Effect of the synthesis conditions of titanium-magnesium catalysts on the composition, structure and performance in propylene polymerization

Supported catalysts synthesized via the interaction of Mg(OEt)2 with TiCl4 in the presence or absence of an internal stereoregulating donor (di-n-butyl phthalate), with different solvents (chlorobenzene, n-undecane, n-heptane) at different titanation temperatures have been studied by a set of physicochemical methods. Data on the chemical composition, X-ray structure and pore structure of these catalysts as well as data on their activity and stereospecificity in polymerization of propylene were obtained. Chemical composition, structure, activity and stereospecificity depend primarily on the presence of an electron donor stereoregulating component and on the solvent nature and titanation temperature. Activity of the catalysts is determined by totality of different characteristics: the chemical composition, in particular, the presence of inactive by-products like TiCl3(OEt), the MgCl2 X-ray structure and pore structure. More active catalyst which was synthesized under optimal conditions in the presence of di-n-butyl phthalate contains the minimal amount of TiCl3(OEt) by-product, and has a more ordered X-ray structure and a homogeneous mesoporous structure with a narrow mesopore size distribution

(Cd,Mn)S in the Composite Photocatalyst: Zinc Blende and Wurtzite Particles or Integrowth of These Two Modifications?

In this study, the crystalline structure and particle shape of Cd1−xMnxS (x~0.3) in the composite photocatalysts prepared by hydrothermal synthesis at different temperatures (T = 80, 100, 120, and 140 °C) were analyzed. Along with mixed Cd–Mn sulfide, the catalysts contain a small amount of β-Mn3O4. XRD patterns of (Cd,Mn)S have features inherent to both cubic zinc blende and hexagonal wurtzite structure. Moreover, XRD peaks are anisotropically broadened. First, the heterogeneous (or two-phased) model was considered by the commonly used Rietveld method. Phase ratio, average crystallite sizes, and strains for both phases were formally determined. However, it was shown that this model is not correct because relatively narrow and broad peaks cannot be fitted simultaneously. Then, the homogeneous model was tested by Debye Function Analysis. This model assumes that particles are statistically homogeneous, but each particle contains lamellar intergrowth of zinc blende and wurtzite modifications. The probability of stacking faults, as well as the average radii of spherical and ellipsoidal particles, were varied. It was shown that nanocrystalline Cd0.7Mn0.3S particles have an ellipsoidal shape. Ellipsoids are elongated along the direction normal to the plane of defects. An increase in the hydrothermal synthesis temperature from 80 °C to 140 °C leads to an enlargement of particles and a gradual decrease in the probability of stacking faults in the wurtzite structure from 0.47 to 0.36. Therefore, with increasing temperature, the structure of (Cd,Mn)S nanoparticles transforms from almost random polytype cubic/hexagonal (ZB:WZ = 47:53) to a preferably hexagonal structure (ZB:WZ = 36:64). Mn2+ ions facilitate CdS phase transformation from zinc blende to wurtzite structure. There is no direct correlation between the structure and photocatalytic activity

TrnL-trnfF cpDNA polymorphism in some representatives of the genus

We present preliminary results of the trnL-trnF cpDNA region analysis for some representatives of the g. Betula, out of which B. procurva can be considered the most interesting. The disjunctive range of this species is confined to the mountain system in southeast Central Asia (Pamir-Alai). The occurrence of the birch is isolated from the main range in the mountainous part of the Urals, in the boreal forests zone and in the Trans-Ural forest-steppe. Due to high interspecific hybridization and population variability of the g. Betula, doubts were raised about the correct identification of the representatives of B. procurva. The studied genetic variability and population structure of B. procurva, B. nana, B. pubescens, B. turkestanica, B. tianshanica and B. pendula species indicate active introgression and hybridization processes. Polymorphism in all groups is significantly reduced, increasing slightly when comparing the most distant groups. The matching of the B. procurva cpDNA haplotypes (GBS) with B. pendula, B. nana, and B. pubescens shows that this trnL-trnF cpDNA site cannot be used for molecular identification of birch species by barcoding as a single marker, but this marker use is possible for determination of certain B. procurva population. Based on the studied cpDNA region (trnL-F) we found a clear geographic subdivision in B. procurva populations of the Trans-Urals and Central Asia,

Nonstoichiometry Defects in Double Oxides of the A₂BO₄-Type

Double oxides with the structure of the Ruddlesden–Popper (R-P) layered perovskite An+1BnO3n+1 attract attention as materials for various electrochemical devices, selective oxygen-permeable ceramic membranes, and catalytic oxidative reactions. In particular, Sr2TiO4 layered perovskite is considered a promising catalyst in the oxidative coupling of methane. Our high-resolution transmission electron microscopy (HRTEM) studies of Sr2TiO4 samples synthesized using various methods have shown that their structure often contains planar defects disturbing the periodicity of layer alternation. This is due to the crystal-chemical features of the R-P layered perovskite-like oxides whose structure is formed by n consecutive layers of perovskite (ABO3)n in alternating with layers of rock-salt type (AO) in various ways along the c crystallographic direction. Planar defects can arise due to a periodicity violation of the layers alternation that also leads to a violation of the synthesized phase stoichiometry. In the present work, a crystallochemical analysis of the possible structure of planar defects is carried out, structures containing defects are modeled, and the effect of such defects on the X-ray diffraction patterns of oxides of the A2BO4 type using Sr2TiO4 is established as an example. For the calculations, we used the method of constructing probabilistic models of one-dimensionally disordered structures. For the first time, the features of diffraction were established, and an approach was demonstrated for determining the concentration of layer alternation defects applicable to layered perovskite-like oxides of the A2BO4 type of any chemical composition. A relation has been established between the concentration of planar defects and the real chemical composition (nonstoichiometry) of the Sr2TiO4 phase. The presence of defects leads to the Ti enrichment of particle volume and, consequently, to the enrichment of the surface with Sr. The latter, in turn, according to the data of a number of authors, can serve as an explanation for the catalytic activity of Sr2TiO4 in the oxidative coupling of methane

Karyotypic differentiation of populations of the common shrew Sorex araneus L. (Mammalia) in Belarus

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Solid solutions of CdS and ZnS: Comparing photocatalytic activity and photocurrent generation

The series of Cd1-xZnxS (x = 0–1.0) photocatalysts and Cd1-xZnxS/FTO thin film photoelectrodes were prepared. The obtained samples were studied by X-ray diffraction method (XRD), diffuse reflectance spectroscopy (DRS), scanning electron microscopy (SEM), and N2 low temperature adsorption. The photocatalysts were tested in the reaction of hydrogen production from Na2S/Na2SO3 solution under visible light irradiation. The photocurrents were measured in aqueous solution of Na2Sn and NaCl. It was shown that the target properties in the hydrogen evolution and photocurrent production are changed differently dependent on the composition of the mixed solid solutions. The highest photocatalytic hydrogen production rate was observed over Cd0.3Zn0.7S while the most effective photoelectrode was Cd0.8Zn0.2S/FTO. The Cd1-xZnxS/FTO samples were studied by the electrochemical methods in details. The factors affecting the photocatalytic activity and the photocurrent generation were found and listed for the first time. Conduction band potential (or flat band potential) and electron lifetime play a crucial role for effective photocatalytic hydrogen production over Cd1-xZnxS. Optimal photoelectrochemical characteristics were obtained in case of high values of electron lifetime. If the electron lifetimes of the tested samples have similar values, the high concentration of charge carriers is required for high photocurrent generation over Cd1-xZnxS/FTO photoelectrodes

Sol-Gel Synthesis and Characterization of the Cu-Mg-O System for Chemical Looping Application

A sol-gel technique was applied to prepare the two-component oxide system Cu-Mg-O, where MgO plays the role of oxide matrix, and CuO is an active chemical looping component. The prepared samples were characterized by scanning electron microscopy, low-temperature nitrogen adsorption, and X-ray diffraction analysis. The reduction behavior of the Cu-Mg-O system was examined in nine consecutive reduction/oxidation cycles. The presence of the MgO matrix was shown to affect the ability of CuO towards reduction and re-oxidation significantly. During the first reduction/oxidation cycle, the main characteristics of the oxide system (particle size, crystallization degree, etc.) undergo noticeable changes. Starting from the third cycle, the system exhibits a stable operation, providing the uptake of similar hydrogen amounts within the same temperature range. Based on the obtained results, the two-component Cu-Mg-O system can be considered as a prospective chemical looping agent

The effect of ruthenium promotion of the Co/d-Al2O3 catalyst on the hydrogen reduction kinetics of cobalt

The effect of ruthenium content on the reductive activation of the Co/δ-Al2O3 catalyst was investigated using thermal analysis and in situ synchrotron radiation X-ray diffraction. Data of thermal analysis and phase transformations can be described by a kinetic scheme consisting of three sequential steps: Co³⁺ → Co²⁺ → (Co⁰Co²⁺) → Co⁰. The first step is the generation of several CoO clusters within one Co3O4 crystallite followed by their further growth obeying the Avrami–Erofeev kinetic equation (An1) with dimensional parameter n1 < 1, which may indicate the diffusion control of the growth. The second step is the kinetically controlled sequential process of the metallic cobalt phase nucleation (An2), which is followed by the third step of slow particle growth limited by mass transport according to the Jander model (D). Ruthenium promotion of Co/δ-Al2O3 catalysts significantly accelerates the reduction of cobalt. As the ruthenium content is raised to 1 wt%, the characteristic temperature of metal phase formation decreases by more than 200 °C and Ea for An2 step decreases by 25%. For step D, a joint decrease in activation energy and pre-exponential factor in case of ruthenium promotion corresponds to a weaker diffusion impediment at the final step of cobalt reduction. In the case of unmodified Co/δ-Al2O3, the characteristic temperature of the metal phase formation reaches very high values, the metallic nuclei rapidly coalesce into larger ones, and the further process is inhibited by diffusion of the reactants through the product layer. For ruthenium promoted catalysts, each CoO crystallite generates one metal crystallite; thus, ruthenium enhances the dispersion of the active component