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.     

A study of the microstructure of La1 − x Ca x MnO3 (x = 0.5, 0.8) solid solutions

HREM and XPD methods are used to study the microstructure of La_{1 − x} Ca _x MnO₃ perovskite-like oxides synthesized by pyrolysis of polymer-salt compositions. The XPD data show that the samples studied are single-phase solid solutions. Morphological transitions are observed in series of samples depending on the substitution parameter x. At 0 ≤ x ≤ 0.4 the samples can be characterized by rhombic symmetry (space group Pnmb); at 0.5 ≤ x ≤ 0.8 the symmetry increases to the tetragonal space group I4/mmm; at 0.9 ≤ x ≤ 1 the symmetry lowers to the monoclinic space group P1121. Heating of samples with x = 0.5 and 0.8 up to 1200°C in air does not lead to noticeable changes in the sample structure, which indicates their high thermal stability. When the La_0.2Ca_0.8MnO₃ sample is heated to 1100°C in vacuum nanocrystalline states with particle disintegration into microphases of different structures form because of a partial decomposition of the solid solution. As a result, a system of Mn₃O₄ nanoparticles appears, which is coherently bound with the perovskite phase of a defective structure.     

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

The phase analysis of La_1-xCa_xFeO_3-0.5x 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.     

Effect of the reaction medium on the physicochemical properties of the oxide monolith catalyst IK-42-1 for ammonia oxidation

The changes in the properties of the oxide monolith catalyst IK-42-1 for ammonia oxidation upon 3540-h-long operation as the second stage of a UKL-7 industrial reactor were studied by X-ray diffraction, chemical analysis, secondary ion mass spectrometry, temperature-programmed reduction, and IR spectroscopy. The spent catalyst shows a lower activity and a lower nitrogen oxide selectivity than the initial catalyst: the decrease in the NO yield is about 25%. As the catalyst operates, the state of its surface changes under the action of the reaction medium. The specific surface area of the catalyst decreases, and the total pore volume increases, which can cause a slight decrease in the ammonia conversion and diminish the mechanical strength of the monoliths. Under the ammonia oxidation conditions, there can be partial reduction of the catalyst surface and the local formation of a spinel structure (spinel-like defects). Oxygen adsorbed on these areas/defects is characterized by a high activation energy of desorption, which favors the reaction route yielding molecular nitrogen. According to IR spectroscopic data, the spent catalyst has a lower concentration of active, coordinatively unsaturated Fe²⁺ sites of adsorption, which are responsible for ammonia oxidation into NO. The decrease in the number of active sites can be due to both the breakdown of the solid solution of iron oxide in aluminum oxide during the reaction and the blocking of these sites by silicon, alkali and alkaline-earth metal, chromium, and rhodium atoms present on the surface of the spent catalyst.     

Synthesis of aluminum oxides from the products of the rapid thermal decomposition of hydrargillite in a centrifugal flash reactor: III. Properties of aluminum hydroxides and oxides obtained via the mild rehydration of the products of the centrifugal thermal activation of hydrargillite

The interaction between the amorphous product of the centrifugal thermal activation of hydrargillite (CTA HG) and aqueous electrolytes (pH 5–11) under mild conditions (15–35°C, atmospheric pressure) has been investigated by a variety of physicochemical methods. This interaction causes material morphologic and phase changes in CTA HG, and the product composition is governed by the pH of the electrolyte and by the hydration temperature and time. The product that forms in a basic medium or water in <24 h contains up to 50% pseudoboehmite. Raising the pH or temperature or extending the hydration time results in the formation of bayerite as the major phase (～80%). An X-ray amorphous hydroxide forms in acid media. The heat treatment of this hydroxide at 550°C yields aluminum oxides differing from alumina prepared via hydroxide reprecipitation. Products with new, unusual properties can thus be obtained.     

Weakly bound oxygen and its role in stability of solid solutions La1−x Sr x FeO3−δ

In the present study high-temperature X-ray diffraction, thermal analysis, and mass-spectrometry have been employed for investigation of samples in the La_1?x Sr _x FeO_3?δ family (0 ≤ x ≤ 1), the materials being solid solutions having perovskite structure. It has been shown that the loss of oxygen by the samples on heating to 1200°C in air (0 ≤ x ≤ 1) or in vacuum (x < 0.75) does not result in structural rearrangement of the solid solutions, but causes an increase in the lattice parameters. Heating of the compositions with x ≥ 0.75 in vacuum affords phases with ordered vacancies. The observed structural evolutions (growth of the unit cell parameter and vacancy ordering) are reversible, and on re-oxidation (on cooling in air or additional heating of the “vacuum” samples in air atmosphere) the original parameters of the oxides are recovered. The amount of oxygen evolved on heating increases in vacuum or in helium atmosphere, as compared to air, and also grows with rising strontium content, but under experimental conditions does not reach the maximum possible value (δ = x/2).     

The kinetics of oxidation of 1-chloro-2-(4-isopropylphenyl)-2-methylpropane by molecular oxygen

The kinetics of formation of 2-[4-(1-chloro-2-methylpropan-2-yl)phenyl]propan-2-yl hydroperoxide during the azobisisobutyronitrile-initiated oxidation of 1-chloro-2-(4-isopropylphenyl)-2-methylpropane by molecular oxygen at 70–100 °C was studied. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1135–1137, June, 2000.     

Oxidative Condensation of Methane on Sr2 –xLaxTiO4 Catalysts: Effect of the Degree of Substitution of Sr and La

Kinetics and Catalysis - The Sr2 –xLaxTiO4 (x = 0–2.0) catalysts were synthesized based on strontium titanate with a layered perovskite structure. The effect of the degree of...     

Low-Temperature Conversion of ortho-Hydrogen to para-Hydrogen over Ni/Al2O3 Supported Catalysts

Russian Journal of Physical Chemistry A - Comparative studies of Ni/Al2O3 catalysts during the low-temperature conversion of ortho-hydrogen to para-hydrogen are performed. Industrial alumina of...