X-ray Absorption Spectroscopy Experiments Research Articles

Upcycling of polyethylene to gasoline through a self-supplied hydrogen strategy in a layered self-pillared zeolite.

Conversion of plastic wastes to valuable carbon resources without using noble metal catalysts or external hydrogen remains a challenging task. Here we report a layered self-pillared zeolite that enables the conversion of polyethylene to gasoline with a remarkable selectivity of 99% and yields of >80% in 4 h at 240 °C. The liquid product is primarily composed of branched alkanes (selectivity of 72%), affording a high research octane number of 88.0 that is comparable to commercial gasoline (86.6). In situ inelastic neutron scattering, small-angle neutron scattering, solid-state nuclear magnetic resonance, X-ray absorption spectroscopy and isotope-labelling experiments reveal that the activation of polyethylene is promoted by the open framework tri-coordinated Al sites of the zeolite, followed by β-scission and isomerization on Brönsted acids sites, accompanied by hydride transfer over open framework tri-coordinated Al sites through a self-supplied hydrogen pathway to yield selectivity to branched alkanes. This study shows the potential of layered zeolite materials in enabling the upcycling of plastic wastes.

A sub-100 nm thickness flat jet for extreme ultraviolet to soft X-ray absorption spectroscopy.

Experimental characterization of the structural, electronic and dynamic properties of dilute systems in aqueous solvents, such as nanoparticles, molecules and proteins, are nowadays an open challenge. X-ray absorption spectroscopy (XAS) is probably one of the most established approaches to this aim as it is element-specific. However, typical dilute systems of interest are often composed of light elements that require extreme-ultraviolet to soft X-ray photons. In this spectral regime, water and other solvents are rather opaque, thus demanding radical reduction of the solvent volume and removal of the liquid to minimize background absorption. Here, we present an experimental endstation designed to operate a liquid flat jet of sub-micrometre thickness in a vacuum environment compatible with extreme ultraviolet/soft XAS measurements in transmission geometry. The apparatus developed can be easily connected to synchrotron and free-electron-laser user-facility beamlines dedicated to XAS experiments. The conditions for stable generation and control of the liquid flat jet are analyzed and discussed. Preliminary soft XAS measurements on some test solutions are shown.

Structural and electronic transformations of GeSe2 glass under high pressures studied by X-ray absorption spectroscopy

Pressure-induced transformations in an archetypal chalcogenide glass (GeSe2) have been investigated up to 157 GPa by X-ray absorption spectroscopy (XAS) and molecular dynamics (MD) simulations. Ge and Se K-edge XAS data allowed simultaneous tracking of the correlated local structural and electronic changes at both Ge and Se sites. Thanks to the simultaneous analysis of extended X-ray absorption fine structure (EXAFS) signals of both edges, reliable quantitative information about the evolution of the first neighbor Ge-Se distribution could be obtained. It also allowed to account for contributions of the Ge-Ge and Se-Se bond distributions (chemical disorder). The low-density to high-density amorphous-amorphous transformation was found to occur within 10 to 30 GPa pressure range, but the conversion from tetrahedral to octahedral coordination of the Ge sites is completed above [Formula: see text] 80 GPa. No convincing evidence of another high-density amorphous state with coordination number larger than six was found within the investigated pressure range. The number of short Ge-Ge and Se-Se "wrong" bonds was found to increase upon pressurization. Experimental XAS results are confirmed by MD simulations, indicating the increase of chemical disorder under high pressure.

Unravelling the amorphous structure and crystallization mechanism of GeTe phase change memory materials

The reversible phase transitions in phase-change memory devices can switch on the order of nanoseconds, suggesting a close structural resemblance between the amorphous and crystalline phases. Despite this, the link between crystalline and amorphous tellurides is not fully understood nor quantified. Here we use in-situ high-temperature x-ray absorption spectroscopy (XAS) and theoretical calculations to quantify the amorphous structure of bulk and nanoscale GeTe. Based on XAS experiments, we develop a theoretical model of the amorphous GeTe structure, consisting of a disordered fcc-type Te sublattice and randomly arranged chains of Ge atoms in a tetrahedral coordination. Strikingly, our intuitive and scalable model provides an accurate description of the structural dynamics in phase-change memory materials, observed experimentally. Specifically, we present a detailed crystallization mechanism through the formation of an intermediate, partially stable ‘ideal glass’ state and demonstrate differences between bulk and nanoscale GeTe leading to size-dependent crystallization temperature.

Unveiling the Degradation of Pt/NbOx/C Catalysts in PEMFCs via In Situ X-ray Absorption Spectroscopy

Among the class of the catalyst that is composed of metal nanoparticles supported on metal oxides (MMO), the Pt/NbOx/C system has shown promising oxygen reduction reaction (ORR) activities as a cathode of proton exchange membrane fuel cells (PEMFCs). Herein, we have studied a series of Pt/NbOx/C catalysts prepared via physical vapor deposition and unraveled the nature of the metal and metal oxide interaction (MMOI) by characterizing this system under reactive conditions. By conducting in situ X-ray absorption spectroscopy (XAS) experiments, we demonstrate that Pt preferably interacts with O but not Nb in the Pt/NbOx/C system. As such, Pt-O interaction benefits the ORR activity via an electronic effect rather than a strain effect. We have also provided clear evidence for the formation of metallic Nb phase at the early stage of PEMFC operation, which led to severe particle growth of Pt after long-term PEMFC operation.

Probing the Local Structure of Na in NaNO3-Promoted, MgO-Based CO2 Sorbents via X-ray Absorption Spectroscopy.

This work provides insight into the local structure of Na in MgO-based CO2 sorbents that are promoted with NaNO3. To this end, we use X-ray absorption spectroscopy (XAS) at the Na K-edge to interrogate the local structure of Na during the CO2 capture (MgO + CO2 ↔ MgCO3). The analysis of Na K-edge XAS data shows that the local environment of Na is altered upon MgO carbonation when compared to that of NaNO3 in the as-prepared sorbent. We attribute the changes observed in the carbonated sorbent to an alteration in the local structure of Na at the NaNO3/MgCO3 interfaces and/or in the vicinity of [Mg2+···CO32-] ionic pairs that are trapped in the cooled NaNO3 melt. The changes observed are reversible, i.e., the local environment of NaNO3 was restored after a regeneration treatment to decompose MgCO3 to MgO. The ex situ Na K-edge XAS experiments were complemented by ex situ magic-angle spinning 23Na nuclear magnetic resonance (MAS 23Na NMR), Mg K-edge XAS and X-ray powder diffraction (XRD). These additional experiments support our interpretation of the Na K-edge XAS data. Furthermore, we develop in situ Na (and Mg) K-edge XAS experiments during the carbonation of the sorbent (NaNO3 is molten under the conditions of the in situ experiments). These in situ Na K-edge XANES spectra of molten NaNO3 open new opportunities to investigate the atomic scale structure of CO2 sorbents modified with Na-based molten salts by using XAS.

Outstanding low-temperature performance for NH3-SCR of NO over broad Cu-ZSM-5 sheet with highly exposed a-c orientation

Five ZSM-5-based catalysts with diverse zeolitic morphologies were prepared to experimentally investigate and computationally analyze the correlation between crystal plane and copper species. The types and numbers of copper species dispersed over Cu-ZSM-5 catalysts were emphatically determined to elucidate the structure-performance relationship of the Cu-ZSM-5 catalyst with broad sheet in the selective catalytic reduction (SCR) of NO by NH3. This study indicates that the broad Cu-ZSM-5 sheet exhibits outstanding low-temperature performance, achieving complete NOx removal at 250–550 °C. Operando X-ray absorption spectroscopy (XAS) experiments demonstrated this nanosheet catalyst has highly dispersed Cu2+ ions as key active sites, which are facilely solubilized by NH3 at low temperatures, further promoting rapid dynamic switching between Cu2+ and Cu+ species. And the density functional theory (DFT) calculations showed the Cu2+ forms a unique interaction with the [010] crystal plane, leading to a lower energy barrier of the rate-determining step, resulting in extraordinary NOx conversion.

Core-alloyed shell structured Fe@FeCo di-atomic nanoclusters loaded in carbon aerogels as efficient bi-functional catalysts towards ORR and HER

In this work, core-alloyed shell structured (CAS) Fe@FeCo di-atomic nanoclusters loaded in carbon aerogels (CAS Fe@FeCo DA-NC/C catalysts) were successfully prepared by utilizing the spatial confinement effect of polyaniline (PANI) hydrogels led by strong chelation among Fe3+ ions, tannic acid (TA), and amino (or imino) groups of PANI polymer chains. The spatial confinement effect can guarantee the formation of CAS Fe@FeCo DA-NCs with sizes between 1.4 and 2.4 nm. Moreover, N4–Fe–O–Co–N4 units are composed of the alloyed shells of the CAS Fe@FeCo DA-NCs, which can act as bi-functional active sites. In addition, N4–Fe–O–Fe–N4 units are the main component of their cores (like Fe-SA-NCs), which would slightly help strengthen the adsorption ability of bi-functional active sites of the CAS Fe@FeCo DA-NCs to the reactants. The core-alloyed shell structure of the CAS Fe@FeCo DA-NCs are demonstrated by results of X-ray photoelectron spectra, X-ray absorption spectroscopy and control experiments. Thus, the CAS Fe@FeCo DA-NC is proposed as a new type of active species with bi-functional active sites for the first time, to the best of our knowledge. Thanks to their unique features as active species (proper size, core-alloyed shell structure, and bi-functional active sites), the as-prepared CAS Fe@FeCo DA-NC/C catalysts show excellent performance as bi-functional catalysts in the oxygen reduction reaction (ORR) with a half-wave potential (E1/2) of 0.895 V (ΔE1/2 = 34 mV vs of Pt/C) in alkaline media and the hydrogen evolution reaction (HER, 133 mV @ 10 mA cm−2) in acidic media.

Chemical and Mechanical Degradation of the Interface between Li7La3Zr2O12 Electrolyte and LiNi0.6Mn0.2Co0.2O2 Cathode as a Result of Electrochemistry

All solid-state Li-ion cells promises high energy density and high safety standards over conventional Li-ion cells based on liquid electrolytes. The first principle calculations shows that the most of the potential candidates for solid electrolytes for Li-ion cells have small electrochemical stability window. The limited electrochemical stability window leads to electrochemical reactivity with the cathode when cycled beyond the voltage limit. The reactions products are often leads to increased overpotential, hence rapid capacity fade during continuous cell operation. In addition to the electrochemical reactions, mechanical incompatibility further makes the cathode-solid-electrolyte interfaces prone to cracking, as cathodes undergoes structural changes during lithiation/delithiation. Indeed many experimental works have also showed limited cyclability of all-solid-state Li-ion cells owing to interfacial side reactions and mechanical cracking. Therefore, electrochemistry induced mechanical failure and electrochemical side reactions at the interface remains the bottleneck for the development of all-soli-state Li-ion cells. This paramount challenge invites us to systematically investigate the aforementioned problem by probing non-destructively near the cathode-solid-electrolyte interface. We introduce a model system that utilizes very thin layer of LiNi0.6Mn0.2Co0.2O2 (NMC622) cathode (60-600 nm) on top of bulk Al-doped-Li7La3Zr2O12 (Al-LLZO) solid-electrolyte pellet. This unique system allows us to non-destructively probe the electrochemical reaction by combined in-operando and ex-situ X-ray absorption spectroscopy (XAS), with probing depth beyond the cathode-solid-electrolyte interfaces.In this work, we elucidate the mechanism of electrochemical failure of NMC622 cathode and Al-LLZO solid-electrolyte interface at different temperature, through combined ex-situ and in-operando characterization techniques. The study revealed that the electrochemical reactivity is dependent on cell operation temperature. For the ex-situ samples that was cycled at higher temperature (80 ⁰C), the reduction of transition metals in cathode, i.e., nickel and cobalt was evident by soft X-ray absorption spectroscopy (XAS) of their respective L-edges. The results match closely with DFT prediction for the interfaces at high voltage. On the other hand, the in-operando XAS experiment, where characterization was done at room temperature, the reduction of transition metals in cathode, and hence electrochemical reactivity, was not observed. These results remarkably outline that electrochemical reactivity of solid-electrolyte and cathode interfaces at high voltage is kinetically inhibited at low or near room temperature, and hence very sensitive to cell operating condition. Nonetheless, it was found that irrespective of operating conditions i.e., temperature, the cell capacity rapidly decays as number of cycles continue. The focused ion beam scanning electron microscopy (FIB-SEM) revealed that this can be mostly attributed to intergranular cracks in NMC622 cathode films as well as to delamination of the cathode-solid-electrolyte interfaces during cell operation. As these cracks, in cathode films and at the interfaces, grows, the overpotential/impedance increases due to limited contacts for effective Li+ migration.The current work contributes to the field by showcasing interfacial degradation pathways in cell operation by novel design of experiments that allows for both in-operando and non-destructive characterization. The findings of this work will guide towards engineering cells of practical use. Figure 1

CatMass: software for calculating optimal sample masses for X-ray absorption spectroscopy experiments involving complex sample compositions.

This paper presents software for calculating the optimal mass of samples with complex compositions (e.g. supported metal catalysts) for X-ray absorption spectroscopy (XAS) and scattering measurements. The ability to calculate the sample mass and other relevant parameters needed for an XAS measurement allows experimentalists to be better prepared in terms of detector selection, energy range of scan and overall time needed to complete the measurement, thus increasing efficiency. CatMass builds on existing sample mass calculators allowing users to determine the optimum sample preparation, collection geometry, usable energy range for a scan and approximate edge step of the absorption event. Visualization tools present the absorption calculation results in a format familiar to XAS experimentalists, with the added ability to save calculations and plots for future reference or recalculation. CatMass is a program broadly applicable in catalysis and is helpful for users with complex samples due to composition/stoichiometry or multiple competing elements.

Operando Laboratory‐based X‐ray Absorption Spectroscopy: Guidelines for Newcomers in the Field

AbstractThe new possibility to perform operando X‐ray absorption spectroscopy (XAS) in the laboratory expands the potential field of applications towards a broad research community. These applications are multidisciplinary at heart and benefit from joint expertise from different fields, most importantly chemistry, physics, geology, and instrumentation. Hence, a development of collaboration networks that combine skills and knowhow from different fields is highly beneficial in this endeavor. As operando laboratory‐based XAS constitutes a highly interesting, advanced, and powerful characterization technique, we provide in this article practical guidelines for newcomers in the field, who would like to employ it. Here, we will describe ten important steps towards a successful operando laboratory‐based XAS experiment, which are not only useful for the catalysis community, but for a much wider audience from other research fields, such as environmental chemistry as well as battery and fuel cell research.

Probing Changes in the Local Structure of Active Bimetallic Mn/Ru Oxides during Oxygen Evolution.

Identifying the active site of catalysts for the oxygen evolution reaction (OER) is critical for the design of electrode materials that will outperform the current, expensive state-of-the-art catalyst, RuO2. Previous work shows that mixed Mn/Ru oxides show comparable performances in the OER, while reducing reliance on this expensive and scarce Pt-group metal. Herein, X-ray photoelectron spectroscopy and X-ray absorption spectroscopy (XAS) are performed on mixed Mn/Ru oxide materials for the OER to understand structural and chemical changes at both metal sites during oxygen evolution. The results show that the Mn-content affects both the oxidation state and local coordination environment of Ru sites. Operando XAS experiments suggest that the presence of MnOx might be essential to achieve high activity likely by facilitating changes in the O-coordination sphere of Ru centers.

QUATI beamline: QUick x-ray Absorption spectroscopy for TIme and space-resolved experiments at the Brazilian Synchrotron Light Laboratory

The QUATI beamline is now under assembly at the Brazilian Synchrotron Laboratory and will be dedicated to high-quality X-ray absorption spectroscopy experiments, with temporal and spatial resolution on a millisecond scale for in situ and operando studies performing: XANES (X-ray Absorption Near Edge Structure), EXAFS (Extended X-ray Absorption Fine Structure) and XES (X-ray Emission Spectroscopy). Furthermore, it will combine X-ray absorption techniques with X-ray microscopy, Raman or Infrared spectrometry, X-ray Diffraction, and Mass Spectrometry to investigate different aspects of the sample in a single experiment to characterize complex materials under real working conditions.

Phosphate Coordination in a Water-Oxidizing Cobalt Oxide Electrocatalyst Revealed by X-ray Absorption Spectroscopy at the Phosphorus K-Edge

In the research on water splitting at neutral pH, phosphorus-containing transition metal oxyhydroxides are often employed for catalyzing the oxygen evolution reaction (OER). We investigated a cobalt–phosphate catalyst (CoCat) representing this material class. We found that CoCat films prepared with potassium phosphate release phosphorus in phosphate-free electrolytes within hours, contrasting orders of magnitude’s faster K+ release. For P speciation and binding mode characterization, we performed technically challenging X-ray absorption spectroscopy experiments at the P K-edge and analyzed the resulting XANES and EXAFS spectra. The CoCat-internal phosphorus is present in the form of phosphate ions. Most phosphate species are likely linked to cobalt ions in Co–O–PO3 motifs, where the connecting oxygen could be a terminal or bridging ligand in Co-oxide fragments (P–Co distance, ~3.1 Å), with additional ionic bonds to K+ ions (P–K distance, ~3.3 Å). The phosphate coordination bond is stronger than the ionic K+-binding, explaining the strongly diverging ion release rates of phosphate and K+. Our results support a structural role of phosphate in the CoCat, with these ions binding at the margins of Co-oxide fragments, thereby limiting the long-range material ordering. The relations of catalyst-internal phosphate ions to cobalt’s redox-state changes, proton transfer, and catalytic activity are discussed.

DANTE Digital Pulse Processor for XRF and XAS experiments

DANTE is a new Digital Pulse Processor (DPP) developed for fluorescence detectors, like Silicon Drift Detectors (SDDs) or High Purity Germanium detectors (HPGe), used in X-ray Fluorescence (XRF) and X-ray Absorption Spectroscopy (XAS) experiments at synchrotron facilities. Its main features are its optimal energy resolution and peak stability for detector count rate values up to 1–2 Mcps, and its enhanced rejection of pile-up events. In this paper, we present the first complete evaluation of DANTE performance in SOLEIL synchrotron facility. DANTE has been tested in laboratory with an X-ray generator source and in different experiments at LUCIA and PUMA beamlines at SOLEIL.

Automatic Feedback System for X-ray Flux at BL08U1A Soft X-ray Spectromicroscopy Beamline of Shanghai Synchrotron Radiation Facility

An online automatic feedback system has been successfully installed and commissioned at the BL08U1A Soft X-ray Spectromicroscopy Beamline of Shanghai Synchrotron Radiation Facility, which can monitor the incident X-ray beam in real time by measuring the blade-edge signals of the exit slit and automatically adjust the elliptical cylindrical mirror parameters to achieve beam calibration and maintain the optimal X-ray flux of the sample. This work provides a comprehensive description of the hardware composition, system implementation, feedback logic, function and software design, system optimization and commission, as well as the online experimental results supported by the system. The experimental results demonstrated that the online automatic feedback system is capable of effectively maintaining the optimal X-ray beam flux for X-ray absorption spectroscopy experiments. Its success can provide valuable technique assistance for the design, construction and optimization of similar systems at various beamlines in synchrotron sources in the future.

Autonomous atomic Hamiltonian construction and active sampling of X-ray absorption spectroscopy by adversarial Bayesian optimization

X-ray absorption spectroscopy (XAS) is a well-established method for in-depth characterization of electronic structure. In practice hundreds of energy-points should be sampled during the measurements, and most of them are redundant. Additionally, it is also tedious to estimate reasonable parameters in the atomic Hamiltonians for mechanistic understanding. We implement an Adversarial Bayesian optimization (ABO) algorithm comprising two coupled BOs to automatically fit the many-body model Hamiltonians and to sample effectively based on active learning (AL). Taking NiO as an example, we find that less than 30 sampling points are sufficient to recover the complete XAS with the corresponding crystal field and charge transfer models, which can be selected based on intuitive hypothesis learning. Further applications on the experimental XAS spectra reveal that less than 80 sampling points give reasonable XAS and reliable atomic model parameters. Our ABO algorithm has a great potential for future applications on automated physics-driven XAS analysis and AL sampling.

X-ray free electron laser studies of electron and phonon dynamics of graphene adsorbed on copper

We report optical pumping and x-ray absorption spectroscopy experiments at the Pohang Accelerator Laboratory free electron laser that probes the electron dynamics of a graphene monolayer adsorbed on copper in the femtosecond regime. By analyzing the results with ab initio theory we infer that the excitation of graphene is dominated by indirect excitation from hot electron-hole pairs created in the copper by the optical laser pulse. However, once the excitation is created in graphene, its decay follows a similar path as in many previous studies of graphene adsorbed on semiconductors, i.e., rapid excitation of strongly coupled optical phonons and eventual thermalization. It is likely that the lifetime of the hot electron-hole pairs in copper governs the lifetime of the electronic excitation of the graphene.Received 21 November 2022Accepted 26 January 2023DOI:https://doi.org/10.1103/PhysRevMaterials.7.024005©2023 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasElectron emissionPhysical SystemsGrapheneTechniquesMolecular dynamicsX-ray absorption spectroscopyCondensed Matter, Materials & Applied Physics

Tracking and Understanding Dynamics of Atoms and Clusters of Late Transition Metals with In-Situ DRIFT and XAS Spectroscopy Assisted by DFT

Dynamic structural changes of single-atom catalysts (SACs) are key to many reactions that were reported to be catalyzed by supported single atoms. To understand these changes, systematic in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and X-ray absorption spectroscopy (XAS) experiments were performed under reducing (1% CO) and oxidizing (1% CO + 1% O2) reaction atmospheres at room temperature over CeO2-supported late transition metals (Ru, Rh, Pd, Ir, and Pt) synthesized via two different methods (wet impregnation and precipitation) to account for the influence of surface properties. As a general trend, the CO vibrational frequencies downshifted under the CO atmosphere, which we assigned to the formation of clusters. Upon changing the gas mixture to more oxidizing (1% CO + 1% O2), single sites are retained as evidenced by the CO vibrational frequencies at higher wavenumbers. Among the investigated metals, Pt2+ and Pd2+ are more prone to cluster formation, and Rh3+ and Ru4+ are found to be stable as single sites following the order Rh > Ru > Ir > Pt > Pd. In combination with the density functional theory (DFT) calculations of CO vibrational frequencies, we were able to assign shifts to changes in the oxidation state of the metals. These findings thus serve as a benchmark for ceria-supported Pd, Pt, Ru, Ir, and Rh SACs.

Synthesis of highly ordered L10 MPt alloys (M = Fe, Co, Ni) from crystalline salts: an in situ study of the pre-ordered precursor reduction strategy

The synthesis of highly ordered magnetic L10 alloys by means of the so-called pre-ordered precursor reduction (PPR) approach is deeply investigated by in situ X-ray absorption spectroscopy experiments.