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.

— We have studied how the initial state of polycrystals of the metastable compounds Bi 2 GeO 5 and Bi 2 SiO 5 with an Aurivillius-type structure and a glass phase with the equimolar composition Bi 2 O 3 : SiO 2 influences the performance of in situ forming catalysts for oxidative dimerization of methane (ODM). The results confirm the effect of the gas phase consisting of the starting components of the catalyzed reaction on improvement of the performance of the catalyst obtained upon thermal decomposition of its initial metastable state: in situ catalyst “self-adjustment” effect. The best catalytic properties for ODM reaction are offered by bismuth silicate (Bi 2 SiO 5 ) decomposition products, and the highest activity is exhibited by the catalyst forming upon crystallization of the glass phase. Bismuth-containing oxide catalysts have been shown to have the best properties if high-purity starting reagents are used to synthesize them.

The effect of the HF modification of ferrospheres separated from fly ash after the combustion of brown coal on their chemical, phase compositions and catalytic properties in the oxidative coupling of methane was studied. The modification led to a change in the phase composition in comparison with that of the initial ferrospheres: a CaF2 phase appeared, the hematite phase content increased, and the ferrospinel content decreased. The yield of C2 hydrocarbons at 750°C increased by a factor of 1.5–2.0, and the fraction of ethylene in them increased to 30 or 65% at 750 or 850°C, respectively. It was assumed that an increase in the efficiency of HF-modified ferrospheres in the formation of ethane and its dehydrogenation into ethylene was due to the formation of oxyfluoride-type active sites. The pyrohydrolysis of fluorine-containing catalyst components at 850°C due to interaction with water vapor in a reaction atmosphere led to the formation of systems active in deep oxidation; this manifested itself in a sharp decrease in selectivity for the formation of C2 hydrocarbons and an increase in selectivity for CO2.

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.

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.

The single-stage separation of fly ash from field 1 of the electrostatic precipitator of Reftinskaya GRES (Regional Thermal Power Plant), which burns coal from the Ekibastuz Basin, was performed by aerodynamic classification with the use of a flow of air. It was found that, at a constant airflow rate of 50 m3/h, the morphologically uniform fractions of small spherical particles with a narrow distribution, for which dmax was 2, 3, and 4 μm and d90 was 4, 6, and 8 μm, can be separated by decreasing the speed of the classifier rotor from 22000 to 10000 rpm. At a minimum speed of 2000 rpm, the target product of separation was a narrow fraction containing large particles, which were characterized by dmax of 109 μm and d90 of 205 μm. It was established that SiO2 and Al2O3 were the basic chemical components of the fractions obtained, and the total concentration of these components was as high as 86–91 wt %. With increasing the size of fractions with dmax from 2 to 116 μm, a decrease in the concentration of SiO2 and an increase in the amounts of Al2O3 and Fe2O3 were observed. The dependence of phase composition on the particle size was established for the small fractions: as dmax was increased in a range of 2–25 μm, a monotonic decrease in the concentration of a quartz phase and an increase in the concentration of mullite were observed; the concentration of a Fe-containing spinel phase also increased.

Проведено сопоставление каталитических свойств ферросфер, содержащих 76–97 мас. % Fe2O3, в процессе окислительной конденсации метана с их фазовым составом и катионным распределением железа по кристаллографическим позициям железосодержащих фаз в стационарном состоянии. Установлено, что при содержании Fe2O3 89 мас. % маршрут окисления метана изменяется. На ферросферах с содержанием Fe2O3 < 88.8 мас. % основным маршрутом реакции является глубокое окисление. При содержании Fe2O3 89 мас. % резко увеличивается выход углеводородов C2 и уменьшается вклад глубокого окисления. Выход углеводородов C2 коррелирует с количеством дефектов в структуре феррошпинели, которые представляют собой ионы железа с тетраэдрическим катионом Ca2+ и октаэдрической катионной вакансией среди ближайших соседей.

The catalytic properties of ferrospheres containing 76–97 wt % Fe2O3 in the oxidative condensation of methane were compared with their phase composition and the distribution of iron cations over the crystallographic positions of iron-containing phases in a steady state. It was established that the reaction route of methane oxidation changed at a Fe2O3 content of 89 wt %. Deep oxidation was the main reaction route on ferrospheres with a Fe2O3 content of <88.8 wt %. At a Fe2O3 content of ≥89 wt %, the yield of C2 hydrocarbons sharply increased and the contribution of deep oxidation decreased. The yield of C2 hydrocarbons correlated with the amount of defects in the structure of iron spinel, which are iron ions with the tetrahedral cation of Ca2+ and the octahedral cation vacancy among the nearest neighbors.

A new possibility of changing the activity and selectivity of perovskite catalysts in the oxidative conversion of methane was demonstrated using the Sr x Gd1–x CoO3–δ (0.5 < x < 0.9) compounds as an example. It was established that, at the same chemical composition, the disordering of Sr2+/Gd3+ ions over the A positions of the crystal structure led to a significant increase in activity in the deep oxidation reaction of CH4, as compared with the samples with an ordered distribution of cations.

A new approach to the preparation of systems that exhibit catalytic activity in the oxidative coupling of methane (OCM) is considered. With the use of ferrospheres separated from power-generation ashes from different sources as an example, it was demonstrated that OCM catalysts can be prepared by the crystallization/solidification of oxide melts with the formation of microspherical particles. The dependence of activity and selectivity for the oxidative reforming of methane on the ferrospheres containing from 36.2 to 92.5 wt % Fe 2 O 3 into the products of deep oxidation and OCM was studied. It was found that deep oxidation reactions on ferrospheres with Fe 2 O 3 contents higher than 85% were suppressed, and the main reaction path of CH 4 conversion was its oxidative coupling with the formation of C 2 products (with selectivity to 60% at 750°C); moreover, the selectivity for C 2 formation in this region was proportional to the concentration of Fe 2 O 3 . Phases responsible for the catalytic conversion of methane into CO x and OCM products were considered, and it was shown that the catalytic activity and selectivity of the oxidative transformation of CH 4 on ferrospheres is determined by the position of the point that corresponds to their composition on a phase diagram of CaO-Fe 2 O 3 -SiO 2 .

Рассмотрен новый подход к получению систем, проявляющих каталитическую активность в реакции окислительной дегидродимеризации метана (ОДМ). На примере ферросфер, выделенных из энергетических зол различных источников, показана возможность получения катализаторов ОДМ путем кристаллизации/отверждения расплавов оксидов с образованием микросферических частиц. Изучена зависимость активности и селективности окислительного превращения метана на ферросферах, содержащих от 36.2 до 92.5 мас. % Fe2O3, в продукты глубокого окисления и ОДМ. Показано, что на ферросферах с содержанием Fe2O3 более 85% реакции глубокого окисления подавлены и основным направлением превращения СН4 является его окислительная дегидродимеризация с образованием продуктов С2 (с селективностью до 60% при 750°С), причем селективность образования С2 в этой области пропорциональна содержанию Fe2O3. Рассмотрены фазы, ответственные за каталитические превращения метана в СОх и продукты ОДМ, и показано, что каталитическая активность и селективность окислительного превращения СН4 на ферросферах определяется положением точки, отвечающей их составу на фазовой диаграмме CaO Fe2O3 SiO2.

Mössbauer, magnetic, and X-ray studies of magnetic microspheres separated from energy ashes have been performed. The major component of the composition of the microspheres is an imperfect magnetite with cation vacancies in which iron is located in trivalent and mixed-valence positions characteristic of stoichiometric magnetite. Moreover, the Fe 3+ and Fe 2.5+ positions with a cation vacancy among the nearest neighbors and the positions with a partial localization of 3 d electrons have been identified.

The application of magnetic cenospheres including ferrospinel nanoparticles as a “transport container” in medicine and biology is quite attractive. Their nanoscale size, high porosity, and well pronounced magnetic properties provide good consumer choice. In this contribution, we report on the magnetic properties of microspheres from energetic ashes of Ekibastuz coals.

Phase composition of six narrow size fractions of microspheres extracted from fly ashes of Ekibastuz coals has been determined by Mössbauer spectroscopy. The basic phase is defect magnetite substituted by Al, Mg, and Ti ions. The magnetic properties of microspheres depend on the content of iron and its distribution over spinel positions.

The photoinduced absorption in a Ca-doped bismuth titanate crystal is measured as a function of temperature in the red (660 nm), yellow (570 nm), and green (505 nm) spectral regions.