Comparative Study of the Oxygen Storage Capacity of Pt/M<Subscript>x</Subscript>O<Subscript>y</Subscript>/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Systems

The oxygen storage capacity of 1% Pt/15% M_xO_y/Al₂O₃ systems containing a rare-earth or an alkaline-earth metal oxide or TiO₂ as the oxygen-storing component was studied. Oxygen storage capacity was evaluated as the amount of C₃H₈ reacting at 400°C with oxygen that was taken up by the catalyst during oxidative treatment. The systems containing a rare-earth metal oxide or TiO₂ possess the highest oxygen storage capacity among the catalysts examined (80 and 75 µmol C₃H₈/g Cat, respectively). Of the BaO and SrO systems, the latter is of interest, although its oxygen storage capacity (∼27 µmol C₃H₈/g Cat) is somewhat lower than the oxygen storage capacity of any rare-earth metal oxide or the TiO₂ system.__________Translated from Kinetika i Kataliz, Vol. 46, No. 4, 2005, pp. 585–589.Original Russian Text Copyright © 2005 by Sinel’nikov, Tolkachev, Stakheev.     

Study on the oxygen exchange capacities of Ce<Subscript>2</Subscript>Sn<Subscript>2</Subscript>O<Subscript>7</Subscript> pyrochlore

Ce₂Sn₂O₇ pyrochlore was synthesized by a hydrothermal method. X-ray powder diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) were used to characterize the composition and valence state of the sample. The oxygen exchange property of the Ce₂Sn₂O₇ phase was measured by an oxidation reaction in sealed air atmosphere and a followed reduction reaction in 5% H₂-95% N₂ atmosphere. Gas chromatography (GC) was used to analyze the oxygen change in the reaction. The results show that Ce₂Sn₂O₇ sample has excellent oxygen absorption capacity at 250°C as Ce³⁺ ions are oxidized to Ce⁴⁺ ions. The oxidized sample can be reduced by 5% H₂-95% N₂. The refreshed sample remains the capacity of oxygen absorption, while the oxygen exchange capacity degrades with the reduction times.     

The electrochemical behavior of the LaSrCuO<Subscript>4 − δ</Subscript>/Ce<Subscript>0.9</Subscript>Gd<Subscript>0.1</Subscript>O<Subscript>2 − δ</Subscript> interface

The electrochemical behavior of the LaSrCuO_{4 − δ}/Ce_0.9Gd_0.1O_{2 − δ} interface is studied by impedance spectroscopy and cyclic voltammetry methods. By analyzing the dependence of the impedance frequency spectra on the oxygen partial pressure, the rate-determining stages of oxygen exchange are determined in the temperature interval of 500–900°C. For temperatures above 700°C, the adsorption of oxygen molecules and their dissociation to oxygen atoms are shown to make a substantial contribution to the polarization resistance of the overall electrode process, besides the charge-transfer resistance.     

Two mechanisms of thermal expansion in perovskite SrCo<Subscript>0.6</Subscript>Fe<Subscript>0.2</Subscript>Nb<Subscript>0.2</Subscript>O<Subscript>3-z</Subscript>

Perovskite SrCo_0.6Fe_0.2Nb_0.2O_3-z attracts attention as a promising material with high oxygen conductivity. The sample was investigated by means of high-temperature X-ray powder diffraction and thermogravimetry. Phase transition was detected near 400 °C and accompanied with significant mass loss. The phase transition affects oxygen mobility, important for the synthesis of oxygen permeable membranes. The unit cell parameters are proved to change with temperature after two effects (1) reversible conventional thermal expansion and (2) irreversible contraction-expansion due to the changes in the oxygen content. In situ high-temperature X-ray diffraction experiments allowed us to separate the contributions and to measure them as a function of temperature.     

The dependence of the oxygen-exchange properties of Ce<Subscript>0.5</Subscript>Zr<Subscript>0.5</Subscript>O<Subscript>2</Subscript> on the method of synthesis

The oxygen-exchange properties of Ce_0.5Zr_0.5O₂ samples prepared by the traditional coprecipitation method with the use of a biotemplate (pine wood sawdust) were studied by the temperature-programmed reduction and oxidation methods. The use of the template and the presence of alkali and alkaline-earth metal cations in it provides a high mobility of oxygen and a high capacity with respect to oxygen in the biomorphic oxide. Impurities cause the segregation of Ce- and Zr-enriched phases under high-temperature treatment conditions, which worsens the redox properties of the biomorphic sample. Nevertheless, the ability of the biomorphic sample to play the role of an “oxygen buffer” remains high compared with the coprecipitated oxide subjected to similar treatments.     

Temperature dependence of the polarization resistance of the O<Subscript>2</Subscript>,Au/La<Subscript>0.88</Subscript>Sr<Subscript>0.12</Subscript>Ga<Subscript>0.82</Subscript>Mg<Subscript>0.18</Subscript>O<Subscript>2.85</Subscript> electrode system

The impedance of a porous gold electrode in contact with solid electrolyte La_0.88Sr_0.12Ga_0.82Mg_0.18O_2.85 and the effect of the manufacture conditions on its polarization resistance are studied at 600–800°C in air. The overall oxygen reaction rate on a gold electrode is described as the sum of two partial constituents, namely, the oxygen exchange at the gas/electrolyte interface at the gold/gas/electrolyte triple-phased boundary.Translated from Elektrokhimiya, Vol. 41, No. 2, 2005, pp. 190–197.Original Russian Text Copyright © 2005 by Shkerin, Sokolova, Khlupin, Beresnev.This revised version was published online in April 2005 with corrections to the article note and article title and cover date.     

Crystal Structure of Sodium Perchloratotriacetatoneptunate(V) Monohydrate Na<Subscript>3</Subscript>NpO<Subscript>2</Subscript>(OOCCH<Subscript>3</Subscript>)<Subscript>3</Subscript>ClO<Subscript>4</Subscript> · H<Subscript>2</Subscript>O

The crystal structure of Na₃NpO₂(OOCCH₃)₃ClO₄ · H₂O was studied by single-crystal X-ray diffraction method. The structure is composed of the complex anions NpO₂(OOCCH₃)₃]²⁻, perchlorate anions, Na cations, and water molecules. The oxygen environment of Np(V) is a hexagonal bipyramid whose equatorial plane is formed by the oxygen atoms of three acetate ions. In addition to the acetate anions, the structure contains the perchlorate ion whose oxygen atoms, except for one, are included in the coordination environment of Na⁺ cations.__________Translated from Koordinatsionnaya Khimiya, Vol. 31, No. 8, 2005, pp. 636–640.Original Russian Text Copyright © 2005 by Charushnikova, Fedoseev, Starikova.     

Kinetics and mechanism of carbon monoxide oxidation on the supported metal complex catalyst PdCl<Subscript>2</Subscript>-CuCl<Subscript>2</Subscript>/Al<Subscript>2</Subscript>O<Subscript>3</Subscript>

The kinetics of carbon monoxide oxidation with atmospheric oxygen on a PdCl₂-CuCl₂/γ-Al₂O₃ catalyst was studied at T = 27°C and an N₂-O₂-CO mixture pressure of 1 atm. The catalyst was prepared by cold impregnation. Three groups of mechanistic hypotheses are considered, and two of them are demonstrated to be consistent with kinetic data, although they differ in the roles of water and oxygen in carbon monoxide oxidation.     

Effect of metal substitution for cobalt on the oxygen adsorption properties of YBaCo<Subscript>4</Subscript>O<Subscript>7</Subscript>

YBaCo₄O₇ compound is capable to intake and release a large amount of oxygen in the temperature range of 200–400°C. In the present study, the effect of Zn, Ga and Fe substitution for Co on the oxygen adsorption/desorption properties of YBaCo₄O₇ were investigated by thermogravimetry (TG) method. Due to fixed oxidation state of Zn2+ ions, the substitution of Zn²⁺ for Co²⁺ suppresses the oxygen adsorption of YBaCo_4−xZn_xO₇. The substitution of Ga³⁺ for Co³⁺ also decreases the oxygen absorption capacity of YBaCo_4−xGa_xO₇. This can be explained by the strong affinity of Ga³⁺ ions towards the GaO₄ tetrahedron. Compared with Zn- and Ga-substituted samples, the drop of oxygen adsorption capacity is smallest for Fe-substituted samples because of the similar changeability of oxidation states of Co and Fe ions.     

Kinetics of oxidation of methane to formaldehyde on Na<Subscript>4</Subscript>[PFeMo<Subscript>11</Subscript>O<Subscript>40</Subscript>]/SiO<Subscript>2</Subscript>

The kinetics of oxidation of CH₄ to formaldehyde on the catalytic system Na₄[PFeMo₁₁O₄₀]/SiO₂ were studied, and a significant role of the redox potential of the CH₄-O₂ system with respect to the catalyst was shown. The density of centers participating in the reaction was determined, and dissociative competitive adsorption of methane and oxygen was established. The equation was deduced in the framework of the Langmuir-Hinshelwood theory taking into account the side conversion of formaldehyde. Possible participation of lattice oxygen in the reaction was suggested.     

Chemical bonding and electronic structure of LaMnO<Subscript>3</Subscript> and La<Subscript>0.75</Subscript>MnO<Subscript>3</Subscript> orthorhombic crystals

The electronic structure of the LaMnO₃ orthorhombic crystal of a stoichiometric composition and of La_0.75MnO₃ crystals with a La vacancy in the unit cell is calculated in the LSDA+U approximation of density functional theory. The calculations showed that LaMnO₃ is an insulator with a forbidden gap of 0.5 eV and with antiferromagnetic ordering of magnetic moments. The magnetic moment on the manganese ions is 3.78 BM. The La atom has ionic bonds in the lattice, while the bond between oxygen and manganese is covalent. After lanthanum has been removed, geometry optimization of the unit cell leads to La_0.75MnO₃ stable structures. In one of the structures, which is lower in energy, the states of manganese may be attributed to Mn⁴⁺ ions. In both structures with removed lanthanum, the oxygen ions have reduced effective charge, so that one can speak about O^? ions appearing along with O^2? in the structure. The oxygen, as well as lanthanum and manganese, ions are nonequivalent in these structures; their nonequivalence is primarily reflected by the local densities of states. This leads to charge and magnetic nonequivalence of ions. In La_0.75MnO₃ crystals, the degree of bond covalence between manganese and oxygen decreases.     

Oxygen redistribution in ZrO<Subscript>2</Subscript>-Y<Subscript>2</Subscript>O<Subscript>3</Subscript> system

This work represents the results of oxygen redistribution studies at quantitative and isotopic levels in synthesis and thermal treatment of ZrO - (0 to 35 mol %) Y₂O₃ solid solution crystals. The crystals were grown by directed melt crystallization method in a cold container using direct high-frequency heating. The crystal oxygen content and isotopic composition data was collected with respect to stabilizer concentration and technological conditions of synthesis. The temperature and frequency relationships of crystal electroconductivity were also studied. Some strength and tribological characteristics of the given materials were represented. The solid state formation by directional melt crystallization was shown to involve oxygen isotopic exchange interaction between the melt, growing crystal, and gas phase.     

High-Temperature X-Ray Diffraction Investigation of the Decomposition Process in Manganese-Gallium Spinel Mn<Subscript>1.5</Subscript>Ga<Subscript>1.5</Subscript>O<Subscript>4</Subscript>

The stability of spinel-type mixed Mn_1.5Ga_1.5O₄ oxide prepared in an inert medium (1000 °C, Ar) is studied by thermogravimetry and high-temperature X-ray diffraction in air in a wide temperature range 30–1000 °C. On heating, reversible decomposition processes of initial spinel are observed. From 30 °C to 600 °C oxygen atoms attach to the surface layer of initial Mn_1.5Ga_1.5O₄ spinel to form a new phase distinct from parent oxide by the oxygen stoichiometry (cation vacancies are formed). The product of decomposition is two oxides: Mn_1.5Ga_1.5O₄ and Mn_1.5–xGa_1.5–x[·]_xO₄. On the contrary, above 600 °C a loss of oxygen occurs, the concentration of cation vacancies decreases in Mn_1.5–xGa_1.5–x[·]_xO₄, and the reverse process of single phase oxide crystallization takes place. At 1000 °C the spinel phase forms again whose composition is similar to that of the initial parent phase Mn_1.5Ga_1.5O₄. On cooling the decomposition of this phase is again observed due to oxygen attachment.     

Mo<Subscript>6</Subscript>Cl<Subscript>12</Subscript>-Incorporated Sol-Gel for Oxygen Sensing Applications

Excitation of hexanuclear molybdenum complexes such as Mo₆Cl₁₂ and its derivatives in the ultraviolet results in a strongly red-shifted luminescence centered at 750nm. Since oxygen efficiently quenches the luminescence, these thermally stable inorganic complexes are candidate lumophores for real-time, high temperature optical fiber based sensing of oxygen. Sol-gel films containing the acetonitrile complex of Mo₆Cl₁₂ were deposited on quartz substrates by dip coating. After drying, the films were heated at 200^∘C for 1 h. The luminescence lineshapes of films before and after heating were unchanged, indicating that heating did not adversely affect the cluster photophysics. Compared to solutions of the acetonitrile complex, quenching by oxygen was smaller in the as-prepared films, but heating at 200^∘C for 1 h increased the quenching, apparently due to increased oxygen permeability resulting from the loss of water or other small molecules from the matrix. These results confirm the potential of hexanuclear molybdenum complexes such as Mo₆Cl₁₂⋅2CH₃CN as the lumophores in fiber optic oxygen sensors that can operate up to 200^∘C.     

Oxygen redistribution during crystal growth of ZrO<Subscript>2</Subscript>-R<Subscript>2</Subscript>O<Subscript>3</Subscript> solid solutions

This paper presents the results of our experimental studies of quantitative redistribution and isotope fractionation of oxygen during the crystal growth of cubic solid solutions based on ZrO₂. The single crystals were grown by directional crystallization of a melt in a cold container. As stabilizing oxides, we used Y₂O₃, Gd₂O₃, and Yb₂O₃ in concentrations of 8–40 mol %. The results showed that the oxygen isotopic growth effects changed depending on the type and content of the stabilizer in the crystals of ZrO₂-R₂O₃ solid solutions.     

The synthesis and properties of solid solutions based on Ba<Subscript>4</Subscript>Ca<Subscript>2</Subscript>Nb<Subscript>2</Subscript>O<Subscript>11</Subscript>

(Ba_{1 ? x} Ca _x )₆Nb₂O₁₁ solid solutions were synthesized. The compositions were shown to be single-phase at 0.23 ≤ x ≤ 0.47 and have a double perovskite cubic structure with an incomplete oxygen sublattice. The interaction of solid solutions with water vapor and their electrical properties were studied. In dry atmosphere, these complex oxides were mixed oxygen-hole conductors. In humid atmosphere, they intercalated water and exhibited protonic conductivity. The influence of Ba/Ca isovalent substitution, the dynamics of the oxygen sublattice, and the concentration of intercalated water on the value and contribution of protonic and hole conductivity was analyzed.     

Microstructure and Conduction of Composites Bi<Subscript>2</Subscript>CuO<Subscript>4</Subscript>-Bi<Subscript>2</Subscript>O<Subscript>3</Subscript> Near the Eutectic Melting Point

Microstructure and conduction of ceramic composites Bi₂CuO₄ + xBi₂O₃ (x = 5, 10, 15, 20 wt %) near the eutectic melting point (770°C) are studied. Bismuth oxide, initially randomly distributed over the ceramics bulk, after quenching from temperatures exceeding the eutectic melting point, becomes localized at triple junctions and grain boundaries in Bi₂CuO₄, which is caused by wetting grain boundaries and forming a liquid-channel structure. The jumpwise change in the composites’ conductivity near 730 and 770°C caused by polymorphic transformation of Bi₂O₃ and the eutectic melting with simultaneous formation of a liquid-channel structure. Transport numbers of the oxygen ion are measured at 770°C by coulomb-volumetric method. The conduction by oxygen ions increases in the composites with decreasing average size of Bi₂CuO₄ crystallites.__________Translated from Elektrokhimiya, Vol. 41, No. 5, 2005, pp. 596–601.Original Russian Text Copyright © 2005 by Lyskov, Metlin, Belousov, Tret’yakov.     

Synthesis,structure, and electric properties of oxygen-deficient perovskites Ba<Subscript>3</Subscript>In<Subscript>2</Subscript>ZrO<Subscript>8</Subscript> and Ba<Subscript>4</Subscript>In<Subscript>2</Subscript>Zr<Subscript>2</Subscript>O<Subscript>11</Subscript>

Perovskite phases Ba₃In₂ZrO₈ and Ba₄In₂Zr₂O₁₁ with the nominal concentration of structural oxygen vacancies 1/9 and 1/12, respectively, were synthesized by solid-phase and solution methods. X-ray diffraction showed cubic symmetry of both phases with the unit cell parameter a = 0.4193(2) and 0.4204(3) nm, respectively. The absence of superstructural lines resulted in the conclusion on statistical arrangement of oxygen vacancies. Thermogravimetry and mass spectrometry proved that both phases can reversibly absorb water from gas phase (pH₂O = 2 × 10⁻² atm) with observed correlation between the concentration of oxygen vacancies and amount of absorbed water. The total water amount was up to 0.9 mol per formula unit or, if recalculated for perovskite unit ABO₃, 0.3 and 0.23 mol H₂O, respectively. The temperature curves of coductivity in the atmosphere with various partial water vapor pressures (pH₂O = 3 × 10⁻⁵ and 2 × 10⁻² atm) showed significantly higher conductivity and lower activation energy (0.52 eV) in humid atmosphere due to proton transfer. The proton conductivity is up to 5 × 10⁻⁴ Ohm⁻¹ cm⁻¹ at 300°C for Ba₃In₂ZrO₈ specimen. IR spectrometry showed that protons in the structure exist primarily in OH-groups.     

The sequence of equilibrium phase states of the Tb-Mn-O system in the thermal dissociation of the TbMn<Subscript>2</Subscript>O<Subscript>5</Subscript> compound

Phase equilibria in the Tb-Mn-O system during the removal of oxygen from the TbMn₂O₅ compound in stages were studied by the static method on a vacuum circulation unit (973–1173 K) with subsequent X-ray analysis of quenched solid phases. The dissociation of TbMn₂O₅ was found to occur in three stages. The temperature dependences of equilibrium oxygen pressure were determined experimentally for the phase equilibria observed. The standard thermodynamic functions of the dissociation and formation from the elements of TbMn₂O₅ and TbMnO₃ were calculated.     

Thermal stability of Bi<Subscript>2</Subscript>O<Subscript>3</Subscript>

Differential thermal analysis (DTA) and thermogravimetric analysis (TGA) of an α-Bi₂O₃ sample revealed staged phase transitions in the range 720–800°C (at 720, 780, and 800°C) and the elimination of oxygen to the composition Bi₂O_2.967 during heating to 895°C in air at 16 K/min. In dynamic vacuum (p = 1.33 Pa) at 780–800°C, Bi₂O₃ consecutively transforms to a phase with the cubic γ-Bi₂O₃ structure and tetragonal Bi₂O_2.3?2.4. In the latter, electron diffraction in a transmission electron microscope (ED/TEM) shows a superstructure with the superstructure vector q ₁₁₀ ≈ 1/9, which indicates an ordered arrangement of oxygen vacancies.