of catalysts.

Slow Magnetic Relaxation in a Palladium-Gadolinium Complex Induced by Electron Density Donation from the Palladium Ion.

Incorporating palladium in the first coordination sphere of acetato-bridged lanthanoid complexes, [Pd2 Ln2 (H2 O)2 (AcO)10 ]⋅2 AcOH (Ln=Gd (1), Y (2), Gd0.4 Y1.6 (3), Eu (4)), led to significant bonding interactions between the palladium and the lanthanoid ions, which were demonstrated by experimental and theoretical methods. We found that electron density was donated from the d8 Pd2+ ion to Gd3+ ion in 1 and 3, leading to the observed slow magnetic relaxation by using local orbital locator (LOL) and X-ray absorption near-edge structure (XANES) analysis. Field-induced dual slow magnetic relaxation was observed for 1 up to 20 K. Complex 3 and frozen aqueous and acetonitrile solutions of 1 showed only one relaxation peak, which confirms the role of intermolecular dipolar interactions in slowing the magnetic relaxation of 1. The slow magnetic relaxation occurred through a combination of Orbach and Direct processes with the highest pre-exponential factor (τo =0.06 s) reported so far for a gadolinium complex exhibiting slow magnetic relaxation. The results revealed that transition metal-lanthanoid (TM-Ln) axial interactions indeed could lead to new physical properties by affecting both the electronic and magnetic states of the compounds.

Disappearance of the Superionic Phase Transition in Sub-5 nm Silver Iodide Nanoparticles.

Bulk silver iodide (AgI) is known to show a phase transition from the poorly conducting ß/γ-phases into the superionic conducting α-phase at 147 °C. Its transition temperature decreases with decreasing the size of AgI, and the α-phase exists stably at 37 °C in AgI nanoparticles with a diameter of 6.3 nm. In this Letter, we investigated the atomic configuration, the phase transition behavior, and the ionic conductivity of AgI nanoparticles with a diameter of 3.0 nm. The combination of pair distribution function (PDF) analysis and reverse Monte Carlo (RMC) modeling based on high-energy X-ray diffraction (XRD) revealed for the first time that they formed the ß/γ-phases with atomic disorder. The results of extended X-ray absorption fine structure (EXAFS) analysis, differential scanning calorimetry (DSC), and AC impedance spectroscopy demonstrated that they did not exhibit the superionic phase transition and their ionic conductivity was lower than that of crystalline AgI. The disappearance of the superionic phase transition and low ionic conductivity in the very small AgI nanoparticles originates from their small size and disordered structure.

In situ study of oxidation states of platinum nanoparticles on a polymer electrolyte fuel cell electrode by near ambient pressure hard X-ray photoelectron spectroscopy.

We performed in situ hard X-ray photoelectron spectroscopy (HAXPES) measurements of the electronic states of platinum nanoparticles on the cathode electrocatalyst of a polymer electrolyte fuel cell (PEFC) using a near ambient pressure (NAP) HAXPES instrument having an 8 keV excitation source. We successfully observed in situ NAP-HAXPES spectra of the Pt/C cathode catalysts of PEFCs under working conditions involving water, not only for the Pt 3d states with large photoionization cross-sections in the hard X-ray regime but also for the Pt 4f states and the valence band with small photoionization cross-sections. Thus, this setup allowed in situ observation of a variety of hard PEFC systems under operating conditions. The Pt 4f spectra of the Pt/C electrocatalysts in PEFCs clearly showed peaks originating from oxidized Pt(ii) at 1.4 V, which unambiguously shows that Pt(iv) species do not exist on the Pt nanoparticles even at such large positive voltages. The water oxidation reaction might take place at that potential (the standard potential of 1.23 V versus a standard hydrogen electrode) but such a reaction should not lead to a buildup of detectable Pt(iv) species. The voltage-dependent NAP-HAXPES Pt 3d spectra revealed different behaviors with increasing voltage (0.6 → 1.0 V) compared with decreasing voltage (1.0 → 0.6 V), showing a clear hysteresis. Moreover, quantitative peak-fitting analysis showed that the fraction of non-metallic Pt species matched the ratio of the surface to total Pt atoms in the nanoparticles, which suggests that Pt oxidation only takes place at the surface of the Pt nanoparticles on the PEFC cathode, and the inner Pt atoms do not participate in the reaction. In the valence band spectra, the density of electronic states near the Fermi edge reduces with decreasing particle size, indicating an increase in the electrocatalytic activity. Additionally, a change in the valence band structure due to the oxidation of platinum atoms was also observed at large positive voltages. The developed apparatus is a valuable in situ tool for the investigation of the electronic states of PEFC electrocatalysts under working conditions.