Showing papers on "Lanthanum published in 1992"

Electron paramagnetic resonance studies of lanthanum-containing C 82

Abstract: THE conjecture that atoms can be trapped inside closed carbon cages such as the fullerenes was first made by Kroto et al1. Mass spectroscopic evidence obtained soon after2 suggested that lanthanum atoms were encapsulated in fullerenes prepared by laser vaporization of a lanthanum-impregnated graphite disk, and these results were later corroborated3,4. Recently, helium atoms have been incorporated into fullerenes through collisions in the gas phase5, and evidence has been obtained for the formation of metal-containing fullerenes during arc burning of composite graphite rods6. All of these studies, however, have produced quantities too small for characterization using standard spectroscopic techniques. We report here the preparation of milligram quantities of lanthanum-containing C82, which can be solvent-extracted in yields of about 2% along with empty C60 and C70 cages. We have measured the electron paramagnetic resonance spectrum of this mixture, both in solution and in the solid state, which reveals that the lanthanum atom has a formal charge of 3+, and the C82a charge of 3-. This runs contrary to some expectations that the doubly charged f ulleride anions would be the most stable species6,7; it also reveals that the fullerene cages have the same formal charge as in the superconducting alkali-metal-doped phases8,9.

Synthesis of Perovskite‐Type (La1−xSrx)MnO3 (O X 0.3) at Low Temperature

Abstract: Perovskite-type (La 1-x Sr x )MnO 3 (0≤x≤0.3) was synthesized through the sol-gel process at low temperature (400 o to 500 o C). Poly(acrylic acid) (PAA) was used to make a gel from an aqueous solution of lanthanum, strontium, and manganese nitrates. The particle-diameter distribution of the manganites had a maximum value of 0.3 to 0.7 μm, and a specific surface area of about 17.5 to 23.5 m 2 /g

Characterization of lanthanum-modified γ-alumina by X-ray photoelectron spectroscopy and carbon dioxide absorption

Abstract: The chemical interaction of dispersed lanthanum with γ-alumina was investigated by combining X-ray photoelectron spectroscopy (XPS) and carbon dioxide absorption. In order to establish the saturation concentration of lanthanum dispersed in the alumina surface layer, a series of materials was formulated at lanthanum concentrations corresponding to La/Al atomic ratios in the range from 0.05 to 2.0. The samples were prepared by impregnation of γ-alumina with aqueous lanthanum nitrate solutions, followed by calcination in air at 925 °C. Upon exposure to carbon dioxide, particles of La2O3 reacted to form bulk La2 (CO3)3, while lanthanum interacting with alumina did not. The reacted carbon dioxide was quantified by observing the carbonate in the C 1s spectra. The La/Al atomic ratio observed by XPS at low loadings from 0.05 to 0.15 was in good agreement with the bulk ratios. The carbon dioxide uptake remained constant at these loadings, while the La 3d5/2 spectral envelope appeared approximately 1.8 eV higher than that expected for La2O3. This shift to higher binding energy and the agreement of theoretical and XPS measurements is consistent with a monolayer dispersion previously reported. As the La loading increased above La/Al =0.15, the ratio measured by XPS increased first slightly, and then substantially over the bulk La/Al ratio, while the amount of carbon dioxide absorbed increased at the same time. In addition, the O 1s, Al 2p and La 3d5/2 spectral envelopes gradually shifted to lower binding energies. As the La/Al bulk ratio approached 2.0, the La 3d5/2 and O 1s spectral envelopes were shifted to what is expected for La2O3, while the Al 2p peak shifted to a binding energy observed for LaAlO3. The differences observed spectrally and quantitatively at low and high loadings imply a change in the coordination of lanthanum.

Modified resin--intermediate processing of perovskite powders:Part II. Processing for fine, nonagglomerated Sr-doped lanthanum chromite powders

Abstract: Nonagglomerated ceramic precursor for Sr-doped lanthanum chromite powder was prepared by a modified resin–intermediate process using equimolar citric acid, ethylene glycol, and metallic nitrates. Pure and well-crystallized submicron powders with a narrow particle-size distribution were calcined from an optimally pulverized resin at a temperature of 750 °C for 2 h in flowing air. The material cost for organics used in the investigated process was considerably lower than the original Pechini process.

The solvent extraction of rare-earth metals by carboxylic acids

Abstract: The solvent extraction of the trivalent lanthanides and yttrium from nitrate media by solutions of carboxylic acids in xylene has been studied. Commercially available carboxylic acids such as Versatic 10 and naphthenic acids were used, as well as model compounds of known structure, such as 2-ethylhexanoic and 3-cyclohexylpropanoic acids. In a few cases, extraction of the metals from sulphate and chloride solutions was also investigated. The dependence of the extraction properties of the carboxylic acids on the atomic number of the lanthanide shows a definite relationship to the steric bulk of the carboxylic acid molecule quantified by means of the steric parameter, Es′ of the substituent alkyl; group. For sterically hindered acids (− Es′ > 2) the pH0.5 of extraction decreases more or less continuously from lanthanum through to lutetium, and the behaviour of yttrium most closely resembles that of the middle lanthanides, such as gadolinium or terbium. For straight-chain and other non-hindered acids...

Large enhancement in oxygen mobility in the superconductors RBa2Cu3O7 with increasing rare-earth size

Abstract: Ultrasonic composite oscillator measurements of the mechanical relaxation in RBa2Cu3O7–8 arising from oxygen hopping in the basal chain layer show enhancements in oxygen mobility of 20, 50, and 100 times for R = gadolinium, neodymium, and lanthanum, respectively, above that for R = yttrium. The use of the larger rare earths offers a practical solution to the major problem of slow oxygen diffusion in single crystals and bulk, dense material for wires and melt-textured monolithic bodies.

Structures in solution and in the solid state of the complexes of lanthanum(III) with 1,10-phenanthroline. X-ray crystallographic and proton, carbon-13, oxygen-17 and lanthanum-139 solution NMR studies

Abstract: The reaction between hexahydrated lanthanum(III) nitrate and 1,10-phenanthroline (phen) in acetonitrile solution was investigated by [sup 1]H, [sup 13]C, [sup 17]O, and [sup 139]La NMR spectroscopy. The crystal structure of trinitratobis(1,10-phenanthroline)lanthanum(III) (La-(NO[sub 3])[sub 3](C[sub 12]H[sub 8]N[sub 2])[sub 2]) was determined by X-ray diffraction as having a monoclinic lattice. In solution, [sup 139]La NMR spectra have shown up to three sites in slow exchange with variable intensities. They were attributed to solvated La(III) ions and to two complex species with one or two phen ligands in the La(III) coordination shell. A more complete structural analysis by [sup 17]O NMR spectroscopy indicated that during the course of the complexation by phen, coordinated water was totally expelled from the La(III) coordination sphere, whereas all three nitrate groups remained coordinated. Proton NMR data have confirmed the total displacement of water. In addition, the proton chemical shifts of phen, in comparison with the [sup 13]C NMR data, clearly showed an anisotropic shielding contribution when two phen ligands were coordinated to La(III). This indicates that the two phen molecules are in a close vicinity in the coordination sphere. From the NMR results, one can conclude that the structure in solution of the 2:1 complex, La(NO[sub 3])[sub more » 3](phen)[sub 2], is very similar to the solid-state one. From [sup 139]La NMR spectroscopy, the populations of the various La(III) species were obtained and the apparent formation constants were calculated. « less

Do low-coordinated Group 1-3 cations Mn+Lm (Mn+ = potassium, rubidium cesium, calcium, strontium, barium, scandium, yttrium, lanthanum; L = ammonia, water, hydrogen fluoride; m = 1-3) with a formal noble-gas electron configuration favor regular or abnormal shapes?

Abstract: The equilibrium structures of the complexes M n+ L m (M n+ =K + , Rb 1+, Cs + , Ca 2+ , Sr 2+ , Ba 2+ , Sc 3+ , Y 3+ , La 3+ , L=NH 3 , H 2 O, HF; m=1-3) have been computed ab initio by using quasirelastivistic pseudopotentials and flexible, polarized basis sets. Many of these species do not obey expectations based on the valence shell electron pair repulsion (VSEPR) rules or simple electrostatic models

Temperature-programmed study of the oxidation of palladium/alumina catalysts and their lanthanum modification

Abstract: Chemical modification with lanthana has previously been found to improve the performance of palladium-based three-way catalysts. In this work, the solid-state oxidation and oxide decomposition behavior of Pd/γ-Al2O3 and Pd/(La2O3/γ-Al2O3) catalysts of varying metal loadings have been assessed using temperature-programmed analysis. For Pd/Al2O3 catalysts with low metal concentrations (⩽= 1 wt.-% Pd), the oxygen consumed during TPO (temperature-programmed oxidation) was significantly in excess of that expected from an O/Pd ratio of one, which suggests the formation of a highly dispersed, oxygen-rich Pd/Al2O3 species. For Pd/Al2O3 catalysts of high metal loading (>=5 wt.-% Pd), there were two oxidation peaks in the TPO spectrum. The one at high temperature, which attenuated with lanthanum addition, is thought to be due to the oxidation of Pd/Al2O3 species of lower dispersion with an O/Pd ratio similar to that of PdO; this species may be an alumina-perturbed state of crystalline PdO. The other peak was assigned to the bulk oxidation of palladium particles. TPO revealed that lanthanum promoted the oxidation of palladium. Lanthanum modification impeded the formation of the highly dispersed, oxygen-rich Pd/Al2O3 species when the palladium concentration was low. Instead, there was TPO evidence for the presence of a new oxidation mode of the metal which was assigned to oxidation of a species comprising a Pd-La2O3 interaction of undetermined character. Oxygen TPD (temperature-programmed desorption) spectra also manifested a new feature upon lanthanum promotion of catalysts of high palladium concentration, which may indicate the presence of a Pd-La2O3 interaction, as well. Hydrothermal aging in steam at 1000° C for 16 h had little effect on the solid-state oxidation processes in lanthanum-free catalysts of low palladium concentration. Metal coalescence appeared to have been modest upon steaming of these catalysts, as indicated by the limited decrease in the oxygen uptake during the formation of the highly dispersed, oxygen-rich Pd/Al2O3 species. Thermal treatment brought about more drastic changes in the TPO profile of catalysts of low palladium concentration when lanthanum was present.

Formation of lanthanum cobalt oxide (LaCoO3) highly dispersed on zirconium dioxide

Abstract: The well-characterized La-Co oxide overlayer was prepared on the surface of ZrO 2 powder by impregnating ZrO 2 with aqueous solutions of the mixtures of La and Co acetates followed by calcination at various temperatures. The preparation processes and the structure of La-Co oxide overlayers were investigated in detail by IR, XRD, XPS, EDX, TEM, and adsorption of NO and pyridine. The resulting catalysts showed very high catalytic activities for complete oxidation of propane.

Air electrode and solid electrolyte fuel cell having the same

Abstract: An air electrode as a component of a solid electrolyte fuel cell. The air electrode is composed of a first layer and a second layer. The first layer has an open porosity of 25% to 57%, pore diameters of 2.5 μm to 12 μm and a resistivity of less than 0.22 Ωcm. The second layer has an open porosity of 8% to 24%, pore diameters of 0.2 μm to 3 μm, and a ratio of the thickness of the second layer to the thickness of the air electrode is 2% to 28%. The sum of the thicknesses of the first and second layers is 0.7 mm to 3.0 mm. The materials of the first and second layers have perovskite structures selected from the group consisting of lanthanum manganate, calcium manganate, lanthanum nickelate, lanthanum cobalate and lanthanum chromate. A solid electrolyte fuel cell includes the air electrode as described above, a solid electrolyte film formed on the surface of the second layer of the air electrode and a fuel electrode film formed on the surface of the solid electrolyte film.

Sol-gel synthesis of Ln2(Ln = La, Nd)Ti2O7

Abstract: Lanthanum and neodymium titanates were prepared by a sol-gel route. Synthesis of La2Ti2O7 from pure alkoxide precursors yielded an intermediate perovskite type phase, La(1–x)TiO3, which partially transformed to La2Ti2O7 on heating to 1500 °C. Substitution of the lanthanum acetylacetonate for alkoxide precursor yielded La2Ti2O7 without any intermediate phase at a very low temperature of 700 °C. Sintering of the La2Ti2O7 gel powder yielded a highly dense ceramic with ∼97% theoretical density. Similar sintering treatment resulted in ∼92% theoretical density for Nd2Ti2O7.

Dielectric properties and defect structure in lanthanum doped SrTiO3 ceramics

Abstract: The dielectric properties and positron annihilation lifetime spectra of the ceramics with the nominal composition Sr1−xLaxTiO3 (0≤x≤0.06) were measured. The dielectric properties and relaxation behaviors, which varied obviously with lanthanum content, were explained by a charge compensation model. The defect structure of the materials was further discussed and the above model proved to be reasonable by the positron annihilation study.

Method and apparatus for separating oxygen from a gaseous mixture

Abstract: A solid composition provides high oxygen ion conductivity, and includes a metal oxide combined with multiple dopants. The oxide may, for example, include zirconia, bismuth trioxide, thoria or halfnia. The dopants are chosen such that they are of similar ionic radius to the oxide, but such that they generally have different valences. For example, zirconium has a +4 valence, while dopants are usually chosen which have +2 or +3 valences. Possible dopants include materials such as magnesia, yttria, and oxides of calcium, barium, strontium, lanthanum, and scandium. It has been found that choosing the dopants such that they exist in the composition at specified ratios results in surprising ability to transport oxygen ions. In the case of a composition which includes magnesia and yttria dopants placed within a zirconia matrix, it has been found desirable to maintain the ratio of the mole percentages of the magnesia to the yttria in the range of from about 6.5:10 to about 9.5:10. Also disclosed are methods and apparatus for using the composition in removing oxygen and water from a mixture of gases. Such a process is valuable in the isolation and purification of both oxygen and the other gases in the mixture.

Beneficiated lanthanum chromite for low temperature firing.

Abstract: A lanthanum chromite ceramic powder mixture that consists essentially of 1 mol of LaCr1-xMxO3, where M is a divalent metal selected from the group consisting of magnesium, calcium and mixtures thereof and x ranges from 0.03 to 0.3; y mols of B2O3, where y ranges from 0.005 to 0.04; and z mols of La2O3, where the ratio z/y ranges from 1.1 to 3. The mixture is formed into powder compacts and sintered to near full density at temperatures as low as 1400 °C.

Rare earth metal coordination compounds as lactone polymerization catalysts

Abstract: A process for polymerizing lactide and up to 20 mole percent of another lactone is disclosed. The catalysts used have the formula MZ3 wherein M is scandium, yttrium, bismuth, a lanthanide series rare earth metal or a mixture thereof and the Zs are independently the same or different highly coordinating ligands. The preferred metals M are lanthanum, cerium, praseodymium and neodymium with lanthanum being especially preferred. The preferred ligands are betadiketones, betaketoesters and malonate anions with the betadiketones such as 2,2,6,6-tetramethyl-3,5-heptanedionates being especially preferred.

Effects of substitution in perovskites La2−xSrxNiO4−λ on their catalytic action for the nitric oxide+carbon monoxide reaction

Abstract: The catalytic activity of the perovskite series La 2− x Sr x NiO 4−γ ( x = 0.00, 0.25, 0.50, 0.75, 1.00, 1.25 and 1.50) has been studied for the reduction of nitric oxide by carbon monoxide in equimolecular mixture. The conversion ( X %) of reactants approaches the value X NO / X CO = 2 at low temperatures and low degree of substitution of lanthanum by strontium but tends to unity at high temperatures and high degree of substitution. This behaviour is due to the change of the rate determining step of the reaction which at low temperature is the oxygen formation through decomposition of nitric oxide while at high temperatures it is the reaction between the adsorbed oxygen species and carbon monoxide. The apparent activation energies for the nitric oxide elimination drop with the process of substitution from x = 0.00 to x = 1.00 and increase thereafter. The controlling factor of the catalytic activity of the series seems to be the strength of adsorption of oxygen on the perovskite surface. Thus it is shown that the sum of the enthalpies of the metal-oxygen bonds in the material is closely related to the temperature where reaction switches mechanism. Besides, the differential mean electronegativity of the solids is related both to the reaction rate as well as to the apparent activation energy for nitric oxide reduction.

Catalysts for the purification of exhaust gas

Abstract: A catalyst for the purification of exhaust gas comprises a combination of palladium carried on cerium oxide and palladium carried on alumina, or further rhodium carried on alumina, in which a weight ratio of palladium carried on cerium oxide to palladium carried on alumina is 2:8-8:2. Preferably, the cerium oxide contains at least one of zirconium, lanthanum, neodymium and praseodymium, or the alumina contains at least one of zirconium, lanthanum, neodymium, cerium, strontium and barium. This catalyst develops excellent purification performances against exhaust gas of oxidizing atmosphere to reducing atmosphere.

Comparison of lanthanum oxide and strontium-modified lanthanum oxide catalysts for the oxidative coupling of methane

Abstract: The activity and selectivity of lanthanum oxide and strontium-modified lanthanum oxide catalysts for the oxidative coupling of methane have been compared. Lanthanum oxide is more active than strontium-modified lanthanum oxide. This is thought to be due to a reduction in the surface concentration of active lanthanum, by replacement with less active strontium carbonate. During methane coupling, catalyst performance varies. This is dependent on calcination temperature and the presence of strontium carbonate. These variations are related to the relative concentration of two types of lanthanum oxycarbonate which may be formed, and their stability, during reaction.

Lanthanum lutetium oxide phosphor with cerium luminescence

Abstract: A phosphor comprising, an ABO3 oxide compound wherein A is about 0.002 to 22 atomic percent cerium and up to about 22 atomic percent lanthanum, and B is about 18 to 22 atomic percent of at least one of lutetium or yttrium.

Infrared study on the hydration of manganese(2+), iron(2+), cobalt(2+), lanthanum(3+), neodymium(3+), dysprosium(3+), and ytterbium(3+) ions in dilute aqueous solution

Abstract: OD stretching vibration spectra of Mn 2+ , Fe 2+ , Co 2+ , La 3+ , Nd 3+ , Dy 3+ , and Yb 3+ perchlorates in dilute (0.2-0.4 M) aqueous solution were obtained using infrared spectroscopy. Intensities, bandwidths, and peak positions of the hydrated cation bands are reported. The currently studied first-row transition-metal ions all have a similar effect on the coordinated water (HDO) molecules and give rise to OD stretching vibration bands at 2427 (2) cm -1

Characterization of high-temperature calcined lanthanum-modified alumina by X-ray photoelectron spectroscopy and X-ray diffraction

Abstract: Previous studies in this laboratory have shown that for the La/γ-Al2O3 system as the atomic ratio La/Al is increased from 0.05 to 2.0 the X-ray photoelectron spectroscopy (XPS) La 3d5/2 binding energy progressively decreases from 835.2 to 832.9 eV. The shirts in binding energy demonstrate a change in the coordination of lanthanum coinciding with a transformation from dispersed to particulate phases. It is of particular interest to understand more completely the chemical interaction between lanthana and alumina at lower lanthanum loadings, which is known to retard the sintering of γ-alumina and promote catalysis. In this study lanthanum-modified alumina was calcined at high temperature (1500 °C). Up to a La/Al ratio of 0.10, X-ray diffraction (XRD) identified the dominant lanthanum phase as lanthanum-β-alumina (La2O3·11Al2O3). The XPS La binding energy for lanthanum-β-alumina was 835.2 eV, which corresponds to that measured previously for dispersed phase lanthanum in γ-alumina. These results suggest that at La/Al ⩽0.10 the highly dispersed La/γ-Al2O3 species is a precursor of crystalline lanthanum-β-alumina formed at higher temperatures. At higher lanthanum concentrations XRD identified highly ordered crystalline LaAlO3 and La2O3, consistent with the phase diagram of the La/Al2O3 system. According to XPS binding energy shifts the precipitation of LaAlO3 requires a higher lanthanum concentration at 925°C (γ-alumina phase) than at 1500°C (β-alumina phase).