► Overview of some recent developments in hard X-ray RXES/RIXS. ► Evaluation of spectral line broadening in RXES/RIXS. ► Modelling of RXES/RIXS by ground state DFT calculations. ► Discussion on when ...HERFD provides a good approximation to XAS.
An increasing community of researchers in various fields of natural sciences is combining X-ray absorption with X-ray emission spectroscopy (XAS–XES) to study electronic structure. With the applications becoming more diverse, the objectives and the requirements in photon-in/photon-out spectroscopy are becoming broader. It is desirable to find simple experimental protocols, robust data reduction and theoretical tools that help the experimentalist to understand their data and learn about the electronic structure. This article presents a collection of considerations on non-resonant and resonant XES with the aim to guide the experimentalist to make good use of this technique.
X-ray absorption spectroscopy (XAS) is a widely used characterization technique to explore the local geometric and electronic structures of materials with element specificity. XAS measurements are ...performed at synchrotron radiation sources that provide brilliant, tunable, and monochromatic energy photons. The advantages of XAS include good elemental, chemical, and orbital sensitivities, which all stem from inherent electron excitation and transition processes. XAS is categorized into soft (<2000 eV) and hard (>5000 eV) x-ray regimes, based on the incident photon energy. Soft x-rays can probe the K-edges of low-Z (atomic number) elements, including Li, C, N, O, and F, and the L-edges of 3d transition metals, whose K-edge is within the hard x-ray regime. All of these elements are essential components of energy materials. This article introduces the principle of XAS and reviews some recent applications in energy storage and energy conversion, illustrating the capabilities of XAS to investigate the fundamental properties of materials from the points of view of atomic and electronic structures, which play crucial roles in understanding the reaction mechanisms in high-performance devices.
XANES and Raman reveal new information on the properties of supported Pt
Sn nanoparticles during propane dehydrogenation–regeneration cycles leading to insight into the dynamics of Pt
Sn alloy ...formation at elevated temperatures.
The catalytic performances of Pt/Al
2O
3 and Pt–Sn/Al
2O
3 catalysts for the dehydrogenation of propane through consecutive reaction–regeneration cycles have been studied under realistic reaction conditions. A 10-fold successive dehydrogenation–regeneration cycling study, similar to that employed in an industrial propane dehydrogenation reactor, was performed in order to examine the catalyst activity and stability as well as the propene selectivity. Combined in situ UV–Vis/Raman spectroscopy measurements were taken in order to follow the coke formation processes during propane dehydrogenation. This approach allowed correlating propane conversion and propene formation with the on-line determined Raman D over G band intensity ratio and amount of coke formed. These in situ measurements on coke formation and related catalyst deactivation were supplemented by in situ high-energy resolution fluorescence detected (HERFD) XANES measurements in order to characterize the structural and electronic properties of the supported Pt and Pt
Sn nanoparticles during the successive dehydrogenation–regeneration cycles. This combination of powerful spectroscopic techniques revealed unique information regarding the activity behavior and deactivation mechanism of Pt- and Pt
Sn-based propane dehydrogenation catalysts, enabling us to identify important structure-electronic-performance relationships as well as fundamental insight into the dynamics of Pt
Sn alloy formation processes in Pt nanoparticles at elevated temperatures.
Electrochemical water splitting constitutes one of the most promising strategies for converting water into hydrogen-based fuels, and this technology is predicted to play a key role in the transition ...towards a carbon-neutral energy economy. To enable the design of cost-effective electrolysis cells based on this technology, new and more efficient anodes with augmented water splitting activity and stability will be required. Herein, we report an active molecular Ru-based catalyst for electrochemically-driven water oxidation (overpotential of ∼395 mV at pH 7 phosphate buffer) and two simple methods for preparing anodes by attaching this catalyst onto glassy carbon through multi-walled carbon nanotubes to improve stability as well as reactivity. The anodes modified with the molecular catalyst were characterized by a broad toolbox of microscopy and spectroscopy techniques, and interestingly no RuO
2
formation was detected during electrocatalysis over 4 h. These results demonstrate that the herein presented strategy can be used to prepare anodes that rival the performance of
state-of-the-art
metal oxide anodes.
Immobilizing ruthenium containing molecular electrocatalyst onto glassy carbon surface through bipyridine linkers and MWCNTs for efficient water oxidation.
A multi-crystal wavelength dispersive hard x-ray spectrometer with high-energy resolution and large solid angle collection is described. The instrument is specifically designed for time-resolved ...applications of x-ray emission spectroscopy (XES) and x-ray Raman scattering (XRS) at X-ray Free Electron Lasers (XFEL) and synchrotron radiation facilities. It also simplifies resonant inelastic x-ray scattering (RIXS) studies of the whole 2d RIXS plane. The spectrometer is based on the Von Hamos geometry. This dispersive setup enables an XES or XRS spectrum to be measured in a single-shot mode, overcoming the scanning needs of the Rowland circle spectrometers. In conjunction with the XFEL temporal profile and high-flux, it is a powerful tool for studying the dynamics of time-dependent systems. Photo-induced processes and fast catalytic reaction kinetics, ranging from femtoseconds to milliseconds, will be resolvable in a wide array of systems circumventing radiation damage.
The bis-benzimidazole derivative (BBM) molecule, consisting of two 2-(2'-hydroxyphenyl) benzimidazole (HBI) halves, has been synthesized and successfully utilized as a ratiometric fluorescence sensor ...for the sensitive detection of Cu
based on enol-keto excited-state intramolecular proton transfer (ESIPT). In this study, we strategically implement femtosecond stimulated Raman spectroscopy and several time-resolved electronic spectroscopies, aided by quantum chemical calculations to investigate the detailed primary photodynamics of the BBM molecule. The results demonstrate that the ESIPT from BBM-enol* to BBM-keto* was observed in only one of the HBI halves with a time constant of 300 fs; after that, the rotation of the dihedral angle between the two HBI halves generated a planarized BBM-keto* isomer in 3 ps, leading to a dynamic redshift of BBM-keto* emission.
Developing light-harvesting and photocatalytic molecules made with iron could provide a cost effective, scalable, and environmentally benign path for solar energy conversion. To date these ...developments have been limited by the sub-picosecond metal-to-ligand charge transfer (MLCT) electronic excited state lifetime of iron based complexes due to spin crossover - the extremely fast intersystem crossing and internal conversion to high spin metal-centered excited states. We revitalize a 30 year old synthetic strategy for extending the MLCT excited state lifetimes of iron complexes by making mixed ligand iron complexes with four cyanide (CN
) ligands and one 2,2'-bipyridine (bpy) ligand. This enables MLCT excited state and metal-centered excited state energies to be manipulated with partial independence and provides a path to suppressing spin crossover. We have combined X-ray Free-Electron Laser (XFEL) Kβ hard X-ray fluorescence spectroscopy with femtosecond time-resolved UV-visible absorption spectroscopy to characterize the electronic excited state dynamics initiated by MLCT excitation of Fe(CN)
(bpy)
. The two experimental techniques are highly complementary; the time-resolved UV-visible measurement probes allowed electronic transitions between valence states making it sensitive to ligand-centered electronic states such as MLCT states, whereas the Kβ fluorescence spectroscopy provides a sensitive measure of changes in the Fe spin state characteristic of metal-centered excited states. We conclude that the MLCT excited state of Fe(CN)
(bpy)
decays with roughly a 20 ps lifetime without undergoing spin crossover, exceeding the MLCT excited state lifetime of Fe(2,2'-bipyridine)
by more than two orders of magnitude.
Until recently, sulfur was known as a "spectroscopically silent" element because of a paucity of convenient spectroscopic probes suitable for
in situ
chemical speciation. In recent years the ...technique of sulfur K-edge X-ray absorption spectroscopy (XAS) has been used extensively in sulfur speciation in a variety of different fields. With an initial focus on reduced forms of organic sulfur, we have explored a complementary X-ray based spectroscopy - sulfur Kβ X-ray emission spectroscopy (XES) - as a potential analytical tool for sulfur speciation in complex samples. We compare and contrast the sensitivity of sulfur Kβ XES with that of sulfur K-edge XAS, and find differing sensitivities for the two techniques. In some cases an approach involving both sulfur K-edge XAS and sulfur Kβ XES may be a powerful combination for deducing sulfur speciation in samples containing complex mixtures.
Sulfur Kβ non-resonant X-ray emission spectroscopy complements sulfur K-edge X-ray absorption spectroscopy in providing information on chemical speciation and electronic structure.
Meteor impacts can induce unique pressure‐dependent structural changes in minerals due to the propagation of shock waves. Plagioclase—ubiquitous throughout the Earth’s crust, extraterrestrial bodies, ...and meteorites—is commonly used for reconstructing the impact history and conditions of the parent bodies. However, there have been unresolved inconsistencies in the interpretation of shock transformations across previous studies: The pressure at which amorphization begins and the process by which it occurs is the subject of ongoing debate. Here, we utilize time‐resolved in situ X‐ray diffraction (XRD) to probe the phase transformation pathway of plagioclase during shock compression at a sub‐nanosecond timescale. Direct amorphization begins at pressures much lower than what was previously assumed, just above the Hugoniot elastic limit of 5 GPa, with full amorphization to a high‐density amorphous phase, observed at 32(10) GPa and 20 ns. Upon release, the material partially recrystallizes back into the original structure, demonstrating a memory effect.
Carbon Core Electron Spectra of Polycyclic Aromatic Hydrocarbons Pomerantz, Andrew E; Crace, Ethan; Weng, Tsu-Chien ...
The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory,
07/2018, Letnik:
122, Številka:
26
Journal Article
Recenzirano
Odprti dostop
Aromaticity profoundly affects molecular orbitals in polycyclic aromatic hydrocarbons. X-ray core electron spectroscopy has observed that carbon 1s−π* transitions can be broadened or even split in ...some polycyclic systems, although the origin of the effect has remained obscure. The π electrons in polycyclic systems are typically classified in the Clar model as belonging to either true aromatic sextets (similar to benzene) or isolated double bonds (similar to olefins). Here, bulk-sensitive carbon core excitation spectra are presented for a series of polycyclic systems and show that the magnitude of the 1s−π* splitting is determined primarily by the ratio of true aromatic sextets to isolated double bonds. The observed splitting can be rationalized in terms of ground state energetics as described by Hückel, driven by the π electron structure described by Clar. This simple model including only ground state energetics is shown to explain the basics physics behind the spectral evolution for a broad set of polycyclic aromatic hydrocarbons, although some residual deviations between this model and experiment can likely be improved by including a more detailed electronic structure and the core hole effect.
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