Showing papers on "Lanthanum published in 2003"

Highly Efficient Water Splitting into H2 and O2 over Lanthanum-Doped NaTaO3 Photocatalysts with High Crystallinity and Surface Nanostructure

Abstract: NiO-loaded NaTaO3 doped with lanthanum showed a high photocatalytic activity for water splitting into H2 and O2 in a stoichiometric amount under UV irradiation. The photocatalytic activity of NiO-loaded NaTaO3 doped with lanthanum was 9 times higher than that of nondoped NiO-loaded NaTaO3. The maximum apparent quantum yield of the NiO/NaTaO3:La photocatalyst was 56% at 270 nm. The factors affecting the highly efficient photocatalytic water splitting were examined by using various characterization techniques. Electron microscope observations revealed that the particle sizes of NaTaO3:La crystals (0.1−0.7 μm) were smaller than that of the nondoped NaTaO3 crystal (2−3 μm) and that the ordered surface nanostructure with many characteristic steps was created by the lanthanum doping. The small particle size with a high crystallinity was advantageous to an increase in the probability of the reaction of photogenerated electrons and holes with water molecules toward the recombination. Transmission electron microsc...

Lithium Lanthanum Titanates: A Review

Abstract: To date, the highest bulk lithium ion-conducting solid electrolyte is the perovskite (ABO3)-type lithium lanthanum titanate (LLT) Li3xLa(2/3)-x□(1/3)-2xTiO3 (0 < x < 0.16) and its related structure materials. The x ≈ 0.1 member exhibits conductivity of 1 × 10-3 S/cm at room temperature with an activation energy of 0.40 eV. The conductivity is comparable to that of commonly used polymer/liquid electrolytes. The ionic conductivity of LLT mainly depends on the size of the A-site ion cation (e.g., La or rare earth, alkali or alkaline earth), lithium and vacancy concentration, and the nature of the B−O bond. For example, replacement of La by other rare earth elements with smaller ionic radii than that of La decreases the lithium ion conductivity, while partial substitution of La by Sr (larger ionic radii than that of La) slightly increases the lithium ion conductivity. The high lithium ion conductivity of LLT is considered to be due to the large concentration of A-site vacancies, and the motion of lithium by a...

Thermal Conductivity and Thermal Expansion Coefficients of the Lanthanum Rare-Earth-Element Zirconate System

Abstract: To enhance the insulating properties of a thermal barrier coating, one has to focus on new materials with lower intrinsic thermal conductivity than established yttria-stabilized zirconia. Substances with pyrochlore structure were investigated. Starting from lanthanum zirconate, substitutions of the lanthanum by other trivalent rare-earth elements were made, and the thermal conductivity and the thermal expansion coefficient of the manufactured materials were measured. A complete substitution of the lanthanum led to increased thermal expansion coefficients, whereas the partial substitution did not show an appreciable effect. The thermal conductivities of the modified materials were lower than that of the pure lanthanum zirconate for temperatures <1000°C for all amounts and elements of substitution. A comparison of the observed values with calculated values of the thermal conductivities showed a relatively good agreement.

Oxygen stoichiometry, ferromagnetism, and transport properties of La 2-x NiMnO 6+δ

Abstract: Nominal La 2 NiMnO 6 prepared by solid-state reaction in air in accordance with earlier reports is shown to contain excess oxygen as well as the coexistence of two ferromagnetic phases of comparable Curie temperatures, onemonoclinic and the other rhombohedral. As originally predicted, ordering of Ni 2 + and Mn 4 + ions gives ferromagnetic Ni 2 + -O-Mn 4 + interactions and transforms the orthorhombic Pbnm space group to monoclinic P2 1 In with β90° and the rhombohedral R3c space group to R3m or R3. Synthesis by the Pechini method in Ar, air, and O 2 atmospheres under different thermal treatments also consistently gave O 6 + δ ; the lowest δ=0.05(1) was attained for a single P2 1 /n phase reacted under Ar at 1350°C. Lowering the mean A-site atomic radius in La 2 - x □ x NiMnO 6 and Nd 2 NiMnO 6 also stabilizes the monoclinic phase, and near oxygen stoichiometry was attained in La 2 - x □ x NiMnO 6 for x0.09. Excess oxygen is accommodated in the perovskite structure by the creation of cation vacancies, and it is shown that lanthanum vacancies create deep three-hole acceptor traps. Comparison with the double perovskite La 2 CoMnO 6 and La 2 NiRuO 6 + δ versus La 2 CoRuO 6 signals that stabilization of lanthanum vacancies is associated with a Ni 3 + /Ni 2 + redox couple that is stabilized by a counter octahedral-site cation M having a strong covalent component to its M-O bonding. It is therefore proposed that in the presence of Ni 2 + , but not Co 2 + , a lanthanum vacancy is stabilized by the formation of two holes trapped deeply in molecular orbitals of an O 1 2 cluster of the oxygen atoms that neighbor a lanthanum vacancy. Transport data also indicate a lowering of the separation of the Mn 4 + /Mn 3 + and Ni 3 + /Ni 2 + redox couples from E g ≥0.8 eV to E g 0.3 eV in the ordered Ni 2 + , Mn 4 + array. This lowering and a motional enthalpy ΔH m n 0.1 eV of electrons is attributed to locally cooperative Jahn-Teller deformations of low-spin Ni 3 + and high-spin Mn 3 + octahedral sites. The magnetization M(5 K, 50 kOe) is lowered by both local atomic disorder and the formation of antiphase boundaries. It is shown that a prolonged anneal at 800 °C reduces the local atomic disorder, but it does not remove the antiphase boundaries. Synthetic strategies to increase the magnetization must be designed to reduce the concentration of antiphase boundaries and cation vacancies as well as the atomic disorder.

Photocatalytic Activity for Water Decomposition of Indates with Octahedrally Coordinated d10 Configuration. I. Influences of Preparation Conditions on Activity

Abstract: The photocatalytic properties for water decomposition of RuO2-dispersed alkaline earth metal and lanthanum indates with an octahedrally coordinated In3+ ion of d10 configuration were studied. The i...

Preparation and characterization of lanthanum-gadolinium monazites as ceramics for radioactive waste storage

Abstract: Several La1 − xGdxPO4 solid solutions were prepared in the monazite- or rhabdophane-type structures for various x values using three methods of preparation (direct evaporation, synthesis in closed PTFE containers on a sand bath or in autoclaves). Samples of rhabdophane-type La1 − xGdxPO4·nH2O (n ≈ 0.5) were prepared at 150 °C only for x ≥ 0.4. For x ≤ 0.3, the solids were precipitated as the monazite-type structure. These results were confirmed by the study of pure rare earth phosphates synthesized under the same conditions. By these means, well-crystallized and monophase samples of MPO4·nH2O (n ≈ 0.5–1) in the monazite (La, Ce), rhabdophane (Nd, Sm, Eu, Gd, Tb, Dy) or churchite (Ho, Er, Tm, Yb, Lu) forms were prepared. On the basis of the variation of the specific area versus the holding temperature and of the dilatometric studies, the optimal temperature of sintering for these solids was found to be between 1250 and 1400 °C. The effective relative densities of the pellets of GdPO4 prepared using a two-step procedure (pressing between 200 and 700 MPa, then heat treatment at 1300 °C) reached 96% of the value calculated from the XRD data. The chemical durability of sintered samples of GdPO4 was evaluated in several acidic media between room temperature and 90 °C. The very low normalized dissolution rates RL (between 10−6 and 10−3 g m−2 day−1) measured even in very acidic media confirmed the very good retention properties of this kind of phosphate-based matrix for the immobilization of radionuclides and especially of trivalent actinides.

Oxidative coupling of methane catalyzed by rare earth oxides: Unexpected synergistic effect of the oxide mixtures

Abstract: Oxidative coupling of methane (OCM) catalyzed by a series of rare earth oxides (REO) and mixtures of REO was studied using a continuous-flow quartz reactor at atmospheric pressure, 650–900 °C and variable gas flow rates of methane and air. The mixtures of the rare earth oxides available industrially (as intermediate products in commercial RE metal manufacturing) revealed good efficiency in the OCM. Catalytic performance of the light group of REO consisting of lanthanum, praseodymium, neodymium and cerium oxides with 5.5 wt.% of ceria is comparable to that of the individual oxides (samaria, ytterbia and europia). Addition of 10 wt.% of ceria to pure lanthana unexpectedly enhanced the efficiency of the catalyst. Raman spectroscopy as well as XRD suggested an oxide–oxide interaction between lanthana and ceria providing formation of additional oxygen vacancies.

Effect of substitution by cerium on the activity of LaMnO3 perovskite in methane combustion

Abstract: This study concerns the effect of lanthanum substitution by cerium on the catalytic activity of La1-xCexMnO3 catalysts and its relation to their physico-chemical characteristics. Samples of pure and cerium-substituted lanthanum manganese perovskites. La1-xCexMnO3 with x = 0.1-0.5 and LaCexMnO3 with x = 0.1, 0.2 and 0.3, were prepared by the citrate method and calcined 5 h at 973 or 1073 K. All samples were characterized by XRD, XPS and oxygen TPD and had their specific surface area (SSA) determined by nitrogen adsorption. The catalytic activity was determined, using 0.1 g catalyst, 1% methane in air at a flow rate of 75 ml/min (GHSV = 45,000 ml/g(cat) h). Substitution with cerium affects significantly the physico-chemical properties of individual compositions. It slows the rate of perovskite phase formation, increases the SSA, has an effect on thermal stability and modifies the oxygen desorption characteristics. However, these changes do not correlate in the expected way with changes in activity for methane combustion. Substitution with cerium or addition of cerium over the formal stoichiometry were positive only for x = 0.1 in samples calcined 5 h at 973 K. Higher x values resulted in lower activity. (C) 2002 Elsevier Science B.V. All rights reserved.

Crystalchemistry and Oxide Ion Conductivity in the Lanthanum Oxygermanate Apatite Series

Abstract: 120.334(3)°, and V ) 620.46(3) A 3 ) and the fits were satisfactory for such complex pseudohexagonal structure. This structure contains 72 variable positional parameters and 24 thermal factors. High-temperature neutron powder diffraction (NPD) data were also collected at 773 and 1173 K for hexagonal La9.60(GeO4)6O2.40. The electrical results suggest that the samples are bulk oxide ion conductors. The plots of the imaginary parts of the impedance, Z′′, and the electric modulus, M′′, vs log(frequency), possess maxima for both curves separated by less than a half decade in frequency with associated capacities of 2 pF. The curvatures observed in the Arrhenius plots are not due to a phase transition. The conductivities are almost independent of the oxygen partial pressure under oxidizing conditions, which suggests pure oxide-ion conduction with negligible electronic contribution.

Reducibility of fast oxide-ion conductors La2−xRxMo2−yWyO9(R = Nd, Gd)

Abstract: The substitutional range and cell parameter evolution of fast oxide-ion conductors La2−xRxMo2−yWyO9 (R = Nd, Gd) are investigated. In the whole series, the cubic β-La2Mo2O9 structural type is stabilized at room temperature. The effects on reducibility of both single and double substitutions are presented. Lanthanum substitution by rare earth appeared to be responsible for an increase in the reducibility and a strong but reversible amorphization under dilute hydrogen. On the contrary, the favourable role of tungsten on the compound stability under reducing conditions is evidenced: it depletes oxygen loss while making the La2Mo2O9 structural type less affected by it.

Perovskites as catalysts precursors: CO2 reforming of CH4 on Ln1−xCaxRu0.8Ni0.2O3 (Ln = La, Sm, Nd)

Abstract: A series of perovskite-like oxide in which the A-site cation of the precursor perovskite, LaRu08Ni02O3, was partially or totally substituted by calcium, samarium and neodymium have been used to produce in situ nanoparticles of Ru(Ni) well dispersed on a stable support for the carbon dioxide reforming of methane Perovskites of the type LnxCa1−xRu08Ni02O3 (Ln=La3+, Sm3+, Nd3+) were synthesized as catalysts precursors The reduced solids of nominal composition (Ru,Ni)/CaO and/or La2O3, Sm2O3, Nd2O3, were used as catalysts The La1−xCaxRu08Ni02O3 series showed a well-defined perovskite structure with surface areas between 3 and 10 m2/g However, when lanthanum was replaced by samarium and neodymium, the presence of pyrochlore structures, together with the perovskites, were obtained After reduction Ru(Ni) crystallites size between 9 and 17 nm were produced The substitution of La by cations of smaller ionic radii (Ca, Nd, Sm) decrease the stability of the perovskites and lower their reduction temperature Among the calcium series, La08Ca02Ru08Ni02O3 and La05Ca05Ru08Ni02O3, proved to be the most active catalysts with the highest selectivity to CO While samarium-containing perovskite was the best among the lanthanide series Correlations between the effect of partial or total substitution of A-site cations of the precursor perovskite and the catalytic activity and stability of in situ formed nickel and ruthenium particles were established

MOCVD of High-Dielectric-Constant Lanthanum Oxide Thin Films

Abstract: Lanthanum oxide thin films were fabricated on Si substrates by the metallorganic chemical vapor deposition (MOCVD) method at substrate temperatures ranging from 400 to 650°C. From the results of X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), cross-sectional scanning transmission electron microscopy (STEM), and energy-dispersive X-ray (EDX) analyses, the enhanced chemical reaction at the interface between the Si substrate and the films was revealed, which results in the generation of an interfacial layer of SiO 2 (1-2 nm thick) and lanthanum silicate at all the experimental substrate temperatures. We found that a thin silicon oxynitride layer on the Si substrate is effective in suppressing the interfacial reaction and in increasing the dielectric constant of the lanthanum oxide deposited on it. The thin silicon oxynitride layer is also effective in reducing the leakage current through the film. For the stacked La-oxide/SiON film, the dielectric constant of the lanthanum oxide film was 19 and the leakage current density was 3 X 10 -6 A/cm 2 at the oxide voltage of 1 V for a film with an equivalent oxide thickness of 2.4 nm.

Study of the CO and humidity interference in La doped tin oxide CO2 gas sensor

Abstract: The influence of lanthanum on SnO2 properties was studied using samples with different La over SnO2 content prepared by the impregnation method. Samples were structurally characterised by XRD, XPS and Raman spectroscopy in order to identify the location of the additive, its oxidation state, and its main phase. Thick film sensors fabricated by screen-printing were tested at concentrations of CO2 from 500 to 2500 ppm, at a wide range of relative humidities. CO was also analysed as main interfering molecule. DRIFTS studies were performed to explore the role of lanthanum in the sensing mechanisms. Finally, a two-sensor array is proposed to differentiate between the contributions of CO2 and its interferents.

Inorganic colors and related nanotechnology

Abstract: A pigment with modified properties because of the powder size being below 100 nanometers. Blue, yellow and brown pigments are illustrated. Nanoscale coated, un-coated, whisker inorganic fillers are included. Stoichiometric and non-stoichiometric composition are disclosed. The pigment nanopowders taught comprise one or more elements from the group actinium, aluminum, antimony, arsenic, barium, beryllium, bismuth, cadmium, calcium, cerium, cesium, cobalt, copper, chalcogenide, dysprosium, erbium, europium, gadolinium, gallium, gold, hafnium, hydrogen, indium, iridium, iron, lanthanum, lithium, magnesium, manganese, mendelevium, mercury, molybdenum, neodymium, neptunium, nickel, niobium, nitrogen, oxygen, osmium, palladium, platinum, potassium, praseodymium, promethium, protactinium, rhenium, rubidium, scandium, silver, sodium, strontium, tantalum, terbium, thallium, thorium, tin, titanium, tungsten, vanadium, ytterbium, yttrium, zinc, and zirconium.

Aqueous Combustion Synthesis of Strontium-Doped Lanthanum Chromite Ceramics

Abstract: An aqueous combustion synthesis is used to produce powders of La0.8Sr0.2CrO3 perovskite. It is shown that interaction between chromium nitrate and glycine controls the process. In addition, it is suggested that glycine reacts with products of nitrate decomposition to yield an intermediate compound, which decomposes exothermically providing high-temperature conditions for complex oxide formation. It is remarkable that although reaction temperature is high (up to 800°C) and characteristic time is small (∼1 s) for synthesis under the self-propagating high-temperature mode, the produced perovskites have high specific surface area (∼40 m2/g) and well-defined crystalline structure. As a result, ceramics sintered by using these powders are dense (∼96% of theoretical) and possess high electronic and low ionic conductivities, important for interconnect applications in solid oxide fuel cells.

Nanoparticles of rare earth oxides

Abstract: Rare earth compositions comprising nanoparticles, methods of making nanoparticles, and methods of using nanoparticles are described. The compositions of the nanomaterials discussed may include scandium (Sc), yttrium (Y), lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu). The nanoparticles can be used to make organometallics, nitrates, and hydroxides. The nanoparticles can be used in a variety of applications, such as pigments, catalysts, polishing agents, coatings, electroceramics, catalysts, optics, phosphors, and detectors.

Removal of fluoride using some lanthanum(III)‐loaded adsorbents with different functional groups and polymer matrices

Abstract: Although fluoride is beneficial for human beings in small quantities, it causes dental fluorosis when consumed in larger quantities over a period of time. In recent years, considerable work has been conducted for the purpose of developing new and low cost absorbents for adsorptive removal of fluoride, especially chelating resins loaded with metal ions. In the present study, several types of adsorbents with different functional groups loaded with lanthanum(III) were prepared to be used for fluoride removal from water. The optimum conditions for loading lanthanum(III) on the adsorbents and the effects of pH and initial fluoride concentration as well as shaking time and solid–liquid ratio on the removal of fluoride have been investigated. Based on these fundamental data, the removal of fluoride from actual hot spring water was also tested as a practical application by comparing the efficiency of different adsorbents for the removal of fluoride from hot spring water. The following conclusions were obtained. (1) The different chemical composition and chemical structure of the polymer matrix play the most important role in fluoride adsorption, (2) strongly acidic adsorbents are more effective on fluoride removal at neutral pH than weakly acidic adsorbents, (3) the order of fluoride removal in the neutral pH range of 4.5–8.0 by the different La(III)-loaded adsorbents employed in the present work is as follows: 200CT resin > POJRgel > IR124resin > SOJR gel ≥ CPAgel ≥ WK11 resin. The column experiments showed that the 200CT resin loaded with lanthanum(III) at pH 6.0 can be successfully employed for the removal of fluoride ions from actual hot spring water. Copyright © 2003 Society of Chemical Industry

Accumulation of rare earth elements in maize plants (Zea mays L.) after application of mixtures of rare earth elements and lanthanum

Abstract: Rare earth elements are applied in China to improve crop production, and the distribution patterns of individual rare earth elements in native plants have widely been reported. But our knowledge is still limited about the dose-dependent accumulation of individual rare earth elements in agricultural crops after application of rare earth elements. Effects of lanthanum and mixtures of rare earth elements were studied in pot experiments on the accumulation of individual rare earth elements in maize plants. All plant samples were divided into plant tops and roots. On addition of mixtures of rare earth elements and lanthanum to the soil, a significant dose-dependent accumulation of individual rare earth element(s) was found in the roots and in the plant tops. Application of mixtures of rare earth elements at >10 mg kg−1 soil, resulted in a significant increase in contents of light rare earth elements in the roots, and at a dose of 50 mg kg−1 soil, a similar phenomenon was found in the plant tops. When mixtures of rare earth elements were replaced by lanthanum alone, at a dose higher than 10 mg La kg−1 soil, a significant increase in La content occurred in the roots and in the plant tops. The content ratio of La to Ce in maize plants appeared to increase as the application doses of rare earth element(s) increased. At a highest dose (50 mg kg−1soil), the transport of the absorbed La from the roots to the plant tops might be substantially reduced after treatment with lanthanum alone, compared with mixtures of rare earth elements. Increasing the application doses of rare earth element(s) appeared to cause a positive Gd and negative Ce anomaly in the roots and in the plant tops, and the anomaly was more obvious in the plant tops than in the roots. The results indicated that the Gd and Ce anomaly in corns might be considered as important parameters for the safety assessment of agricultural application of rare earth elements.

Solid oxide fuel cell components and method of manufacture thereof

Abstract: A solid oxide fuel cell comprises a dense electrolyte disposed between a porous anode and a porous cathode wherein the dense electrolyte comprises doped lanthanum gallate or yttria stabilized zirconia, the porous anode comprises yttrium-doped strontium titanate, yttrium-doped strontium titanate and nickel, lanthanum-doped ceria and nickel or yttria stabilized zirconia and nickel and the porous cathode comprises doped lanthanum ferrite or strontium-doped lanthanum manganite. The fuel cell may further comprise an interlayer(s) comprising lanthanum-doped ceria disposed between an electrode (anode, cathode or both) and the electrolyte. An interconnect layer comprising doped lanthanum chromate may be disposed between the anode of a first single fuel cell and the cathode of a second single fuel cell. The anode, cathode, electrolyte and optional interlayer(s) are produced by thermal spray.