New Insights into the Nature of Co‐components and Their Impact on Pd Structure: X‐ray Absorption Studies on Toluene Acetoxylation Catalysts

Co‐components are a powerful tool to tune the performance of catalysts, but their nature and their impact on the catalysts is often controversially discussed. In this study X‐ray absorption spectroscopy (XAS) was employed to elucidate the nature of co‐components and their impact on the catalytic reaction. In anatase‐supported Pd‐based catalysts for the gas‐phase acetoxylation of toluene, less noble co‐components (e.g., Mn, Co, and Sb) spread over the support in their oxidic form and changed their valence state on stream. Incorporated atoms such as C or a small part of the Sb affect the electronic structure of Pd. For the noble Au, only a weak interaction with the support and Pd was observed during time on stream. Only XAS at the K‐edges together with investigations of the Pd L‐edge for a better understanding of the electronic structure, supplemented by STEM for elemental mapping, allow such detailed insights.     

Microsecond X‐ray Absorption Spectroscopy Identification of CoI Intermediates in Cobaloxime‐Catalyzed Hydrogen Evolution

Rational development of efficient photocatalytic systems for hydrogen production requires understanding the catalytic mechanism and detailed information about the structure of intermediates in the catalytic cycle. We demonstrate how time‐resolved X‐ray absorption spectroscopy in the microsecond time range can be used to identify such intermediates and to determine their local geometric structure. This method was used to obtain the solution structure of the Co^I intermediate of cobaloxime, which is a non‐noble metal catalyst for solar hydrogen production from water. Distances between cobalt and the nearest ligands including two solvent molecules and displacement of the cobalt atom out of plane formed by the planar ligands have been determined. Combining in situ X‐ray absorption and UV/Vis data, we demonstrate how slight modification of the catalyst structure can lead to the formation of a catalytically inactive Co^I state under similar conditions. Possible deactivation mechanisms are discussed.     

Evidence of CuI/CuII Redox Process by X‐ray Absorption and EPR Spectroscopy: Direct Synthesis of Dihydrofurans from β‐Ketocarbonyl Derivatives and Olefins

The Cu^I/Cu^II and Cu^I/Cu^III catalytic cycles have been subject to intense debate in the field of copper‐catalyzed oxidative coupling reactions. A mechanistic study on the Cu^I/Cu^II redox process, by X‐ray absorption (XAS) and electron paramagnetic resonance (EPR) spectroscopies, has elucidated the reduction mechanism of Cu^II to Cu^I by 1,3‐diketone and detailed investigation revealed that the halide ion is important for the reduction process. The oxidative nature of the thereby‐formed Cu^I has also been studied by XAS and EPR spectroscopy. This mechanistic information is applicable to the copper‐catalyzed oxidative cyclization of β‐ketocarbonyl derivatives to dihydrofurans. This protocol provides an ideal route to highly substituted dihydrofuran rings from easily available 1,3‐dicarbonyls and olefins.     

Ultrathin Hexagonal Hybrid Nanosheets Synthesized by Graphene Oxide‐Assisted Exfoliation of β‐Co(OH)2 Mesocrystals

In the present study, we report the synthesis of a high‐quality, single‐crystal hexagonal β‐Co(OH)₂ nanosheet, exhibiting a thickness down to ten atomic layers and an aspect ratio exceeding 900, by using graphene oxide (GO) as an exfoliant of β‐Co(OH)₂ nanoflowers. Unlike conventional approaches using ionic precursors in which morphological control is realized by structure‐directing molecules, the β‐Co(OH)₂ flower‐like superstructures were first grown by a nanoparticle‐mediated crystallization process, which results in large 3D superstructure consisting of ultrathin nanosheets interspaced by polydimethoxyaniline (PDMA). Thereafter, β‐Co(OH)₂ nanoflowers were chemically exfoliated by surface‐active GO under hydrothermal conditions into unilamellar single‐crystal nanosheets. In this reaction, GO acts as a two‐dimensional (2D) amphiphile to facilitate the exfoliation process through tailored interactions between organic and inorganic molecules. Meanwhile, the on‐site conjugation of GO and Co(OH)₂ promotes the thermodynamic stability of freestanding ultrathin nanosheets and restrains further growth through Oswald ripening. The unique 2D structure combined with functionalities of the hybrid ultrathin Co(OH)₂ nanosheets on rGO resulted in a remarkably enhanced lithium‐ion storage performance as anode materials, maintaining a reversible capacity of 860 mA h g^?1 for as many as 30 cycles. Since mesocrystals are ubiquitous and rich in morphological diversity, the strategy of the GO‐assisted exfoliation of mesocrystals developed here provides an opportunity for the synthesis of new functional nanostructures that could bear importance in clean renewable energy, catalysis, photoelectronics, and photonics.     

X‐ray Absorption Near‐Edge Structure and Nuclear Magnetic Resonance Study of the Lithium–Sulfur Battery and its Components

Understanding the mechanism(s) of polysulfide formation and knowledge about the interactions of sulfur and polysulfides with a host matrix and electrolyte are essential for the development of long‐cycle‐life lithium–sulfur (Li–S) batteries. To achieve this goal, new analytical tools need to be developed. Herein, sulfur K‐edge X‐ray absorption near‐edge structure (XANES) and ^6,7Li magic‐angle spinning (MAS) NMR studies on a Li–S battery and its sulfur components are reported. The characterization of different stoichiometric mixtures of sulfur and lithium compounds (polysulfides), synthesized through a chemical route with all‐sulfur‐based components in the Li–S battery (sulfur and electrolyte), enables the understanding of changes in the batteries measured in postmortem mode and in operando mode. A detailed XANES analysis is performed on different battery components (cathode composite and separator). The relative amounts of each sulfur compound in the cathode and separator are determined precisely, according to the linear combination fit of the XANES spectra, by using reference compounds. Complementary information about the lithium species within the cathode are obtained by using ⁷Li MAS NMR spectroscopy. The setup for the in operando XANES measurements can be viewed as a valuable analytical tool that can aid the understanding of the sulfur environment in Li–S batteries.     

Wet‐Chemical Synthesis of Nanoscale Iron Boride,XAFS Analysis and Crystallisation to α‐FeB

The reaction of lithium tetrahydridoborate and iron bromide in high boiling ether as reaction medium produces an ultrafine, pyrophoric and magnetic precipitate. X‐ray and electron diffraction proved the product to be amorphous. According to X‐ray absorption fine structure spectroscopy (XAFS) the precipitate has FeB structure up to nearly two coordination spheres around an iron absorber atom. Transmission electron microscopy (TEM) confirms the ultrafine powder to be nanoscale. Subsequent annealing at 450 °C causes the atoms to arrange in a more distinct FeB structure, and further thermal treatment to 1050 °C extends the local structure to the α‐modification of FeB. Between 1050 °C and 1500 °C α‐FeB is transformed into β‐FeB.     

Environmentally Friendly γ‐MnO2 Hexagon‐Based Nanoarchitectures: Structural Understanding and Their Energy‐Saving Applications

Although about 200,000 metric tons of γ‐MnO₂ are used annually worldwide for industrial applications, the γ‐MnO₂ structure is still known to possess a highly ambiguous crystal lattice. To better understand the γ‐MnO₂ atomic structure, hexagon‐based nanoarchitectures were successfully synthesized and used to elucidate its internal structure for the present work. The structural analysis results, obtained from the hexagon‐based nanoarchitectures, clearly show the coexistence of akhtenskite (ε‐MnO₂), pyrolusite (β‐MnO₂), and ramsdellite in the so‐called γ‐MnO₂ phase and verified the heterogeneous phase assembly of the γ‐MnO₂ state, which violates the well‐known “De Wolff” model and derivative models, but partially accords with Heuer's results. Furthermore, heterogeneous γ‐MnO₂ assembly was found to be a metastable structure under hydrothermal conditions, and the individual components of the heterogeneous γ‐MnO₂ system have structural similarities and a high lattice matches with pyrolusite (β‐MnO₂). The as‐obtained γ‐MnO₂ nanoarchitectures are nontoxic and environmentally friendly, and the application of such nanoarchitectures as support matrices successfully mitigates the common problems for phase‐change materials of inorganic salts, such as phase separation and supercooling‐effects, thereby showing prospect in energy‐saving applications in future “smart‐house” systems.     

Hollow and Yolk‐Shell Iron Oxide Nanostructures on Few‐Layer Graphene in Li‐Ion Batteries

We report a simple and template‐free strategy for the synthesis of hollow and yolk‐shell iron oxide (FeO_x) nanostructures sandwiched between few‐layer graphene (FLG) sheets. The morphology and microstructure of this material are characterized in detail by X‐ray diffraction, X‐ray absorption near‐edge structure, X‐ray photoelectron spectroscopy, Raman spectroscopy, scanning and transmission electron microscopy. Its properties are evaluated as negative electrode material for Li‐ion batteries and compared with those of solid FeO_x/FLG and two commercial iron oxides. In all cases, the content of carbon in the electrode has a great influence on the performance. The use of pristine FLG improves the capacity retention and further enhancement is achieved with the hollow structure. For a low carbon loading of 18 wt. %, the presence of metallic iron in the hollow and yolk‐shell FeO_x/FLG composite significantly enhances the capacity retention, albeit with a relatively lower initial reversible capacity, retaining above 97 % after 120 cycles at 1000 mA g^?1 in the voltage range of 0.1–3.0 V.     

High‐Temperature Elastic Moduli of Flux‐Grown α‐GeO2 Single Crystal

From high‐precision Brillouin spectroscopy measurements, six elastic constants (C₁₁, C₃₃, C₄₄, C₆₆, C₁₂, and C₁₄) of a flux‐grown GeO₂ single crystal with the α‐quartz‐like structure are obtained in the 298–1273 K temperature range. High‐temperature powder X‐ray diffraction data is collected to determine the temperature dependence of the lattice parameters and the volume thermal expansion coefficients. The temperature dependence of the mass density, ρ, is evaluated and used to estimate the thermal dependence of its refractive indices (ordinary and extraordinary), according to the Lorentz–Lorenz equation. The extraction of the ambient piezoelectric stress contribution, e₁₁, from the C′₁₁–C₁₁ difference gives, for the piezoelectric strain coefficient d₁₁, a value of 5.7(2) pC N^?1, which is more than twice that of α‐quartz. As the quartz structure of α‐GeO₂ remains stable until melting, piezoelectric activity is observed until 1273 K.     

The Role of Pd2+/Pd0 in Hydrogenation by [Pd(2‐pymo)2]n: An X‐ray Absorption and IR Spectroscopic Study

The metal–organic framework (MOF) [Pd(2‐pymo)₂]_n (2‐pymo=2‐pyrimidinolate) was used as catalyst in the hydrogenation of 1‐octene. During catalytic hydrogenation, the changes at the metal nodes and linkers of the MOF were investigated by in situ X‐ray absorption spectroscopy (XAS) and IR spectroscopy. With the help of extended X‐ray absorption fine structure and X‐ray absorption near edge structure data, Quick‐XAS, and IR spectroscopy, detailed insights into the catalytic relevance of Pd²⁺/Pd⁰ in the hydrogenation of 1‐octene could be achieved. Shortly after exposure of the catalyst to H₂ and simultaneously with the hydrogenation of 1‐octene, the aromatic rings of the linker molecules are hydrogenated rapidly. Up to this point, the MOF structure remained intact. After completion of linker hydrogenation, the linkers were also protonated. When half of the linker molecules were protonated, the onset of reduction of the Pd²⁺ centers to Pd⁰ was observed and the hydrogenation activity decreased, followed by fast reduction of the palladium centers and collapse of the MOF structure. Major fractions of Pd⁰ are only observed when the hydrogenation of 1‐octene is almost finished. Consequently, the Pd²⁺ nodes of the MOF [Pd(2‐pymo)₂]_n are identified as active centers in the hydrogenation of 1‐octene.     

X‐ray Absorption Spectroscopy as a Probe of Photo‐ and Thermally Induced Valence Tautomeric Transition in a 1:1 Cobalt–Dioxolene Complex

The role of the cobalt ion in the entropy‐ and optically‐driven valence tautomeric (VT) interconversion exhibited by the [Co(Me₂tpa)(DTBdiox)](PF₆)?C₆H₅CH₃ complex (Me₂tpa=bis (6‐methyl‐(2‐pyridylmethyl))(2‐pyridylmethyl)amine, DBdiox=3,5‐ditertbutyl‐dioxolene) is established by means of X‐ray absorption spectroscopy (XAS). Analysis of the pre‐edge features at 6 and 300 K in the Co K‐edge XAS spectra using a ligand field multiplet approach allows us to obtain detailed information on the electronic structures of the metal ion in the two redox isomers. The temperature dependence of the spectra confirms the occurrence of a thermally induced VT transition and suggests that nucleation and distortion of the phase boundaries take place during the process. Moreover, optically induced metastable state formation is monitored at low temperatures—with a high degree of reproducibility—without changing the position of measurement on the sample during the experiment. This result paves the way for the use of such a highly sensitive technique for the investigation of photoswitchable materials in non‐crystalline and nanostructured environments.     

Online wide‐angle X‐ray diffraction/small‐angle X‐ray scattering measurements for the CO2‐laser‐heated drawing of poly(ethylene terephthalate) fiber

Fiber‐structure‐development in the poly(ethylene terephthalate) fiber drawing process was investigated with online measurements of wide‐angle and small‐angle X‐ray scattering with both a high‐luminance X‐ray source and a CO₂‐laser‐heated drawing system. The intensity profile of the transmitted X‐ray confirmed the location of the neck‐drawing point. The diffraction images had a time resolution of several milliseconds, and this still left much room for improvement. Crystal diffraction appeared in the wide‐angle X‐ray images almost instantaneously about 20 ms after necking, whereas a four‐point small‐angle X‐ray scattering pattern appeared immediately after necking. With the elapse of time after necking, the four‐point scattering pattern changed into a meridional two‐point shape. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 1090–1099, 2005     

The Hydrogen Bond of Water from the Perspective of Soft X‐Ray Spectroscopy

Its importance for life and its unusual properties keep water within the focus of ongoing research; this focus especially applies to water in the liquid phase. Scientists agree that the hydrogen‐bond network, which is formed by interactions between the water molecules, is key for understanding the anomalies of water. However, a better understanding of the structure of this network, as well as its dynamics, must yet be established. Soft X‐ray spectroscopy allows the investigation of the local electronic structure of water by probing the occupied and unoccupied valence molecular orbitals. In this Focus Review, we present soft‐X‐ray‐based techniques, their development in terms of liquid spectroscopy, and recent studies on the hydrogen‐bond network of liquid water.     

In Situ Observation of the Thermally Induced Growth of Platinum‐Nanoparticle Catalysts Using High‐Temperature X‐ray Diffraction

Fundamental understanding about the thermal stability of nanoparticles and deliberate control of structural and morphological changes under reactive conditions is of general importance for a wide range of reaction processes in heterogeneous and electrochemical catalysis. Herein, we present a parametric study of the thermal stability of carbon‐supported Pt nanoparticles at 80 °C and 160 °C, with an initial particle size below 3 nm, using in situ high‐temperature X‐ray diffraction (HT‐XRD). The effects on the thermal stability of carbon‐supported Pt nanoparticles are investigated with control parameters such as Brunauer–Emmet–Teller (BET) surface area, metal loading, temperature, and gas environment. We demonstrate that the growth rate exhibits a complex, nonlinear behavior and is largely controlled by the temperature, the initial particle size, and the interparticle distance. In addition, an ex situ transmission electron microscopy study was performed to verify our results obtained from the in situ HT‐XRD study.