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.     

Nitrous‐Acid‐Mediated Synthesis of Iron–Nitrosyl–Porphyrin: pH‐Dependent Release of Nitric Oxide

Two iron–nitrosyl–porphyrins, nitrosyl[meso‐tetrakis(3,4,5‐trimethoxyphenylporphyrin]iron(II) acetic acid solvate ( 3 ) and nitrosyl[meso‐tetrakis(4‐methoxyphenylporphyrin]iron(II) CH₂Cl₂ solvate ( 4 ), were synthesized in quantitative yield by using a modified procedure with nitrous acid, followed by oxygen‐atom abstraction by triphenylphosphine under an argon atmosphere. These nitrosyl porphyrins are in the {FeNO}⁷ class. Under an argon atmosphere, these compounds are relatively stable over a broad range of pH values (4–8) but, under aerobic conditions, they release nitric oxide faster at high pH values than that at low pH values. The generated nitric‐oxide‐free iron(III)–porphyrin can be re‐nitrosylated by using nitrous acid and triphenylphosphine. The rapid release of NO from these Fe^II complexes at high pH values seems to be similar to that in nitrophorin, a nitric‐oxide‐transport protein, which formally possesses Fe^III. However, because the release of NO occurs from ferrous–nitrosyl–porphyrin under aerobic conditions, these compounds are more closely related to nitrobindin, a recently discovered heme protein.     

Mapping of the Photoinduced Electron Traps in TiO2 by Picosecond X‐ray Absorption Spectroscopy

Titanium dioxide (TiO₂) is the most popular material for applications in solar‐energy conversion and photocatalysis, both of which rely on the creation, transport, and trapping of charges (holes and electrons). The nature and lifetime of electron traps at room temperature have so far not been elucidated. Herein, we use picosecond X‐ray absorption spectroscopy at the Ti K‐edge and the Ru L₃‐edge to address this issue for photoexcited bare and N719‐dye‐sensitized anatase and amorphous TiO₂ nanoparticles. Our results show that 100 ps after photoexcitation, the electrons are trapped deep in the defect‐rich surface shell in the case of anatase TiO₂, whereas they are inside the bulk in the case of amorphous TiO₂. In the case of dye‐sensitized anatase or amorphous TiO₂, the electrons are trapped at the outer surface. Only two traps were identified in all cases, with lifetimes in the range of nanoseconds to tens of nanoseconds.     

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.     

X‐Ray Near‐Edge Absorption Study of Temperature‐Induced Low‐Spin‐to‐High‐Spin Change in Metallo‐Supramolecular Assemblies

X‐ray absorption near the iron K edge (XANES) was used to investigate the characteristics of temperature‐induced low‐spin‐to‐high‐spin change (SC) in metallo‐supramolecular polyelectrolyte amphiphile complexes (PAC) containing FeN₆ octahedra attached to two or six amphiphilic molecules. Compared to the typical spin‐crossover material Fe(phen)₂(NCS)₂ XANES spectra of PAC show fingerprint features restricted to the near‐edge region which mainly measures multiple scattering (MS) events. The changes of the XANES profiles during SC are thus attributed to the structure changes due to different MS path lengths. Our results can be interpreted by a uniaxial deformation of FeN₆ octahedra in PAC. This is in agreement with the prediction that SC is originated by a structural phase transition in the amphiphilic matrix of PAC, but in contrast to Fe(phen)₂(NCS)₂, showing the typical spin crossover being associated with shortening of all the metal–ligand distances.     

In Situ Study of the Surface Oxidation of FeCr Alloys Using Grazing Incidence X‐ray Absorption Spectroscopy (GIXAS)

The surface oxidation of FeCr alloys with 18, 28, and 43 mass‐% Cr was investigated in situ using grazing‐incidence X‐ray absorption spectroscopy (GIXAS) at the chromium and iron K‐edges. Oxidation in air was monitored in situ in the temperature range from 290 K to 680 K. The standard GIXAS data analysis is extended for the treatment of a single layer model in order to estimate the chromium concentrations of the oxide layer and of the near‐interface substrate as well as the oxide layer thickness. XANES analysis shows transitions from b.c.c. Fe to corundum type Fe₂O₃ and from b.c.c. Cr to corundum type Cr₂O₃. The initial oxide layers are 1.1‐1.4 nm thick and contain 60‐90 mass‐% chromium, while the near‐interface substrate is depleted in Cr. During heating, iron oxide growth dominates up to 560‐600 K. Then the chromium oxide layer loses its passivation effect and Cr oxidation sets in.     

Fe and Cr K‐edges EXAFS Study of Double Perovskite (Sr2‐xCax)FeMoO6 (0 ≤ x ≤ 2.0) and Sr2CrMO6 (M = Mo,W) Systems

The local structure of the double perovskite (Sr_2‐xCa_x)FeMoO₆ (0 ≤ × ≤ 2.0) and Sr₂CrMO₆ (M = Mo, W) systems have been probed by extended X‐ray absorption fine structure (EXAFS) spectroscopy at the Fe and Cr K‐edges. We found Fe‐O (ave) distance apparently decreases from 1.999 Å (x = 0) to 1.991 Å (x = 1.0) in (Sr_2‐xCa_x)FeMoO₆ (tetragonal structure). When x is increased further from 1.5 to 2.0, the Fe‐O bond distance decreased from 2.034 Å to 2.012 Å (monoclinic structure). In addition, Cr‐O, Sr‐Cr, and Cr‐Mo bond distances in Sr₂CrWO₆ are all slightly larger than the bond distances of Sr₂CrMoO₆, which is due to the ionic radius of the W⁵⁺ (0.62 Å) which is larger than the ionic radius of Mo⁵⁺ (0.61 Å). The results are consistent with our XRD refinements data.     

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.