I. S. Yakovleva

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.

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.

Oxygen species and their reactivity in the mechanochemically prepared substituted perovskites La1 − xSrxCoO3 − y (x = 0–1)

The oxygen species and their reactivity in the mechanochemically prepared substituted perovskites La1 − xSrxCoO3 − y were studied using temperature-programmed reduction (TPR) of the samples with hydrogen. The experimental data were compared with data on the catalytic activity of the series of La1 − xSrxCoO3 − y catalysts in the oxidation of CO, as well as with the real structures and surface compositions of the samples, which were studied in detail previously. As the strontium content was increased, the degree of reduction of the samples increased in the course of TPR and the TPR peaks shifted to the region of lower temperatures, except for the last sample containing no lanthanum (x = 1). An increase in the calcination temperature and time resulted in a decrease in TPR peak intensities and in a shift of the peaks to the region of higher temperatures. A reaction scheme was proposed for the reduction. In accordance with this reaction scheme, Co4+ in substituted cobaltites was reduced to Co0 at temperatures lower than 400°C. In the temperature region of 400–500°C, the Co3+ → Co2+ bulk reduction, as well as the deep reduction processes Co3+ → Co0 and Co4+ → Co0, occurred; substitution facilitated the above processes. At temperatures higher than 500°C, Co2+ → Co0 bulk reduction occurred. The observed reduction of the mechanochemically prepared samples depended on their microstructure, which was described previously. It was found that the activity of the samples in the oxidation of CO depends on the amount of the most weakly bound reactive surface oxygen species, which were removed in TPR with hydrogen to 150°C. No correlation between the amount of strongly bound (lattice) oxygen removed upon TPR and the activity of La1 − xSrxCoO3 − y samples in the oxidation of CO was found.

Mechanochemical synthesis and physicochemical properties of La1 − xBaxFeO3 − δ (0 ≤ x ≤ 1) perovskites

La1 − xBaxFeO3 − δ (x = 0–1) perovskites were prepared by the mechanical activation of mixtures of binary oxides in a centrifugal planetary ball mill for 3 min followed by calcination at 1100°C for 4 h, and their phase composition was determined by X-ray diffraction. All samples up to x = 0.8 are single-phase oxides with a perovskite structure. The x = 1 sample is a mixture of perovskite-and brownmillerite-type phases. An orthorhombic-to-cubic morphotropic transition is observed at x = 0.3. The catalytic activity of the perovskite samples in CO oxidation, chosen as a model reaction, depends nonmonotonically on the barium content of the catalyst. High catalytic activities are shown by the x = 0.8 and 0.3 samples.

Microwave synthesis of LaMO3 (M = Mn, Co, Fe) perovskites from crystalline hydrates of nitrates

The possibility of synthesizing perovskite-type LaMO3 (M = Mn, Co, Fe) oxides by microwave irradiation of crystalline hydrates of nitrates was studied. Oxides with the perovskite structure form at the microwave irradiation stage; however, the resulting product is not singe-phase. Additional thermal treatment of the microwave synthesis product at 600 to 900°C for 5 h is needed for a single-phase oxide to be formed in the case of M = Mn. In the case of M = Co or Fe, the samples contain considerable amounts of the simple oxides La2O3 and Fe2O3 or Co3O4 along with the perovskite. The synthesized products were investigated in nitrous oxide decomposition and methane oxidation as model reactions. As compared to the samples obtained by other techniques, they have a larger specific surface area and are more active.

Microwave synthesis of LaBO3 (B = Co, Fe) perovskites using graphite and citric acid additions

We studied the effects of graphite and citric acid additions on the formation of LaFeO3 and LaCoO3 perovskites upon microwave treatment of crystalline hydrates or nitrate solutions, respectively. The addition of graphite was shown to increase the yield of LaCoO3 perovskite and to result in the formation of crystalline LaFeO3 perovskite even at the microwave treatment stage. Subsequent thermal treatment at 800°C yields single-phase lanthanum ferrite with a high specific surface area (11 m2/g). Due to the addition of citric acid to nitrate solutions, a highly viscous gel forms, which allows preparation of single-phase perovskites with a high specific surface area (up to 34 m2/g) after microwave treatment and calcination. The samples obtained using the admixtures are characterized by a high catalytic activity in methane oxidation. No single-phase oxides form without introduction of these admixtures.