Cation Distribution and Interatomic Interactions in Oxides with Heterovalent Isomorphism: X. Structure of the Ho2SrAl2O7 Oxide at 100, 298, and 673 K

Crystallochemical data for the Ho2SrAl2O7 oxide at 100, 293, and 673 K were obtained by full-profile X-ray analysis. Analysis of the thermal expansion anisotropy showed that increased temperature gives rise to equalization of metal-oxygen bond lengths in all oxygen polyhedra at the expense of lengthening of some bonds and virtual invariability of others.

Interaction between carbon oxides, hydrogen and Fe2O3 and An + 1FenO3n + 1 (A = Gd, Sr, n = 1, 2, …, ∞)

The interaction between carbon oxides and hydrogen and surfaces of iron(III) oxide and An + 1FenO3n + 1 (where A = Gd, Sr, and n = 1, 2, …, ∞ is the number of perovskite layers) complex oxides is studied for the first time by means of thermal programmed desorption. It is shown that carbon oxides are adsorbed in molecular form with the formation of carbonate–carboxylate complexes, and in dissociative form. The ratios of the adsorption forms of both oxides are determined by the structure of ferrites, the number of perovskite layers, and the valence state and coordinative saturation of iron. The presence of weakly and strongly bonded hydrogen forms is established, and it is suggested that hydrogen dissolves in the bulk of a perovskite. © 2016, Pleiades Publishing, Ltd

Double-layered Aurivillius-type ferroelectrics with magnetic moments

We have synthesized the double-layer Aurivillius phase Bi2LnNbTiO9 where Ln = Nd–Gd, Bi. All compounds adopt the orthorhombic polar space group A21am. The magnetic Ln-ion occupies the cuboctahedral position in the middle of the perovskite double-layer, and thus controls the octahedral tilt of the perovskite block. This tilt modifies the ferroelectric displacement of the (Nb,Ti) towards the octahedral faces. This provides a mechanism to systematically modify the polarization. Mn-substitution on the (Nb,Ti) site for Ln = Bi is homogeneous up to x = 0.2. The magnetic moments of Ln do not order down to 5 K.

Interaction between carbon oxides, hydrogen and Fe2O3 and An + 1FenO3n + 1 (A = Gd, Sr, n = 1, 2, …, ∞)

The interaction between carbon oxides and hydrogen and surfaces of iron(III) oxide and An + 1FenO3n + 1 (where A = Gd, Sr, and n = 1, 2, …, ∞ is the number of perovskite layers) complex oxides is studied for the first time by means of thermal programmed desorption. It is shown that carbon oxides are adsorbed in molecular form with the formation of carbonate–carboxylate complexes, and in dissociative form. The ratios of the adsorption forms of both oxides are determined by the structure of ferrites, the number of perovskite layers, and the valence state and coordinative saturation of iron. The presence of weakly and strongly bonded hydrogen forms is established, and it is suggested that hydrogen dissolves in the bulk of a perovskite. © 2016, Pleiades Publishing, Ltd

Study of effect of preparation method and composition on the catalytic properties of complex oxides (Gd,Sr)n + 1FenO3n + 1 for dry reforming of methane

The catalytic activity of nanostructured perovskite-type ferrites of gadolinium and strontium, has been studied. The intercorrelation between the catalytic activity and the method used to obtain ferrites has been shown: the products formation rates are higher in the samples obtained by a ceramic technology, while the sol-gel method makes it possible to obtain ferrites with a greater hydrogen selectivity, which is accounted for by their nanocrystalline state and porous structure. It is shown that the specific catalytic activity of the catalysts, which are prepared by the same method and are of approximately the same chemical composition, is constant. It is shown that a nonisovalent replacement of gadolinium by strontium causes symmetry lowering in the perovskite structure and the appearance of a heterovalent state of iron atoms, which affects ferrite catalytic activity. © 2017, Pleiades Publishing, Ltd

New mixed perovskite-type Gd2–xSr1+xFe2O7 catalysts for dry reforming of methane, and production of light olefins

The catalytic properties of complex perovskite-type gadolinium and strontium oxides in carbon dioxide reforming of methane and the production of gaseous olefins by carbon monoxide hydrogenation have been studied. Samples of Gd2SrFe2O7 and Gd2–xSr1+xFe2O7 (х = 0.1; 0.2; 0.3; and 0.4) have been obtained by the sol–gel method and ceramic technology, and have been characterized by means of X-ray diffraction, scanning electron microscopy, photon correlation spectroscopy, Mössbauer spectroscopy, and N2 adsorption–desorption analysis. It has been shown that the sol–gel method allows us to produce samples with better catalytic characteristics than ceramic systems. The nonisovalent substitution of Gd3+ for Sr2+ distorts the structure of complex oxide, resulting in the emergence of the heterovalent state of iron atoms (Fe3+ and Fe4+) reflected in the values of reactant conversion and selectivity for the target products. A sample of Gd2–xSr1+xFe2O7 with х = 0.3 displays the highest catalytic activity in dry reforming of methane reforming, along with the highest selectivity for unsaturated hydrocarbons (ethylene and propylene) in hydrogenation of carbon monoxide. © 2017, Pleiades Publishing, Ltd