XAS Study Research Articles

Hierarchical porous carbon-graphene-based Lithium–Sulfur batteries

Rechargeable lithium-sulfur battery with a high theoretical energy density attracts intense attention, but it still often suffers from poor cyclability. Herein, a hierarchical porous carbon/graphene hybrid is designed to host sulfur for Li–S battery. The batteries exhibit good performances including a specific capacity of 1178 mA h g−1 after 70 cycles at 0.2C, a stable reversible capacity of 400 mA h g−1 after 600 cycles at 1.0C (80% retention). Extensive characterizations including adsorption-desorption isotherms, XRD, Raman spectroscopy, HAADF-STEM combining with soft XAS study the relationship between the structure and the performance of as-synthesized cathode material. The synergistic effect of graphene and hierarchical porous carbon provides the pathway of the electron and Li+ ingress/egress to react with the confined sulfur clusters, retard the loss of the polysulfide anions from the cathode and suppress the shuttle effect during long-term cycling process.

XAS study of Mo-based dispersed catalysts for upgrading of heavy oil

Results of X-ray absorption spectroscopy study of Mo-based dispersed catalysts in the coke residue after catalytic steam cracking, catalytic cracking in the absence of water, and hydrocracking processes are reported. It was found that the catalyst precursor (ammonium heptamolybdate) is transformed to three different forms with the different ratios: MoO3, MoO2, and MoS2. After the catalytic steam cracking, the catalyst contains mainly MoO2 (∼74%), whereas after hydrocracking, the prevalent form is MoS2 (∼75%). The precursor after water-free catalytic cracking is transformed into 37% MoO2 and 63% MoS2. The catalyst genesis scheme in the processing of heavy oil was suggested, taking into account that reaction media affect the active phase formation, resulting in different composition and properties of products.

Multi-Site Redox Activities in Chemically Complex Sodium Layered Oxide Cathode Materials

Sodium layered oxide cathode materials with high specific capacity, energy density, and long cycle life have been the bottleneck for the widespread market implementation of sodium-ion batteries. Even though sodium layered oxide materials share many similarities with their Li-ion counterpart, there are many dissimilarities between the two which can be utilized to design high performance sodium layered oxide cathode materials. Sodium layered oxide materials can incorporate almost all of the first row transition metals in the transition metal oxide layer. Herein, we have synthesized a sodium layered oxide material with five transition metals to demonstrate such concept. The material is synthesized following a simple co-precipitation method and is a mixture of P2 and P3 type crystal structures (P2/P3=1). The material has good electrochemical performance with excellent stability on long term cycling. It delivers a specific capacity in the range of 95 mAh/g at C/10 with no capacity fading after 100 cycles, 72 mAh/g at 1C with 95% capacity retention after 500 cycles, and 55 mAh/g at 5C with 85% capacity retention after 1000 cycles. We further demonstrate the surprisingly good performance of the material at low temperatures. For example, the material delivers a 60 mAh/g capacity at 1C and 0°C with no capacity fading after 100 cycles. We further studied the structural evolution and redox reaction mechanism of the cathode material at different states of charge. The reversibility of the Na (de)insertion is evident from the reversibility of the XRD pattern upon cycling. Hard XAS study has revealed that most transition metals contribute to some degree towards the redox reaction of the cathode material, with most energy derived from the redox of Ni2+ and Co3+. This study has demonstrated that selecting the ideal transition metal hosts and building up the compositional complexity can be a path towards further improving sodium cathode materials.

Investigation of multi-mode spin–phonon coupling and local B-site disorder in Pr2CoFeO6 by Raman spectroscopy and correlation with its electronic structure by XPS and XAS studies

Electronic structure of Pr2CoFeO6 (at 300 K) was investigated by x-ray photoemission spectroscopy (XPS) and x-ray absorption spectroscopy techniques. All three cations, i.e. Pr, Co and Fe were found to be trivalent in nature. XPS valance band analysis suggested the system to be insulating in nature. The analysis suggested that Co3+ ions exist in low spin state in the system. Moreover, Raman spectroscopy study indicated the random distribution of the B-site ions (Co/Fe) triggered by same charge states. In temperature-dependent Raman study, the relative heights of the two observed phonon modes exhibited anomalous behaviour near magnetic transition temperature TN ~ 270 K, thus indicating towards interplay between spin and phonon degrees of freedom in the system. Furthermore, clear anomalous softening was observed below TN which confirmed the existence of strong spin–phonon coupling occurring for at least two phonon modes of the system. The line width analysis of the phonon modes essentially ruled out the role of magnetostriction effect in the observed phonon anomaly. The investigation of the lattice parameter variation across TN (obtained from the temperature-dependent neutron diffraction measurements) further confirmed the existence of the spin–phonon coupling.

XAS and TEM studies on ferromagnetic GeMn prepared by Mn ion implantation and helium ion beam induced epitaxial crystallization

In this study, we prepared a p-type Ge thin film and then implanted Mn ions in it at room temperature. P-type Ge thin film was made by gallium ion implantation in to an n-type Ge substrate to provide additional hole carriers. Mn ions were implanted into the same layer with Ga ions. Helium ion beam induced epitaxial crystallization with 2 MeV was used to growth GeMn thin films. The results show the beam annealed GeMn sample has been regrowth and has magnetoresistance.

Synchrotron XAS study on the As transformations during the weathering of sulfide-rich mine wastes

This paper describes the weathering processes that occurred across two tailing dumps in the Concordia mine (Puna of Argentina) with the purpose of evaluating the formation of secondary As-bearing minerals due to arsenopyrite alteration. After 30 years of exposure, the gradual oxidation of the wastes produced a number of secondary mineral phases containing As in different chemical arrangements. Synchrotron-based X-ray absorption spectroscopy was used to determine both, As and Fe solid speciation and to identify the formed As-bearing minerals. The results reveal that in the first stages of oxidation, As released from arsenopyrite is adsorbed/substituted in the jarosite structure partially inhibiting its dissolution. When pH values in the system slightly increase As-jarosite transforms into schwertmannite, where the released As could be re-adsorbed or co-precipitated. When the available adsorption sites become oversaturated with As, the precipitation of amorphous ferric arsenates may occur. The latter, likely constitute the more labile As fractions in the sediments and are therefore the main phases contributing As to the nearby environments. These amorphous and labile phases are more abundant in the uppermost layers of the profiles, where oxidation has taken place for a more prolonged time-lapse. The described transformations are enhanced by the acidic pH, the absence of minerals attenuating the acidity and the high sulfate and As concentrations in pore water.

Influence of the number of alkali cation on the photo-induced [formula omitted] charge transfer in [formula omitted] CoFe PBAs – A Co K-edge XANES study

In CoFe Prussian Blue Analogs (PBAs), a small difference in the number of alkali cation per unit cell can lead to drastic differences in the properties of the PBA. We investigate the origin of this effect in order to be able to control it and design new functional materials for data storage devices. Here we characterize the Cs0.7Co4[Fe(CN)6]2.8.17 H2O and Cs2Co4[Fe(CN)6]3.3.11H2O PBAs by Co K-edge in situ XAS. For both PBAs, the photo-induced (PI) CoIIIFeII→CoIIFeIII charge transfer and the different involved states (room temperature, ground and PI) are described. In the case of the Cs0.7 PBA, the relaxation from the CoIIFeIII PI state towards the CoIIIFeII ground state is monitored and quantified. All the results, fully consistent with previous magnetometry and Co L3-edge XAS studies, are discussed with respect to the electronic structure of the Co ions in the different involved states.

In situ XAS study of the local structure and oxidation state evolution of palladium in a reduced graphene oxide supported Pd(ii) carbene complex during an undirected C–H acetoxylation reaction

In situ XAS is used to reveal the evolution of palladium species during an undirected C–H acetoxylation reaction.

Local structure of Cu(I) ions in the MOR zeolite: A DFT-assisted XAS study

We combine in situ XAS and DFT to determine the local structure of Cu(I) ions in the Cu-exchanged mordenite (MOR) zeolite, one of the most promising materials for the selective oxidation of methane to methanol. Data analysis points to a quasi-linear coordination geometry in the MOR side pocket and suggests that ca. 20% of the Cu-ions exist as coupled di-copper(I) species, paving the way for more detailed structural refinements.

Cobalt oxide-based materials as non-PGM catalyst for HER in PEM electrolysis and in situ XAS characterization of its functional state

• Evaluation of cobalt oxide-based materials as cathode catalyst in PEM electrolyzer. • Optimum amount of carbon additive results in high performance. • Co 3 O 4 -based catalyst outperforms Pt at high cell potentials. • In situ XAS of Co 3 O 4 -electrode catalysts under PEM electrolysis conditions. • Dynamic changes of Co 2+ /Co 3+ fractions with temperature and applied potential. The polymer electrolyte membrane (PEM)-based electrolysis technology is a promising mean to split water and store renewable energy in the form of clean fuel, hydrogen. However, its high price due to the use of platinum group metals (PGMs) as catalyst materials generally makes the technology cost-restrictive. Herein we present the performance evaluation of cobalt oxide in PEM electrolysis as cathode catalyst where hydrogen evolution reaction (HER) takes place. Performance comparison of non-PGM catalysts (CoO, Co 3 O 4 and MoS 2 ) revealed that better performance was attained with Co 3 O 4 and it was further improved by mixing with an electrically conducting carbon material (Vulcan). An optimum amount of the carbon additive was found, and the best performance was recorded at 47 wt% Co 3 O 4 over the total amount. At 2.05 V, 47 wt% Co 3 O 4 -based catalyst (0.55 A/cm 2 ) outperformed 47 wt% MoS 2 -based one (0.51 A/cm 2 ). More importantly, the performance of the former at 2.3 V (1.12 A/cm 2 ) surpassed even that of Pt-black (1.11 A/cm 2 ). In situ XAS study of the Co 3 O 4 -based material under PEM electrolysis conditions revealed dynamic interchange of Co 3+ and Co 2+ fractions, which was attributed to ultimately boost the HER performance.

Structure of liquid In20Sn80 at high temperature: a XAS study

X-ray absorption measurements of the liquid metal alloy In20Sn80 have been carried out between 573 and 1073 K. The temperature evolution of the structure is studied through reverse Monte Carlo simulations, combining EXAFS data and partial pair distribution functions. From the equilibrated configurations we were able to obtain information about bond-angle distributions and local geometries, probed by common neighbor analysis. Our study shows a gradual change of the structure with the temperature in the range considered.

CuFeAl-composite catalysts of oxidation of gasification products of solid fuels: In situ XAS and XRD study

The reduction of high-active FeAl and CuFeAl composite catalysts was investigated in situ by XRD and XAS in the temperature range between 100 and 600 °C. It was found that the fresh catalysts consist of Fe2O3 and Al2O3 phases only. The Al3+ cations partially dissolute in the Fe2O3 lattice. The addition of copper to the Fe-Al catalyst leads to formation of highly dispersed CuO nanoparticles and mixed oxide with a spinel-type crystalline structure similar to that of CuFe2O4. The low-temperature reduction of Cu2+ to Cu0 accelerates the Fe2O3 → Fe3O4 and FeO → Fe0 transformations. In the presence of oxygen we did not observe the reduction of Cu and Fe to the metallic state. This means that the oxidation of CO over FeAl and CuFeAl composite catalysts proceeds via a redox mechanism.

Probing local structures in (Ni/Co)‐codoped ZnO/PVDF composite flexible and freestanding films by using XAS and XPS studies

Extended X‐ray absorption fine‐structure and X‐ray absorption near‐edge structure studies have been performed on five Ni/Co‐codoped ZnO/polyvinylidene fluoride composite flexible and freestanding films to probe the local structure surrounding the Zn, Co, and Ni sites. X‐ray photoelectron spectroscopy studies are also performed for throwing insight in charge states of Co and Ni being incorporated. X‐ray absorption near‐edge structure studies confirmed the incorporation of Co atoms at tetrahedral sites of the wurtzite host matrix. They acted as substitutional impurities without forming any detectable impurity phase or cobalt clusters.

Insights into Mg2+ Intercalation in a Zero-Strain Material: Thiospinel MgxZr2S4

The Mg battery cathode material, thiospinel MgxZr2S4 (0 ≤ x ≤ 1), exhibits negligible volume change (ca. 0.05%) during electrochemical cycling, providing valuable insight into the limiting factors in divalent cation intercalation. Rietveld refinement of XRD data for MgxZr2S4 electrodes at various states of charge, , coupled with EDX analysis, demonstrates that Mg2+ can be inserted into Zr2S4 at 60 °C up to x = 0.7 at a C/10 rate (up to x = 0.9 at very slow rates) and cycled with a high Coulombic efficiency of 99.75%. HAADF-STEM studies provide clear visual evidence of Mg-ion occupation in the lattice, whereas XAS studies show that Zr4+ was reduced upon Mg2+ intercalation. Operando and synchrotron XRD studies reveal the creation of two phases during the latter stages of discharge (x > 0.5) as the lattice fills and Mg2+ ions begin occupying tetrahedral (8a) sites in addition to octahedral (16c) interstitial sites. Compared to the isostructural Ti2S4 thiospinel, Zr2S4 presents a slightly larger cell volume a...

Cover Feature: Solution and Solid‐State Ligand K‐Edge XAS Studies of PdCl2 Diphosphine Complexes with Phenyl and Cyclohexyl Substituents (Eur. J. Inorg. Chem. 20‐21/2018)

Cover Feature: Solution and Solid‐State Ligand K‐Edge XAS Studies of PdCl₂ Diphosphine Complexes with Phenyl and Cyclohexyl Substituents (Eur. J. Inorg. Chem. 20‐21/2018)

Impact of Al sites on the methane co-aromatization with alkanes over Zn/HZSM-5

The presented work investigates the co-aromatization of methane and propane over Zn/HZSM-5 catalysts. The impact of Al sites, such as framework and extra-framework aluminum (EFAL) sites, on the methane participation pathway during the co-aromatization reaction is explored. The Al sites in the catalysts are manipulated by using ZSM-5 support materials with variable SiO2/Al2O3 molar ratios. Post-synthetic treatments, such as the ammonium hexafluorosilicate (AHFS) treatment to remove EFAL and the steam treatment to convert some of the framework Al sites to EFAL sites, are also employed to prepare catalysts and evaluate the contribution of these Al sites to the methane participation pathway. A higher concentration of Al in the catalysts would facilitate the methane participation in the phenyl ring formation. An average carbon number increment of 0.93, a substitution index reduction of 0.028, and an enhanced aromatic carbon fraction by 0.036 among the product molecules are witnessed over the Zn/HZSM-5 (23:1) catalyst during the co-aromatization reaction. This phenomenon is further enhanced when some of the framework Al atoms are converted to the EFAL sites by the steam treatment. The changes of substitution index and aromatic carbon fraction are expanded to 0.045 and 0.055. When the total Al concentration is low in the catalyst, however, the increased EFAL site number at the cost of framework Al sites does not improve methane participation, indicating that the framework Al sites are also crucial for the methane participation. The chemical environment of the Al sites in the catalysts is probed by 27Al solid-state NMR and XAS studies. The NH3-TPD and DRIFTS investigations of the catalysts demonstrate that the framework Al sites and the EFAL sites are closely related to the Brönsted and Lewis acid sites, respectively. The TEM and XRD studies reveal the intact crystalline structure of the catalysts upon the AHFS and steam treatments. The impact of the Al environments on the methane participation pathway in the co-aromatization reaction may be due to the acid properties of the catalysts.

Bifunctional catalysts based on colloidal Cu/Zn nanoparticles for the direct conversion of synthesis gas to dimethyl ether and hydrocarbons

Hybrid catalysts were prepared using well-defined, colloidal Cu/Zn-based nanoparticles as building units. The nanoparticles were immobilized on acidic supports (i.e., γ-Al2O3, HZSM-5, and HY) to yield a series of bifunctional catalysts with a close proximity of active sites for both methanol synthesis and its further conversion to dimethyl ether (DME) or hydrocarbons (HCs). By this model kit principle, a high comparability of the bifunctional catalysts was ensured. The catalysts were characterized in depth regarding their structure and catalytic performance in the conversion of CO-rich synthesis gas. In situ XAS studies demonstrated the formation of the active phase under reducing conditions. The present study revealed important material parameters to control activity and selectivity of the bifunctional catalysts either towards DME or liquefied petroleum gas (LPG) products in the direct conversion of simulated biomass-derived synthesis gas. In particular, Cu loading, pore structure and Si:Al ratio were investigated.

The role of yttrium and titanium during the development of ODS ferritic steels obtained through the STARS route: TEM and XAS study

Oxide Dispersion Strengthened Ferritic Steels (ODS FS) are candidate materials for structural components in future fusion reactors. Their high strength and creep resistance at elevated temperatures and their good resistance to neutron radiation damage is obtained through extremely fine microstructures containing a high density of nanometric precipitates, generally yttrium and titanium oxides.This work shows transmission electron microscopy (TEM) and extended X-ray absorption fine structure (EXAFS) characterization of Fe-14Cr-2W-0.3Ti-0.24Y ODS FS obtained by the STARS route (Surface Treatment of gas Atomized powder followed by Reactive Synthesis), an alternative method to obtain ODS alloys that avoids the mechanical alloying to introduce Y2O3 powder particles. In this route, FS powders already containing Ti and Y, precursors of the nanometric oxides, are obtained by gas atomization. Then, a metastable Cr- and Fe-rich oxide layer is formed on the surface of the powder particles. During consolidation by HIP at elevated temperatures, and post-HIP heat treatments above the HIP temperature, this oxide layer at Prior Particle Boundaries (PPBs) dissociates, the oxygen diffuses, and Y-Ti-O nano-oxides precipitate in the ferritic matrix.TEM characterization combined with XAFS and XANES analyses have proven to be suitable tools to follow the evolution of the nature of the different oxides present in the material during the whole processing route and select appropriate HIP and post-HIP parameters to promote profuse and fine Y-Ti-O nanometric precipitates.

Structural characterization of metal complexes in aqueous solutions: a XAS study of stannous fluoride.

The identity and structure of tin(ii)-fluoride complexes formed in aqueous solutions is determined by combining X-ray absorption spectroscopy, thermodynamic modeling and quantum mechanical calculations. Spectroscopic measurements confirm the presence of 3 stannous fluoride complexes, SnF+, SnF02 and SnF3-, with mean Sn-F bond distances that increase linearly, from 1.98 to about 2.04 Å, as a function of the coordination number. Computational ab initio calculations indicate that the stannous fluoride complexes form localized σs-p bonds, with the stereochemically active lone pair of the Sn(ii) atom distorting the geometry of the complexes. In addition, the SnF3- complex exhibits loosely coordinated water, which is removed upon addition of glycerol to lower the solvent activity. Our results provide spectroscopic confirmation of the stannous fluoride complexes proposed in the literature, and explain why glycerol additions stabilize solutions of Sn(ii) against oxidation.

Operando XAS studies on catalysts for energy-related processes

<i>Operando</i> XAS studies on catalysts for energy-related processes