Showing papers on "Lanthanum published in 2013"

Lanthanum–Strontium–Manganese Perovskites as Redox Materials for Solar Thermochemical Splitting of H2O and CO2

Abstract: A thermodynamic and experimental investigation of a new class of solar thermochemical redox intermediates, namely, lanthanum–strontium–manganese perovskites, is presented. A defect model based on low-temperature oxygen non-stoichiometry data is formulated and extrapolated to higher temperatures more relevant to thermochemical redox cycles. Strontium contents of x = 0.3 (LSM30) and x = 0.4 (LSM40) in La1–xSrxMnO3−δ result in favorable reduction extents compared to ceria in the temperature range of 1523–1923 K. Oxidation with CO2 and H2O is not as thermodynamically favorable and largely dependent upon the oxidant concentration. The model is experimentally validated by O2 non-stoichiometry measurements at high temperatures (>1623 K) and CO2 reduction cycles with commercially available LSM35. Theoretical solar–fuel energy conversion efficiencies for LSM40 and ceria redox cycles are 16 and 22% at 1800 K and 13 and 7% at 1600 K, respectively.

Lead Lanthanum Zirconate Titanate Ceramic Thin Films for Energy Storage

Abstract: An acetic-acid-based sol–gel method was used to deposit lead lanthanum zirconate titanate (PLZT, 8/52/48) thin films on either platinized silicon (Pt/Si) or nickel buffered by a lanthanum nickel oxide buffer layer (LNO/Ni). X-ray diffraction and scanning electron microscopy of the samples revealed that dense polycrystalline PLZT thin films formed without apparent defects or secondary phases. The dielectric breakdown strength was greater in PLZT thin films deposited on LNO/Ni compared with those on Pt/Si, leading to better energy storage. Finally, optimized dielectric properties were determined for a 3-μm-thick PLZT/LNO/Ni capacitor for energy storage purposes: DC dielectric breakdown strength of ∼1.6 MV/cm (480 V), energy density of ∼22 J/cc, energy storage efficiency of ∼77%, and permittivity of ∼1100. These values are very stable from room temperature to 150 °C, indicating that cost-effective, volumetrically efficient capacitors can be fabricated for high-power energy storage.

Reactivity of Doped Ceria-Based Mixed Oxides for Solar Thermochemical Hydrogen Generation via Two-Step Water-Splitting Cycles

Abstract: Ceria-type materials were investigated as reactive chemical intermediates, in view of solar thermochemical hydrogen production via two-step water-splitting. Ceria/zirconia mixed oxides and ceria doped with yttrium, lanthanum, praseodymium, or gadolinium were studied using a thermobalance to evaluate their thermal reduction capacity in inert atmosphere and their subsequent reactivity with water steam to generate hydrogen. Ceria/zirconia materials present the highest reduction yields with a noticeable linear increase as a function of the zirconium content (in the range 0–54% Zr), while the gravimetric amount of O2 released during reduction tends to level off for Zr atomic contents above 25%. Temperature-programmed reduction experiments demonstrate that the zirconium insertion favors the bulk reduction. The addition of different dopants (among Y, La, Pr, and Gd) did not affect the global materials reducibility, although it should favor the material thermal stability during repeated cycles. The marked effect ...

The effect of A-site substitution by Sr, Mg and Ce on the catalytic performance of LaMnO3 catalysts for the oxidation of vinyl chloride emission

Abstract: Catalytic oxidation of vinyl chloride (VC) emission was carried out over LaMnO3 and La0.8A0.2MnO3 (A = Sr, Mg and Ce) perovskite oxides synthesized via co-precipitation method. Numerous characterization techniques were performed to investigate the relationship between the catalytic performance and its physicochemical properties. It was found that the partial substitution of lanthanum by cerium and magnesium had a positive effect on the catalytic performance for VC oxidation, whereas strontium involved a negative effect. Under the reaction conditions (VC concentration = 1000 ppm, GHSV = 15,000 h−1), the overall catalyst ranking in terms of the catalytic activity from the best to the worst performance was La0.8Ce0.2MnO3 > La0.8Mg0.2MnO3 > LaMnO3 > La0.8Sr0.2MnO3 with regard to the temperature of T50 and T90. The Ce-doped perovskite catalyst showed the optimum catalytic performance due to its higher specific surface area and its ability to promote the low-temperature reducibility. Moreover, as the active species, the increased surface adsorbed oxygen was also responsible for the enhancement of the catalytic performance.

Encyclopedia of metalloproteins

Abstract: Period 2: Lithium, Beryllium.- Period 3: Sodium, Magnesium, Aluminium.- Period 4: Potassium, Calcium, Scandium, Titanium, Vanadium, Chromium, Manganese, Iron, Cobalt, Nickel, Copper, Zinc, Gallium.- Period 5: Rubidium, Strontium, Yttrium, Zirconium, Niobium, Molybdenum, Technetium, Ruthenium, Rhodium, Palladium, Silver, Cadmium, Indium, Tin.- Period 6: Caesium, Barium, Lanthaninds (Lanthanum, Cerium, Praseodymium, Neodymium, Promethium, Samarium, Europium, Gadolinium, Terbium, Dysprosium, Holmium, Erbium, Thulium, Ytterbium), Lutetium, Hafnium, Tantalum, Wolfram, Rhenium, Osmium, Iridium, Platinum, Gold, Mercury, Thallium, Lead, Bismuth.- Period 7: Francium, Radium, Actinides (Actinium, Thorium, Protactinium, Uranium).

Electrocatalytic Activity of Transition Metal Oxide-Carbon Composites for Oxygen Reduction in Alkaline Batteries and Fuel Cells

Abstract: Conductive transition metal oxides (perovskites, spinels and pyrochlores) are attractive as catalysts for the air electrode in alkaline rechargeable metal-air batteries and fuel cells. We have found that conductive carbon materials when added to transition metal oxides such as calcium-doped lanthanum cobalt oxide, nickel cobalt oxide and calcium-doped lanthanum manganese cobalt oxide increase the electrocatalytic activity of the oxide for oxygen reduction by a factor of five to ten. We have studied rotating ring-disk electrodes coated with (a) various mass ratios of carbon and transition metal oxide, (b) different types of carbon additives and (c) different types of transition metal oxides. Our experiments and analysis establish that in such composite catalysts, carbon is the primary electro- catalyst for the two-electron electro-reduction of oxygen to hydroperoxide while the transition metal oxide decomposes the hydroperoxide to generate additional oxygen that enhances the observed current resulting in an apparent four-electron process. These findings are significant in that they change the way we interpret previous reports in the scientific literature on the electrocatalytic activity of various transition metal oxide- carbon composites for oxygen reduction, especially where carbon is assumed to be an additive that just enhances the electronic conductivity of the oxide catalyst. (C)more » 2013 The Electrochemical Society. All rights reserved.« less

Low temperature stabilization of cubic (Li7−xAlx/3)La3Zr2O12: role of aluminum during formation

Abstract: The lithium lanthanum zirconium oxide garnet, Li7La3Zr2O12 (LLZ), has received significant attention in recent years due to its high room temperature lithium ion conductivity and its stability against lithium metal. Together these features make it a promising electrolyte candidate for a high energy all solid-state battery. Previous studies have shown that incorporation of aluminum cations during the synthesis stabilizes the higher conductivity cubic phase of LLZ; however the incorporation process and its effect on the phase transition are still unclear. In the present study, we have combined powder X-ray diffraction (XRD), 27Al and 7Li MAS NMR and high-resolution X-ray diffraction (HRXRD) to determine the disposition of Al cations during the formation of low temperature cubic LLZ. At temperatures as low as 700 °C, the aluminum is incorporated into amorphous or nanocrystalline grain boundary phases. Above 700 °C, the Al cations are associated with a poorly crystalline anti-fluorite phase Li5AlO4, composed of molecular [AlO4]5− anions. This phase then reacts with tetragonal LLZ to form cubic LLZ over a 25 hour period at 850 °C. Although the reaction appears complete by powder X-ray diffraction, 27Al NMR spectra showed overlapping resonances suggesting multiple Al environments due to uneven substitution of the 24d Li(1) site. This was confirmed by high-resolution XRD and was consistent with a series of closely related cubic LLZ phases with slightly different Al concentrations, indicating the slower Al(III) diffusion within the lattice has not reached equilibrium in the time allotted. The disorder over the two crystallographic tetrahedral sites by lithium and aluminum cations at this temperature contributes to the observed lattice enlargement associated with the low temperature cubic phase.

Oxidative Coupling of Methane by Nanofiber Catalysts

Abstract: The direct utilization of methane, the main component in natural gas (NG), as an alternate chemical feedstock to petroleum is a highly desirable but difficult goal in industrial catalysis. Many direct and indirect methods have been proposed and studied to convert CH4 into more-useful products, including olefins (e.g. C2H4, C3H6) and higher-molecular-weight hydrocarbons and liquids (e.g. benzene and gasoline), as discussed in a recent review. The production of ethylene (C2H4) from NG represents a particularly desirable process because of its massive worldwide use as an intermediate in the production of plastics, such as polyethylene and polyvinylchloride (PVC). In addition, ethylene can be oligomerized into liquid hydrocarbons, thereby enabling the efficient utilization of natural gas in remote parts of the world. The global production rate of ethylene is over 100 million tons per year, which represents an annual business in excess of $110 billion (July 2012). All indirect NG-conversion routes utilize a high temperature, endothermic, and costly steam-reforming process as the first step, from which synthetic gas (H2/CO mixtures) is produced. This step is followed by the synthesis of useful products through various catalytic processes. Although direct methods avoid the use of costly syngas, they remain uneconomical, owing, in part, to low yields of C2+ compounds, high temperatures, and low throughputs. High temperatures are particularly detrimental because they result in catalyst deactivation and create problems for reactors. In the oxidative coupling of methane (OCM), CH4 is directly converted into C2H6, C2H4, and water in the presence of O2 and a suitable catalyst. The first step involves the abstraction of H from CH4 by the catalyst to form methyl radicals (CH3C). [2, 3] The coupling of two CH3C radicals creates C2H6, followed by its dehydrogenation to afford C2H4. Some C3 hydrocarbons are also formed by the addition of a CH3C radical to C2H4. [4] However, undesirable surface reactions and gas-phase combustion reactions also lead to CO and CO2 (COx). Because high temperatures promote homogeneous gas-phase free-radical reactions, which are detrimental for C2+ products, the development of new catalysts that can operate at low temperatures is crucial for the economic viability of the OCM. Since the pioneering works of Keller and Bhasin, Hinsen and Baerns, and Ito and Lunsford, the OCM has received immense global attention, as evidenced by the large number of catalysts that have been investigated for this transformation: The oxides of almost all of the metals on the periodic table, either individually or in various combinations, have been considered and analyzed as OCM catalysts. Even the best catalysts have been reported to require feed-gas temperatures (Tf) of 700–850 8C and reaction times of 0.2–5.5 s, with C2+ yields of less than 25 %. Gas temperatures within the catalytic zones were found to be 100–200 8C higher than this range, owing to the exothermicity of this process. The actual catalyst-surface temperature is likely to be even higher still because of heattransfer considerations. An important common feature of all of these OCM catalysts is that they are based on quasi-spherical nanoparticles (powders) 2] and, thus, are prone to metal dispersion, agglomeration, and sintering. All of these problems retard catalytic activity. Herein, we report a new fabric catalyst that is comprised of La2O3 CeO2 nanofibers; this catalyst is capable of negating these problems and promoting the ignition of the OCM at a Tf value of 470 8C. The nanofibers were prepared by electrospinning. An analogous La2O3 CeO2 powder catalyst was also prepared by co-precipitation for comparison. Powders of this binary system have previously been studied by Dedov et al. and have been reported to show OCM activity at significantly higher Tf values (715–830 8C). Shown in Figure 1 (left) is the calcined La2O3 CeO2 nanofiber fabric (La/Ce, 15:1 w/w) that was used in the OCM experiments. A SEM image of this fabric (Figure 1, right) shows the formation of highly uniform La2O3 CeO2 nanofibers with diameters in the range 50–75 nm. This fabric had a low BET area of about 26 m g , which suggested that the nanofibers were dense and did not possess any internal porosity. The SEM image also shows the presence of large voids between the fibers, which enhances fabric diffusivity and decreases sintering. The XRD data for La2O3 CeO2 nanofibers and powders, both before and after the test conditions, are presented in Figure 2 (A, B and C, D), along with those for individual La2O3 (E) and CeO2 (F) fibers and powders. From these data, several interesting features are revealed: First, both the La2O3 fibers (Fig-

Influence of Impurities on Crystallization Kinetics of Calcium Sulfate Dihydrate and Hemihydrate in Strong HCl-CaCl2 Solutions

Abstract: The effects of inorganic impurities on the crystallization of calcium sulfates in strong HCl (6.3 mol L–1)-CaCl2 (1.8 mol L–1) solutions were investigated. The impurities considered relate to hydrochloric acid leaching of apatite-type ores for the extraction of rare earth elements. The impurities investigated were K+, Mg2+, Sr2+, Ba2+, Al3+, Fe2+, Fe3+, La3+, Y3+, F– (fluoride), and PO43– (phosphate). The investigation was done in the context of a continuous steady-state crystallization process. Therefore, temperature-controlled, semibatch crystal growth experiments with regulated reagent addition, to ensure nearly constant supersaturation, were performed. The experiments were conducted at 40 and 80 °C corresponding, respectively, to crystallization of calcium sulfate dihydrate (DH) and calcium sulfate hemihydrate (HH). Among all impurities investigated, phosphate and strontium were found to have the most significant effects, with La3+ and Y3+ having some modest effects. Phosphate (added as phosphoric aci...

The effect of lanthanum doping on activity of Zn-Al spinel for transesterification

Abstract: The transesterification of soybean oil with methanol to biodiesel using lanthanum doped zinc aluminate with spinel structure as catalyst is studied. The catalyst was characterized by X-ray diffraction (XRD), N-2 adsorption/desorption, temperature programmed desorption of CO2 (CO2-TPD) and infrared spectroscopy of CO2 adsorption (CO2-IR). The results of CO2-IR show that the basic sites of La doped zinc aluminate are surface isolated O2- anions and OH- groups. The results of CO2-TPD show that the total amount of basic sties of the catalyst improves with the increase of the La doping amount until which is to 18.5 wt%. Doping La also enlarges the pore volume and diameter of zinc aluminate. It gets the largest pore volume (0.30 cm(3) g(-1)) and pore diameter (7.9 nm) when doped with 5.5 wt% La. Zinc aluminate doped with 5.5 wt% La (5.5% La/ZnAl2O4) exhibits the highest activity, with which the biodiesel yield exceeds 95% at 160 degrees C, 2.0 MPa and 0.9h(-1). Furthermore, the transesterification reaction of about 500h in a fixed-bed reactor was carried out over 5.5% La/ZnAl2O4 catalyst. No deactivation and leaching of catalyst components were happened on the catalyst in reaction. (C) 2013 Elsevier B.V. All rights reserved.

Low-cost and large-scale synthesis of functional porous materials for phosphate removal with high performance

Abstract: A facile spray drying technique has been developed for large-scale and template-free production of nanoporous silica with controlled morphology, large pore size, and high pore volume, using commercially available fumed silica, Aerosil 200, as a sole precursor. This approach can be applied to the preparation of functional nanoporous materials, in this study, lanthanum oxide functionalised silica microspheres by introducing lanthanum nitrate in situ during the spray drying process and followed by a post-calcination process. The resultant lanthanum functionalised Aerosil microspheres manifest high phosphate adsorption capacity (up to 2.317 mmol g−1), fast kinetics, and excellent adsorption performance at a low phosphate concentration (1 mg L−1). In virtue of the easy and scalable synthesis method, low cost and high performances of the product, the materials we reported here are promising for water treatment. Our approach may be general and extended to the synthesis of other functional nanoporous materials with versatile applications.

Influence of lanthanum on borosilicate glass structure: A multinuclear MAS and MQMAS NMR investigation

Abstract: The physical and chemical properties of silicate glasses containing rare-earth elements (REEs), either as dopants or at higher concentrations, are sensitive to the REE structural and chemical environment. An unambiguous role of REEs in the glass structure still remains difficult to define because many configurations may exist and are strongly composition-dependent. The structural configuration of lanthanum and its interactions with sodium and calcium are examined here in borosilicate glasses. The impact of lanthanum and calcium substituted for sodium on the boron speciation is investigated by 11B MAS NMR. The resulting 29Si MAS NMR spectra and their interpretations are discussed. A quantitative approach of 17O MQMAS NMR data with the reconstruction of 17O NMR parameter distributions provides an overview of lanthanum distribution and its interactions with the other cations in the vitreous network. No clustering of lanthanum atoms is observed; they are uniformly distributed in the glass structure, surrounded by about 6 non-bridging oxygen atoms and mixed with sodium and calcium atoms to the detriment of the number of BO4 groups. These data provide a better understanding of the addition of rare earths in the glass and of the conditions favorable to their uniform distribution in soda-lime borosilicate glass matrices.

Highly active and stable (La0.24Sr0.16Ba0.6)(Co0.5Fe0.44Nb0.06)O3−δ (LSBCFN) cathodes for solid oxide fuel cells prepared by a novel mixing synthesis method

Abstract: Lanthanum and/or barium strontium cobalt ferrite perovskite oxide materials are highly active cathodes for solid oxide fuel cells (SOFCs) operated at intermediate temperatures of 600–900 °C. However, they are vulnerable to degradation by chromium deposition and poisoning by volatile Cr species from chromia-forming metallic interconnects. Here we report the development of a new mixed ionic and electronic conductor, lanthanum strontium barium cobalt ferrite niobium perovskite, (La0.24Sr0.16Ba0.6)(Co0.5Fe0.44Nb0.06)O3−δ (LSBCFN), prepared by a novel direct mixing synthesis of (La0.6Sr0.4)(Co0.2Fe0.8)O3−δ (LSCF) and Ba(Co0.7Fe0.2Nb0.1)O3−δ (BSCN). The electrical conductivity of LSBCFN is 124 S cm−1 at 600 °C, which is significantly higher than 11 S cm−1 measured on BSCN at the same temperature. The new LSBCFN cathode which combines the structural stability and activity of BCFN and the high conductivity of LSCF not only exhibits better electrochemical activity for the O2 reduction reaction than either LSCF or BCFN in the temperature range of 600–900 °C, but most importantly, it shows excellent stability and tolerance toward chromium deposition and poisoning under SOFC operation conditions. A semi-quantitative analysis indicates that the Cr deposition at the electrode is closely related to the surface segregation of Sr and Ba.

Synthesis and Characterization of Nonsubstituted and Substituted Proton-Conducting La6–xWO12–y

Abstract: Mixed proton–electron conductors (MPEC) can be used as gas separation membranes to extract hydrogen from a gas stream, for example, in a power plant. From the different MPEC, the ceramic material lanthanum tungstate presents an important mixed protonic–electronic conductivity. Lanthanum tungstate La6–xWO12–y (with y = 1.5x + δ and x = 0.5–0.8) compounds were prepared with La/W ratios between 4.8 and 6.0 and sintered at temperatures between 1300 and 1500 °C in order to study the dependence of the single-phase formation region on the La/W ratio and temperature. Furthermore, compounds substituted in the La or W position were prepared. Ce, Nd, Tb, and Y were used for partial substitution at the La site, while Ir, Re, and Mo were applied for W substitution. All substituents were applied in different concentrations. The electrical conductivity of nonsubstituted La6–xWO12–y and for all substituted La6–xWO12–y compounds was measured in the temperature range of 400–900 °C in wet (2.5% H2O) and dry mixtures of 4% H...

Emergence of superconductivity at 45 K by lanthanum and phosphorus co-doping of CaFe₂As₂.

Abstract: Co-doping of lanthanum and phosphorus in CaFe2As2 induces superconductivity at 45 K. This superconducting transition temperature is higher than the 38 K transition in Ba1−xKxFe2As2, which is the maximum found thus far among the 122 phases. Superconductivity with a substantial shielding volume fraction was observed at 0.12 ≤ x ≤ 0.18 and y = 0.06 in Ca1−xLaxFe2(As1−yPy)2. The superconducting phase of the present system seems to be not adjacent to an antiferromagnetic phase.

Emergence of superconductivity at 45 K by lanthanum and phosphorus co-doping of CaFe2As2

Abstract: Co-doping of lanthanum and phosphorus in CaFe2As2 induces superconductivity at 45 K. This superconducting transition temperature is higher than the 38 K transition in Ba1-xKxFe2As2, which is the maximum found thus far among the 122 phases. Superconductivity with a substantial shielding volume fraction was observed at 0.12 $\leq$ x $\leq$ 0.18 and y = 0.06 in Ca1-xLaxFe2(As1-yPy)2. The superconducting phase of the present system seems to be not adjacent to an antiferromagnetic phase.