Showing papers on "Lanthanum published in 2009"

Simultaneous transesterification and esterification of unrefined or waste oils over ZnO-La2O3 catalysts

Abstract: A single-step method was developed for biodiesel production from unrefined or waste oils using a series of heterogeneous zinc and lanthanum mixed oxides. Effects of metal oxide molar ratio, free fatty acids (FFA) and water content in feedstock, molar ratio of methanol and oil, and reaction temperature on the yield of biodiesel were investigated. A strong interaction between Zn and La species was observed with enhanced catalyst activities. Lanthanum promoted zinc oxide distribution, and increased the surface acid and base sites. The catalyst with 3:1 ratio of zinc to lanthanum was found to simultaneously catalyze the oil transesterification and fatty acid esterification reactions, while minimizing oil and biodiesel hydrolysis. A reaction temperature window of 170–220 °C was found for the biodiesel formation. A high yield (96%) of fatty acid methyl esters (FAME) was obtained within 3 h even using unrefined or waste oils.

Oil transesterification over calcium oxides modified with lanthanum

Abstract: Investigations were conducted on a series of calcium and lanthanum oxides catalyst for biodiesel production. Mixed oxides catalyst showed a superior transesterification activity over pure calcium or pure lanthanum oxide catalysts. The catalyst activity was correlated with surface basicity and specific surface areas. The effects of water and free fatty acids (FFA) levels in oil feedstock, water and CO2 in air, mass ratio of catalyst, molar ratio of oil to methanol, and reaction temperature on fatty acid methyl ester (FAME) yield were investigated. Under optimal conditions, FAME yields reached 94.3% within 60 min at 58 °C. Mixed CaO-La2O3 catalyst showed a high tolerance to water and FFA, and could be used for converting pure or diluted unrefined/waste oils to biodiesel.

New crystal structure and characterization of lanthanum tungstate "La6WO12" prepared by freeze-drying synthesis.

Abstract: Lanthanum tungstates with a La/W atomic ratio between 6 and 4.8 have been synthesized as polycrystalline materials using the freeze-drying wet-chemical precursor method. Our results show that a single phase material is obtained when the La/W ratio is between 5.3 and 5.7 (T = 1500 °C). Outside this compositional range, segregation of either La2O3 (La/W ≥ 5.8) or La6W2O15 (La/W ≤ 5.2) are found. We have solved the crystal structure for the composition with a La/W nominal atomic ratio of 5.6 by combining powder X-ray and powder neutron diffraction techniques. This structure substantially differs from that previously reported for Ln6WO12 (Ln = Y, Ho). The main differences between the two structure types are the crystal symmetry, the different coordination environment of the cations and the formula unit. The formula unit can be written as La6.63W1.17O13.43 (Z = 4; calculated density = 6.395 g/cm3), well in accordance with the diffraction techniques, He-pycnometry and electron probe microanalysis. These materials can be described as a face centred cubic structure with space group F3m. Lattice parameters vary between 11.173 and 11.188 A, depending on composition. Dense ceramic materials are obtained at 1400 °C, and microanalyses measurements indicate that no significant tungsten evaporation occurs compared to the nominal values. Compositions with La2O3 segregation show similar conductivity values as the single phase ones, but those containing segregation of W-rich phases show a considerable drop in conductivity with increasing content of the secondary phase.

Influence of lanthanum doping on the dielectric, ferroelectric and relaxor behaviour of barium bismuth titanate ceramics

Abstract: Barium lanthanum bismuth titanate (Ba1−(3/2)xLaxBi4Ti4O15, x = 0–0.4) ceramics were fabricated using the powders synthesized via the solid-state reaction route. X-ray powder diffraction analysis confirmed the above compositions to be monophasic and belonged to the m = 4 member of the Aurivillius family of oxides. The effect of the partial presence of La3+ on Ba2+ sites on the microstructure, dielectric and relaxor behaviour of BaBi4Ti4O15 (BBT) ceramics was investigated. For the compositions pertaining to x ≤ 0.1, the dielectric constant at both room temperature and in the vicinity of the temperature of the dielectric maximum (Tm) of the parent phase (BBT) increased significantly with an increase in x while Tm remained almost constant. Tm shifted towards lower temperatures accompanied by a decrease in the magnitude of the dielectric maximum (em) with an increase in the lanthanum content (0.1 < x ≤ 0.4). The dielectric relaxation was modelled using the Vogel–Fulcher relation and a decrease in the activation energy for frequency dispersion with increasing x was observed. The frequency dispersion of Tm was found to decrease with an increase in lanthanum doping, and for compositions corresponding to x ≥ 0.3, Tm was frequency independent. Well-developed P(polarization)–E(electric field) hysteresis loops were observed at 150 °C for all the samples and the remanent polarization (2Pr) was improved from 6.3 µC cm−2 for pure BBT to 13.4 µC cm−2 for Ba0.7La0.2Bi4Ti4O15 ceramics. Dc conductivities and associated activation energies were evaluated using impedance spectroscopy.

Diffusion Path and Conduction Mechanism of Oxide Ions in Apatite-Type Lanthanum Silicates

Abstract: Apatite-type lanthanum silicates of general formula La9.33+2x/3(SiO4)6O2+x have appeared recently as a new promising class of oxide ion conductors with potential applications as electrolytes for solid oxide fuel cells (SOFCs). They have been shown to demonstrate relatively high oxide ion conductivity at moderate temperatures as well as at low oxygen partial pressures. In this paper, the diffusion pathways and the conduction mechanism of oxide ions in these phases are reinvestigated. This is done by means of atomic scale computer modeling techniques with both semiempirical and bond valence methods. Our results support that oxide ion conduction along the c-axis proceeds by an interstitial mechanism. They also support the presence of interstitial sites located within the conduction channel. However, contrarily to recent research, it is shown that the channel oxide ions are involved in the conduction process by a push−pull type mechanism. This mechanism brings into play a cooperative movement of both two adja...

Effect of some rare earth elements on the growth and lanthanide accumulation in different Trichoderma strains

Abstract: Accumulation of rare earth elements (REE) in the soil may be due to the use of REE enriched fertilizers and to contamination by REE containing wastes. Although widely used in China for soil and foliar dressing of crops, little is known about the effect of REE applications on the soil microbial community. The effect of REE on the growth of biological control strains of Trichoderma atroviride and Trichoderma harzianum was investigated in vitro using either a mix of different REE containing different amounts of lanthanum, cerium, praseodymium, neodymium, gadolinium nitrate and lanthanum nitrate alone in comparison to treatments with potassium nitrate and water. In plate tests applied concentrations ranged from 0.1 mM to 300 mM for lanthanum and REE mix and from 0.1 mM to 900 mM for the potassium solution. In liquid culture tests applied concentrations ranged from 0.001 mM to 100 mM for lanthanum and REE mix and from 0.003 mM to 900 mM for the potassium solution. ICP-MS, TEM and TEM X-ray microanalysis were used to study the accumulation of REE in fungal biomass. All the Trichoderma strains showed a good tolerance to the presence of REE in the culture media. Some growth enhancing effects were observed in liquid cultures of T. harzianum strains but not in T. atroviride. Accumulation of REE in fungal biomass, both at intracellular level and in the extracellular matrix, was observed.

Metal–metal bonds in f-element chemistry

Abstract: The molecular chemistry of the f-elements is traditionally dominated by the use of carbon-, nitrogen-, oxygen-, or halide-ligands. However, the use of metal-based fragments as ligands is underdeveloped, which contrasts to the fields of d- and p-block metal–metal complexes that have developed extensively over the last fifty years. This perspective outlines the development of compounds, which possess polarised covalent or donor–acceptor f-element–metal bonds. For this review, the f-element is defined as (i) a group 3 or lanthanide metal: scandium, yttrium, lanthanum to lutetium, or (ii) an actinide metal: thorium, or uranium, and the metal is defined as a d-block transition metal, or a group 13 (aluminium or gallium), a group 14 (silicon, germanium, or tin), or a group 15 (antimony, or bismuth) metal. Silicon, germanium, and antimony are traditionally classified as metalloids but they are included for completeness. This review focuses mainly on complexes that have been structurally authenticated by single-crystal X-ray diffraction studies and we highlight novel aspects of their syntheses, properties, and reactivities.

In Situ Infrared Characterization during Atomic Layer Deposition of Lanthanum Oxide

Abstract: Mechanisms of atomic layer deposition (ALD) growth of lanthanum oxide on H-terminated Si(111) using lanthanum tris(N,N′-diisopropylacetamidinate) (La(iPr-MeAMD)3) are investigated using infrared (I...

Lanthanum Silicate and Lanthanum Zirconate Nanoparticles Co-Doped with Ho3+ and Yb3+: Matrix-Dependent Red and Green Upconversion Emissions

Abstract: Excitation of Ho3+ and Yb3+ co-doped lanthanum silicate and lanthanum zirconate nanoparticles with 980 nm diode laser light gave a red and green glow, respectively, observable by naked eye. Spectroscopic investigations on these materials revealed green (540 nm), red (660 nm), and near-infrared (750 nm) upconversion emissions with the green to red ratio varying with the matrix type and the dopant ion (Yb3+) concentration. The emission was predominantly red for Ho3+:Yb3+ (1:3) in lanthanum silicate nanoparticles, while it was predominantly green for Ho3+:Yb3+ (1:7) in lanthanum zirconate nanoparticles. A mechanism involving cross-relaxations and energy back transfer has been proposed to explain the observed behavior. The predominance of red emission has been attributed to a strong quenching of Ho3+ green emitting level mainly by energy back transfer from Ho3+ to Yb3+ on the basis of the near-infrared (NIR) emission spectral analysis.

Characteristics of lanthanum loaded TiO2-ZSM-5 photocatalysts: Decolorization and degradation processes of methyl orange

Abstract: Novel photocatalytic materials were prepared by incorporation of lanthanum ions on TiO 2 supported ZSM-5. These materials were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), UV–vis diffuse reflectance spectra (UV–vis DRS), atomic force microscopy (AFM), scanning electron microscopy SEM with energy dispersive X-ray analysis (EDX) and surface area (BET) measurements. XRD and FTIR results showed adsorption of TiO 2 and La 2 O 3 nanoparticles on the surface of ZSM-5 support. AFM and SEM images revealed obvious variations in the surface morphology of raw ZSM-5 after TiO 2 and La 2 O 3 loading. Photocatalytic activities of the supported catalysts were examined for decolorization and degradation processes of methyl orange (MO) solution under UV irradiation. Incorporation of lanthanum ions improved the photocatalytic activities of TiO 2 supported ZSM-5 catalysts. Effects of TiO 2 content, lanthanum ion content and calcination temperature were investigated. Quantum efficiencies, relative photonic efficiencies as well as reuse performances of the supported catalysts were also demonstrated. A rough model was postulated for TiO 2 and La 2 O 3 loading on the surface of ZSM-5.

Hydrothermally fabricated single-crystalline strontium-substituted lanthanum manganite microcubes for the catalytic combustion of toluene

Abstract: La 1− x Sr x MnO 3− δ ( x = 0.4, 0.5, and 0.6) catalysts were fabricated hydrothermally from KMnO 4 , MnCl 2 (KMnO 4 /MnCl 2 molar ratio = 3/7), and stoichiometric amounts of lanthanum and strontium nitrates in KOH solution. The physicochemical properties of the materials were characterized by a number of analytical techniques. It was found that the La 1− x Sr x MnO 3− δ samples fabricated at 250 °C are single-crystalline perovskite-type oxides in the form of microcubes. The as-fabricated La 1− x Sr x MnO 3− δ materials display various states of oxygen nonstoichiometry. The total amount of oxygen vacancies and Mn 4+ in the catalysts decreases in the order of La 0.5 Sr 0.5 MnO 3− δ > La 0.4 Sr 0.6 MnO 3− δ > La 0.6 Sr 0.4 MnO 3− δ . It has been found that the trends of catalyst reducibility and catalytic performance of the three catalysts follow a similar order. We observed 100% toluene conversion at 255 °C over the La 0.5 Sr 0.5 MnO 3− δ catalyst, 113 °C lower than that observed over a polycrystalline La 0.5 Sr 0.5 MnO 3− δ catalyst prepared by calcination at 950 °C. The excellent performance of the former can be related to the high Mn 4+ /Mn 3+ ratio, distinct oxygen nonstoichiometry, and single-crystalline structure of the catalyst.

Diffusion pathway of mobile ions and crystal structure of ionic and mixed conductors - A brief review

Abstract: A brief review on the field of Solid State Ionics, including the diffusion pathway of mobile ions, crystal structure and materials, is presented. In the fluorite-structured ionic conductors such as ceria solid solution Ce0.93Y0.07O1.96, bismuth oxide solid solution δ-Bi1.4Yb0.6O3 and copper iodide CuI, a similar curved diffusion pathway along the directions is observed. In the ionic and mixed conductors with the cubic ABO3 perovskite-type structure such as lanthanum gallate and lanthanum cobaltite solid solutions, the mobile ions diffuse along a curved line keeping the interatomic distance between the B cation and O2- anion to some degree. The structure and diffusion path of double-perovskite-type La0.64Ti0.92Nb0.08O2.99, K2NiF4-type (Pr0.9La0.1)2(Ni0.74Cu0.21Ga0.05)O4+δ , and apatite-type La9.69(Si5.70Mg0.30)O26.24 are described. The diffusion paths of Li+ ions in La0.62Li0.16TiO3 and Li0.6FePO4 are two- and one-dimensional, respectively.

Luminescent Properties of Eu-Doped Lanthanum Oxyfluoride Sol−Gel Thin Films

Abstract: Highly luminescent Eu3+-doped LaOF thin films have been prepared by a sol−gel procedure using La(hfa)3 diglyme (Hhfa = 1,1,1,5,5,5-hexafluoro-2,4-pentanedione; diglyme = bis(2-metoxyethyl)ether) as a single-source precursor for lanthanum and fluorine. Europium doping (Eu/La = 3 and 10 atom %) has been achieved by adding the proper amount of europium acetate hydrate (Eu(CH3CO2)3·xH2O, x ≤ 4) to the precursor solution. After single-layer deposition, the obtained coatings were heat treated in air up to 700 °C. All the samples, under UV irradiation, presented a bright red luminescence clearly visible to the naked eye notwithstanding the low europium content and the limited film thickness (≤50 nm). The microstructure, composition, and morphology of the samples and their interplay with the synthesis conditions were investigated by glancing incidence X-ray diffraction (GIXRD), X-ray photoelectron spectroscopy (XPS), secondary ion mass spectrometry (SIMS), and field emission scanning electron microscopy (FE-SEM)....