XANES Study Research Articles

Understanding highly active and durable Fe-decorated Co(OH)2 catalysts in alkaline oxygen evolution reaction by in situ XANES studies

The activation and degradation mechanisms of CoFe layered double hydroxide (CoFe-LDH) and Fe-decorated Co(OH)2 (Fe-Co(OH)2) have been investigated using in situ X-ray absorption spectroscopy coupled with cyclic voltammetry (CV) in 0–0.9 V (vs Hg/HgO). A series of CoFe-LDH samples have been prepared by co-electrodeposition with varying Co/Fe ratios. Fe-decorating on Co(OH)2 has been applied using 10 ppm of Fe in 1.0 M KOH. Electrochemical impedance spectroscopy (EIS) and CV cycles confirm the degradation of Co(OH)2 and CoFe-LDH due to the irreversible redox of Co(OH)2/CoOOH, and Fe extraction. In contrast, Fe-decorated Co(OH)2 shows high activity and stability in the OER. In situ XANES analysis suggests that the Fe-decorating allows the Co(OH)2 to remain as γ-CoOOH even under potential fluctuations.

Identifying a superior Ptδ+ species for the hydrogen-borrowing amination of alcohol at low temperature

The chemical state of metallic site is crucial for the performance of supported catalysts. Due to the difference of metal-support interaction, the chemical states of the supported metallic species are usually various and identifying the most active site is therefore very important for the design of efficient catalysts. In this work, a superior Ptδ+ species with high positive charge is identified as the active site in the hydrogen-borrowing amination of cyclopentanol with cyclopentylamine via structure and performance relationship study over Pt/CeO2 catalyst. It is found that the lower Pt loaded catalyst (0.74 %Pt/CeO2) has much higher activity than that of the higher Pt loaded one (1.5 %Pt/CeO2). In-situ CO-DRIFTs and XANES studies show that Pt in 0.74 %Pt/CeO2 catalyst has higher positive charge, favoring the dehydrogenation of cyclopentanol to cyclopentanone, the rate-determining step in the whole reaction. Furthermore, a 0.38 %Pt/CeO2 catalyst was prepared, which shows excellent performance at lower temperature (80 °C), the mildest reaction conditions investigated up to now. This work provides a new sight in the design of excellent catalysts for the hydrogen-borrowing amination under mild reaction conditions.

Electronic structure and composition of tin oxide thin epitaxial and magnetron layers according to synchrotron XANES studies

The materials of the tin-oxygen system and thin-film structures based on them are modern and actual for the creation of a wide range of electronic devices, for example, resistive gas sensors of high sensitivity and short response time with low energy consumption and high manufacturability. An important direction in the study of such materials and structures is the control of properties with variations in technological formation regimes. Information on the composition, local atomic and electronic structure of thin layers of the tin-oxygen system with varying approaches to their production is in demand. The work is devoted to the study of the electronic structure of thin layers of tin oxides obtained by modern methods of molecular beam epitaxy and magnetron sputtering. A study of the local partial density of electronic states in the conduction band by X-ray absorption near edge structure spectroscopy of tin and oxygen has been carried out. The data were obtained using high-intensity synchrotron radiation, which allows varying the monochromatized radiation quantum energy without loss in intensity, that is necessary to obtain high-resolution X-ray spectral data. It is shown that the composition, local atomic surrounding, electronic spectrum and their features depend on the technology of formation and storage conditions of the studied structures. Synchrotron X-ray spectroscopy data show the presence of intermediate oxides of the tin-oxygen system in the studied materials after prolonged storage in laboratory conditions. The data obtained indicate the possibility of controlled variation in the composition, local atomic surrounding and electronic spectrum of thin-film structures of tin oxides of small thickness. The results of the work can be used in the formation and subsequent modification of thin and ultrathin layers of tin oxides by magnetron sputtering and molecular beam epitaxy, as well as in their further application as active layers of microelectronics devices

Syngas Production by Chemical Looping Dry Reforming of Methane over Ni-modified MoO3/ZrO2.

We investigated supported-MoO3 materials effective for the chemical looping dry reforming of methane (CL-DRM) to decrease the reaction temperature. Ni-modified molybdenum zirconia (Ni/MoO3/ZrO2) showed CL-DRM activity under isothermal reaction conditions of 650 °C, which was 100-200 °C lower than the previously reported oxide-based materials. Ni/MoO3/ZrO2 activity strongly depends on the MoO3 loading amount. The optimal loading amount was 9.0 wt.% (Ni/MoO3(9.0)/ZrO2), wherein two-dimensional polymolybdate species were dominantly formed. Increasing the loading amount to more than 12.0 wt.% resulted in a loss of activity owing to the formation of bulk Zr(MoO4)2 and/or MoO3. In situ Mo K-edge XANES studies revealed that the surface polymolybdate species serve as oxygen storage sites. The Mo6+ species were reduced to Mo4+ species by CH4 to produce CO and H2. The reduced Mo species reoxidized by CO2 with the concomitant formation of CO. The developed Ni/MoO3(9.0)/ZrO2 was applied to the long-term CL-DRM under high concentration conditions (20 % CH4 and 20 % CO2) at 650 °C, with two pathways possible for converting CH4 and CO2 to CO and H2 via the redox reaction of the Mo species and coke formation.

Reductive hydrothermal conversion of uranyl oxalates into UO2+x monitored by in situ XANES analyses.

Hydrothermal conversion of actinide oxalates has recently gained attention as an innovative fabrication route for nuclear fuels but has remained mainly limited to tetra- or tri-valent cations. We report herein the reductive conversion of mixtures of uranyl and oxalate ions into UO2+x oxides under mild hydrothermal conditions (T = 250 °C). A multi-parametric study first led to specifying the optimal conditions in terms of pH, oxalate/U ratio and duration to provide a quantitative precipitation of uranium in the hyper-stoichiometric dioxide form with pH = 0.8; R = noxalate/nU = 3, and t = 72 hours. Particularly, pH was evidenced as a key parameter, with 3 different compounds obtained over a range of only 0.4 units. The mechanism leading to the formation of UO2+x was then investigated thanks to an in situ XANES study. Analysis of the supernatant showed that U(VI) was quickly reduced into U(IV) thanks to the presence of oxalates and/or their decomposition products in solution, following first-order kinetics. Tetravalent uranium was then hydrolysed, leading to the precipitation of UO2+x as the only crystalline phase. This study thus demonstrates that the hydrothermal conversion of actinide oxalates into oxides is an extremely versatile tool that can be implemented in a large variety of chemical systems, particularly in terms of the oxidation state of the cations initially present in solution.

The Origin of the Size Effect in the Oxidation of CO on Supported Palladium Nanoparticles

Two Pd/TiO2 catalysts with mean particle sizes of 1 and 3 nm were prepared and tested in the low-temperature oxidation of CO. It was found that the first catalyst with higher dispersion is more active. Turnover frequencies varied for these catalysts by almost six times. In contrast, the apparent activation energy of the oxidation of CO on the catalyst with smaller Pd nanoparticles was estimated at 76 kJ/mol, and for the catalyst with larger Pd nanoparticles at 58 kJ/mol. According to in situ XANES studies, the particle size effect originates from the oxidation of small palladium nanoparticles under reaction conditions, whereas larger nanoparticles are stable and consist of palladium atoms mainly in the metallic state. Palladium oxide is more active in the low-temperature oxidation of CO than metallic palladium. This means that the origin of size-dependent activity of Pd nanoparticles in the low-temperature oxidation of CO is associated with the change in the chemical composition of nanoparticles that leads to a change in the reaction mechanism and, as a result, in their activity.

Towards the Operational Window for Nitridic and Carbidic Palladium Nanoparticles for Directed Catalysis

AbstractThe reactions under which interstitial structures of Pd form are profoundly important and prevalent in catalysis; the formation and stability of Pd hydride structures are well understood, however, interstitial structures of the carbide and nitride are relatively under explored. This work reports a systematic study of the formation and stability of PdCx and PdNx at elevated temperatures and different atmospheres using in situ Pd L3 edge XANES spectroscopy. These studies were further complemented by the application of 14N MAS‐NMR experiments and computational DFT investigations. The experiments confirmed that PdCx was significantly more stable than PdNx; 14N MAS‐NMR provided direct confirmation on the formation of the nitride, however, the XANES studies evidenced very limited stability under the conditions employed. Moreover, the results suggest that the formation of the nitride imparts some structural changes that are not entirely reversible under the conditions used in these experiments. This work provides important insights into the stability of interstitial structures of Pd and the conditions in which they could be employed for directed catalytic processes.

XANES, XPS and Raman Studies of Hafnium Oxide Thin Films fabricated by RF Magnetron Sputtering at Different Power

Hafnium oxide layer was deposited on unheated silicon wafer and glass substrates at different power by using RF magnetron sputtering technique. The structural property was investigated by Raman spectroscopy. Moreover, HfO2 structure in monoclinic major phase especially at high RF power was found from the Raman spectra in vibrational modes. In addition, oxidations state of hafnium oxide thin films was gained by synchrotron-based X-ray absorption spectroscopy (XAS) using Hf L3-edge of XANES techniques and X-ray photoelectron spectroscopy (XPS) as well. The XANES and XPS results show that the oxidation state of HfO2 films is unchanged at different powers. The thin film prepared at higher power tends to have lower of oxygen vacancy.

Entrapping aliphatic and heterocyclic gaseous amines on copper-exchanged montmorillonite: A straight and green process to reduce air pollution

The capture of a number of aliphatic (APAs) and heterocyclic (HCAs) amines, known as widespread pollutants, by Cu2+-exchanged montmorillonite (Mt-Cu) was studied through elemental analysis diffuse-reflectance ultraviolet-visible and infra-red spectroscopies, X-rays diffraction, thermal analysis coupled with mass spectrometry of the evolved gases, XANES and EXAFS spectroscopy, and solid-state NMR. The investigated APAs and HCAs were: ethylamine (EtNH2), triethylamine (Et3N), ethylenediamine (En) and morpholine (Morph), piperidine (Piper), and pyridine (Py), respectively. Mt-Cu saturation occurred within 25 h at worst, the immobilized amount was relevant and involved the binding of the amino nitrogen to the interlayer Cu2+ ions. While for APAs the entrapped amount changed with the structure of the molecule, for HCAs it did not. The EXAFS and XANES studies showed that the interlayer Cu2+ ions are strongly coordinated to the amino groups. This resulted in the coordination number changing from four to five coordination, with the exception of En for which the CuII ions remained tetra-coordinated as in pristine Mt-Cu. Residual water molecules were found to be bound to the copper also after exposure to the amines. For EtNH2, En, Morph, and Piper the coordination to CuII occurred without the deprotonation of the amino group. NMR measurements showed the presence of physisorbed amines whose amount depended on the structure: while En was mostly bound to CuII (physisorption was negligible), for EtNH2, Et3N, Morph, and Piper the physisorbed amount ranged from 55 to 30% and for Py it was even the 70%. In addition, En was found to be the amine entrapped with the highest strength and covalent character. The obtained results supported the effectiveness of Cu2+-modified layered silicates as an air/gas purification system. The exploitation of devices based on Cu2+-exchanged montmorillonite could therefore become a very appealing future challenge as the interlayer modification was very simple and fast and the entire process turned out to be green and economic.

Effect of B2O3 concentration on the local atomic structure of lanthanum in lanthanum-borate glasses: XANES study and the principle of crystal-chemical similarity of the short-range order in glasses and crystals

Dependence of lanthanum local atomic structure in borate glasses La2O3-Nb2O5-B2O3 upon component composition was studied combining XANES, computer simulations using crystal chemical similarity (CCS) of a short-range order in glass and crystal, and multidimensional interpolation of spectra as the function of structural parameters. By this approach, the values of La-O bond lengths and angles in chains O-La-O were determined. Comparison of the obtained radial and angle distribution functions of oxygen atoms around La in the studied glasses with the same functions of the reference crystals revealed that within the existing CCS of La local structure in glasses and in B3LaO6, there is a decrease in the average length of La-O bonds by ∼ 0.1 Å and decrease of FWHF by ∼ 0.2 Å in the radial distribution of nearest O atoms, under retaining angle distribution of these atoms. It was shown that the ratio of intensities of the first two peaks in experimental La L3-edge XANES is an effective fingerprint of the number of nearest oxygen neighbors of lanthanum. Using the dependence of this ratio upon the component composition of glasses, the changes in the local structure of La under decreasing of B2O3 fraction from 72.5 to 47.5% were determined.

Enhancement in rate performance and high voltage structural stability of P3/O3 Na0.9Fe0.5Mn0.45Ni0.05O2 layered oxide cathode

We report synthesis and sodium storage performances of two biphasic P3/O3 materials, Na0.9Fe0.5Mn0.5O2 (P3/O3 NFM) and Na0.9Fe0.5Mn0.45Ni0.05O2 (P3/O3 NFNM). The choice of composition and Ni doping context have been justified. On replacing 5 % Mn by Ni, we found improvements in (de)sodiation capacity and rate performance. The delivered storage capacities of P3/O3 NFM and P3/O3 NFNM in the first cycle between 4.4 and 2.0 V are 118 mAh g−1 and 148 mAh g−1 at C/10 (0.015Ag−1) rate, respectively. The subsequent cycles have a narrow voltage window of 4.2 to 1.9 V for optimized cycling performance. In this voltage window, P3/O3 NFNM demonstrates more than twice the discharge capacity of P3/O3 NFM at 3C rate (0.45Ag−1); further it retains 74 % of its capacity after 200 cycles at 1C rate (0.15Ag−1) as compared to 57 % in P3/O3 NFM. X-ray diffraction studies confirm that Ni doping helps in retaining the P3 phase throughout the charge cycle in P3/O3 NFNM which helps in achieving a higher discharge capacity and better cycling. A greater reduction in the internal resistance, specifically the charge transfer resistance along with enhanced hole conductivity is observed from impedance studies of fully desodiated P3/O3 NFNM due to the maintenance of the P3 phase. XANES studies reveal that the charge compensation mechanism of Fe and Mn changes upon Ni doping which is attributed to the structural stability of P3/O3 NFNM at higher voltages.

In situ Ga K-edge XANES study of Ga-exchanged zeolites at high temperatures under different atmospheres including vacuum, CO, and pressurized H2

In situ Ga K-edge XANES analysis of Ga-exchanged zeolite was performed under actual operating temperatures and different atmospheres. The main absorption peak intensity was responsible for the presence or absence of a hydride source in the gas phase.

Redox-induced crystallisation in Ti-bearing glass-forming melts: A Ti K-edge XANES study

Ti is usually tetravalent in silicate glasses. It may be trivalent under very reduced conditions, but this remains unclear. In this study, in situ Ti K-edge XANES experiments were performed on four different Ti-bearing melts, equilibrated at high temperatures using a micro-furnace with controlled atmosphere. No formation of Ti3+ was observed, even at fO2 ∼ 10−15 (estimated). Instead, systematic crystallisation of the melts was observed. In experiments at fO2 ≤ 10−12, the system favours Ti4+ crystallisation instead of reduction to Ti3+ in the studied melts. Fluxing melts with reduced gas mixtures, like Ar-2%H2, may be a novel way to induce surface crystallisation and produce ceramic-coated glass materials.

Crystal structure, XANES and charge distribution investigation of krennerite and sylvanite: analysis of Au-Te and Te-Te bonds in Au1-xAgxTe2 group minerals.

The structure refinement and XANES study of two gold-silver-tellurides [Au1+xAgxTe2, krennerite (x = 0.11-0.13) and sylvanite (x = 0.29-0.31)] are presented and the structures are compared with the prototype structure of calaverite (x = 0.08-0.10). Whereas the latter is well known for being incommensurately modulated at ambient conditions, neither krennerite nor sylvanite present any modulation. This is attributed to the presence of relatively strong Te-Te bonds (bond distances < 2.9 Å) in the two minerals, which are absent in calaverite (bond distances > 3.2 Å). In both tellurides, trivalent gold occurs in slightly distorted square planar coordination, whereas monovalent gold, partly substituted by monovalent silver, presents a 2+2+2 coordination, corresponding to distorted rhombic bipyramids. The differentiation between bonding and non-bonding contacts is obtained by computation of the Effective Coordination Number (ECoN). The CHARge DIstribution (CHARDI) analysis is satisfactory for both tellurides but suggests that the Te-Te bond in the [Te3]2- anion is not entirely homopolar. Both tellurides can therefore be described as Madelung-type compounds, despite the presence of Te-Te in both structures.

Achieving Bright Reddish-Orange Luminescence in CaSnO3 Ceramics through Doping Manipulation

The reddish-orange phosphors of rare-earth activator have attracted intense interest due to their applications in lighting and display devices, including plasma display panel, field emission display and white light diode. In this work, Eu-doped CaSnO3 phosphor materials were prepared and examined. Their X-ray diffraction patterns were used for lattice parameters calculations and show that there was no phase transformation in all doping conditions. XANES studies suggest the stabilization of Sn and Eu ions in 4+ and 3+ oxidation states in the prepared phosphors, respectively. A broad emission band, with a major peak at 614 nm upon excitation wavelength of 395 nm, was observed in the range of 550 to 730 nm at room temperature due to 5D0 → 7FJ transition of Eu3+ ions. The luminescent spectra of the samples showed the intense light emission in the red-orange light region.

Temperature and Time-resolved XANES Studies of Novel Valence Tautomeric Cobalt Complex

VT transition from the high-spin CoII to low-spin CoIII state was observed for complex (N-cyclo-hexyl-2-iminopyridine)(3,6-di-tert-butyl-o-benzosemiquinonato)(3,6-di-tert-butyl-catecholato)cobalt in both solid and liquid form. For the first time we demonstrate a low-temperature nanosecond pump-probe XANES experiment revealing the valence tautomeric transition in a solution of Co complex. Reversible transition was induced both under the temperature and laser radiation stimuli.

Temperature- and time-resolved XANES studies of novel valence tautomeric cobalt complex

Temperature- and time-resolved XANES studies of novel valence tautomeric cobalt complex

Methane C-H Activation by [Cu2O]2+ and [Cu3O3]2+ in Copper-Exchanged Zeolites: Computational Analysis of Redox Chemistry and X-ray Absorption Spectroscopy.

There is an ongoing debate regarding the role of [Cu3O3]2+ in methane-to-methanol conversion by copper-exchanged zeolites. Here, we perform electronic structure analysis and localized orbital bonding analysis to probe the redox chemistry of its Cu and μ-oxo sites. Also, the X-ray absorption near-edge structure, XANES, of methane activation in [Cu3O3]2+ is compared to that of the more ubiquitous [Cu2O]2+. Methane C-H activation is associated with only the Cu2+/Cu+ redox couple in [Cu2O]2+. For [Cu3O3]2+, there is no basis for the Cu3+/Cu2+ couple's participation at the density functional theory ground state. In [Cu3O3]2+, there are many possible intrazeolite intermediates for methane activation. In the nine possibilities that we examined, methane activation is driven by a combination of the Cu2+/Cu+ and oxyl/O2- redox couples. Based on this, the Cu 1s-edge XANES spectra of [Cu2O]2+ and [Cu3O3]2+ should both have energy signatures of Cu2+ → Cu+ reduction during methane activation. This is indeed what we obtained from the calculated XANES spectra. [Cu2O]2+ and [Cu3O3]2+ intermediates with one Cu+ site are shifted by 0.9-1.7 eV, while those with two Cu+ sites are shifted by 3.0-4.2 eV. These are near a range of 2.5-3.2 eV observed experimentally after contacting methane with activated copper-exchanged zeolites. Thus, activation of methane by [Cu3O3]2+ will lead to formation of Cu+ sites. Importantly, for future quantitative XANES studies, involvement of O- + e- → O2- in [Cu3O3]2+ implies a disconnect between the overall reactivity and the number of electrons used in the Cu2+/Cu+ redox couple.

Rapid reduction of basaltic glasses in piston-cylinder experiments: a XANES study

Graphite capsules are commonly used in high-temperature, high-pressure experiments, particularly for nominally anhydrous experiments and iron-bearing silicate samples. Due to the presence of graphite in the sample assembly, the oxygen fugacity for these experiments is thought to be relatively low, typically at or below the graphite-CO-CO2 buffer (CCO). The detailed mechanism and kinetics of redox control in graphite capsule experiments are, however, poorly understood. This is especially problematic for short duration experiments (e.g. kinetic experiments), because it is uncertain whether the experimental product will preserve its initial oxygen fugacity, or become reduced during the experiment. In this study, a set of basaltic glasses after high-temperature experiments in graphite capsules were analyzed by micro X-ray absorption near-edge structure (µ-XANES) to obtain their Fe3+/ΣFe profiles near the graphite–melt interface. The results show rapid reduction of ferric iron in the basaltic melt, reaching near-equilibrium in half an hour for samples of 2 mm diameter and 1.3–1.9 mm thickness. Even for a “time-zero” experiment, which was quenched immediately after reaching the target temperature, the reduction profile is over 100 µm in length. By comparing experiments at the same temperature and pressure but with different durations, the reduction reaction progress is found to be linear to the square root of duration, indicating that the reduction process is diffusion-controlled. Such a rapid reduction of the basaltic melt requires a mechanism that is significantly faster than divalent cation diffusion or oxygen diffusion, and is best explained by molecular hydrogen diffusion. It has been shown by previous studies that nominally anhydrous high-pressure experiments could contain significant amounts of water. Thousands of ppm of H2O could remain in the graphite capsule even after drying at 120 °C for an extended time period. At high temperatures, H2O reacts with graphite to produce molecular hydrogen, which then diffuses into the basaltic glass and causes reduction. This mechanism is also supported by a compensating H2O profile of equivalent length in the basaltic glass, showing evidence for H2O produced by molecular hydrogen reacting with ferric iron. A quantitative model is proposed and it successfully reproduces the Fe3+/ΣFe profiles in our experiments. The model helps explain the kinetics of the reduction process and demonstrates that for a basaltic glass with reasonable initial FeO* content, Fe3+/ΣFe ratios, and thicknesses, the equilibrium oxidation state can usually be reached in one hour at ~ 1300 °C and ~ 0.5 GPa. Although extrapolating our conclusion to the large range of graphite capsule experiments requires knowledge on how H2 solubility and diffusivity varies as a function of silicate composition, temperature, and pressure, the reduction process is expected to be rapid in general because H2 diffusivity is high in silicate melts. Our study elucidates the mechanism and rate of oxygen fugacity change in graphite capsule experiments. Based on thermodynamic calculations, the reaction between graphite capsule and H2O is expected to produce a C-O-H fluid with an intrinsic oxygen fugacity of CCO −0.9, which agrees well with the measured Fe3+/ΣFe ratios in the basaltic glasses and the estimated oxygen fugacity for graphite and Pt–graphite capsule experiments from a previous study. Future studies are needed to better constrain the kinetics of this process at different temperature, pressure, and in silicate melts of different compositions. The dynamic process of H2 diffusing and reducing Fe3+ to Fe2+ shown in our experiments also provides a potential way to determine the diffusivity and solubility of molecular hydrogen in silicate melts, which are crucial in understanding volatile behaviors on reducing planetary bodies, such as the Moon and Mercury.

EXAFS and XANES study of cobalt complexes synthesized with ligands of chloroaniline and fluoroaniline dithiocarbamate

The cobalt [Co(II)] complexes (03 nos) were synthesized using various ligands of chloroaniline and fluoroaniline dithiocarbamate. The synthesized Co(II) complexes were studied for different physical, structural and chemical parameters using XANES and EXAFS conducted on DXAFS beamline BL-8, Indus 2, Synchrotron source. The EXAFS spectra recorded for Co(II) complexes were used for computation of the bond length, chemical shift, effective nuclear charge, principal absorption maxima and edge width using Athena and Origin 6.0 software. The results of the study revealed that, the synthesized cobalt complexes showed higher value for K-absorption edge (Ek) and Absorption maxima (EA) than Co metal. The Chemical shift (&Delta;EK) value for Co(II) complexes were ranged 1.17-4.52 eV. The Shift of principal absorption maxima (&Delta;EA) value for Co(II) complexes were ranged 0.65-4.18 eV. The Edge width of three different complexes of cobalt [Co(II)] synthesized with ligands of chloroaniline/fluoroaniline dithocarbamate ranged 6.60-11.14 eV. The bond length calculated by LSS method ranged 1.41-2.20 &Aring; for different complexes whereas the bond length calculated by FT method ranged 1.26-3.06 &Aring;. The Lytle&rsquo;s method and Levy&rsquo;s method gives the bond length ranged 2.44-3.08 &Aring; and 1.97-2.58 &Aring;, respectively for studied complexes.