High-energy Resolution Fluorescence-detected X-ray Research Articles

Investigating Degradation Mechanisms in PtCo Alloy Catalysts: The Role of Co Content and a Pt-Rich Shell Using Operando High-Energy Resolution Fluorescence Detection X-ray Absorption Spectroscopy.

Low Pt-based alloy catalysts are regarded as an efficient strategy in achieving high activity for the oxygen reduction reaction (ORR) in proton-exchange membrane fuel cells (PEMFCs). However, the desired durability for the low Pt-based catalysts, such as the Pt1Co3 catalyst, has still been considered a great challenge for PEMFCs. In this study, we investigate sub-2.5 nm PtxCoy alloy catalysts with varying Co content and Pt1Co3@Pt core-shell (CS) nanostructure catalysts obtained through a simple displacement reaction. The Pt1Co3@Pt_H catalysts showed a high mass activity (MA) of 1.46 A/mgPt at 0.9 V and 14% MA loss after 10k accelerated degradation test (ADT) cycles, which suggested the improved stability compared with Pt1Co3 catalysts (52% MA loss). To clarify the degradation mechanism, operando high-energy resolution fluorescence detection X-ray absorption spectroscopy (XAS) was applied in addition to conventional advanced measurement techniques, including operando conventional XAS, to analyze the electronic state and structure changes during operation potentials. We found that introducing Co improves the catalysts' activity mainly from the strain effect, but an excessive amount of Co leads to increased Pt-oxidation, which accelerates the degradation of the catalysts. The Pt1Co3@Pt_H catalyst shows high tolerance to Pt-oxidation, benefiting both the stability and activity. Our findings demonstrate an in-depth understanding of the degradation mechanism and the importance of designing PtCo CS nanostructures with optimal Co content for enhanced performance in PEMFCs.

Mapping the Initial Stages of a Protective Pathway that Enhances Catalytic Turnover by a Lytic Polysaccharide Monooxygenase.

Oxygenase and peroxygenase enzymes generate intermediates at their active sites which bring about the controlled functionalization of inert C-H bonds in substrates, such as in the enzymatic conversion of methane to methanol. To be viable catalysts, however, these enzymes must also prevent oxidative damage to essential active site residues, which can occur during both coupled and uncoupled turnover. Herein, we use a combination of stopped-flow spectroscopy, targeted mutagenesis, TD-DFT calculations, high-energy resolution fluorescence detection X-ray absorption spectroscopy, and electron paramagnetic resonance spectroscopy to study two transient intermediates that together form a protective pathway built into the active sites of copper-dependent lytic polysaccharide monooxygenases (LPMOs). First, a transient high-valent species is generated at the copper histidine brace active site following treatment of the LPMO with either hydrogen peroxide or peroxyacids in the absence of substrate. This intermediate, which we propose to be a CuII-(histidyl radical), then reacts with a nearby tyrosine residue in an intersystem-crossing reaction to give a ferromagnetically coupled (S = 1) CuII-tyrosyl radical pair, thereby restoring the histidine brace active site to its resting state and allowing it to re-enter the catalytic cycle through reduction. This process gives the enzyme the capacity to minimize damage to the active site histidine residues "on the fly" to increase the total turnover number prior to enzyme deactivation, highlighting how oxidative enzymes are evolved to protect themselves from deleterious side reactions during uncoupled turnover.

Unraveling the Dynamic Structural Evolution of Phthalocyanine Catalysts during CO2 Electroreduction

Understanding the atomic and electronic changes of active sites promotes the whole new sight into electrochemical carbon dioxide reduction reaction (CO2RR), which provides a feasible strategy to achieve carbon neutrality. Here we employ operando high-energy resolution fluorescence-detected X-ray absorption spectroscopy (HERFD-XAS) to track the structural evolution of Ni(II) phthalocyanine (NiPc), considered as the model catalysts with uniform Ni-N4-C8 moiety, during the CO2RR. The HERFD-XAS method is in favor of elucidating the interaction of the reactant/catalyst interface from the atomic electronic structure dimension, facilitating the establishment of the catalytic mechanism and the dynamic structure changes. Based on operando measurement, surface sensitive difference spectra (Δµ) and spectroscopy simulation, the interfacial interactions between the active sites of NiPc and reactants are monitored and the Ni species gradually reduced by increasing the applied potential is discovered. HERFD-XAS method offers an advanced and powerful tool for elucidating the complex catalytic mechanism in further various systems.

Bacterial Metabolites and Particle Size Determine Cerium Oxide Nanomaterial Biotransformation.

Soil is a major receptor of manufactured nanomaterials (NMs) following unintentional releases or intentional uses. Ceria NMs have been shown to undergo biotransformation in plant and soil organisms with a partial Ce(IV) reduction into Ce(III), but the influence of environmentally widespread soil bacteria is poorly understood. We used high-energy resolution fluorescence-detected X-ray absorption spectroscopy (HERFD-XAS) with an unprecedented detection limit to assess Ce speciation in a model soil bacterium (Pseudomonas brassicacearum) exposed to CeO2 NMs of different sizes and shapes. The findings revealed that the CeO2 NM's size drives the biotransformation process. No biotransformation was observed for the 31 nm CeO2 NMs, contrary to 7 and 4 nm CeO2 NMs, with a Ce reduction of 64 ± 14% and 70 ± 15%, respectively. This major reduction appeared quickly, from the early exponential bacterial growth phase. Environmentally relevant organic acid metabolites secreted by Pseudomonas, especially in the rhizosphere, were investigated. The 2-keto-gluconic and citric acid metabolites alone were able to induce a significant reduction in 4 nm CeO2 NMs. The high biotransformation measured for <7 nm NMs would affect the fate of Ce in the soil and biota.

Metal-insulator transition in CuIr2S4 observed by Cu Kα resonant x-ray emission spectroscopy

We have investigated the Cu-derived electronic structure of iridium spinel compound ${\mathrm{CuIr}}_{2}{\mathrm{S}}_{4}$ with a metal-insulator transition (MIT) at ${T}_{\mathrm{MIT}}\ensuremath{\sim}230$ K by means of high-energy resolution fluorescence-detected x-ray absorption spectroscopy (HERFD-XAS) and Cu $K\ensuremath{\alpha}$ resonant x-ray emission spectroscopy (RXES) around the Cu $K$ absorption edge. The Cu $K$ HERFD-XAS spectrum in the metallic phase exhibits a peak in the pre-edge region at an incident photon energy of $h{\ensuremath{\nu}}_{\mathrm{in}}=8981$ eV. The peak is ascribed to the Cu $1s--3d$ quadrupole transition and is abruptly shifted to higher energy by about 0.5 eV on going from metallic to insulating phases. The energy shift of the unoccupied Cu $3d$ states is related to the insulator gap formation of ${\mathrm{CuIr}}_{2}{\mathrm{S}}_{4}$ via the hybridization with the Ir $5d$--S $3p$ bands. A hysteresis behavior is observed in the Cu $1s--3d$ peak intensity between cooling and heating processes around ${T}_{\mathrm{MIT}}$, consistent with those observed in the transport properties. The abrupt change in the unoccupied Cu $3d$ states across MIT is also clearly detected in the Cu $K\ensuremath{\alpha}$ RXES spectra.

Valence transition of the intermetallic compound Ce2Rh2Ga probed by resonant x-ray emission spectroscopy

We have investigated the Ce valence of an intermetallic compound ${\mathrm{Ce}}_{2}{\mathrm{Rh}}_{2}\mathrm{Ga}$ with a phase transition at ${T}_{t}=128.5$ K by means of high-energy resolution fluorescence-detected x-ray absorption spectroscopy (HERFD-XAS) and resonant x-ray emission spectroscopy (RXES) around the Ce ${L}_{3}$ edge. The HERFD-XAS spectra in the whole temperature range from 300 to 3 K exhibit weak ${f}^{0}({\mathrm{Ce}}^{4+})$ and ${f}^{2}({\mathrm{Ce}}^{2+})$ peaks as well as the ${f}^{1}({\mathrm{Ce}}^{3+})$ main peak, indicating that the Ce $4f$ electrons hybridize with the conduction electrons. With decreasing temperature from 125 to 100 K across ${T}_{t}$, the ${f}^{1}$ peak is reduced, while the ${f}^{0}$ peak is enhanced. Concomitantly, the Ce $L{\ensuremath{\alpha}}_{1}$ RXES intensity taken at the ${f}^{2}$ peak of the HERFD-XAS spectrum decreases. These spectroscopic results indicate that the phase transition at ${T}_{t}$ of ${\mathrm{Ce}}_{2}{\mathrm{Rh}}_{2}\mathrm{Ga}$ is driven by a valence transition of the Ce ions.

A Theory-Guided X-ray Absorption Spectroscopy Approach for Identifying Active Sites in Atomically Dispersed Transition-Metal Catalysts.

Atomically dispersed supported metal catalysts offer new properties and the benefits of maximized metal accessibility and utilization. The characterization of these materials, however, remains challenging. Using atomically dispersed platinum supported on crystalline MgO (chosen for its well-defined bonding sites) as a prototypical example, we demonstrate how systematic density functional theory calculations for assessing all the potentially stable platinum sites, combined with automated analysis of extended X-ray absorption fine structure (EXAFS) spectra, leads to unbiased identification of isolated, surface-enveloped platinum cations as the catalytic species for CO oxidation. The catalyst has been characterized by atomic-resolution imaging and EXAFS and high-energy resolution fluorescence detection X-ray absorption near edge spectroscopy. The proposed platinum sites are in agreement with experiment. This theory-guided workflow leads to rigorously determined structural models and provides a more detailed picture of the structure of the catalytically active site than what is currently possible with conventional EXAFS analyses. As this approach is efficient and agnostic to the metal, support, and catalytic reaction, we posit that it will be of broad interest to the materials characterization and catalysis communities.

Charge Distribution in U1-xCexO2+y Nanoparticles.

In view of safe management of the nuclear wastes, a sound knowledge of the atomic-scale properties of U1-xMxO2+y nanoparticles is essential. In particular, their cation valences and oxygen stoichiometries are of great interest as these properties drive their diffusion and migration behaviors into the environment. Here, we present an in-depth study of U1-xCexO2+y, over the full compositional domain, by combining X-ray diffraction and high-energy resolution fluorescence detection X-ray absorption near-edge structure. We show, on one hand, the coexistence of UIV, UV, and UVI and, on the other hand, that the fluorite structure is maintained despite this charge distribution.

X-ray studies bridge the molecular and macro length scales during the emergence of CoO assemblies

The key to fabricating complex, hierarchical materials is the control of chemical reactions at various length scales. To this end, the classical model of nucleation and growth fails to provide sufficient information. Here, we illustrate how modern X-ray spectroscopic and scattering in situ studies bridge the molecular- and macro- length scales for assemblies of polyhedrally shaped CoO nanocrystals. Utilizing high energy-resolution fluorescence-detected X-ray absorption spectroscopy, we directly access the molecular level of the nanomaterial synthesis. We reveal that initially Co(acac)3 rapidly reduces to square-planar Co(acac)2 and coordinates to two solvent molecules. Combining atomic pair distribution functions and small-angle X-ray scattering we observe that, unlike a classical nucleation and growth mechanism, nuclei as small as 2 nm assemble into superstructures of 20 nm. The individual nanoparticles and assemblies continue growing at a similar pace. The final spherical assemblies are smaller than 100 nm, while the nanoparticles reach a size of 6 nm and adopt various polyhedral, edgy shapes. Our work thus provides a comprehensive perspective on the emergence of nano-assemblies in solution.

Operando HERFD-XANES and surface sensitive Δμ analyses identify the structural evolution of copper(II) phthalocyanine for electroreduction of CO2

The quantitative understanding of how atomic-level catalyst structural changes affect the reactivity of the electrochemical CO2 reduction reaction is challenging. Due to the complexity of catalytic systems, conventional in situ X-ray spectroscopy plays a limited role in tracing the underlying dynamic structural changes in catalysts active sites. Herein, operando high-energy resolution fluorescence-detected X-ray absorption spectroscopy was used to precisely identify the dynamic structural transformation of well-defined active sites of a representative model copper(II) phthalocyanine catalyst which is of guiding significance in studying single-atom catalysis system. Comprehensive X-ray spectroscopy analyses, including surface sensitive Δμ spectra which isolates the surface changes by subtracting the disturb of bulk base and X-ray absorption near-edge structure spectroscopy simulation, were used to discover that Cu species aggregated with increasing applied potential, which is responsible for the observed evolution of C2H4. The approach developed in this work, characterizing the active-site geometry and dynamic structural change, is a novel and powerful technique to elucidate complex catalytic mechanisms and is expected to contribute to the rational design of highly effective catalysts.

New insights on Br speciation in volcanic glasses and structural controls on halogen degassing

Abstract The volcanic degassing of halogens, and especially of the heavier Br and I, received increased attention over the last 20 years due to their significant effect on atmospheric chemistry, notably the depletion of stratospheric ozone. While the effect of melt composition on halogen diffusion, solubility, or fluid-melt partitioning in crustal magma chambers has been thoroughly studied, structural controls on halogen incorporation in silicate melts remain poorly known, with only few studies available in simplified borosilicate or haplogranite compositions. Here, we demonstrate that high-energy resolution fluorescence detection X-ray absorption spectroscopy (HERFD-XAS) with a crystal analyzer spectrometer (CAS) is well-suited for the study of Br speciation in natural volcanic glasses which can contain lower Br concentrations than their laboratory analogs. Especially, HERFD-XAS results in sharper and better-resolved XANES and EXAFS features than previously reported and enables detection limits for EXAFS analysis down to 100 ppm when previous studies required Br concentrations above the 1000 ppm level. XANES and EXAFS analyses suggest important structural differences between synthetic haplogranitic glass, where Br is surrounded by Na and next-nearest oxygen neighbors, and natural volcanic glasses of basaltic to rhyodacitic compositions, where Br is incorporated in at least three distinct sites, surrounded by Na, K, or Ca. Similar environments, involving both alkali and alkaline earth metals have already been reported for Cl in Ca-bearing aluminosilicate glass and our study thus underlines that the association of Br with divalent cations (Ca2+) has been underestimated in the past due to the use of simplified laboratory analogs. Overall, similarities in Cl and Br structural environments over a large array of compositions (46–67 wt% SiO2) suggest that melt composition alone may not have a significant effect on halogen degassing and further support the coupled degassing of Cl and Br in volcanic systems.

The nature and partitioning of invisible gold in the pyrite-fluid system

The most characteristic feature of hydrothermal deposits of gold is its intimate association with pyrite. Microscopically visible gold occurs in pyrite ore as metal particles of >0.1 µm in size, together with so called “invisible” gold, undetectable by conventional microscopic methods. The chemical, redox and structural state of this invisible gold and the mechanisms of its incorporation into pyrite remain both inconsistent and controversial since the dawn of economic geology. To clarify these issues, we performed laboratory experiments to simulate interactions of gold-bearing sulfur-rich hydrothermal fluids with arsenic-free pyrite at temperatures from 350 to 450 °C and pressures from 400 to 700 bar, typical of the formation conditions of many types of gold deposits. Gold solubility was measured in these fluids as a function of sulfur speciation and acidity. Gold redox and structural state in pyrite was characterized by high-energy resolution fluorescence-detected x-ray absorption spectroscopy (HERFD-XAS), together with more traditional analytical techniques such as scanning electron microscopy (SEM), x-ray diffraction (XRD), electron probe micro analysis (EPMA), laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS), and inductively coupled plasma atomic emission spectrometry (ICP-AES). Results show that dissolved Au in sulfide-sulfate solutions forms complexes with hydrogen sulfide, and tri- and di-sulfur radical ions whose amounts depend mostly on the fluid pH and total sulfur concentration. Invisible gold in pyrite occurs as Au metal submicron- to nano-sized particles and chemically bound Au(I) in the form of (poly)sulfide clusters composed on S-Au-S linear units, similar to those in aqueous complexes. Our findings contest the common belief that Au(I) substitutes for Fe and/or S in the structure of As-poor pyrite. The partition coefficient of Au(I) between pyrite and the fluid, Dpy/fl, is determined to be 0.15 ± 0.07 at 450 °C in a wide range of Au fluid phase concentrations (10–1000 ppm), but much higher Dpy/fl values, between 10 and 50, are found at 350 °C. These Au partitioning trends coupled with the new data on Au molecular environment in pyrite suggest a control of Au(I) incorporation in the mineral by a chemisorption step. Extrapolated to Au contents of hydrothermal fluids of the Earth’s crust which are typically below 1 ppm, our Dpy/fl values reproduce fairly well the natural Au tenors in As-poor pyrites (∼0.1–1 ppm Au), which are 100–1000 times lower than those typically observed in arsenian pyrites and arsenopyrites (10–1000 ppm Au at As tenors of 0.01–10 wt%). Our results thus indirectly highlight a key role played by arsenic in gold enrichment in As-bearing iron sulfide ore, a role that yet remains to be fully understood and quantified.

Temperature-induced valence transition in EuNi2(Si1–xGex)2 investigated by high-energy resolution fluorescence detection X-ray absorption spectroscopy

Electronic structures of the temperature-induced valence transition system EuNi2(Si1–xGex)2 with x = 0.70, 0.79, 0.82 have been investigated by means of high-energy resolution fluorescence detection X-ray absorption spectroscopy (HERFD-XAS). The HERFD-XAS spectra clearly change their intensities of Eu2+ and Eu3+ components which directly reflect the temperature-induced valence transition. For x = 0.70, gradual spectral change exhibits a continuous valence transition, while drastic changes for x = 0.79 and 0.82 indicate first-order valence transitions. High-energy resolution measurements made it possible to observe additional fine structures which are recognized more clearly below the transition temperature. Existence of these fine structures suggests that many-body effect plays an important role in the temperature-induced valence transition of this system. The variation of Eu mean valence estimated from the HERFD-XAS spectra for each x correlates with that of magnetic susceptibility.

Erratum to: Influence of Local Atomic and Electronic Structures of Magnetite on Subtle Effects in HERFD-XANES Spectra

Large iron oxide nanoparticles are studied by high-energy resolution fluorescence-detected X-ray absorption spectroscopy (HERFD XANES). A theoretical procedure within the finite-difference method framework is tested to calculate Fe K-edge X-ray absorption spectra for the magnetite structure. The influence of structural and non-structural calculation parameters on the subtle effects in the X-ray absorption spectra are considered. Tetrahedral positions of Fe3+ ions are shown to provide the main contribution to the spectral feature at 20–21 eV whose intensity grows together with the content of these ions.

Identification of Stabilizing High-Valent Active Sites by Operando High-Energy Resolution Fluorescence-Detected X-ray Absorption Spectroscopy for High-Efficiency Water Oxidation.

Composite electrocatalysts have exhibited high activities toward water electrolysis, but the catalytically active sites really in charge of the reaction are still debatable while the conventional in situ X-ray spectroscopies are not capable of conclusively identifying the interaction of these materials with the electrolyte because of the complexity of catalysis. In this work, by utilization of operando Kβ1,3 high-energy resolution fluorescence-detected X-ray absorption spectroscopy (HERFD-XAS) with a small incident angle, the operando quadrupole transition obviously showed that oxygen directly interacted with 3d orbitals of Co ions rather than that of Fe ions. Most importantly, Fe ions can promote the stabilization of the Co ions under a higher valent state during water oxidation, which may lead to a stable intermediate of reactant and its superior intrinsic activity. Accompanied by the first-principle calculations, the intermediates between 3d orbitals for surface Co ions and O 2p orbitals for the attaching hydroxide ions were ascribed to this orbital hybridization. Because of the unvaried structural features in conventional in situ techniques, operando HERFD-XAS revealed the remarkable change of chemical status to correlate with the orbital interaction rather than with the structural variation. This operando HERFD-XAS approach corresponding to the local orbital interaction in reactant/catalyst interface can potentially offer synergetic strategies toward realizing the chemical reactions or reaction pathways in various fields.

Tuning the Selectivity of Single-Site Supported Metal Catalysts with Ionic Liquids

1,3-Dialkylimidazolium ionic liquid coatings act as electron-donors increasing the selectivity for partial hydrogenation of 1,3-butadiene catalyzed by iridium complexes supported on high-surface-area γ-Al2O3. High-energy resolution fluo-rescence detection X-ray absorption near edge structure (HERFD XANES) measurements quantify the electron dona-tion and are correlated with the catalytic activity and selectivity. The results demonstrate broad opportunities to tune electronic environments and catalytic properties of atomically dispersed supported metal catalysts.

A three-crystal spectrometer for high-energy resolution fluorescence-detected X-ray absorption spectroscopy and X-ray emission spectroscopy at SSRF

A Johann-type spectrometer for the study of high-energy resolution fluorescence-detected X-ray absorption spectroscopy, X-ray emission spectroscopy and resonant inelastic X-ray scattering has been developed at BL14W1 X-ray absorption fine structure spectroscopy beamline of Shanghai Synchrotron Radiation Facility. The spectrometer consists of three crystal analyzers mounted on a vertical motion stage. The instrument is scanned vertically and covers the Bragg angle range of 71.5–88°. The energy resolution of the spectrometer ranges from sub-eV to a few eV. The spectrometer has a solid angle of about 1.87 × 0−3 of 4π sr, and the overall photons acquired by the detector could be 105 counts per second for the standard sample. The performances of the spectrometer are illustrated by the three experiments that are difficult to perform with the conventional absorption or emission spectroscopy. Copyright © 2016 John Wiley & Sons, Ltd.

Core-shell-like Au sub-nanometer clusters in Er-implanted silica.

The very early steps of Au metal cluster formation in Er-doped silica have been investigated by high-energy resolution fluorescence-detected X-ray absorption spectroscopy (HERFD-XAS). A combined analysis of the near-edge and extended part of the experimental spectra shows that Au cluster nucleation starts from a few Au and O atoms covalently interconnected, likely in the presence of embryonic Au-Au correlation. The first Au clusters, characterized by a well defined Au-Au coordination distance, form upon 400 °C inert annealing. The estimated upper limit of the Gibbs free energy for the associated heterogeneous nucleation is 0.06 eV per atom, suggesting that the Au nucleation is assisted by matrix defects, most likely non-bridging oxygen atoms. The experimental results indicate that the formed subnanometer Au clusters can be applied as effective core-shell systems in which the Au atoms of the 'core' develop a metallic character, whereas the Au atoms in the 'shell' can retain a partially covalent bond with O atoms of the silica matrix. High structural disorder at the Au site is found upon neutral annealing at a moderate temperature (600 °C), likely driven by the configurational disorder of the defective silica matrix. A suitable choice of the Au concentration and annealing temperature allows tailoring of the Au cluster size in the sub-nanometer range. The interaction of the Au cluster surface with the surrounding silica matrix is likely responsible for the infrared luminescence previously reported on the same systems.

Electronic structure and local distortions in epitaxial ScGaN films

High energy resolution fluorescence-detected x-ray absorption spectroscopy and density functional theory calculations were used to investigate the local bonding and electronic structure of Sc in epitaxial wurtzite-structure ScxGa1−xN films with x ≤ 0.059. Sc atoms are found to substitute for Ga atoms, accompanied by a local distortion involving an increase in the internal lattice parameter u around the Sc atoms. The local bonding and electronic structure at Sc are not affected strongly by the strain state or the defect microstructure of the films. These data are consistent with theoretical predictions regarding the electronic structure of dilute ScxGa1−xN alloys.

V oxidation state in Fe–Ti oxides by high-energy resolution fluorescence-detected X-ray absorption spectroscopy

The oxidation state of vanadium in natural and synthetic Fe–Ti oxides is determined using high-energy resolution fluorescence-detected X-ray absorption spectroscopy (HERFD-XAS). Eleven natural magnetite-bearing samples from a borehole of the Main Magnetite Layer of the Bushveld Complex (South Africa), five synthetic Fe oxide samples, and three natural hematite-bearing samples from Dharwar supergroup (India) are investigated. V K edge spectra were recorded on the ID26 beamline at the European Synchrotron Radiation Facility (Grenoble, France), and the pre-edge features were used to determine the local environment and oxidation state of vanadium. In the case of the magnetite samples (natural and synthetic), we show that vanadium is incorporated in the octahedral site of the spinel structure under two oxidation states: +III and +IV. The variations of the pre-edge area are interpreted as various proportions in V3+ and V4+ (between 9.5 and 16.3% of V4+), V3+ being the main oxidation state. In particular, the variations of the V4+/V3+ ratio along the profile of the Main Magnetite Layer seem to follow the crystallization sequence of the layer. In the case of the hematite samples from India, the pre-edge features indicate that vanadium is substituted to Fe and mainly incorporated as V4+ (between 40 and 72% of V4+). We also demonstrate the potentiality of HERFD-XAS for mineralogical studies, since it can filter out the unwanted fluorescence and give better resolved spectra than conventional XAS.