The catalysts of the CaO‒Fe2O3 system with Fe2O3 content in the range of 0‒100 wt.% were synthesized by the solid state method at 900 and 1000 °C. The catalysts were characterized by XRD and SEM‒EDX methods, and their activity in the oxidative conversion of methane at 750 °C was studied. The phase composition of the catalysts corresponds to the CaO‒Ca2Fe2O5, Ca2Fe2O5‒CaFe2O4 and CaFe2O4‒a-Fe2O3 regions of the phase diagram. The specific catalytic activity dependence on the Fe2O3 content has an extremum. The CaO‒Ca2Fe2O5 catalysts have the highest reactivity and the active centers in them are localized at the phase interface. The activity of Ca2Fe2O5–CaFe2O4 samples decreases with an increase in the CaFe2O4 content. The CaFe2O4‒a-Fe2O3 catalysts have a core-shell structure and exhibit the least activity, which is determined by the CaFe2O4 shell.

Using the methods of X-ray diffraction, Mössbauer spectroscopy, and temperature-programmed reduction with hydrogen, the relationship between the phase composition, structural characteristics of the phases, and the reactivity with respect to hydrogen was investigated for calcium ferrites-based catalysts. The catalyst samples were prepared via solid-state synthesis from CaO and Fe2O3 at 900 and 1000°C by varying the Fe2O3 content in the CaO–Fe2O3 system. The phase composition of the resultant samples corresponds to the CaO‒Ca2Fe2O5, Ca2Fe2O5‒CaFe2O4, and CaFe2O4‒α-Fe2O3 regions. In the CaO–Ca2Fe2O5 samples the lattice parameters of Ca2Fe2O5 and its activity with respect to hydrogen depend on the phase ratio. The activity of CaFe2O4 is higher in Ca2Fe2O5‒CaFe2O4 compared to CaFe2O4‒α-Fe2O3 catalysts.

The effect of temperature and duration of calcination of single-phase samples of hematite with the alpha-Fe2O3 structure on the oxidizing ability with respect to hydrogen in the temperature-programmed reaction mode in the temperature range of 40-900 degrees C was studied. It is shown that the calcination temperature is a significant factor affecting the reactivity of lattice oxygen in the oxidation of hydrogen. Samples of alpha-Fe2O3, calcined at 800-900 degrees C, show the highest activity, the process of alpha-Fe2O3 reduction in these samples proceeds through the stage of reduction to magnetite, followed by complete reduction to metal through the combination of reduction stages of oxides. The calcination of alpha-Fe2O3 samples at 1000-1100 degrees C leads to a significant decrease in the oxidizing ability, the alpha-Fe2O3 reduction initiates at temperatures 50-100 degrees C higher, proceeds without separating individual reduction stages of oxide, there is no complete reduction of alpha-Fe2O3 under the studied conditions. It has been established that with an increase in the calcination temperature of the hematite samples, the X-ray density of alpha-Fe2O3 increases, which indicates a decrease in the degree of crystal lattice disorder and an increase in the binding energy of lattice oxygen and manifests itself in a significant decrease in the reactivity of alpha-Fe(2)O(3)in the oxidation of hydrogen.

In this paper, the relationship between the structural features of hematite samples calcined in the interval of 800–1100 °C and their reactivity regarding hydrogen studied in the temperature-programmed reaction (TPR-H2) was studied. The oxygen reactivity of the samples decreases with the increasing calcination temperature. The study of calcined hematite samples used X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), X-ray Photoelectron Spectroscopy (XPS), and Raman spectroscopy, and their textural characteristics were studied also. According to XRD results, hematite samples calcined in the temperature range under study are monophase, represented by the α-Fe2O3 phase, in which crystal density increases with increasing calcination temperature. The Raman spectroscopy results also register only the α-Fe2O3 phase; the samples consist of large, well-crystallized particles with smaller particles on their surface, having a significantly lower degree of crystallinity, and their proportion decreases with increasing calcination temperature. XPS results show the α-Fe2O3 surface enriched with Fe2+ ions, whose proportion increases with increasing calcination temperature, which leads to an increase in the lattice oxygen binding energy and a decrease in the α-Fe2O3 reactivity regarding hydrogen.

The samples with the CaFe2O4-type crystal structure were obtained by the solid-state reaction method at 1000 degrees C in the air and the helium atmosphere for the first time. We investigated the modification of the structural and electronic properties of the obtained samples. Mossbauer, XAFS-, XPS-spectroscopies, and dc-, ac-conductivity measurements were carried out. Mossbauer and XAFS-spectroscopies showed that the local environment of Fe and Ca cations does not change in the case of the inert atmosphere synthesis. Nevertheless, a sharp six-order increase in the electrical resistance observed at room temperature for the sample obtained in the in the helium atmosphere. Moreover, calculated from dc-conductivity data activation energy rises from 0.327 for the air-synthesized sample to 0.585 eV for helium-obtained one. This behavior indicates significant modification of in-band-gap energy structure, which correlated with thermally activated charge carriers. Our ac-conductivity measurements in the frequency range of 1 kHz -2 MHz for the CaFe2O4 obtained in the air showed the presence of defect levels in the energy band structure. Oxygen pressure reduction during the synthesis results in levels vanishing. Therefore, we suppose the key role of oxygen atoms in the transport properties of the material, which is indirectly confirmed by XPS data. In prospect, CaFe2O4 can be used in promising gas analyzers. (C) 2019 Elsevier B.V. All rights reserved.

In this paper, α-Fe2O3–CaFe2O4 composite materials obtained by high-temperature solid-phase synthesis from Ca and Fe (III) oxides with varying molar ratio CaO/Fe2O3 in the range 0.15-1.00 were investigated. The materials are characterized by Х-ray diffraction (XRD), scanning electron microscopy with energy-dispersive X-ray microanalysis (SEM-EDS) and simultaneous thermal analysis (STA) in the hydrogen temperature-programmed reduction mode (H2-TPR). SEM-EDS studies of the specimens were revealed a formation of the “core-shell” type complex microstructure of material with the hematite phase as the “core”. H2-TPR of the specimens allowed to establish a decrease of the contribution of low-temperature forms of lattice oxygen in areas of 350-510 °С (up to 2.6 times) and 510-650 °С (up to 1.7 times), and the growth of the contribution of the high-temperature oxygen form in the range of 650-900 °С (up to 2 times) with an increase in the content of the phase CaFe2O4 from 33.4 to 97.5 wt. %. Relying on the assessment of lattice oxygen mobility, it was suggested, that the samples with content of CaFe2O4 phase more than 55.4 wt. % are promising for use as oxygen carriers in chemical looping processes of syngas production.

Методом твердофазного синтеза при температуре 1000 °C получены структурированные композиционные материалы СaFe2O4—α-Fe2O3 (содержание α-Fe2O3 2—82 мас.%). Изучение фазового состава образцов выполнено методом рентгеновской дифракции. Показано, что в зависимости от состава шихты изменение содержания фаз СaFe2O4 и α-Fe2O3 происходит по линейному закону. С помощью сканирующей электронной микроскопии показано формирование двухфазной системы α-Fe2O3—СaFe2O4. Методом мёссбауэровской спектроскопии при комнатной температуре установлено формирование катионных вакансий железа в кристаллической структуре СaFe2O4 при отсутствии структурных дефектов α-Fe2O3. Показано, что катионные вакансии могут формироваться при синтезе образцов в воздушной атмосфере.

Structured composite materials CaFe 2 O 4 -α-Fe 2 O 3 (α-Fe 2 O 3 content is 2–82 wt.%) are obtained with the method of solid-phase synthesis at 1000 °C. The phase composition of the samples is studied using powder X-ray diffraction. It is shown that the content of CaFe 2 O 4 and α-Fe 2 O 3 phases changes linearly, depending on the composition of the starting material. The scanning electron microscopy data indicate the formation of a two-phase system α-Fe 2 O 3 -CaFe 2 O 4 . The Mössbauer spectroscopy data at room temperature testify the formation of cationic iron vacancies in the CaFe 2 O 4 crystal structure in the absence of α-Fe 2 O 3 structural defects. Cationic vacancies can be formed during the synthesis in the atmosphere of air.

The phase transformation behavior of a single-phase tetragonal Sr0.8Gd0.2CoO3-delta(with ordered distribution of Sr2+/Gd3+ cations and anion vacancies) was investigated by TG-DSC and XRD at 1100-1473 K and oxygen partial pressure p(O-2) from 1 to 5.10(4) Pa. The first-order smeared order-disorder (o-d) phase transition involving heat absorption was observed at about 1383 K under O-2-Ar flow with p(O-2) > 2.5 kPa. The crystal structure of the high-temperature phase was found to be cubic perovskite with disordered Sr2+/Gd3+ cations and anion vacancies. The temperature of o-d transition at p(O-2) > 2.5 kPa was not influenced by the heating rate or oxygen partial pressure. It was shown that at p(O-2) < 2.5 kPa the o-d transition gives rise to Sr0,8Gd0.2CoO3-delta decomposition to form CoO and Sr0.8Gd0.2CoO3-delta - Ruddlesden-Popper type phase with novel composition. The phase boundaries between the ordered tetragonal Sr0.8Gd0.2CoO3-delta, disordered cubic Sr0.8Gd0.2CoO3-delta and CoO + Sr0.8Gd0.2CoO3-delta composite as a function of temperature and p(O-2) values were determined.

Magnetic Ni(2+)-zeolite/ferrosphere and Ni(2+)-silica/ferrosphere beads (Ni-ferrosphere beads - NFB) of a core-shell structure were synthesized starting from coal fly ash ferrospheres having diameters in the range of 0.063-0.050 mm. The strategy of NFB fabrication is an oriented chemical modification of the outer surface preserving the magnetic core of parent beads with the formation of micro-mesoporous coverings. Two routes of ferrosphere modification were realized, such as (i) hydrothermal treatment in an alkaline medium resulting in a NaP zeolite layer and (ii) synthesis of micro-mesoporous silica on the glass surface using conventional methods. Immobilization of Ni(2+) ions in the siliceous porous shell of the magnetic beads was carried out via (i) the ion exchange of Na(+) for Ni(2+) in the zeolite layer or (ii) deposition of NiO clusters in the zeolite and silica pores. The final NFB were tested for affinity in magnetic separation of the histidine-tagged green fluorescent protein (GFP) directly from a cell lysate. Results pointed to the high affinity of the magnetic beads towards the protein in the presence of 10 mM EDTA. The sorption capacity of the ferrosphere-based Ni-beads with respect to GFP was in the range 1.5-5.7 mg cm(-3).

We have investigated the x-ray diffraction (XRD) structure, magnetic susceptibility, and heat capacity of GdCoO3 in a wide temperature range. A model of phase separation of the low-spin (LS) and high-spin (HS) states has been proposed based on the analysis of XRD peak shape anomalies in the temperature range 200-800 K. From magnetic measurements we separated the HS Co3+ contribution and fitted it with the temperature-dependent spin gap. We found a smooth LS-HS crossover at T = 800 K. The possible contribution of the intermediate spin (IS) state to the thermodynamics is excluded by the calculation IS-LS excitation energy within the modified crystal-field approach. In the two-phase model, with HS/LS probabilities calculated from the found spin gap and the LS and HS volumes calculated by the DFT-GGA method, we were able to reproduce the temperature dependence of the unit-cell volume and thermal expansion. Thus, we conclude that in GdCoO3 the main mechanism of the lattice expansion is not the conventional lattice anharmonicity, but the HS/LS fluctuations. The electronic structure has been calculated by the LDA+GTB method. At zero temperature, we have obtained the charge-transfer insulator with the charge gap E-g = 0.5 eV. The thermal population of the HS term results in the in-gap band formation inside the insulator gap and smooth insulator-metal transition at T-IMT = 780 K. Heat-capacity measurements revealed a smooth maximum near the T-IMT.

Hollow microsphere zeolite materials with a bilayered zeolite/glass crystalline shell bearing NaP1 zeolite were synthesized by the hydrothermal treatment of coal fly ash cenospheres (Si/Al = 2.7) in an alkaline medium. Cs+ and/or Sr2+ of zeolitized cenospheres with the different Cs+ and/or Sr2+ loading were prepared by the ion exchange from nitrate solutions. The resulted (Cs,Na)P1, (Sr,Na)P1 and (Cs,Sr,Na)P1 bearing microsphere zeolites were converted to glass ceramics by heating at 900-1000 degrees C.The differential scanning calorimetry and quantitative phase analysis were used to monitor the solid-phase transformation of the initial and ion exchanged zeolite materials. It was established that the final solidified forms of Cs+ and/or Sr2+ are glass-crystalline ceramic materials based on pollucite-nepheline, Sr-feldspar-nepheline and Sr-feldspar-pollucite composites including similar to 60 wt.% of the major host phases (pollucite, Sr-feldspar) and 10-20 wt.% of glass.The Cs-137 leaching rate of 4.1 x 10(-7) g cm(-2) day(-1) was determined for the pollucite glass-ceramic according to Russian State Standard (GUST) No. 52126 P-2003 (7 day, 25 degrees C, distilled water). (C) 2013 Elsevier B.V. All rights reserved.

One-step procedure to fabricate hollow aluminosilicate microspheres with a composite NaA(NaX)/glass/mullite composite shell was demonstrated by the direct conversion of low-silica cenospheres (Si/Al=1.8) separated from coal fly ash. The reported method does not involve seeding and structure-directing reagents. Composition of cenospheres and the stirring of reactants at the optimal synthesis temperature (T=80°C) and alkaline molarity (2.5M NaOH) are parameters affected the formation of a monozeolitic phase (NaA or NaX). The influence of the silica–alumina composition of cenospheres on the type of zeolite phase formation is only revealed under stirring conditions. In the case of the low-silica cenospheres, NaA (LTA) zeolite crystallizes as a dominant phase (43wt.%) (Si/Al=1.8) under stirring of reactants, while the static conditions favor formation of the NaX (FAU) zeolite phase (45wt.%).

The use of the products formed in combustion of the mineral part of coal (cenospheres) that are close in the ratio SiO2/Al2O3 to crystalline aluminosilicates of the structural type of rock-forming granitoid minerals, in processing and burying radioactive wastes (RAW) allows one to solve the problems of waste minimization both in nuclear and fuel power engineering. Due to the application of microspehric zeolites and porous materials based on cenospheres of the ash from the combustion of the Kuznetsk coal for solidification of liquid RAW containing cesium and strontium radionuclides, it becomes possible to obtain glass crystal compounds under rather soft conditions (750-900 degrees C). Under these conditions, mineral-like phases of feldspar and feldspathoids fixing radionuclides in their lattice are formed. The target phases content reaches 66-80 %.

AbstractThe paper describes the studies of the transformation of Cs+- and Sr2+-containing zeolite sorbents synthesized from fly ash cenospheres to crystalline mineral composition, suitable for the long-term disposal. Series of Cs+- and Sr2+-exchanged NaP1-containing sorbents were subjected to the thermochemical transformation in the temperature range 40-1100°C at atmospheric pressure in air and the progress of reaction was monitored by DSC and XRD analysis. It was shown that initial sodium zeolite undergoes two-step transformation at 736-785°C and 892-982°C forming nepheline as the principle product, with the conversion temperatures being dependant on the heating rate.The thermal treatment of Cs+-bearing zeolite sorbent led to formation of a complex multiphase system, the principal components of which were nepheline and pollucite. Increasing cesium content in the samples led to a monotonous shift of crystallization peak to the higher temperature range (1005-1006°C). A more complicated behavior was observed for Sr2+-containing samples, for which the crystallization temperature tends to increase (compared with NaP1) at lower Sr contents, but it starts decreasing parallel to the Sr2+ content at Sr2+ loadings >10 mg/g. The principal crystalline phases in Sr-NaP1 sample conversion were nepheline and Sr2+-containing feldspar, the quantity of which increased parallel to the increase of strontium content in zeolite.Apparent activation energies of thermochemical transformations were calculated and possible approaches to reduce transformation temperature are discussed and experimentally illustrated.

The possibility of immobilizing liquid radioactive wastes into polyfunctional microspherical materials of the block and powdered types is demonstrated. These materials are intended for reprocessing radioactive wastes of different compositions and make it possible to perform a multistage process of conditioning radioactive wastes under relatively mild conditions (at temperatures below 1000°C) with the conversion of water-soluble cesium and strontium compounds into water-insoluble mineral forms in the course of solid-phase transformations. Owing to the aluminosilicate composition of microspherical components of energy ashes (cenospheres), the cenospheres can serve as precursors of aluminosilicate granitoid minerals. Different techniques are proposed and conditions are experimentally determined for transforming precursors into final mineral-like materials of the predicted structure types chosen in the framework of the geoecological approach. The framework structures of aluminosilicates and phosphates thus formed can fix cesium and strontium in the crystal lattice and ensure the geochemical equilibrium between the matrix and incorporating granitoid rocks under conditions of long-term disposal.

For the first time, adding no seeding agent and no structure-forming components, the synthesis of microspheric zeolite sorbents is realized basing on vitrocrystalline cenospheres from power station ashes. As the result of hydrothermal processing the silica-alumina material of the wall of cenospheres undergoes transformation into low-module zeolites conserving the morphology of initial particles. The final product represents hollow spheres 80-200 mu m in size whose walls entirely or partially consist of the crystals of NaA, NaX and NaP1 zeolites.

Oxidative conversion of methane, ethane, propane, benzene, hydrogen and their binary mixtures R1–R2–N2O–He (where R1, R2 are substances under study) were studied on HZSM-5 at 350–450°C. Relative reactivities were estimated, rate of conversion of hydrocarbons correlating with the strength of CH bond. 13C label distribution in the product of 13CH4–C6H6–N2O feed was studied by GC-MS, 1H and 13C NMR. It was shown that methane was capable to alkylate the aromatic ring under reaction condition, giving toluene and xylenes from benzene.