L. A. Isupova

Influence of the mobility of oxygen on the reactivity of La1 − xSrxMnO3 perovskites in methane oxidation

Radically different dependences of the activity of La1 − xSrxMnO3 (x = 0−0.5) perovskites in methane oxidation on the degree of substitution of strontium for lanthanum are observed for low and high temperatures. Unsubstituted LaMnO3 exhibits the highest activity in the temperature range from 300 to 500°C, while the sample with the maximum degree of substitution (La0.5Sr0.5MnO3) shows the highest activity at higher temperatures of 700–900°C. In the low temperature region, the activity of La1t - xSrxMnO3 is determined by the amount of weakly bound (overstoichiometric) oxygen, which is formed in cation-deficient lattices and is characterized by a thermal desorption peak with Tmax = 705°C. At higher temperatures (800–900°C), the strongly bound oxygen of the catalyst lattice is involved in the formation of the reaction products under both unsteady- and steady-state conditions. As a consequence, the catalytic activity in methane oxidation correlates with the apparent rate constant of oxygen diffusion in the oxide bulk.

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.

Role of defect structure in structural sensitivity of the oxidation reactions catalyzed by dispersed transition metal oxides

For dispersed transition metal oxides, the specific catalytic activity in the reactions of CO and hydrocarbons oxidation was compared with the densities of bulk and surface defects that were estimated using a combination of diffraction and spectroscopic methods. On this basis, the oxide systems were classified with respect to the scale and origin of the structural sensitivity manifestation. The results were discussed from the point of view of the atomic structure of the most developed surface faces of these oxides and their stoichiometry ranges. The most active surface sites were found to be associated with the surface extended defects including those located at the outlets of bulk extended defects.

Mössbauer studies of the phase composition and microstructure of the La1-xCaxFeO3-y system as related to the reactivity of surface and bulk oxygen

The phase composition and microstructure of samples of the La1-xCaxFeO3-y system prepared via a ceramic route were characterized by Mössbauer spectroscopy. In all cases, iron was found in the 3+ state. The ordering of anion vacancies in the samples with the composition in the range of 0.8 > x ≥ 0.4, which corresponds to a microheterogeneous solid solution, generates new distorted octahedral and fivefold/tetrahedral sites revealed by two typical sextets. The disordering of this solid solution and small (10-100 Å) sizes of domains with a perovskite, braunmillerite or Grenier phase structure caused the appearance of a superparamagnetic doublet, which grows with the Ca content up to x = 0.8 but disappears in the sample of pure braunmillerite. The appearance of Fe cations in a distorted coordination correlates with the increased activity of the samples with a microheterogeneous structure in the CO catalytic oxidation and with their reducibility by H2.

Forms of oxygen in La1 − xCaxMnO3 + δ (x = 0–1) perovskites and their reactivities in oxidation reactions

The effects of substitution in the cationic sublattice and of the synthesis procedure on the reactivity of different forms of oxygen in La1 − xCaxMnO3 + δ perovskites synthesized by mechanochemical and ceramic processing was studied by temperature-programmed reduction (TPR) with hydrogen. As the calcium content of the perovskite is raised, the maxima of the TPR peaks shift to lower temperatures and the extent of reduction of the perovskite increase, implying an increase in the reactivity of the system. Conversely, raising the calcination temperature or extending the calcination time shifts the maxima of the peaks to higher temperatures and diminishes the extent of reduction of the sample. TPR data for the intermediate-composition samples can be explained in terms of the dependence of microstructure on the synthesis procedure (near-surface calcium segregation in the mechanochemically synthesized samples and the microheterogeneous structure of the ceramic samples). The reduction process Mn4+ → Mn2+ takes place in the low- and medium-temperature regions. According to the literature, the bulk reduction process Mn3+ → Mn2+ occurs at high temperatures. The activity of the system in CO oxidation is correlated with the amount of the most reactive surface oxygen, which is eliminated in hydrogen TPR runs below 250–300°C.

Real structure and catalytic activity of La1−xSrxCoO3 perovskites

Abstract Mechanoceramical synthesis of La 1− x Sr x CoO 3 (0≤ x ≤1) perovskites was made from simple oxides. Samples calcined at 900 and 1100°C for 4 h are nearly monophase and well crystallized. Sr adding was found to cause a structure rearrangement from the hexagonal (at x ≤0.4) to the cubic one (at 0.8> x >0.4) and back to the hexagonal at x >0.8. There are two maxima of the catalytic activity versus chemical composition: at x =0.3 and at x =0.8. TEM data for these samples were obtained and disordered surface layers were detected. There is a correlation between the catalytic activity and surface layers microstructure.

Physicochemical and catalytic properties of La1-xCaxFeO3-0.5x perovskites prepared using mechanochemical activation

The phase composition of La1 – xCaxFeO3 – 0.5x perovskites synthesized from preactivated oxides was studied by powder X-ray diffraction analysis and differential dissolution. The system does not form a continuous series of homogeneous solid solutions. No intermediate samples from this series are monophasic. It was found that the synthesis under nonequilibrium conditions (mechanical activation + calcination at 900°С for 4 h) resulted in nonequilibrium microheterogeneous solid solutions with degrees of calcium substitution for lanthanum of no higher than 0.5. A longer calcination (for 16 h) or an increase in the calcination temperature of solutions up to 1100 °С decreased the calcium content of the samples down to x ∼ 0.2 because of the formation of a brownmillerite phase. The catalytic activity of the test samples in the oxidation of CO changed nonmonotonically with x, and it was maximum at x = 0.5–0.6, which correlates with the maximum density of interphase boundaries in these samples.

Synthesis of aluminum oxides from the products of the rapid thermal decomposition of hydrargillite in a centrifugal flash reactor: II. Physicochemical properties of the products obtained by the centrifugal thermal activation of hydrargillite

A variety of physicochemical methods were used to characterize the product of the rapid thermal decomposition of hydrargillite in a centrifugal flash reactor under the following conditions: the average particle size of the reactant, 80–120 μm; the temperature of the solid heating surface (plate or cylinder), 300–700°C; hot-zone residence time, ∼1 s; transfer of the product to the cooled zone of the reactor. The composition of the product and the extent of decomposition of hydrargillite were determined as a function of the processing temperature. The centrifugal thermal activation (CTA) of hydrargillite affords an X-ray-amorphous, highly reactive product with a developed surface and a disordered and inhomogeneous porous structure. This structure is capable of forming different modifications of aluminum hydroxide and oxide. The properties of the CTA product are compared with the properties of the earlier reported hydrargillite rapid decomposition products obtained using a gaseous heat-transfer agent (thermochemical activation product) or a fluidized bed of a granular heat-transfer agent (thermal dispersion product).

Influence of the temperature of calcination of bayerite-containing aluminum hydroxide pellets on the water vapor adsorption capacity and acid-base properties of alumina

The paper considers the influence of the temperature of calcination (350–550°C) of alumina-based water adsorbents prepared from bayerite-containing aluminum hydroxide on the static water sorption capacity at high and low relative air humidities and on the acid-base properties of the surface. The acid-base properties of aluminas have been determined by means of IR spectroscopy of adsorbed CO probe molecule (Lewis sites) and by the spin probe method, which was used to determine the concentrations of electron acceptor and electron donor sites. It has been demonstrated that, at a high relative air humidity (60%), there is no correlation between the static capacity of the drier and its acid-base properties, while at low relative humidities (1–1.5%), an increase in the concentration of electron donor sites and a decrease in the concentration of electron donor and Lewis acid sites enhance the water sorption capacity of alumina.