G. N. Kryukova

Microstructural and spectroscopic investigations of the supported copperalumina oxide system: Nature of aging in oxidizing reaction media

Abstract By means of electron microscopy, infrared spectroscopy of adsorbed CO, XPS, and ESR the state of the copper oxide component of the Cu Al O system (CuO/γ-Al 2 O 3 and bulk CuAl 2 O 4 catalysts) has been investigated. Several types of copper Al oxygen entities were detected on the catalyst surfaces: isolated ions, weak magnetic associates, two- and three-dimensional clusters, defect CuO phase. The dependence of their distribution on composition, preparation, and pretreatment conditions and their reducibility by carbon monoxide have been analyzed. The enthalpy of oxygen adsorption on copper atoms in various structures has been calculated by a semiempirical, interacting-bond method. The most reactive species are three-dimensional clusters reducing to Cu 0 by CO even at 163 K. Prolonged treatment in reaction media (CH 3 OH, CH 2 O, O 2 at 673–873 K) annealed extended defects in CuO particles and destroyed the clusters, forming perfect CuO particles and bulk aluminate.

Preparation and Microstructural and Textural Characterization of Single-Phase Aluminum Oxides

Conditions for the preparation of single-phase η-, θ-, and χ-aluminas were determined. The structures of η- and χ-aluminas were characterized. With the use of high-resolution electron microscopy, it was found that η-Al2O3 particles exhibited the most developed {111} face and consisted of coherently joined domains with a pronounced platelet shape. Planar defects in the (111) plane occurred in the η-Al2O3 particles. Microstructural differences between single-phase η-Al2O3 and γ-Al2O3 with a defect spinel structure were revealed. It was found that the η-Al2O3, χ-Al2O3, and θ-Al2O3 oxides are characterized by uniformly porous structures with average pore diameters of 47, 55, and 110 Å, respectively.

Mechanochemical Synthesis and Catalytic Properties of the Calcium Ferrite Ca2Fe2O5

The formation of the real structure of calcium ferrite prepared by the calcination of a mechanochemically activated hydroxide mixture at 600–1100°C was studied by X-ray diffraction analysis, electron microscopy, thermal analysis, Moessbauer spectroscopy, IR spectroscopy, small-angle X-ray scattering, and secondary-ion mass spectrometry. It was found that low-temperature calcium ferrite is an anion-modified oxide, in which the ordering of oxygen vacancies was incomplete. Regions with a disordered structure were detected on the surface of crystallites. As the calcination temperature was increased, the brownmillerite crystal structure was improved and the intercrystalline boundaries were formed and then annealed. At the surface, these processes were accompanied by a change in the predominant form of adsorbed NO from nitrosyl to dinitrosyl species. An increase in the specific catalytic activity of samples with calcination temperature can be associated with the perfection of the brownmillerite structure and with a change in the state of adsorption centers.

Physicochemical and catalytic properties of La1-xCax FeO3-0.5x perovskites

The phase analysis of La1-xCaxFeO3-0.5x perovskites prepared by a ceramic process from oxides is studied by X-ray diffraction and differential dissolution methods. Atx < 0.5, the system does not form a continuous series of homogeneous solid solutions and does not consist of the members of a homological series. Atx < 0.5, the system contains two phases and calcium ferrite nanoparticles located on the surface of lanthanum ferrite. Atx > 0.5, the formation of the structures of a brownmillerite-based homological series is found. The catalytic activity of perovskites depends nonmonotonically on thex value and reaches the maximum atx = 0.6.

Real structure of metastable forms of aluminum oxide

Differences in the real structure of γ-Al2O3 samples obtained by the thermal decomposition of pseudoboehmite and boehmite prepared by the hydrothermal treatment of bayerite were found. The transformations of these structures during their conversion to δ-Al2O3 as the treatment temperature increased were studied. The rate of conversion of metastable alumina species into the stable α-Al2O3 phase significantly depends on the real structure of samples. The rate of this transformation is drastically retarded in the presence of extended defects in the oxides originated from boehmite, and the stability of metastable alumina species increased as the degree of surface dehydroxylation increased.

The structure, energetics, and reactivity of extended defects of copper oxide

Abstract For a discrimination of extended defects of copper oxide in terms of their stability and reactivity a sample of CuO was annealed in the vacuum thermochamber of an electron microscope and in a stream of helium at 200–400°C. The relaxation of the defect structure was studied by electron microscopy and X-ray small-angle scattering. The removal of oxygen from copper oxide was accompanied by rapid disappearance of microstrains. Simultaneously, the development of a dislocation network and the rearrangement of twins enhanced the reactivity of the oxide (the rates of reduction by CO and catalytic oxidation of CO by O 2 ). Therefore, the active centers of the surface may be associated with dislocations and twins. One of the reasons for high reactivity of such defect centers is that they stabilize the highly reactive weakly bound oxygen.

Real structure and catalytic activity of La1−xCaxMnO3+δ perovskites

Abstract Calcium-substituted lanthanum manganites La 1− x Ca x MnO 3+ δ (0≤ x ≤1) were synthesized from the solid oxide precursors using two routes: a traditional ceramic method and the mechanical activation of the mixture of oxides in high power planetary ball mills, followed by the product annealing at different temperatures. The samples structure was studied by XPD and TEM methods, while their surface composition was characterized by the differential dissolution method. A pronounced effect of the preparation method on samples bulk and surface properties, reflected in their reactivity characterized by the rate of CO catalytic oxidation, was observed. For samples prepared via the ceramic route by sintering at 1100°C, continuous solid solutions are formed. In the case of mechanical activation route, by annealing of activated mixture of oxides at 700–100°C, solid solutions are formed only at the Ca content within 0≤ x ≤0.4 range. At higher Ca content, microheterogeneous composites are formed, in which the nucleus of particles is formed by the perovskite phase with x ≤0.4, while the surface layer is enriched by Ca. The catalytic activity in CO oxidation appears to correlate with the density of extended defects, being decreased when Ca is segregated in the surface layer.

Structural Aspect of a Thermal Activation Effect in the MnOx/γ-Al2O3 System

Solid-phase reactions in the aluminum–manganese oxide system, including the structural mechanism of the thermal activation of catalysts, were studied at temperatures up to 1100°C. It was found that the solid-phase reaction at 900–1000°C occurred via two pathways because of the diffusion of manganese ions to aluminum oxide and aluminum ions to manganese oxide. Nanoheterogeneous state of the active component, which was observed in the range 25–600°C, is the product of incomplete decomposition of the high-temperature aluminum–manganese phase Mn2.1 – xAl0.9 + xO4 (0 ≤ x ≤ 0.6) with a cubic spinel structure; this phase was equilibrium at the synthesis temperature but metastable below 650°C.

Effect of structural disorder on the catalytic activity of mixed La−Sr−Co−Fr−O perovskites

For a perovskite system La0.7Sr0.3Co1-xFexO3, the effect of partial substitution of Co by Fe on the particle microstructure and chemical composition of the surface layer has been studied. Iron incorporation was found to cause a structure rearrangement from the hexagonal (at x<0.05) to the cubic (atx>0.05) type, which was not accompanied by any appreciable variation of the surface layer composition. A transient sample with x=0.05 was characterized by a high degree of misorientation of microblocks and had the highest activity in the CO catalytic oxidation.

On the γ-Fe2O3 superstructure observation by high resolution electron microscopy

A pronounced development of the superstructure in slightly reduced γ-Fe2O3 particles has been observed with the aid of high resolution electron microscopy. The model of superstructure formation as a result of vacancy ordering has been proposed on the basis of computer simulation images. Twins of the (110) type have also been detected.