Lyubov A. Isupova

The influence of correlations in the distribution of Ruddlesden–Popper (RP) faults on the diffraction patterns of tetragonal A ₂ B O ₄ -type structures is investigated, using Sr ₂ TiO ₄ as a case study. A combination of X-ray diffraction and high-resolution transmission electron microscopy reveals the presence of both isolated RP faults and coherently intergrown nanoscale domains structurally similar to Sr ₃ Ti ₂ O ₇ within the Sr ₂ TiO ₄ matrix. Simulations based on a one-dimensionally disordered structural model, performed using the DIFFaX and DEFECT programs, show that increasing P ₂₂ , the conditional probability of one RP fault being immediately followed by another, from 0.1 to 0.95 gives rise to characteristic diffraction features, including anisotropic broadening and intensity redistribution of diffraction maxima with certain Miller indices, as well as the appearance of a diffuse scattering maximum. These features cannot be reproduced by conventional two-phase models. The results highlight the necessity of accounting for coherent intergrowths in structural analysis and provide a practical framework for their identification using diffraction-based methods.

Supported LaFe_{1 – x}Ni_xO₃/α-Al₂O₃ (x = 0, 0.6, 1) perovskite containing catalysts are prepared by impregnation of a high-temperature block carrier based on α-Al₂O₃ with a suspension of the active component followed by heat treatment at 900°C. The catalysts were characterized by X-ray diffraction analysis (XRD), scanning electron microscopy (SEM), Brunauer–Emmett–Teller (BET), and temperature-programmed reduction with H₂ (H₂-TPR) methods and studied in the reaction of high-temperature decomposition of nitrous oxide. It is shown that all the supported catalysts are two-phase and contain only corundum and perovskite phases. The parameters of the crystal structure and the X-ray size of the corundum phase in the supported catalysts do not differ from the carrier. The observed slight increase in the cell volume for nickel-containing perovskites in supported catalysts may be due to an increase in the vacancy content in them. According to H₂-TPR data, the temperatures of the reduction peaks of the supported catalysts do not differ from the reduction peaks of the unsupported perovskites, which indicates that the active component is not modified by the carrier. In terms of activity, the supported catalysts can be arranged in a row LaNiO₃/α-Al₂O₃ > LaFe_{1 – x}Ni_xO₃/α-Al₂O₃ > LaFeO₃/α-Al₂O₃, which corresponds to the row activity of unsupported perovskites. Catalyst LaNiO₃/α-Al₂O₃ with LaNiO₃ content ~7%, monoliths height 20 mm and contact time 0.06 s demonstrates ~94% N₂O conversion degree at 900°С.

Using the Pechini method, LaFe_{1 – x}Ni_xO₃ (x = 0, 0.6, 1) perovskite-containing catalysts supported on a monolith corundum support were prepared and studied in the reaction of high-temperature decomposition of nitrous oxide. It is shown that the supported catalysts can be arranged in the following order by activity: LaFeO₃/α-Al₂O₃ > LaFe_0.4Ni_0.6O₃/α-Al₂O₃ > LaNiO₃/α-Al₂O₃, which differs from the activity order of perovskites without a support. The change in the activity order is due to the interaction of the impregnating solution with the support, as a result of which an admixture of the nickel aluminate phase, inactive in this reaction, can be formed in nickel-containing catalysts. The LaFeO₃/α-Al₂O₃ catalyst with an active component content of ~5%, even with a fragment height of 20 mm (and a contact time of 0.06 s), provides a high (~84%) conversion of nitrous oxide, due to the possible presence of an impurity of the dispersed phase of active Fe₂O₃ as well.

The catalytic activity of LaFe_{1 – x}Co_xO₃ perovskites obtained by the Pechini method was studied in reactions of high-temperature decomposition of N₂O, oxidation of CH₄ and CO. The samples were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), adsorption and temperature-programmed hydrogen reduction (H₂-TPR). The obtained single-phase solids with a specific surface area (S_BET) of 6.1–9.7 m²/g are porous aggregates of 10 nm to 1 micron in pore size, consisting of dendrite-like agglomerates. It has been shown that the increase in Co content leads to increase of the amount of weakly bound oxygen in perovskites. Analysis of the relationship between the activity of LaFe_{1 – x}Co_xO₃ perovskites and the amount of weakly bound oxygen revealed a good correlation for the entire range of compositions only for the N₂O decomposition reaction; a limited correlation (up to x = 0.6) for the CO oxidation reaction and the absence of such a correlation for the CH₄ oxidation reaction. The influence of key stages of reaction mechanisms affecting activity is discussed. The advantage of using the Pechini method for the preparation of high active N₂O decomposition catalysts was revealed.

LaFe_1−xNiO₃ are used as catalysts for the decomposition of nitrous oxide (N₂O).

Niobium oxide supported on ZrO₂ and mixed oxide of NbO_x-ZrO₂ was prepared and characterized. Mechanical treatment was followed by the microwave heating procedure of catalysts with more advanced textural parameters. The amount of Lewis (LAS) and Brønsted (BAS) acid sites rose with the increasing Nb content in the catalysts. The catalytic properties of samples of niobia-zirconia (NbZr samples, NbZr catalysts) were studied in a cellulose hydrolysis–dehydration reaction at 453 K under an inert Ar atmosphere in a batch reactor. Glucose and 5-hydroxumethylfurfural (5-HMF) were the major products. The initial reaction rate could be tuned by the density of acid sites on the surface of solid. At a low density of acid sites (0.1–0.3 µmol·m⁻²), the initial reaction rate had a pronounced inverse correlation. Increasing the LAS/BAS from 0.3 to 2.5 slightly stimulated the formation of the target products. The catalytic properties of NbZr catalysts prepared by microwave treatment were studied in cellulose transformation in a flow set-up. Glucose was found to be the major product. The maximum yield of glucose was observed in the presence of the sample of 17%Nb/ZrO₂. Increasing Nb content resulted in the formation of Nb-associated acid centers and, in turn, increasing catalyst acidity and activity.

Cesium-promoted cobalt spinel is promising as a catalyst for the low-temperature decomposition of nitrous oxide for use in the second stage of a single-reactor unit for the combined removal of nitrogen oxides. This work studied the effect of the conditions for the preparation of granular and block bulk catalysts based on Co₃O₄ by extrusion molding.

This study revealed an increased reactivity of centrifugally thermoactivated products of gibbsite toward aqueous solutions of nickel nitrate at room temperature as well as under hydrothermal conditions. X-ray, thermal, microscopy, adsorption and chemical analysis methods were used to investigate and demonstrate the possibility of obtaining highly loaded mixed aluminum–nickel oxide systems, with a nickel content of ca. 33 wt.%, using a hydrochemical treatment at room temperature or a hydrothermal treatment of suspensions of the product of the centrifugal thermal activation of gibbsite in aqueous solutions of nickel nitrate. It was shown that the thermal treatment of xerogels—hydrochemical interaction products—in the range of 350–850 °C led to the formation of NiO phases and highly dispersed solid solutions of nickel based on the NiAl₂O₄ spinel structure, with different ratios and a high specific surface area of 140–200 m²/g. A hydrochemical treatment of suspensions at room temperature ensures that the predominant formation of the NiO phase is distributed over the surface of the alumina matrix after calcination, whereas hydrothermal treatment at 150 °C leads to a deeper interaction of the suspension components at the treatment step, which occurs after the thermal treatment of the formed xerogel in the predominant formation of poorly crystallized NiAl₂O₄ spinel (“protospinel”). The considered method makes it possible to obtain complex aluminum–nickel oxide systems with different phase ratios, decreases the number of initial reagents and synthesis steps, completely excludes waste and diminishes the total amount of nitrates by 75 wt.% compared to the classical nitrate scheme for the coprecipitation of compounds with a similar elemental composition.

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