Hard x-ray photon-in-photon-out spectroscopy with lifetime resolution – of XAS, XES, RIXSS and HERFD

Spectroscopic techniques that aim to resolve the electronic configuration and local coordination of a central atom by detecting inner-shell radiative decays following photoexcitation using hard X-rays are presented. The experimental setup requires an X-ray spectrometer based on perfect crystal Bragg optics. The possibilities arising from non-resonant (X-Ray Emission Spectroscopy - XES) and resonant excitation (Resonant Inelastic X-Ray Scattering Spectroscopy – RIXSS, High-Energy-Resolution Fluorescence Detected (HERFD) XAS) are discussed when the instrumental energy broadenings of the primary (beamline) monochromator and the crystal spectrometer for x-ray emission detection are on the order of the core hole lifetimes of the intermediate and final electronic states. The small energy bandwidth in the emission detection yields line-sharpened absorption features. In transition metal compounds, electron-electron interactions as well as orbital splittings and fractional population can be revealed. Combination with EXAFS spectroscopy enables to extent the k-range beyond unwanted absorption edges in the sample that limit the EXAFS range in conventional absorption spectroscopy

Species diversity of the Anatidae family as an indicator of sustainable development of territories (on the example of reservoirs of the Ulagan Plateau (Eastern Altai)

The article presents the results of ornithological studies carried out on the Ulagan plateau (Eastern Altai) due to increasing anthropogenic influence. This paper presents a species overview and features of the nesting biology of some species of the Anatidae Family. Considerable attention is paid to the features of spring arrivals, the course of nesting processes, spring and autumn migrations. The paper analyzes data on egg laying and nesting dates. This process includes such periods as: the beginning of oviposition, the period of mass oviposition and the latest ovipositions, the dates of the meetings of downy jackets, flaps, the dates of flights and the beginning of accumulations in preparation for long-distance migration are described. In the territory of the Altai Republic, this problem has been little studied and requires further research

Reaction between Thiophene and Ni Nanoparticles Supported on SiO2 or ZnO: In Situ Synchrotron X-Ray Diffraction Study.

International audienc

Reaction between Thiophene and Ni Nanoparticles Supported on SiO2 or ZnO: In Situ Synchrotron X-Ray Diffraction Study.

International audienc

Valence, exchange interaction, and location of Mn ions in polycrystalline MnxGa1−xN (x = 0.04)

We present an experimental study for polycrystalline samples of the diluted magnetic semiconductor MnxGa1−xN (x 0.04) in order to address some of the existing controversial issues. X-ray and neutron diffraction, x-ray absorption near-edge structure, and electron paramagnetic resonance experiments were used to characterize the structural, electronic, and magnetic properties of the samples, and inelastic neutron scattering was employed to determine the magnetic excitations associated with Mn monomers and dimers. Our main conclusions are as follows: (i) The valence of the Mn ions is 2+. (ii) The Mn2+ ions experience a substantial single-ion axial anisotropy with parameter D = 0.027(3) meV. (iii) Nearest-neighbor Mn2+ ions are coupled antiferromagnetically. The exchange parameter J = −0.140(7) meV is independent of the Mn content x; i.e., there is no evidence for hole-induced modifications of J towards a potentially high Curie temperature postulated in the literature

Activation of oxygen on gold-alumina catalysts: in situ high-energy resolution

Gold catalysis has received considerable attention in recent years. Particles of gold that are unsupported or supported on oxidic carriers have been reported to be very active in various oxidation reactions. Examples are the oxidation of CO to CO2, water-gas shift reaction (CO + H2O ? 2CO2 + H2) and oxidation of hydrocarbons. The origin of the high catalytic activity of gold catalysts has been strongly debated and various models have been presented. The major question to be answered is how the oxygen molecule is activated

Low-Temperature CO Oxidation over Combustion Made Fe- and Cr-Doped Co3O4 Catalysts: Role of Dopant’s Nature toward Achieving Superior Catalytic Activity and Stability

Co3O4 with a spinel structure shows unique activity for CO oxidation at low temperature under dry conditions; however the active surface is not very stable. In this study, two series of Fe- and Cr-doped Co3O4 catalysts were prepared by a single-step solution combustion technique. Fe was chosen because of its redox activity corresponding to the Fe2+/Fe3+ redox couple and compared to Cr, which is mainly stable in the Cr3+ state. The catalytic activity of new materials for low-temperature CO oxidation was correlated to the nature of the dopant. As a function of dopant concentration, the temperature corresponding to the 50% CO conversion (T50) demonstrated significant differences. The maximal activity was achieved for 15% Fe-doped Co3O4 with T50 of −85 °C and remained almost constant up to 25% Fe. In the case of Cr, the activity was observed to be maximum for 7% of Cr with T50 of −42 °C and significantly decreased for higher Cr loadings. Similarly, there was a contrasting behavior in catalyst stability too. 100% CO conversion was achieved below −60 °C for 15% Fe/Co3O4 catalyst and remained unchanged even after calcination at 600 °C. In contrast, Co3O4 or 15% Cr/Co3O4 catalysts strongly deactivated after the same treatment. These differences were correlated to the oxidation states, coordination numbers, the nature of surface planes, and the redox properties. We observed that both Cr and Fe were typically present in the +3 oxidation state, occupying octahedral sites in the spinel structure. The catalysts were mainly exposed to (111) and (220) planes on the surface. H2-TPR indicated clear differences in the redox activity of materials due to Fe and Cr substitutions. The reducibility of surface Co3+ species remained similar in all Fe-doped Co3O4 catalysts in contrast to nonreducible Cr-doped analogs, which shifted the reduction temperature to the higher values. As the Fe3+/Fe2+ redox couple partly substituted the Co3+/Co2+ redox couple in the spinel structure, similar bond strength of Fe–O keep redox activity of Co3+ species almost unchanged leading to higher activity and stability of Fe/Co3O4 catalysts for low-temperature CO oxidation. In contrast, nonreducible Cr3+ species characterized by strong Cr–O bond substituting active Co3+ sites can make the Cr/Co3O4 surface less active for CO oxidation

Identification of the active species in the working alumina-supported cobalt catalyst under various conditions of Fischer–Tropsch synthesis

International audienc

Stability and Reactivity of ϵ−χ−θ Iron Carbide Catalyst Phases in Fischer−Tropsch Synthesis: Controlling μC

The stability and reactivity of ϵ, χ, and θ iron carbide phases in Fischer−Tropsch synthesis (FTS) catalysts as a function of relevant reaction conditions was investigated by a synergistic combination of experimental and theoretical methods. Combined in situ X-ray Absorption Fine Structure Spectroscopy/X-ray Diffraction/Raman Spectroscopy was applied to study Fe-based catalysts during pretreatment and, for the first time, at relevant high pressure Fischer−Tropsch synthesis conditions, while Density Functional Theory calculations formed a fundamental basis for understanding the influence of pretreatment and FTS conditions on the formation of bulk iron carbide phases. By combining theory and experiment, it was found that the formation of θ-Fe3C, χ-Fe5C2, and ϵ-carbides can be explained by their relative thermodynamic stability as imposed by gas phase composition and temperature. Furthermore, it was shown that a significant part of the Fe phases was present as amorphous carbide phases during high pressure FTS, sometimes in an equivalent amount to the crystalline iron carbide fraction. A catalyst containing mainly crystalline χ-Fe5C2 was highly susceptible to oxidation during FTS conditions, while a catalyst containing θ-Fe3C and amorphous carbide phases showed a lower activity and selectivity, mainly due to the buildup of carbonaceous deposits on the catalyst surface, suggesting that amorphous phases and the resulting textural properties play an important role in determining final catalyst performance. The findings further uncovered the thermodynamic and kinetic factors inducing the ϵ−χ−θ carbide transformation as a function of the carbon chemical potential μC

Stability and Reactivity of ϵ−χ−θ Iron Carbide Catalyst Phases in Fischer−Tropsch Synthesis: Controlling μC

The stability and reactivity of ϵ, χ, and θ iron carbide phases in Fischer−Tropsch synthesis (FTS) catalysts as a function of relevant reaction conditions was investigated by a synergistic combination of experimental and theoretical methods. Combined in situ X-ray Absorption Fine Structure Spectroscopy/X-ray Diffraction/Raman Spectroscopy was applied to study Fe-based catalysts during pretreatment and, for the first time, at relevant high pressure Fischer−Tropsch synthesis conditions, while Density Functional Theory calculations formed a fundamental basis for understanding the influence of pretreatment and FTS conditions on the formation of bulk iron carbide phases. By combining theory and experiment, it was found that the formation of θ-Fe3C, χ-Fe5C2, and ϵ-carbides can be explained by their relative thermodynamic stability as imposed by gas phase composition and temperature. Furthermore, it was shown that a significant part of the Fe phases was present as amorphous carbide phases during high pressure FTS, sometimes in an equivalent amount to the crystalline iron carbide fraction. A catalyst containing mainly crystalline χ-Fe5C2 was highly susceptible to oxidation during FTS conditions, while a catalyst containing θ-Fe3C and amorphous carbide phases showed a lower activity and selectivity, mainly due to the buildup of carbonaceous deposits on the catalyst surface, suggesting that amorphous phases and the resulting textural properties play an important role in determining final catalyst performance. The findings further uncovered the thermodynamic and kinetic factors inducing the ϵ−χ−θ carbide transformation as a function of the carbon chemical potential μC