Fischer-Tropsch Catalysts Based on Zr-Fe Intermetallides Encapsulated in an Al2O3/Al Matrix

ZrFe and ZrFe₂ intermetallides in an Al₂O₃/Al cermet matrix are reported as catalysts for the fixed-bed Fischer-Tropsch synthesis, and the effects of some preparation conditions on their texture, structural, mechanical, and catalytic properties are discussed. A nonmonotonic dependence of their catalytic activity on the size of interametallide particles is observed. The selectivity, activity, and mechanical strength of the composites depend on the calcination temperature and on the place of the hydriding step in the catalyst preparation procedure. In terms of volumetric efficiency, the catalysts prepared are comparable with bulk, unencapsulated intermetallides and are among the most efficient iron-containing catalysts known to date.     

Catalytic and physico-chemical properties of La1?xSrxCoO3 perovskites

Catalytic properties of strontium-substituted lanthanum cobaltites La_1–xSr_xCoO₃ in the reaction of CO oxidation were compared with the data about chemical composition of the surface and bulk defect structure. It was found that (1) the initial catalytic activity correlates with the cobalt concentration on the surface; (2) surface composition differs from the bulk one; (3) the steady-state activity is proportional to the density of bulk extended defects. Under our experimental conditions no role of point defects was noted.     

X-ray diffraction analysis of ultradisperse systems: The Debye formula

Problems of X-ray diffraction analysis of ultradisperse systems with the particle size up to 5 nm are discussed in the review. A method for calculating X-ray diffraction patterns using the Debye formula within the kinematic theory of X-ray scattering (Debye Function Analysis-DFA) is presented. The DFA method makes it possible to obtain information on the atomic structure, shape and size of nanoparticles, and verify hypotheses on the presence of deformations, stacking faults and other structural features of ultradisperse and nanostructured systems. The method is applicable for modeling the diffraction patterns of objects with an arbitrary (not necessarily crystal) structure. Applications of the method for examining the structure of various objects are discussed.     

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.     

Formation of vanadia-titania oxide catalysts

The formation of vanadia-titania catalysts was studied with a complex of physicochemical methods. The use of highly dispersed anatase with a defect structure results in the formation of coherent boundaries of coalescence of the V₂O₅ and TiO₂ crystallites with the ratio V : Ti =1 : 1 in a wide range of vanadium and titanium concentrations. The catalysts containing coherent boundaries are active and selective in Β-picoline oxidation to nicotinic acid.     

Effect of violations of the structure and layer packing on the diffraction pattern of gibbsite and bayerite

Using the simulation method for the diffraction patterns of one-dimensionally disordered materials, the effect of different violations in the layered structure of aluminum trihydroxides (gibbsite and bayerite) on their diffraction patterns is analyzed. The features of the diffraction patterns of a disordered gibbsite sample obtained by intercalation-deintercalation of lithium salts are considered.     

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.     

On <Emphasis Type="Italic">T</Emphasis> -spaces and relations in relatively free,lie nilpotent,associative algebras

The purpose of this work is to obtain the commutator relations and Frobenius relations in a relatively free algebra F ^(l) specified by the identity [x ₁ , . . . , x _l ] = 0 over a field of characteristic p > 0. These relations for l > 3 are analogous to the relations in the algebra F ⁽³⁾ and are applied to the T-spaces in the algebra F ^(l). In order to study the relations in F ^(l) in more detail, we construct a model algebra analogous to the Grassmann algebra.     

Formation of a real structure of perovskites (La1−x , Sr x )CoO3−δ in the region of the morphotropic phase transition

Precision X-ray diffractometry with synchrotron radiation and transmission electron microscopy are used to show that the extent of hexagonal distortion of the perovskite cell of the (La_1−x, Sr _x )CoO₃−δ system decreases and the microstructure of the samples changes when La is substituted by Sr; these changes are due to increased microstrains in the structure and subsequent formation of microunits. The microunit structure of particles in the region of the morphotropic transition 0.3≤x≤0.4, which is a consequence of the metastable state of the system, may be the reason for the high catalytic activity of the samples which we found previously. G. K. Boreskov Institute of Catalysis, Siberian Branch, Russian Academy of Sciences. Translated fromZhurnal Strukturnoi Khimii, Vol. 39, No. 1, pp. 92–97, January–February, 1998. This work was supported by RFFR grants 96-03-33103 and 97-03-33497.