State Of Iron Atoms Research Articles

Insights into the Reactivity of Gd2−xSrxFe2O7 (x = 0 ÷ 0.4) in CO Radical Hydrogenation

The effect of strontium substitution in the structure of the complex oxide Gd2SrFe2O7 on the production of light olefins by CO hydrogenation was investigated. Perovskite-type oxides Gd2−xSr1+xFe2O7 (x = 0; 0.1; 0.2; 0.3; 0.4) were synthesized by sol–gel technology and characterized by XRD, Mössbauer spectroscopy, BET specific area, acidity testing, and SEM. The experimental data revealed a correlation between the state of iron atoms, acidity, and catalytic performance. It was found that with an increase in the content of Sr2+ in the perovskite phase, the basicity of the surface and the oxygen diffusion rate increased. This contributed to the CO dissociative adsorption, formation of active carbon, and its further interaction with atomic hydrogen.

Crystal structure and optical properties of the Bi–Fe–W–O pyrochlore phase synthesized via a hydrothermal method

The present work investigated the influence of the hydrothermal synthesis conditions (hydrothermal fluid pH) on the composition, structural parameters, morphology, and optical properties of nanocrystalline particles based on a phase of variable composition that forms in the Bi2O3–Fe2O3–WO3 system and has the cubic pyrochlore structure A2B2O6O′δ = (Bi, Fe, VA)2(Fe, W)2O6O′δ , where VA are cation vacancies at the A-site (hereinafter BFWO). The BFWO phase structural parameters were refined by the Rietveld method, and the distribution of Fe over the structurally nonequivalent sites was established. It was shown that the fraction of Fe at the A-site out of the total amount of Fe depends on the BFWO chemical composition and can vary from 0 to ~15%. The results of the Mössbauer study showed that Fe atoms are characterized by two states that differ significantly in terms of the nearest environment, but it was found impossible to explain the differences in the state of iron atoms by their simultaneous presence at the sites A and B. According to the X-ray diffraction (XRD) data, the average crystallite size was shown to change depending on the conditions of hydrothermal synthesis. When the values of hydrothermal fluid pH increased, the average crystallite size first increases from ~78 nm at pH 2 up to 181 nm at pH 5, and then decreases to ~133 nm at pH 7. The scanning electron microscopy (SEM) data have shown that crystallites in all samples grow together into aggregates, the shape of which is close to spherical. The average aggregate size depends on the hydrothermal fluid pH in a similar way: when pH increases, the aggregate size first increases from ~330 nm at pH 2 up to 880 nm at pH 5, and then decreases to ~340 nm at pH 7. The band gap (Eg) of the obtained materials for direct allowed electronic transitions was calculated from the diffuse reflectance spectrum data and found to be in the 2.16–2.41 eV range. The Eg value was shown to decrease with the enrichment of the BFWO composition in Bi and Fe and with a decrease in the volume fraction of crystallites that make up the surface layer of aggregates and have a size of about 30–40 nm.

Template Synthesis of Iron Microstructures Based on Track Membranes

Iron nanowires (NWs) were obtained by template synthesis using polymer track membranes in the framework of development of a general method for the synthesis of regular metal nanostructures. The surface topography of NWs was studied by scanning electron microscopy, and the local state of iron atoms was determined by Mossbauer spectrometry.

Magnetic susceptibility of solid solutions Bi2SrNb2−2xFe2xO9−δ

The state of iron atoms and their interatomic interactions were investigated by the method of magnetic dilution in Bi2SrNb2−2xFe2xO9−δ with a layered perovskite-like structure. It was found that the solid solutions are characterized by increased values of the magnetic moment of iron atoms, compared with purely spin values of Fe(III). This was explained by the presence of exchange-bound aggregates of Fe(III) atoms with antiferro- and ferromagnetic types of exchange. The dependencies of the exchange parameters and cluster distribution on the iron content of Bi2SrNb2−2xFe2xO9−δ solid solutions were calculated.

Iron state in iron nanoparticles with and without zirconium

Mossbauer and X-ray methods are used for investigations of structure, stability and characteristics of pure-iron grain and two iron-zirconium alloys such as Fe + 5 wt.% Zr and Fe + 10 wt.% Zr. The used powder was ground for 24 h in a SPEX Model 8000 mill shaker. Complex nanoparticles are found, which change their properties under milling. Mossbauer spectral parameters are obtained for investigated materials. Milling results in formation of nanosized particles with two states of iron atoms: one main part is pure α-Fe and another part of iron atoms displaced in grain boundaries or defective zones in which hyperfine magnetic splitting decrease to ∼ 30.0 T. In alloys with Zr three iron states are formed in each alloy, main part of iron is in the form of α-Fe and another two states depend on the concentration of Zr and represent iron in grain boundaries with Zr atoms in nearest neighbor. The changing of iron states is discussed.

Structure, magnetic, and electrical properties of bismuth niobates doped with d-elements: XV. Exchange interactions and state of iron atoms in the Bi5Nb3–3x Fe3x O15–δ solid solutions

Magnetic susceptibility of Bi5Nb3–3x Fe3x O15–δ (х ≤ 0.06) iron-containing solid solutions with perovskite-like structure was studied. Exchange-bound aggregates of Fe(III) atoms with antiferro- and ferromagnetic exchange types are formed in the solid solutions. The exchange parameters in the clusters and the distribution of tetramers and dimers of Fe(III) atoms over the solid solutions as functions of paramagnetic atom content were calculated.

New Mixed Perovskite-Like Catalysts Gd2–xSr1+xFe2O7 for Carbon Dioxide Conversion of Methane and for Production of Light Olefins

Catalytic properties of complex perovskite-like gadolinium and strontium oxides were studied during carbon dioxide conversion of methane and production of gaseous olefins via hydrogenation of carbon monoxide. The Gd 2 SrFe 2 O 7 and Gd 2– x Sr 1+x Fe 2 O 7 ( x = 0.1; 0.2; 0.3; 0.4) samples were prepared by the sol-gel method and ceramic technology, and characterized using XPS, SEM, photon correlation spectroscopy, Moessbauer spectroscopy and low-temperature nitrogen desorption. Better catalytic performance was observed with the catalysts prepared by the sol-gel method but not with the ceramic systems. Non-isovalent substitution of Sr 2+ for Gd 3+ resulted in distortion of the complex oxide structure and in the appearance of heterovalent states of iron atoms (Fe 3+ and Fe 4+ ) that affected the reactant conversions and selectivities to the target products. The Gd 2– x Sr 1+x Fe 2 O 7 ( x = 0.3) catalyst was most active to carbon dioxide conversion of methane and most selective to unsaturated hydrocarbons (ethylene and propylene) during hydrogenation of carbon monoxide.

Exchange interactions and the state of iron atoms in Bi3Nb1−xFexO7−δ

On the basis of the results of magnetic susceptibility and ESR studies of the Bi3Nb1−xFexO7−δ solid solutions iron atoms in the solid solutions of cubic modification of bismuth niobate were found to exist as Fe(III) monomers and exchange bound Fe(III)-O-Fe(III) dimers with antiferro- and ferromagnetic type of superexchange. The exchange parameters and the distribution of monomers and dimers in the solid solutions were calculated as a function of paramagnetic atom content.

Structure, magnetic, and electrical properties of bismuth niobates doped with d-elements: XIII. State of iron atoms in the Bi3Nb1–xFexO7–δ solid solutions

From the data of magnetochemical study and ESR spectra of samples of the Bi3Nb1–xFexO7–δ (х ≤ 0.06) solid solutions iron atoms in the solid solutions of cubic modification of bismuth niobate were found to be located as Fe(III) monomers and exchange-bonded Fe(III)–O–Fe(III) dimers with anti- and ferromagnetic exchange types. The exchange parameters and the distribution of monomers and dimers over the solid solution were calculated as functions of the paramagnetic atom fraction.

Mössbauer investigation of a nepheline substrate used for preparing a water treatment reagent

The aim of this work is to reveal the results of the reaction of nepheline mineral with concentrated sulfuric acid. The valence states of iron atoms are determined in the nepheline and the reacted nepheline. The existence of iron containing magnetic particles in the sediment is shown. The differences in the lineshape are discussed also and are associated with a transfer from one phase to another of the magnetic particles.

Phonons and the electronic gap in FeSi

The partial phonon densities of states of iron atoms in the intermetallic compound FeSi have been measured in the temperature range 46–297 K using nuclear resonant inelastic scattering of synchrotron radiation. A significant phonon softening with increasing temperature has been established. The greatest phonon softening for iron atoms is shown to occur in the region of long-wavelength acoustic phonons, for the acoustic branches near the boundary of the Brillouin zone, and for the low-lying weakly dispersive optical branches. The results obtained are analyzed in terms of the views that relate the change in the phonon density of states of iron atoms to the temperature evolution of the electronic density of state for the compound.

Changes in the state of iron atoms in Zr alloys during corrosion tests in an autoclave

Mossbauerinvestigations were carried out on oxide films formed on specimens of zirconium alloys Zr-1.0 %wtFe-1.2 %wtSn-0.5 %wtCr subjected to corrosion in steam-water environment at a temperature of 360 °C and at a pressure of 16.8 MPa with lithium and boron additions, and on Zr-1.4 %wtFe-0.7 %wtCr corroded in steam-water environment at 350 °C and 16.8 MPa as well as in steam-water environment at 500 °C and 10 MPa. In the metal part of the samples, under the oxide film, the iron atoms are in form of intermetallic precipitates of Zr(Fe, Cr)2. The corrosion process decomposes the intermetallic precipitates and particles are formed of metallic iron with inclusions of chromium atoms –Fe(Cr), α–Fe2O3 and Fe3O4 compounds. Part of the iron ions are in divalent and part in trivalent paramagnetic states. It is proposed that some part of the iron containing oxide precipitates in the oxide film may be in the form of nanoparticles which pass from the superparamagnetic to the ferromagnetic state with decreasing temperature.

Local atomic and magnetic structures of nanocrystalline Fe75Cr10B15 alloy

The crystal, local atomic and magnetic structures of Fe75Cr10B15 alloys annealed at 440–473°C for 5 min have been studied using X-ray diffraction and 57Fe Mossbauer spectroscopy. At the annealing temperature T a = 440°C, nanocrystals of the α-Fe phase (∼1%) precipitate in the amorphous matrix of the alloy. The complete crystallization of the amorphous alloy occurs at T a = 473°C with the formation of α-Fe nanocrystals 26 ± 2 nm in size and nanocrystals of tetragonal boride t-Fe3B 47 ± 2 nm in size. It has been found that chromium atoms are located in nanocrystals of the α-Fe and y-Fe3B types. The distribution functions of hyperfine fields in the nanocrystalline Fe75Cr10B15 alloy reconstructed from the Mossbauer spectra (at T a = 473°C) show that there are three allowed states of iron atoms in the α-Fe phase and three equally probable crystallographic nonequivalent states of iron in the t-(Fe,Cr)3B phase. The chromium concentration x in the α-Fe(Cr) phase is found to be ∼10 at %. The substitution of chromium atoms for iron atoms in t-Fe3B substantially decreases local magnetic moments of the iron atoms.

Changes in the iron atomic state in the near-surface layers of zirconium alloys irradiated by Ar+ ions

The effect of irradiation by an argon ion beam with a broad energy spectrum and an average energy of 10 keV in a dose range of (1−10) × 1018 ion cm−2 on the state of iron atoms in the near-surface layers of zirconium alloys Э635, NSF, and Zry-2 was studied by means of conversion electron Mossbauer spectroscopy (CEMS).

Fe3s X-ray photoelectron spectra of polynuclear iron complexes

Specific features of the electronic structure and spin magnetic state of iron atoms in bi-, tri-, and hexanuclear iron trimethylacetate complexes were studied by X-ray photoelectron spectroscopy. A correlation was found between the ionicity (of the spin state of iron atoms) and Fe3s binding energies, exchange splitting of the final photoionization state, and the energy position and intensity of charge-transfer satellites. Nonequivalent iron states were identified in tri- and hexanuclear complexes. The overall magnetic moment of the complexes was found to decrease with an increase of the individual magnetic moments of iron atoms, which is evidence of complicated mutual orientation of atomic magnetic moments in the complexes.

Short-range order in amorphous ferromagnetic Fe-B alloys

Amorphous and quenched crystalline Fe-B alloys in the composition range of 4–25 at % B were prepared by melt spinning and investigated by 57Fe Mossbauer spectroscopy at T = 87 K. The states of iron atoms in the α-Fe phases, including iron atoms having boron atoms in the nearest coordination sphere, and in the orthorhombic (o) and tetragonal (T) Fe2B phases are detected in the microcrystalline alloys. The short-range order and the local atomic structure of the amorphous Fe-B alloys are determined. The amorphous alloys consist of microregions (clusters) with short-range order of the t- and o-Fe2B and α-Fe types. The dependence of the content of various types of clusters on the alloy composition is quantitatively estimated.

Synthesis of Fe,Al-pillared clays starting from the Al,Fe-polymeric precursor: Effect of synthesis parameters on textural and catalytic properties

Effect of OH/(Fe+Al) ratio, aging time of Fe,Al-containing pillaring solution on physicochemical properties of Fe,Al-pillared clays (Fe,Al-PILCs) obtained from Mukhortala (Buryatia) montmorillonites and the change of these properties with the calcination temperature were examined. Characterization studies were performed by use of XRD, FTIR, DR-UV-vis, 27Al NMR and N2-adsorption/desorption analysis. It was found that the prolonged aging time of the Fe,Al-solution (Al/Fe=10/1) favours the increase in total surface area, total pore-volume and micropore volume of Fe,Al-PILCs. The relationships between preparation conditions, iron state, catalytic activity and stability to leaching were revealed in hydrogen peroxide oxidation of phenol and monoazo dye acid chrome dark-blue (ACDB). The state of iron atoms can be controlled by Al/Fe and OH/(Fe+Al) ratios, aging period of Fe,Al-pillaring solution and calcination temperature. The increase in Al/Fe ratio and calcination temperature from 400°C to 500°C reduces to the decrease in the formation of oligomeric iron species. The increase in aging time of Fe,Al-pillaring solution reduced to decrease in iron leaching and increase in reaction rate due to the formation of isolated iron species.

State of iron atoms in synthetic ferrite garnets and zirconolites—matrices for disposal of actinide-containing waste

Synthetic ferrite garnets and zirconolites containing La, Ce, Gd, and Th as simulators of radioactive actinides have been investigated by 57Fe Mossbauer spectroscopy. The state of iron atoms in the samples studied and their distribution over crystallographically nonequivalent structural positions are established. The crystallochemical formulas are refined and space groups are determined for a number of ferrite garnets and zirconolites.

Full phenol peroxide oxidation over Fe-MMM-2 catalysts with enhanced hydrothermal stability

Iron-containing mesoporous mesophase materials Fe-MMM-2 have been synthesized by a sol–mesophase route under mild acidic conditions and characterized by DRS-UV–vis, XRD, and N2 adsorption measurements. It was found that pH of the synthesis solution and iron content in the samples affect both the textural characteristics and the state of iron atoms. Isolated iron species predominate in silica framework under Fe<2wt% and pH∼1.0 or Fe∼1wt% and pH<2.0. These species are stable to leaching and highly active in full H2O2-based phenol oxidation. The increase in iron loading and pH of the synthesis solution lead to the agglomeration and formation of oligomeric iron species, which, in turn, results in the reduction of the catalytic activity of Fe-MMM-2 and the increase of iron leaching.

Specific Features of the Structural State of Iron Ions in Iron-Containing Composites Irradiated by Fast Electrons

The present paper analyzes the valence-coordination state of iron atoms in polymer composite materials fabricated from polystyrene and iron-ore concentrates – magnetite and hematite – upon exposure to fast electrons with energy of 6.2 MeV and fluence of 1018 electrons/cm2. Changes in the phase content and valence-coordination and magnetic states of iron atoms are established. After irradiation, the hematite phase is transformed into the magnetite phase with a predominance of Fe3+ ions with [FeO4] coordination, and the magnetite phase forms the wustite-type (FeO) phase in which Fe2+ ions are arranged in [FeO6] coordination.