Showing papers on "Lanthanum published in 2000"

Low-temperature water-gas shift reaction over Cu- and Ni-loaded cerium oxide catalysts

Abstract: In this paper we report on the activity of Cu- and Ni-containing cerium oxide catalysts for low-temperature water-gas shift (WGS). Bulk catalysts were prepared in nanocrystalline form by the urea co-precipitation–gelation method. Lanthanum dopant (10 at.%) was used as a structural stabilizer of ceria, while the content of Cu or Ni was in the range of 5–15 at.% (2–8 wt.%). At low metal loadings, Cu or Ni were present in the form of highly dispersed oxide clusters, while at high loadings, clusters as well as particles of CuO or NiO (>10 nm in size) were present on ceria. Both Cu and Ni increased the reducibility of ceria, as evidenced by H2-TPR experiments. The WGS reaction activity of Ce(La)Ox was increased significantly by addition of a small amount (2 wt.%) of Cu or Ni. The catalysts were not activated prior to testing. Steady-state WGS kinetics were measured over the temperature range of 175–300 and 250–300°C, respectively, for Cu- and Ni–Ce(La)Ox. The activation energy of the reaction over Ce(La)Ox was 58.5 kJ/mol, while it was 38.2 and 30.4 kJ/mol, respectively, over the 5 at.% Ni–Ce(La)Ox and 5 at.% Cu–Ce(La)Ox catalysts in CO-rich conditions. A co-operative redox reaction mechanism, involving oxidation of CO adsorbed on the metal cluster by oxygen supplied to the metal interface by ceria, followed by H2O capping the oxygen vacancy on ceria, was used to fit the kinetics. Parametric studies were mainly performed with the 5 at.% Cu–(La)Ox catalyst. Notably, this material requires no activation and retains high WGS activity and stability at temperatures up to 600°C.

Surface properties and catalytic performance in methane combustion of Sr-substituted lanthanum manganites

Abstract: Perovskite type La1 − xSrxMnO3 (x = 0–0.5) oxides were prepared by the amorphous citrate process, characterised by X-ray diffraction, oxygen desorption, temperature-programmed reduction, infrared and X-ray photoelectron spectroscopic techniques, and tested for methane combustion within the 473–1073 K temperature range. Since catalyst activity was found to depend strongly on BET areas and to a lesser extent, on the degree of substitution (x), intrinsic activities were computed for La1 − xSrxMnO3 catalyst series. Among the compositions investigated, the degree of substitution x = 0.2 showed the highest intrinsic activity within the temperatures explored. Characterisation techniques made possible to correlate catalytic performance with the structural characteristics of the oxides. The stability of Mn4+ is probably the most important parameter, but excess of oxygen and atomic surface composition should also be taken into account.

Doped zirconia, or zirconia-like, dielectric film transistor structure and deposition method for same

Abstract: A high-k dielectric films is provided, which is doped with divalent or trivalent metals to vary the electron affinity, and consequently the electron and hole barrier height. The high-k dielectric film is a metal oxide of either zirconium (Zr) or hafnium (Hf), doped with a divalent metal, such as calcium (Ca) or strontium (Sr), or a trivalent metal, such as aluminum (Al), scandium (Sc), lanthanum (La), or yttrium (Y). By selecting either a divalent or trivalent doping metal, the electron affinity of the dielectric material can be controlled, while also providing a higher dielectric constant material then silicon dioxide. Preferably, the dielectric material will also be amorphous to reduce leakage caused by grain boundaries. Also provided are sputtering, CVD, Atomic Layer CVD, and evaporation deposition methods for the above-mentioned, doped high dielectric films.

Characterization of Lanthanide Ions Binding Sites in the Cell Wall of Pseudomonas aeruginosa

Abstract: Earlier studies have shown that Pseudomonas aeruginosa can adsorb selectively La3+, Eu3+, and Yb3+ from aqueous solution. These bacterial cells may find promising applications for removal and separation of lanthanide ions from contaminated effluents. In this work, potentiometric titrations and time-resolved laser-induced fluorescence spectroscopy were used to determine the binding sites of the biomass and, consequently, to elucidate the underlying mechanisms involved in the biosorption of lanthanide ions. Around 90 ± 5% of the adsorbed lanthanum was easily desorbed with an EDTA 0.1 M solution. In most instances, lanthanides seemed to concentrate extracellularly. The diversity of potential metal-binding groups was revealed by potentiometric titrations of the biomass. The amount of strong and weaker acidic functional groups in the wet biomass was estimated at 0.24 ± 0.04 and 0.86 ± 0.02 mequiv/g, respectively. Time-resolved laser-induced fluorescence spectroscopy on europium-loaded P. aeruginosa biomass sug...

Perovskite catalysts for the catalytic flameless combustion of methane

Abstract: A new method was employed for the preparation of a set of lanthanum cobaltites of general formula La1−xMxCoO3+δ with M=Ce, Eu and x=0, 0.05, 0.1, 0.2. All the samples thus prepared were nanostructured, thermally very stable and characterised by highly crystalline perovskite-like structure and high surface area. Their activity as catalysts for the catalytic flameless combustion (CFC) of methane was by ca. one order of magnitude higher than that of their analogues, prepared through the usual calcination-milling (CM) procedure. Adsorption of oxygen was accompanied by formation of paramagnetic species. Desorption of preadsorbed oxygen was dependent on the nature of the doping element and on the value of the stoichiometric coefficient x of their formula. A correlation between the temperature of the maximal rate of oxygen release and catalytic activity was found. The following scale of activity for the title reaction versus x values could be set up: 0.1Ce>0.05Ce>0>0.05Eu>0.1 Eu≅0.2Ce. The higher activity of Ce-doped catalysts as compared to those doped with Eu was found to be related to the strength of the bond between oxygen and Co ions.

Cation ordering at fluoride sites in silicate glasses: a high-resolution 19F NMR study

Abstract: High-resolution 19 F NMR, with sample spinning rates to 25 kHz, can provide quantitative, new information about the structure of fluoride sites in F-doped silicate glasses. Here we present results for sodium-, potassium-, calcium-, lanthanum- and mixed cation silicate glasses, containing 1–2 wt% F. In the end member compositions, most fluoride is in sites that are structurally similar to the corresponding simple metal fluoride, although a few percent of Si–F bonds can be detected in the sodium silicates. In the mixed cation glasses, partially resolved peaks are observed for series of distinct mixed cation coordination environments for fluoride. Fluoride ions seem to be preferentially bonded to higher field-strength modifier cations, but considerable disorder is present for all cation pairs (Na–K, Na–Ca, Na–La) studied.

Phosphate diester hydrolysis by mono- and dinuclear lanthanum complexes with an unusual third-order dependence.

Abstract: A series of mono- and dinuclear lanthanum complexes of 15,31-dimethyl-3,11,19,27,33,35-hexaazapentacyclo[27.3.1.15,91.13,17.121,25]hexatriaconta-5,7,9(33),13,15,17(34),21,23,25(35),29,31,1(36)-dodecaene-34,36-diol (24RBPyBC, L) have been defined in solution. Their ability to hydrolyze bis(4-nitrophenyl) phosphate, a phosphate diester, was studied. The various metal-coordinated hydroxide nucleophiles that form in solution attack the substrate in the hydrolysis reaction. The dihydroxo dilanthanum complex, L-2La-2(OH), is the most effective catalyst. Its rate constant is 75 times larger than the rate constant for the monohydroxo dilanthanum complex, L-2La-OH. The mononuclear complexes are not as successful as the dinuclear complexes because they have fewer metal ions per complex to act as Lewis acids. They also cannot generate hydroxide nucleophiles at low pH values like the dinuclear complexes can. The reaction has an unusual third-order dependence on the catalyst concentration which is valid for the dinucl...

Steric effects control the structure of the solvated lanthanum(III) ion in aqueous, dimethyl sulfoxide, and N,N'-dimethylpropyleneurea solution. An EXAFS and large-angle X-ray scattering study.

Abstract: The structure of the solvated lanthanum(III) ion has been determined in aqueous, dimethyl sulfoxide, and N,N‘-dimethylpropyleneurea solution by means of the EXAFS and large-angle X-ray scattering (LAXS) techniques. The close agreement between the EXAFS spectra of solid nonaaqualanthanum(III) trifluoromethanesulfonate and of an aqueous lanthanum(III) perchlorate solution shows that the hydrated lanthanum(III) ion in aqueous solution most probably has the same structure as in the solid, i.e., nine water molecules coordinated in a tricapped trigonal prismatic configuration. The data analysis from EXAFS and LAXS measurements of the aqueous solution resulted in the La−O bond distances 2.52(2) and 2.65(3) A to the water molecules in the prism and the capping positions, respectively. The LAXS study shows a second hydration sphere consistent with approximately 18 water molecules at 4.63(2) A. The EXAFS spectra of solid octakis(dimethyl sulfoxide)lanthanum(III) trifluoromethanesulfonate and a dimethyl sulfoxide so...

Stabilization of calcium oxyapatites with lanthanum(III)-created anionic vacancies

Abstract: Two series of samples of the system Ca 10-x La x (PO 4 ) 6 (OH) y (0 ≤ x ≤ 2), have been synthesized by solid-state reaction at high temperature. In this composition range the apatite structure is maintained. Cationic distribution was analyzed by Rietveld refinement of the XRD data, showing a preferential substitution of La(III) ions in the Ca(2) position. The introduction of the trivalent ion provokes the transformation of OH - to O 2- ions. Stable oxy- or oxyhydroxyapatites with numerous anionic vacancies have been synthesized as a function of the lanthanum content or the synthesis conditions.

Effect of lanthanum on aged seed germination of rice

Abstract: Attempts were made to promote germination of natural aged rice seeds by treating them with lanthanum nitrate. In tests to measure the germination rate, germination index, and vigor index of natural aged rice seeds were found to be significantly increased by lanthanum. It is treating aged rice seed with lanthanum nitrate that enhanced the respiratory rate and activities of superoxide dismutase, catalase, and peroxidase, and decreased superoxide O2*- and malondidehyde contents, and therefore reduced plasma membrane permeability. It suggests that lanthanum may be used to pretreat seed before sowing.

Cobalt oxide catalysts for ammonia oxidation activated with cerium and lanthanum

Abstract: The effect of cerium and lanthanum oxides in cobalt oxide catalysts for both activity and selectivity in ammonia oxidation was studied. The molar ratios of Ce/Co and La/Co in the catalysts were 0.02. All the catalysts were characterized by XRD. The activity was measured in a fixed bed reactor at a temperature range of 720–820°C. Cerium oxide proved to be a more efficient promoter than lanthanum oxide, both being advantageous. The selectivity of ammonia oxidation to NO over Ce-containing sample was even as high as 97.7% and the selectivity to N2O was as low as 0.5%.

The role of lanthanum oxide on Pd-only three-way catalysts prepared by co-impregnation and sequential impregnation methods

Abstract: The effect of lanthanum oxides on the catalytic performance and the physicochemical properties of Pd‐only three‐way catalysts prepared by co‐impregnation and sequential impregnation methods was studied by using hydrogen chemisorption, BET surface area, X‐ray diffraction and X‐ray photoelectron spectroscopy. It was found that the roles of La closely depended on the order of La introduction in the preparation of the Pd catalysts. Pd–La/Al2O3 prepared by co‐impregnation of La and Pd, kept its superior activity in spite of the significant loss of surface area of the alumina support after thermal aging at 1273 K, indicating that the primary role of La was a Pd stabilizer through the intimate interaction between La and Pd. However, on Pd/La/Al2O3, in which Pd was consecutively impregnated after the impregnation of La, La preferentially interacted with the alumina support as a form of LaxAlyO2, resulting in the stabilization of the alumina support during thermal aging. XPS results indicated that lanthanum oxide suppressed the formation of PdO interacting with alumina during thermal aging. In the case of Pd/La–Ce/Al2O3, the formation of the solid solution of (CexLa1−x)O2 was not strong enough to maintain the high activity and the good textural property after thermal aging.

Crystallization of gadolinium- and lanthanum-containing phases from sodium alumino-borosilicate glasses

Abstract: Lanthanide-containing glasses, commonly used for optical and laser applications, are also important in the vitrification of actinide-bearing radioactive wastes. In previous studies, we measured the glass forming regions of La2O3 and Gd2O3 in some sodium alumino-borosilicate glasses. Above their highest concentrations in these glasses, lanthanide silicate crystals with an apatite structure were found. In this paper, we characterize these crystals using powder X-ray diffraction (XRD), electron microscopy, energy dispersive spectroscopy (EDS), and selective area diffraction (SAD) to evaluate baseline glass composition effect and mixed La/Gd effect on the structure and chemistry of these crystals. When different lanthanide elements (lanthanum and gadolinium) co-exist in the glasses, complete lanthanide silicate solid solution is observed. Small amounts of boron can enter the gadolinium silicate structure if aluminum is present in the melt. The boron is probably substituting for the silicon in the crystal lattice. This substitution will cause a decrease in the unit cell parameters a0 and c0. A small amount of Na can also enter the crystal lattice, causing a decrease in the cell parameter a0, but an increase in c0. These results may help us to develop better understanding on the solution mechanism of lanthanide oxides in these glasses.

Aqueous alteration of lanthanum alumino-silicate glasses

Abstract: The aqueous alteration behavior of glasses in the system Y–La–Al–Si–O–N has been investigated using the Soxlhet test. The altered surface was then studied by means of solid and solution analyses. In the case of La–Al–Si–O–N glasses, the formation of a crystallized lanthanum hydroxycarbonate was determined by X-ray diffraction (XRD) analysis. In the case of Y–La–Al–Si–O–N, X-ray photoelectron spectroscopy analysis (XPS) reveals a complete disappearance of silicon and aluminum at the glass surface (depth analyzed: 20 A). Rutherford backscattering spectrometry (RBS) gives some evidence that the glass surface is covered with a phase consisting mainly of yttrium and lanthanum compounds. Solution and solid analyses provide evidence that yttrium increases the chemical durability of the glass. Nitrogen seems to enhance glass corrosion by increasing the pH of the solution.