Characteristics optimization of activated alumina desiccants based on product of a centrifugal thermal activation of gibbsite

Activated alumina desiccants modified with NaOH and KOH were synthesized from the product of a centrifugal thermal activation of gibbsite, with the subsequent hydration in an acid or alkaline medium, and their properties were studied. It was shown that the modification makes it possible to raise the dynamic capacity of desiccants produced from pseudoboehmite by up to a factor of 2 via formation of new super-strong basic centers the concentration of which grows with increasing content of an alkaline oxide. A correlation was found between the total concentration of basic centers on the surface of the desiccants and their dynamic capacity in drying of both dry and humid air. Use of the modified desiccants with high static and dynamic capacity will make it possible to improve the drying efficiency.     

Effect of modifying alumina desiccants with sulfuric acid on their physicochemical properties

In the production of alumina desiccants by extrusion, the introduction of sulfuric acid at the stage of preparing a mouldable paste based on hydroxides containing bayerite or pseudoboehmite increases the sorption capacity of the product. This effect is most pronounced for the pseudoboehmite-based materials. The dynamic capacity of these desiccants increases to the level characteristic of the bayerite-containing hydroxide (>5 g H₂O/100 cm³) for a dew point of ?40°C and a contact time of 1.5 s), and their static capacity exceeds this value (increasing from 21.13 to 23.1 g H₂O/100 cm³). This procedure changes the phase composition and textural characteristics of the pseudoboehmite-based desiccants and increases Brønsted acidity and generates strong Lewis acid sites on the surface of all oxides. The dynamic capacity of desiccants with similar textural characteristics depends on the acid-base properties of their surface.     

Local Chemical Analysis of the Grain Surface,Cauliflowers, and Pores on Pt–Pd–Rh–Ru Gauzes after the Oxidation of NH3 at 1133 K

Kinetics and Catalysis - The local chemical composition of the most characteristic regions of a rough etched layer on the front side of the Pt–Pd–Rh–Ru gauze with a composition of...     

Study of acid-modified aluminum oxides produced by centrifugal thermal activation in dehydration of ethanol

Possibility of using products formed in centrifugal thermal activation of hydrargillite to obtain alumina catalysts washed to remove admixtures of alkali metals was considered. A comparison of the physicochemical and catalytic properties of the samples demonstrated that washing with water is more favorable than that with nitric acid; the catalytic activity and acid-base properties of the catalyst surface are determined not only by the content of Na, but also by the whole set of catalyst preparation conditions. The most active of the samples obtained in the study has acidity close to that of industrial aluminum oxide produced by the reprecipitation method, but surpasses it in activity: at 370°C, the total yield of ethylene and diethyl ether reaches a value of 88.8 mol %, which is 4% higher than that for the reference sample.     

Influence of the mobility of oxygen on the reactivity of La<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Sr<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>MnO<Subscript>3</Subscript> perovskites in methane oxidation

Radically different dependences of the activity of La_{1 − x} Sr _x MnO₃ (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 (La_0.5Sr_0.5MnO₃) shows the highest activity at higher temperatures of 700–900°C. In the low temperature region, the activity of La_{1t - x} Sr _x MnO₃ 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 T _max = 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.     

Effect of the reaction medium on the structure of the La1 ? xCaxMnO3 (x = 0–1) solid solutions prepared by the pechini method

The phase composition and microstructure of La_{1 − x} Ca _x MnO₃ (x = 0–1) materials prepared by the Pechini method from polymer-salt stocks were studied after testing these materials in methane oxidation. According to X-ray diffraction data, the reaction medium causes no significant changes in the samples, while high-resolution transmission electron microscopy indicates that the x > 0.3 samples are unstable. Under the action of the reaction medium, the perovskite structure of these samples undergoes partial decomposition accompanied by the formation of planar defects having a lower manganese content. The number and degree of segregation of these defects increase with increasing calcium content. The calcium oxide and manganese oxide phases as segregated nanoparticles are observed on the particle surface. These changes are caused by the decrease in the oxygen content of the manganites under the action of the reaction medium $ (T,P_{O_2 } ) $ (T,P_{O_2 } ) , by the formation of vacancies, and by the variation of the charge of the manganese cations, as well as by the charge ordering tendency of the manganese cations. Therefore, the observed changes in catalytic activity under the action of the reaction medium for x > 0.3 can be due to perovskite decomposition accompanied by the formation of planar defects, the release of the manganese oxide and calcium oxide phases, and their subsequent sintering.     

Resonance features of the conductance of open billiards with the spin-orbit interaction

The transport properties of a circular billiard with attached channels, which is an open system, have been studied in the presence of the Dresselhaus and Rashba spin-orbit interactions. It has been shown that this interaction leads to the appearance of additional Fano resonances in the energy dependence of the conductance, the width of which is proportional to the fourth power of the spin-orbit coupling constant.     

Structural features of the formation of La1?xCaxFeO3?δ (0 ≤x ≤ 0.7) hetero valent solid solutions

A synthesis method with the use of polymer-salt compositions (calcination temperature 800°C) provides the preparation of various solid solutions of a La_1−x Ca _x FeO_3−δ series in the 0≤ x≤ 0.7 range, which belong to the perovskite structure type. A morphotropic phase transition occurs from the orthorhombic perovskite modification (0≤ x ≤ 0.4) to the cubic one (0.5 ≤ x≤ 0.7). A growing number of microdistortions in the perovskite structure and the formation of a microblock structure in the morphotropic phase transition region are observed with increasing degree of calcium substitution for lanthanum. Calcination of solid solutions with x = 0.6 and 0.7 at temperatures above 1000°C in the air or under conditions of reduced oxygen partial pressure (laboratory vacuum of 10⁻³ Torr) results in the formation of a nanostructured state with coherently grown blocks of perovskite and Grenier phase, which is due to irreversible oxygen loss.     

Kinetic Aspects of the Adsorption on Aluminum Oxide Drying Agents Doped with Alkali Metal Ions

Fundamental dynamic (kinetic) aspects of the process in which water vapor interacts with the surface of drying agents that are synthesized on the basis of low-temperature modification of aluminum oxide produced from a pseudoboehmite-containing hydroxide and are modified (doped) with alkali atoms (K, Na). It is shown that the kinetics of adsorption on the samples under study, formed from the fine fraction (0.5–1.0 mm) of aluminum oxide adsorbents, can be described with the Glueckauf equation, which rather well describes the dynamics of water vapor absorption in the course of time. The equation parameters were determined: adsorption rate constants and the equilibrium adsorption capacities (a*). It was found that the alkaline modification of the surface of aluminum oxide adsorbents results in that a* increases (by ~40%) as compared with the unmodified drying agent. A correlation is observed between the equilibrium adsorption capacity of the samples under study and the acid-base properties of the surface.     

M?ssbauer 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 La_1-xCa_xFeO_3-y system prepared via a ceramic route were characterized by M?ssbauer 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 ?) 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. This revised version was published online in August 2006 with corrections to the Cover Date.