N2O Decomposition on Singly and Doubly (K and Li)-Doped Co3O4 Nanocubes─Establishing Key Factors Governing Redox Behavior of Catalysts.

The intimate mechanism of N2O decomposition on bare and redox-tuned Co3O4 nanocubes (achieved by single (Li or K) and double (Li and K) doping) was elucidated. The catalysts synthesized by the hydrothermal method were characterized by X-ray electron absorption fine structure measurements, X-ray diffraction, Raman spectroscopy, scanning electron microscopy, transmission electron microscopy, and Kelvin Probe techniques. TPSR and steady-state isothermal catalytic tests reveal that the N2O turnover frequencies are critically sensitive to the work function of the catalysts, adjusted purposely by doping. For the catalysts obtained by one-pot hydrothermal synthesis, lithiation of the Co3O4 nanocubes leads to the formation of {Li'8a, Co·16d} species, decreasing steadily the work function and the activity, while for the catalysts prepared by postsynthesis impregnation, formation of {Li'8a, Co'16d, Co··16c} species leads to a volcano-type dependence of the catalytic activity and the work function in parallel. The beneficial effect of potassium was discussed in terms of mitigation of surface potential buildup due to the accumulation of ionosorbed oxygen intermediates (surface electrostatics), which hinders the interfacial electron transfer. Analysis of the catalytic activity response to the redox tuning of Co3O4, substantiated by DFT calculations, allowed for a straightforward conceptualization of the redox nature of the N2O decomposition in terms of the lineup of frontier orbitals of the N2O/N2O- and O2-/O2 reactants with the surface DOS structure and the resultant molecular orbital interactions. The positions of the virtual bonding 3πg0(N2O)-α-3dz2 and the occupied 2πg1(O2-)-α-3dz2 states relative to the Fermi energy level play a crucial role in the regulation of the forward and backward interfacial electron transfer events, which drive the redox process.

Correlation of refractive index based and THz streaking arrival time tools for a hard X-ray free-electron laser.

To fully exploit ultra-short X-ray pulse durations routinely available at X-ray free-electron lasers to follow out-of-equilibrium dynamics, inherent arrival time fluctuations of the X-ray pulse with an external perturbing laser pulse need to be measured. In this work, two methods of arrival time measurement were compared to measure the arrival time jitter of hard X-ray pulses. The methods were photoelectron streaking by a THz field and a transient refractive index change of a semiconductor. The methods were validated by shot-to-shot correction of a pump-probe transient reflectivity measurement. An ultimate shot-to-shot full width at half-maximum error between the devices of 19.2 ± 0.1 fs was measured.

Spin cascade and doming in ferric hemes: Femtosecond X-ray absorption and X-ray emission studies.

The structure-function relationship is at the heart of biology, and major protein deformations are correlated to specific functions. For ferrous heme proteins, doming is associated with the respiratory function in hemoglobin and myoglobins. Cytochrome c (Cyt c) has evolved to become an important electron-transfer protein in humans. In its ferrous form, it undergoes ligand release and doming upon photoexcitation, but its ferric form does not release the distal ligand, while the return to the ground state has been attributed to thermal relaxation. Here, by combining femtosecond Fe Kα and Kß X-ray emission spectroscopy (XES) with Fe K-edge X-ray absorption near-edge structure (XANES), we demonstrate that the photocycle of ferric Cyt c is entirely due to a cascade among excited spin states of the iron ion, causing the ferric heme to undergo doming, which we identify. We also argue that this pattern is common to a wide diversity of ferric heme proteins, raising the question of the biological relevance of doming in such proteins.

Femtosecond X-ray spectroscopy of haem proteins.

We discuss our recently reported femtosecond (fs) X-ray emission spectroscopy results on the ligand dissociation and recombination in nitrosylmyoglobin (MbNO) in the context of previous studies on ferrous haem proteins. We also present a preliminary account of femtosecond X-ray absorption studies on MbNO, pointing to the presence of more than one species formed upon photolysis.

Reduction Mechanisms of Anticancer Osmium(VI) Complexes Revealed by Atomic Telemetry and Theoretical Calculations.

Resonant X-ray emission spectroscopy (RXES) has developed in the past decade as a powerful tool to probe the chemical state of a metal center and in situ study chemical reactions. We have used it to monitor spectral changes associated with the reduction of osmium(VI) nitrido complexes to the osmium(III) ammine state by the biologically relevant reducing agent, glutathione. RXES difference maps are consistent with the proposed DFT mechanism and the formation of two stable osmium(IV) intermediates, thereby supporting the overall pathway for the reduction of these high-valent anticancer metal complexes for which reduction by thiols within cells may be essential to the antiproliferative activity.

Determination of Crystal-Field Splitting Induced by Thermal Oxidation of Titanium.

The electronic structure of transition-metal oxides is a key component responsible for material's optical and chemical properties. Specifically for metal-oxide structures, the crystal-field interaction determines the shape, strength, and occupancy of electronic orbitals. Consequently, the crystal-field splitting and resulting unoccupied state populations can be foreseen as modeling factors of the photochemical activity. Herein, we study the formation of crystal-field effects during thermal oxidation of titanium in an ambient atmosphere and range of temperatures. The X-ray absorption spectroscopy is employed for quantitative analysis of average t2g-eg crystal-field splitting (Δoct) and relative t2g/eg bands occupancy. The obtained result shows that Δoct changes as a function of temperature from 1.97 eV for a passive oxide layer created on a Ti metal surface at room temperature to 2.41 eV at 600 °C when the material changes into the TiO2 rutile phase. On the basis of XAS data analysis, we show that the Δoct values determined from L2 and L3 absorption edges are equal, indicating that the 2p1/2 and 2p3/2 core holes screen the t2g and eg electronic states in a similar manner.

Comparative study of the around-Fermi electronic structure of 5d metals and metal-oxides by means of high-resolution X-ray emission and absorption spectroscopies.

The composition of occupied and unoccupied electronic states in the vicinity of Fermi energies is vital for all materials and relates to their physical, chemical and mechanical properties. This work demonstrates how the combination of resonant and non-resonant X-ray emission spectroscopies supplemented with theoretical modelling allows for quantitative analysis of electronic states in 5d transition metal and metal-oxide materials. Application of X-rays provides element selectivity that, in combination with the penetrating properties of hard X-rays, allows determination of the composition of electronic states under working conditions, i.e. non-vacuum environment. Tungsten metal and tungsten oxide are evaluated to show the capability to simultaneously assess composition of around-band-gap electronic states as well as the character and magnitude of the crystal field splitting.

Implementation of a crossed-slit system for fast alignment of sealed polycapillary X-ray optics.

A new modification of a table-top laser-driven water-jet plasma X-ray source has been successfully implemented and commissioned at the Extreme Light Infrastructure (ELI) Beamlines user facility. In order to preserve the broadband nature of the source for spectroscopic experiments, a polycapillary lens was initially chosen as the focusing element. Generally, polycapillary X-ray optics have a narrow photon acceptance angle and small field of view, making alignment complicated and time-consuming. This contribution demonstrates a straightforward, reliable and reproducible procedure for aligning polycapillary focusing optics with broadband X-rays. The method involves a pre-alignment step where two X-ray slits are mounted orthogonally on opposite sides of a 3D-printed cylindrical polycapillary holder. This helps to precisely determine the optical axis of the X-ray beam. Subsequent mounting of the polycapillary in the pre-aligned holder with the slits removed allowed for immediate transmission of the X-ray photons through the optics and has provided a good starting point for fine alignment.

In situ observation of charge transfer and crystal field formation via high energy resolution X-ray spectroscopy during temperature programmed oxidation.

Herein, it has been demonstrated how resonant X-ray emission spectroscopy can be employed to study the charge transfer dynamics in real-time during the temperature-induced oxidation of metallic tungsten. Application of high energy resolution schemes allowed distinguishing charge transfer to separate orbitals resulting from crystal field splitting. Based on the time-resolved studies, it was possible to determine the corresponding charge transfer rates. From the experimental data, we determined that the electron transfer during the thermal oxidation of the metal dominates in the temperature range of 470-570 °C, reaching a maximum of 0.036 electrons per °C.

The influence of nitrogen doping on the electronic structure of the valence and conduction band in TiO2.

X-ray emission spectroscopy (XES) and X-ray absorption spectroscopy (XAS) provide a unique opportunity to probe both the highest occupied and the lowest unoccupied states in matter with bulk sensitivity. In this work, a combination of valence-to-core XES and pre-edge XAS techniques are used to determine changes induced in the electronic structure of titanium dioxide doped with nitrogen atoms. Based on the experimental data it is shown that N-doping leads to incorporation of the p-states on the occupied electronic site. For the conduction band, a decrease in population of the lowest unoccupied d-localized orbitals with respect to the d-delocalized orbitals is observed. As confirmed by theoretical calculations, the N p-states in TiO2 structure are characterized by higher binding energy than the O p-states which gives a smaller value of the band-gap energy for the doped material.

A compact and versatile tender X-ray single-shot spectrometer for online XFEL diagnostics.

One of the remaining challenges for accurate photon diagnostics at X-ray free-electron lasers (FELs) is the shot-to-shot, non-destructive, high-resolution characterization of the FEL pulse spectrum at photon energies between 2 keV and 4 keV, the so-called tender X-ray range. Here, a spectrometer setup is reported, based on the von Hamos geometry and using elastic scattering as a fingerprint of the FEL-generated spectrum. It is capable of pulse-to-pulse measurement of the spectrum with an energy resolution (ΔE/E) of 10-4, within a bandwidth of 2%. The Tender X-ray Single-Shot Spectrometer (TXS) will grant to experimental scientists the freedom to measure the spectrum in a single-shot measurement, keeping the transmitted beam undisturbed. It will enable single-shot reconstructions for easier and faster data analysis.

Investigating DNA Radiation Damage Using X-Ray Absorption Spectroscopy.

The biological influence of radiation on living matter has been studied for years; however, several questions about the detailed mechanism of radiation damage formation remain largely unanswered. Among all biomolecules exposed to radiation, DNA plays an important role because any damage to its molecular structure can affect the whole cell and may lead to chromosomal rearrangements resulting in genomic instability or cell death. To identify and characterize damage induced in the DNA sugar-phosphate backbone, in this work we performed x-ray absorption spectroscopy at the P K-edge on DNA irradiated with either UVA light or protons. By combining the experimental results with theoretical calculations, we were able to establish the types and relative ratio of lesions produced by both UVA and protons around the phosphorus atoms in DNA.

Resonant X-ray emission spectroscopy of platinum(II) anticancer complexes.

Platinum-based drugs are commonly used in cancer treatment. The biological activity of a metallodrug is obviously closely related to its chemical and stereochemical characteristics. An overlooked aspect is the effect of the ligand to the electronic structure of the metal atom (coordinated atom). We report herein a Resonant X-ray Emission Spectroscopy (RXES) study on the chemical speciation of chiral platinum complexes in which diastereomers are distinguished on the basis of their metal electronic configuration. This demonstrates RXES high chemical speciation capabilities, a necessary property to further investigate the reactivity of the Pt atom towards nucleophiles or bionucleophiles, and an important complement the previously reported RXES abilities, namely that it can be employed for in situ studies at physiological concentrations.

An operando emission spectroscopy study of Pt/Al2O3 and Pt/CeO2/Al2O3.

In situ time-resolved spectroscopic examination of catalysts based on well dispersed nanoparticles on metal oxides under transient conditions significantly facilitates the elucidation of reaction mechanisms. In this contribution, we demonstrate the level of structural information that can be obtained using high-energy resolution off-resonant spectroscopy (HEROS) to study 1.3 wt% Pt/Al2O3 and 1.3 wt% Pt/20 wt% CeO2/Al2O3 catalysts subjected to redox pulsing. First, HEROS is compared with XANES in a temperature programmed reduction experiment to demonstrate the increased sensitivity and time resolution of HEROS. Second, modulation excitation spectroscopy is exploited by redox pulsing to enhance the sensitivity of HEROS to structural changes by the application of phase sensitive detection (PSD) to the time-resolved HEROS data set. The HEROS measurements were complemented by resonant X-ray emission (RXES) and diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy measurements performed under identical conditions and in a single reactor cell in order to probe different aspects of the catalyst materials under the selected experimental conditions.

Depth-Resolved X-ray Absorption Spectroscopy by Means of Grazing Emission X-ray Fluorescence.

Grazing emission X-ray fluorescence (GEXRF) is well suited for nondestructive elemental-sensitive depth-profiling measurements on samples with nanometer-sized features. By varying the grazing emission angle under which the X-ray fluorescence signal is detected, the probed depth range can be tuned from a few to several hundred nanometers. The dependence of the XRF intensity on the grazing emission angle can be assessed in a sequence of measurements or in a scanning-free approach using a position-sensitive area detector. Hereafter, we will show that the combination of scanning-free GEXRF and fluorescence detected X-ray absorption spectroscopy (XAS) allows for depth-resolved chemical speciation measurements with nanometer-scale accuracy. While the conventional grazing emission geometry is advantageous to minimize self-absorption effects, the use of a scanning-free setup makes the sequential scanning of the grazing emission angles obsolete and paves the way toward time-resolved depth-sensitive XAS measurements. The presented experimental approach was applied to study the surface oxidation of an Fe layer on the top of bulk Si and of a Ge bulk sample. Thanks to the penetrating properties and the insensitivity toward the electric conduction properties of the incident and emitted X-rays, the presented experimental approach is well suited for in situ sample surface studies in the nanometer regime.

Chemical State Analysis of Phosphorus Performed by X-ray Emission Spectroscopy.

An experimental and theoretical study of phosphorus electronic structure based on high energy resolution X-ray emission spectroscopy was performed. The Kα and Kß emission spectra of several phosphorus compounds were recorded using monochromatic synchrotron radiation and megaelectronvolt (MeV) proton beam for target excitation. Measured spectra are compared to the results of ab initio quantum chemical calculations based on density functional theory (DFT). Clear correlation between energy position of the Kα emission line and the phosphorus formal oxidation state as well as DFT-calculated number of valence electrons is obtained; measured energy shifts are reproduced by the calculations. Chemical sensitivity is increased further by looking at the Kß emission spectra probing directly the structure of occupied molecular orbitals. Energies and relative intensities of main components are given together with the calculated average atomic character of the corresponding molecular orbitals involved in transitions.

Insights into the structure-activity relationships of chiral 1,2-diaminophenylalkane platinum(II) anticancer derivatives.

The structure-activity relationships of chiral 1,2-diaminophenylalkane platinum(II) anticancer derivatives are studied, including interactions with telomeric- and genomic-like DNA sequences, the pKa of their diaqua species, structural properties obtained from DFT calculations and resonant X-ray emission spectroscopy. The binding modes of the compounds to telomeric sequences were elucidated, showing no major differences with conventional cis-platinum(II) complexes like cisplatin, supporting that the cis-square planar geometry governs the binding of small Pt(II) complexes to G4 structures. Double-stranded DNA platination kinetics and acid-base constants of the diaqua species of the compounds were measured and compared, highlighting a strong steric dependence of the DNA-binding kinetics, but independent to stereoisomerism. Structural features of the compounds are discussed on the basis of dispersion-corrected DFT, showing that the most active series presents conformers for which the platinum atom is well devoid of steric hindrance. If reactivity indices derived from conceptual DFT do not show evidences for different reactivity between the compounds, RXES experiments provide new insight into the availability of platinum orbitals for binding to nucleophiles.

Study of the reactivity of silica supported tantalum catalysts with oxygen followed by in situ HEROS.

We report on the reactivity of grafted tantalum organometallic catalysts with molecular oxygen. The changes in the local Ta electronic structure were followed by in situ high-energy resolution off-resonant spectroscopy (HEROS). The results revealed agglomeration and formation of Ta dimers, which cannot be reversed. The process occurs independently of starting grafted complex.

The use of Resonant X-ray Emission Spectroscopy (RXES) for the electronic analysis of metal complexes and their interactions with biomolecules.

This review presents a new application of Resonant X-ray Emission Spectroscopy (RXES) to study the mechanism of action of metal containing anticancer derivatives and in particular platinum in situ and in vivo. The technique is an example of a photon-in photon-out X-ray spectroscopic approach, which enables chemical speciation of drugs to be determined and therefore to derive action mechanisms, and to determine drug binding rates under physiological conditions and therapeutic concentrations. This is made feasible due to the atomic specificity and high penetration depth of RXES. The review presents examples of the three main types of information that can be obtained by RXES and establishes an experimental protocol to perfect the measurements within cells.

Catalytically Active and Spectator Ce(3+) in Ceria-Supported Metal Catalysts.

Identification of active species and the rate-determining reaction steps are crucial for optimizing the performance of oxygen-storage materials, which play an important role in catalysts lowering automotive emissions, as electrode materials for fuel cells, and as antioxidants in biomedicine. We demonstrated that active Ce(3+) species in a ceria-supported platinum catalyst during CO oxidation are short-lived and therefore cannot be observed under steady-state conditions. Using time-resolved resonant X-ray emission spectroscopy, we quantitatively correlated the initial rate of Ce(3+) formation under transient conditions to the overall rate of CO oxidation under steady-state conditions and showed that ceria reduction is a kinetically relevant step in CO oxidation, whereas a fraction of Ce(3+) was present as spectators. This approach can be applied to various catalytic processes involving oxygen-storage materials and reducible oxides to distinguish between redox and nonredox catalytic mechanisms.