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.     

Fe K‐Edge X‐ray Absorption Fine Structure Determination of γ‐Al2O3‐Supported Iron‐Oxide Species

The structure of FeO_x species supported on γ‐Al₂O₃ was investigated by using Fe K‐edge X‐ray absorption fine structure (XAFS) and X‐ray diffraction (XRD) measurements. The samples were prepared through the impregnation of iron nitrate on Al₂O₃ and co‐gelation of aluminum and iron sulfates. The dependence of the XRD patterns on Fe loading revealed the formation of α‐Fe₂O₃ particles at an Fe loading of above 10 wt %, whereas the formation of iron‐oxide crystals was not observed at Fe loadings of less than 9.0 wt %. The Fe K‐edge XAFS was characterized by a clear pre‐edge peak, which indicated that the Fe?O coordination structure deviates from central symmetry and that the degree of Fe?O?Fe bond formation is significantly lower than that in bulk samples at low Fe loading (<9.0 wt %). Fe K‐edge extended XAFS oscillations of the samples with low Fe loadings were explained by assuming an isolated iron‐oxide monomer on the γ‐Al₂O₃ surface.     

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.     

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.     

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.     

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.     

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.     

Maximum Spin Polarization in Chromium Dimer Cations as Demonstrated by X‐ray Magnetic Circular Dichroism Spectroscopy

X‐ray magnetic circular dichroism spectroscopy has been used to characterize the electronic structure and magnetic moment of Cr₂⁺. Our results indicate that the removal of a single electron from the 4sσ_g bonding orbital of Cr₂ drastically changes the preferred coupling of the 3d electronic spins. While the neutral molecule has a zero‐spin ground state with a very short bond length, the molecular cation exhibits a ferromagnetically coupled ground state with the highest possible spin of S=11/2, and almost twice the bond length of the neutral molecule. This spin configuration can be interpreted as a result of indirect exchange coupling between the 3d electrons of the two atoms that is mediated by the single 4s electron through a strong intraatomic 3d‐4s exchange interaction. Our finding allows an estimate of the relative energies of two states that are often discussed as ground‐state candidates, the ferromagnetically coupled ¹²Σ and the low‐spin ²Σ state.     

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.     

Direct In Situ Investigation of Milling Reactions Using Combined X‐ray Diffraction and Raman Spectroscopy

The combination of two analytical methods including time‐resolved in situ X‐ray diffraction (XRD) and Raman spectroscopy provides a new opportunity for a detailed analysis of the key mechanisms of milling reactions. To prove the general applicability of our setup, we investigated the mechanochemical synthesis of four archetypical model compounds, ranging from 3D frameworks through layered structures to organic molecular compounds. The reaction mechanism for each model compound could be elucidated. The results clearly show the unique advantage of the combination of XRD and Raman spectroscopy because of the different information content and dynamic range of both individual methods. The specific combination allows to study milling processes comprehensively on the level of the molecular and crystalline structures and thus obtaining reliable data for mechanistic studies.     

Direct Synthesis of an α,ω‐Diester from 2,7‐Octadienol as Bulk Feedstock in Three Tandem Catalytic Steps

A new tandem catalytic process was designed and developed as a tool for the direct conversion of the widely available feedstock 2,7‐octadienol into an α,ω‐diester. This innovative auto‐tandem catalysis is atom efficient and consists of three consecutive palladium‐catalysed reactions: ether formation, ether carbonylation and alkoxycarbonylation. By using the design of experiments (DoE) approach, significant parameters were determined and the yield of the desired α,ω‐diester was optimised. Model substrates allowed deeper insight into the progress of the reaction to be gained and, as a result, the reaction sequence was uncovered. Furthermore, by simply applying other ligands, a different reaction path was followed, allowing other, new tandem catalytic sequences to be explored and enabling new compounds to be obtained.     

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.     

Synthesis and conjugation of a triiodohydroxylamine for the preparation of highly X‐ray opaque poly(ε‐caprolactone) materials

To address a need for highly X‐ray opaque biodegradable materials, a series of poly(ε‐caprolactone) derivatives were prepared using oxime post‐polymerization modification to conjugate two different iodinated hydroxylamines. These materials were synthesized with up to 32 wt. % iodine content and demonstrated improved X‐ray contrast relative to both the ketone precursors and previously reported monoiodinated materials. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 787–793