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[en] In various transition metals with different crystal structures, we have demonstrated the validity of sparse modeling (SpM) for the analysis of extended X-ray absorption fine structure (EXAFS). The target data are EXAFS oscillations of Ni, Co, and Fe foils, and these materials have face centered cubic, hexagonal close-packed, and body centered cubic structures, respectively. The SpM method provides the sparse radial distribution functions associated with the respective crystal structures. Although an assumption of crystal structure is required to extract insightful information concerning structural fluctuations in the conventional analysis method, the SpM method allows us to estimate more accurate Debye–Waller factors without such assumption. (author)
[en] Full text: Cerium dioxide (CeO2, ceria) is one of the materials with widest range of applications. When doping with Zr, resulting materials could be used as active support for anodes of solid oxide fuel cells (SOFC), or as active catalysts for methane oxidation. Materials used as anodes in a SOFC should have a good level of electronic conductivity, and for that reason many researchers have studied ceria-based anodes with transition metals. Inclusion of transition metals into the crystal lattice of ceria is not a trivial issue. It strongly depends on preparation method and thermal treatment, and there are some disagreements in the literature about solubility limits. In this work we analyzed the system (Ce0.9Zr0.1)1-xNixO2-d (x = 0, 0.03, 0.05 y 0.1), with the aim to determine whether Ni is included or not into the ceria-zirconia lattice. The crystal structure of the samples was analyzed by X-Ray diffraction and by means of HR-TEM (images and elemental maps of the samples were obtained). The local order of Ni was studied by extended X-ray absorption fine structure (EXAFS) around the Ni K-edge, in the D08B-XAFS2 beamline of the Brazilian Synchrotron Light Source (Proposal 20180396). The results of this study give some insight of the existence of an amorphous phase of Ni in samples with low content of Ni and low temperature thermal treatment. Results of XRD show that the lattice parameter remains constant in all the range of Ni contents analyzed, and there is no evidence of phase segregation for samples calcined at 600ºC and x ≤ 0.05. This is supported by TEM observations, and it was concluded by EXAFS that there is no long range order. Absorption as a function of wavenumber and Fourier transformed plots exhibit the typical behaviour of an amorphous phase. Samples heat treated at T 800°C or with Ni contents higher than 5%mol, exhibit a crystalline NiO phase detected by XRD, HR-TEM and EXAFS. (author)
[en] Full text: XANES studies, performed at XAFS2 beamline, reveal that Cr(2-x)TixO3 (x=0.2 - CTO) particles present Ti ions in the +3 & +4 oxidation state. Even more, XPS showed that the Ti ions located at the surface of the CTO are in the +4 state. The resistance (R) of CTO is modeled by small polarons hopping (SPH). R-curve presents a change in the activation energy associated with a variation in the conduction mechanism (750-790 K). This change is not observed in the magnetic and structural characterization. The intensity of selected peaks of the Ti K-edge XANES spectra vs. temperature (T) exhibits a break as occurred in the R-curve. This change is associated with variations in the electron-phonon interactions, but there is no theory that explains this intriguing phenomenon at the present. The Ti ions in the particle lead to SPH between the transition metal sites. In addition, the CTO was evaluated as acetone vapor sensor. In-situ XANES experiments shown that the CTO changes its spectra with the incorporation of the vapor. The configuration of the new QUATI beamline will allow us to improve the previous experiment, performing EXAFS with better time resolution (<1 sec). Fitting the spectra we expect to obtain the Debye-Waller factor vs. T. This will let us to better understand the transition observed in the R-curve and perform theoretical calculations to explain the observed results on the EXAFS + XANES vs. T, and be able to explain this new electron-phonon interaction in this kind of system. In addition, the simultaneous measurement of complementary techniques such as Raman spectroscopy (RS) will allow the study the material in one single experiment increasing the reliability and quality of the data. It is important to have tools, like RS, that let monitoring of the surface and volume processes in-situ or in-operando conditions. The measurement of EXAFS and RS simultaneously bring a unique opportunity to understand the sensing mechanism and design new material. (author)
[en] Full text: QUATI (QUick X-Ray Absorption Spectroscopy for TIme-Resolved experiments) is a beamline dedicated to high quality X-ray absorption spectroscopy experiment sand in-situ/in operando studies, in XANES (X-Ray Absorption Near Edge Structure) and EXAFS (Extended X-ray Absorption Fine Structure) modes, allowing measurements in the time scale of milliseconds. A multi technique approach will be performed: XRD, Raman, and IR coupled with XAFS experiments. X-ray Emission Spectroscopy (XES) and related techniques will be accessible by a Von Hamos spectrometer, this choice is to enable fast scan acquisition. (author)
[en] The heavy baryon system bounded by the strong interaction has a rich internal structure, so its mass spectra can have the fine structure similar to the line spectra of atom bounded by the electromagnetic interaction. We systematically study the internal structure of P-wave Ω baryons and calculate their D-wave decay properties. The present study, together with our previous studies on their mass spectra and S-wave decay properties, suggest that all the four excited Ω baryons recently discovered by LHCb can be well explained as P-wave Ω baryons, and their beautiful fine structure is directly related to the rich internal structure of P-wave Ω baryons.
[en] We propose a new method to extract the informations of microscopic structure from the extended X-ray absorption fine structure by the application of sparse modeling based on a simplified single-scattering approximation of photo-electron waves. This method can extract the sparse radial distribution function of the atoms located nearby the target atom and can estimate the Debye-Waller factor without any assumption of the micro-structures. Therefore, this method is expected to exhibit considerable ability in the crystallographic researches on new materials and such cooperative researches of data-driven science and crystallographic measurements are strongly expected to extend the frontiers in various research fields. (author)
[en] Precision spectroscopy of the Muonium Lamb shift and fine structure requires a robust source of 2S Muonium. To date, the beam-foil technique is the only demonstrated method for creating such a beam in vacuum. Previous experiments using this technique were statistics limited, and new measurements would benefit tremendously from the efficient 2S production at a low energy muon (<20 keV) facility. Such a source of abundant low energy μ has only become available in recent years, e.g. at the Low-Energy Muon beamline at the Paul Scherrer Institute. Using this source, we report on the successful creation of an intense, directed beam of metastable Muonium. We find that even though the theoretical Muonium fraction is maximal in the low energy range of 2–5 keV, scattering by the foil and transport characteristics of the beamline favor slightly higher μ energies of 7–10 keV. We estimate that an event detection rate of a few events per second for a future Lamb shift measurement is feasible, enabling an increase in precision by two orders of magnitude over previous determinations.
[en] Full text: Sub-nanometer metal structures, usually known as Atomic Quantum Clusters (AQCs) exhibit unique electronic and chemical properties, different from bulk material and even nanoparticles. Because of this, AQCs have attracted much interest recently in the field of catalysis. In particular, Cu clusters have shown important catalytic properties e.g. for the oxidation of CO and the selective hydrogenation of olefin and carbonyl groups. Moreover, it has been observed in many cases that AQCs are able to catalyze reactions at lower temperatures and pressures compared to bulk and conventional nanosized materials. We have also shown that the electronic properties of Cu atoms in the AQCs can be tuned when changing the material on which AQCs are supported what makes them suitable for specific-designed catalysts. It is then expected that Cu AQCs can be used for reactions in which cooper is not active. In particular, we have tested the CO2 hydrogenation, for which bulk metallic copper is inactive, and our catalytic studies have shown that TiO2 supported Cu5-AQCs can catalyze the reduction of CO2 by molecular hydrogen to produce methane. In this work, we perform an in-situ XAFS study of Cu5-AQCs/TiO2 catalysts for the CO2 reduction reaction at the ISS beamline of the National Synchrotron Light Source II (NSLS-II), Upton, New York. We analyze changes in the electronic structure of the AQCs through XANES studies at the Cu K-edge in CO2, H2, He atmosphere, and also under reaction conditions (stoichiometric ratios of CO2 and H2 for methane synthesis; 1:4) at different temperatures (300-700 K) in order to elucidate the interaction of Cu atoms with the different reactants and to understand the reaction pathway. (author)
[en] Full text: Layered double hydroxides (LDH) are minerals from the family of anionic clays. Here, we show a partial result, which is the synthesis of methanol, an intermediate in DME production, an important raw material, using catalysts prepared from LDH. MnCuAl LDH precursors were prepared by a co-precipitation method, using Na free reactants. The ratio of Al was fixed at 0.19 and the Cu/Mn ratio at 0.5 to 2.0. For Cu/Mn = 0.5, two pH values were used (6.5 e 8.0). The catalysts were obtained by calcining the precursors at 400 ºC and then reducing them with H2 at 265 °C. With the use of the in-situ XRD (XPD beamline of LNLS) it was possible to see the disappearance of the poorly crystalline CuO-like phase in the calcined materials and the appearance of a metallic Cu phase in reduced ones. Transition begins near the end of the temperature ramp (265 °C) and metallic Cu stays well defined and crystalline during the temperature plateau (for 40 min). X-ray absorption spectroscopic characterization (XANES and EXAFS) were performed on the Cu K-edge at the XAFS2 beamline of LNLS. In the calcined material with pH 8 and Cu/Mn = 0.5, copper appeared to be more electro-deficient than in CuO, probably because of the presence of manganese at a higher oxidation state and/or Al3+ in its vicinity or due to its existence as a defective nanocrystalline phase. In this catalyst, copper appeared to be fourfold coordinated by oxygen probably in a planar configuration, with considerable structure disorder from the second coordination sphere onwards. In the reduced catalysts, no difference in the electronic properties of copper between catalyst and a copper foil standard were observed. The catalyst with the highest manganese content and prepared at higher pH, presented the highest methanol productivity and selectivity. This was credited to the larger surface area of its oxide precursor, which resulted in smaller Cu crystallites and larger copper surface area. (author)
[en] Full text: The damage caused by organic pollutants has increased the search for high efficiency and low-cost processes, that can treat large volumes of effluents. Advanced oxidative processes stand out as an excellent alternative, being extremely useful in the case of substances resistant to conventional technologies. The main characteristic of the processes is to transform organic pollutants into compounds that are not harmful to the environment through the use of semiconductors as TiO2 under ultraviolet radiation. In this work, oxidation processes involving semiconductors and UV radiation will be discussed. Heterogeneous photocatalysis and the behavior of several semiconductors are of great interest, therefore study solar energy-absorbing materials is of great interest for technological development and maintenance of life. Niobia has been studied as a semiconductor for photocatalysis due to its high thermal stability and because it presents a bandgap in the UV region. When doped with other elements, changes in its optical, structural and morphological properties are observed. This addition of dopants to the structure can cause an increase in photocatalytic activity, causing the material to absorb energies close to the visible, enabling solar absorption. Herein, the dopants used were titanium, as it is known in the literature as an excellent photocatalyst, and cerium that has a high oxidation and reduction capacity. The materials were synthesized by the hydrothermal method, for 20 h at 150 °C, and doped proportion were 0.5%, 0.8% and 1% mol. The final solids were evaluated concerning their structural (FTIR, Raman, XRD, TGA), morphological (SEM), optical (DRS in UV-Vis) properties. Doped niobia compounds are ready to be submitted to the photocatalytic study for pollutants degradation under solar radiation. The SIRIUS synchrotron facility will be essential to high-quality characterization of the compounds by means of EXAFS, XANES techniques to study the local structure of Ti species accommodated in Nb2O5 and XRD technique to characterize the structural changes caused by de dopant and the purity of the samples. (author)