Catalytic Effects of Lanthanide Oxides on the Thermal Decomposition of Barium Perchlorate

Catalytic activities exerted by the lanthanide oxides Ln₂O₃ (Ln=La, Sm, Gd and Dy) (0.25 mol%) on the thermal decomposition of barium perchlorate were studied gasometrically at 718 K. The α vs. t plot for the salt alone displays (i) initial gas evolution (ii) an induction period, (iii) a short acceleratory stage and (iv) a long decay stage. For the mixtures with Ln₂O₃, phenomena (i) and (ii) are not observed. Ln₂O₃ enhances the rate of reaction in both the acceleratory and the decay stage, and increases the fraction decomposed, α, in the sequence La₂O₃>Gd₂O₃>Sm₂O₃,>Dy₂O₃. The influence of Dy₂O₃ (0.25–2.0 mol%) on the decomposition of Ba(ClO₄)₂ at 718 K indicates that such admixture facilitates the process and the effect increases with increasing concentration. The salt alone and the mixtures decompose through the same stages in the temperature range 703–733 K as at 718 K. The data on both types of samples fit well to the Prout-Tompkins and the Avrami-Erofeev mechanism, suggesting that nucleation takes place in a chain-branching manner and that the two-dimensional growth of the nuclei occurs during the process. Admixture enhances the rate of reaction marked without affecting the energy of activation . This revised version was published online in August 2006 with corrections to the Cover Date.     

Preparation and characterization of ceria-Based electrolytes for intermediate temperature solid oxide fuel cells (IT-SOFC)

Solid-oxide fuel cells (SOFCs) can be used for clean, efficient and environment-friendly energy conversion with a variety of fuels at high temperature (1273 K). The high temperature operation accelerates unwanted reactions and creates materials challenges; so, intermediate-temperature SOFCs (IT-SOFCs) have been developed. Reduction of the operating temperature (between 873–1073 K) requires solid electrolyte materials with higher conductivities. In this study, partially substituted ceria as solid electrolyte is experimented systematically for use in solid oxide fuel cells operating below 1073 K (intermediate temperature range). Nine compositions namely, CeO₂, Ce_0.95Gd_0.05O_2-δ (CGO9505), Ce_0.90Gd_0.10O_2-δ (CGO9010), Ce_0.85Gd_0.15O_2-δ (CGO8515), Ce_0.80Gd_0.20O_2-δ (CGO8020), Ce_0.95Sm_0.05O_2-δ (SDC9505), Ce_0.90Sm_0.10O_2-δ (SDC9010), Ce_0.85Sm_0.15O_2-δ (SDC8515) and Ce_0.80Sm_0.20O_2-δ (SDC8020) were synthesized by Glycine Nitrate (GN) combustion technique and investigated. The physical properties and the other relevant features of the data obtained are analyzed with a view to use these alternate electrolyte materials in IT-SOFC.     

Effect of oxide additives on radiolytic decomposition of potassium nitrate

Gamma-ray induced decomposition of binary mixtures of potassium nitrate with 90, 70, 50, 30 and 10 mol% SiO₂, Al₂O₃, MnO₂, V₂O₅, La₂O₃, CeO₂, Sm₂O₃, Eu₂O₃, Gd₂O₃ and Dy₂O₃ has been studied at different doses up to 500 kGy. Radiolytic decomposition of the nitrate is affected by the concentration of the oxide in the binary mixture as well as by the absorbed dose. The enhancement is up to 10³ times at 90 mol% of the additive.G(NO₂ ⁻) values calculated on the basis of electron fraction of the nitrate decrease with the increasing concentration of the nitrate. A comparison ofG(NO₂ ⁻) for 90 mol% oxides shows decreasing trend as Gd₂O₃>Sm₂O₃≈Dy₂O₃> Eu₂O₃>CeO₂>Al₂O₃>V₂O₅>SiO₂>MnO₂. ESR and TL measurements suggest the formation of radical species which interact with the radical species of nitrate causing enhanced decomposition by energy transfer mechanism.     

Electrochemical characteristics of La0.6Sr0.4CoO3-δ, Pr0.6Sr0.4CoO3-δ and Gd0.6Sr0.4CoO3-δ on Ce0.85Sm0.15O1.925 electrolyte

Cyclic voltammetry, chronoamperometry and electro-chemical impedance have been used for the analysis of the following medium temperature half-cells: Ce_0.85Sm_0.15O_1.925| La_0.6Sr_0.4CoO_3-δ, Ce_0.85Sm_0.15O_1.925| Pr_0.6Sr_0.4CoO_3-δ and Ce_0.85Sm_0.15O_1.925| Gd_0.6Sr_0.4CoO_3-δ. The influence of the atomic mass of the A–site cation in the perovskite cathode on the oxygen reduction kinetics has been discussed. The total polarisation resistance, obtained from the Z′′, Z′-plots, increases with the rise of atomic mass of the cation in the A-site position. Two different time constants have been obtained for the oxygen electroreduction process, and the replacement of La³⁺ by Gd³⁺ in the cathode material decreases somewhat the surface catalytic activity, but the noticeably higher low-frequency series resistance, i.e. mainly diffusion-like mass transfer resistance, values have been obtained. However, the mainly diffusion-limited process at T≤773 K for Gd_0.6Sr_0.4CoO_3-δ and the kinetically mixed process (diffusion + charge transfer) for Pr_0.6Sr_0.4CoO_3-δ and La_0.6Sr_0.4CoO_3-δ have been established. At higher temperature (T≥993 K) and more negative potentials, the O₂ reduction process is limited mainly by the heterogeneous charge transfer step. Presented at the fourth Baltic Conference on Electrochemistry, Greifswald, March 13–16, 2005.     

Formation of Bilayer Thin-Film Electrolyte on Cathode Substrate by Electrophoretic Deposition

Potentialities of the method of bilayer thin-film electrolyte electrophoretic deposition onto cathodic substrate are analyzed. Ce_0.8Sm_0.2O_1.9–δ (SDC) nanopowder and BaCe_0.89Gd_0.1Cu_0.01O_3–δ BCGCuO) micropowder are prepared by the methods of laser evaporation–condensation and pyrolysis, respectively. The effect of ultrasonic treatment on the SDC and BCGCuO particle distribution in suspensions and their electrokinetic properties are studied. The using of the ultrasonic treatment combined with centrifugation allowed obtaining an aggregative-stable suspension of the BaCe_0.89Gd_0.1Cu_0.01O_3–δ micron particles in the isopropanol–acetylacetone mixed medium (70/30 v/v) that is characterized by high zeta potential. Ce_0.8Sm_0.2O_1.9–δ and BaCe_0.89Gd_0.1Cu_0.01O_3–δ thin films are obtained at the La₂NiO_{4 +δ} cathode substrate using electrophoretic deposition; microstructure and electric properties of the prepared thin-film structures are studied. The conductivity and electric properties of the bilayer electrolyte were found to be determined by the Ce_0.8Sm_0.2O_1.9–δ film properties. Despite the sintering high temperature, the grain structure of the BaCe_0.89Gd_0.1Cu_0.01O_3–δ film is underdeveloped; this is determined by the micron powder properties.     

Microstructure–conductivity relationship in Gd- and Sm-doped ceria-based electrolytes prepared by the acrylamide sol–gel-related method

We have prepared pure electrolytes of Ce_0.8Gd_0.2O_1.9 (CGO) and Ce_0.8Sm_0.2O_1.9 (CSO), useful for SOFCs, by a sol–gel-related technique like the acrylamide method. This method consists of preparing a solution from the single oxides followed by gelation. Then, the combustion or decomposition of the organic molecules is initiated, producing nanometric calcined powders of the above-mentioned compounds. Thermal treatments were optimized in order to obtain good electrochemical properties of the electrolytes. We have observed that the synthesis temperature to obtain the pure phase is lower for the sol–gel samples than for the pellets prepared by solid-state reaction, and the final density is higher. The microstructure and composition of the powders were characterized by TEM, SEM, and EDX analysis. The electrical properties of the electrolytes were measured by impedance spectroscopy at different temperatures and oxygen partial pressures.Presented at the OSSEP Workshop Ionic and Mixed Conductors: Methods and Processes, Aveiro, Portugal, 10–12 April 2003     

Role of rare earth (RE) semiconductors on the decomposition of barium bromate

Barium bromate decomposes in the solid state through various stages: (i) initial gas evolution, (ii) induction period, (iii) slow linear reaction, (iv) acceleratory and decay stages. Rare earth semiconductors (p-type), Gd₂O₃ and Dy₃O₃ facilitate the acceleratory and decay stages without affecting the induction and linear periods. The extent of decomposition is higher in the case of mixture than that of the pure salt. The data are discussed in the light of theories of Prout—Tompkins and Avarami-Erofeev mechanisms exploring that nucleation occurs in a chain branching manner and there is two dimensional growth of nuclei.

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Zusammenfassung Bariumbromat zersetzt sich im festen Zustand über verschiedene Schritte: (i) anfängliche Gasentwicklung, (ii) Induktionsperiode, (iii) langsame lineare Reaktion, (iv) Beschleunigungs- und Abschlußschritt. Seltenerdenhalbleiter (vomp-Typ) erleichtern die Beschleunigungs- und Abschlußschritte, ohne die Induktions- und die lineare Periode zu beeinflussen. Das Ausmaß der Zersetzung ist im Falle von Gemischen höher als bei reinen Salzen. Die Angaben wurden im Hinblick auf die Mechanismentheorien von Prout-Tompkins und Avarami-Erofeev besprochen, die besagen, daß die Keimbildung auf kettenverzweigende Weise erfolgt und daß ein zweidimensionales Keimwachstum stattfindet.

     

Selective extraction of gadolinium from Sm2O3 and Gd2O3 mixtures in a single step assisted by MgCl2 in LiCl–KCl melts

The chloration of MgCl₂ was studied in the LiCl–KCl–MgCl₂–Gd₂O₃–Sm₂O₃ melts. Gd(III) and Sm(III) ions were observed by cyclic voltammetry and square wave voltammetry assisted by MgCl₂ in melts. X-ray diffraction (XRD) patterns of melts indicated Gd₂O₃ and Sm₂O₃ were chlorinated by MgCl₂ and formed GdCl₃ and SmCl₃. XRD patterns of non-dissolved residues, which were left after the melts were washed with water to remove the soluble salt, showed that the new compounds (i.e., GdOCl, SmOCl, MgO, Gd(OH)₃, and Sm(OH)₃) were produced. Potentiostatic electrolysis experiments were performed to extract Gd from Gd₂O₃ and Sm₂O₃ mixtures assisted by MgCl₂. Separation between Gd₂O₃ and Sm₂O₃ was also achieved in a single step with the formation of Mg–Li–Gd alloys. XRD patterns of alloys indicated that Mg₃Gd phase was formed. Scanning electron microscope images with energy dispersive X-ray spectroscopy showed Gd elements were mainly distributed in the grain boundary.     

On the sol–gel synthesis and catalytic activity of Ce1−xAxO2−δ (A = Pr, Sm, Gd) SOFCs anode materials for reforming of methane

A sol–gel route to synthesize nanocrystalline praseodymium-, samarium- and gadolinium-doped ceria powders for solid oxides fuel Cells SOFCs is presented. The method involves metal nitrates with propionic acid (both as chelating ligand and solvent), gel formation, liquid nitrogen quenching, drying at 150 °C/24 h, and finally decomposition at 450 °C in nitrogen followed by calcination at 650 °C in air. TG–DTA, BET, XRD, FTIR, UV–vis and catalytic tests were used to characterize the samples. Ce_0.8Pr_0.2O_2?δ sample exhibited the best catalytic performance in methane steam reforming under water deficient conditions, closely followed by Ce_0.9Gd_0.1O_2?δ, Ce_0.8Sm_0.2O_2?δ and Ce_0.8Gd_0.2O_2?δ catalysts. The superior catalytic performance of Ce_0.8Pr_0.2O_2?δ sample was attributed to the existence of praseodymium species (Pr⁴⁺/Pr³⁺) strongly interacting with ceria. The two systems act synergistically in the catalytic steam reforming of methane.     

Physico-chemical studies on the chelation behaviour of biologically active 2-hydroxy-1-naphthaldehyde semicarbazone (HNAS) with some lanthanons

Summary The chelation behaviour of the complexes of La³⁺, Ce³⁺, Pr³⁺, Nd³⁺, Sm³⁺, Eu³⁺, Gd³⁺, Y³⁺, Tb³⁺, Dy³⁺, Ho³⁺ with biologically active 2-Hydroxy-1-naphthaldehyde semicarbazone (HNAS) has been studied potentiometrically in 75% (v/v) dioxane-water medium at various ionic strengths. The method of Bjerrum and Calvin, as modified by Irving and Rossotti has been used to find out the values of n andpL. The formation constants of metal chelates and the values ofS _min have been calculated. The order of formation constants of chelates was found to be: La³⁺3+3+3+3+3+3+3+3+3+3+.

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Physikochemische Untersuchungen zur Komplexierung von biologisch aktivem 2-Hydroxy-1-naphthaldehyd-Semicarbazon (HNAS) mit Lanthanoiden

Zusammenfassung Das Chelierungsverhalten von La³⁺, Ce³⁺, Pr³⁺, Nd³⁺, Sm³⁺, Eu³⁺, Gd³⁺, Y³⁺, Tb³⁺, Dy³⁺ und Ho³⁺ mit 2-Hydroxy-1-naphthaldehydsemicarbazon (HNAS) wurde potentiometrisch in 75% (v/v) Dioxan-Wasser bei verschiedenen Ionenstärken untersucht. Die Werte für n undpL wurden nach der Methode von Bjerrum und Calvin in der Modifikation von Irving und Rossotti bestimmt. Die Komplexbildungskonstanten der Metallchelate und die WerteS _min wurden ermittelt. Die Reihung der Komplexbildungskonstanten war La³⁺3+3+3+3+3+3+3+3+33+.

     

Measurement of the enhancement effect in different series in X-ray fluorescence analysis

In this study the enhancement effect depending on the changing of the sample of the matrix element has been investigated using energy dispersive X-ray fluorescence analysis. The present measured values of the enhancement effect factors for eight series (La₂O₃−Sm₂O₃, La₂O₃−Eu₂O₃, La₂O₃−Gd₂O₃, La₂O₃−Tb₄O₇, La₂O₃−Dy₂O₃, La₂O₃−Ho₂O₃, La₂O₃−Er₂O₃ and La₂O₃−Tm₂O₃) at 59.5 keV incident photon energy have been studied. It has been observed that changing of the matrix element influence the enhancement effect.     

Sm2O3Ga2O3 and Gd2O3Ga2O3 phase diagrams

Four definite compounds exist in the Sm₂O₃Ga₂O₃ binary phase diagram, namely: Sm₃GaO₆, Sm₄Ga₂O₉, SmGaO₃, and Sm₃Ga₅O₁₂. The $\text{3}\text{1}$ compound is orthorhombic (space group Pnna - Z.4) with the cell parameters: a = 11.40₀Å, b = 5.51₅Å, c = 9.07Å and belongs to the oxysel family. Sm₃GaO₆ and SmGaO₃ melt incongruently at 1715 and 1565°C; Sm₄Ga₂O₉ and Sm₃Ga₅O₁₂ have a congruent melting point at 1710 and 1655°C. With regard to the Gd₂O₃Ga₂O₃ system three definite compounds have been identified: Gd₃GaO₆, Gd₄Ga₂O₉, and Gd₃Ga₅O₁₂. Only the garnet melts congruently at 1740°C with the following composition: Gd_3.12Ga_4.88O₁₂. Gd₃GaO₆, and Gd₄Ga₂O₉ melt incongruently at 1760 and 1700°C. GdGaO₃ is only obtained by melt overheating which may yield an equilibrium or a metastable phase diagram.     

Study on Gd3+ and Sm3+ co-doped ceria-based electrolytes

Doped ceria electrolytes of Ce_1-aGd_a-ySm_yO_2–0.5a, wherein a=0.15 or 0.2, and 0ya, were prepared with the citrate method, and characterized by inductively coupled plasma–atomic emission spectrometry, energy dispersive spectrometry, scanning electron microscopy, powder X-ray diffraction, and AC impedance spectroscopy. The effect of composition on the structure and conductivity was studied. All the samples were fluorite-type ceria-based solid solutions. For the singly doped samples, the optimal composition was Ce_0.85Gd_0.15O_1.925 for Gd³⁺-doped ceria (CGO), which showed higher ionic conductivity than the best Sm³⁺-doped ceria (CSO) at 773–973 K. For the co-doped samples, the ionic conductivities were higher than those of the singly doped ones in the temperature range 673–973 K when a=0.15, but only better in 673–773 K when a=0.2. For the samples of Ce_0.85Gd_0.15-ySm_yO_1.925, wherein 0.05y0.1, much higher ionic conductivity was observed than those of the singly doped ceria at 773K~973 K. Therefore, these co-doped samples would be better than CGO and CSO to be the electrolytes of intermediate-temperature solid oxide fuel cells.     

Catalytic transfer hydrogenation of 2-butanone over oxide catalysts

Catalytic transfer hydrogenation of 2-butanone with 2-propanol was studied in gas phase over a series of oxides of different acid-base properties. Although the basic oxides (MgO, La₂O₃) gave high initial conversions, these oxides underwent deactivation during the reaction. This deactivation could be partially prevented by a previous treatment with chloroform of the oxide. The amphoteric oxides (TiO₂, ZrO₂, Al₂O₃) were also active in this reaction. Increasing the acidic character of the catalyst (Nb₂O₅, WO₃) led to a pronounced dehydration of 2-propanol. The results obtained over a series of rare earth oxides (La₂O₃, Sm₂O₃, Gd₂O₃, Dy₂O₃, Er₂O₃) revealed that beside the role of basic and acid sites a correlation seems to exist between the number of unpaired electrons of the metal ion and the catalytic activity, indicating the role of one electron donor sites.     

The promoter effect of lanthana on MgO supported ruthenium catalysts for ammonia synthesis

When activated at high temperature (873 K) doped lanthana such as La₂O₃, Ce₂O₃, Sm₂O₃ derived from nitrate showed remarkable effect on the MgO-supported ruthenium activity for ammonia synthesis. The activity treated with hydrogen at 873 K became 5 times as active as that treated at 623 K. The activity levels were almost the same as those of the Ru-lanthana (support) system under the same activation condition. It was thought that the lanthana was partly reduced and a strong metal support interaction occurred. The partially reduced lanthana was thought to donate electrons to the contacting Ru atoms forming the active centers. The other characters such as the stability were also discussed by comparing with alkali promoters.     

Ce4+替代Pu4+的模拟固化体(Gd1-xCex)2Zr2O7+x的合成及结构演变

Gd₂Zr₂O₇中Gd具有很大的中子吸收截面, 其烧绿石结构-缺陷萤石结构的转变能较低, 使其成为理想的核废料固化基材. 使用硝酸盐为原料, 添加少量NaF作助熔剂, 在较低温度下(和传统高温固相反应相比), 合成了烧绿石型Gd₂Zr₂O₇. 以Ce⁴⁺模拟Pu⁴⁺, 研究了Gd₂Zr₂O₇对锕系核素的固化, 并合成了系列模拟固化体(Gd_1-xCe_x)₂Zr₂O_7+x (0≤x≤0.6). 采用粉末X射线衍射(XRD)对系列样品进行了表征. 结果表明: 随着x值的增大,样品从烧绿石结构向缺陷萤石结构转变, 且晶胞大小基本保持恒定, 但当x=0.6时, 衍射峰明显宽化, 晶格畸变比较严重, 晶格稳定性降低. 当x=1时, 即用Ce⁴⁺完全取代Gd³⁺进行合成, 不能得到Ce₂Zr₂O₈, 产物发生了相分离, 为四方结构的(Zr_0.88Ce_0.12)O₂和萤石结构的(Ce_0.75Zr_0.25)O₂的混合物. 模拟固化体的浸出率测试表明: 当x≤0.2时, 各元素浸出率均很低, 但当x≥0.4时, 各元素的浸出率明显升高, 说明以Gd₂Zr₂O₇作为固化Pu⁴⁺的基材, Pu⁴⁺掺入量不宜高于40%.     

Electrochemical evaluation of La2NiO4+δ -based composite electrodes screen-printed on Ce0.8Sm0.2O1.9 electrolyte

La₂NiO_4+δ , 60 wt.% La₂NiO_4+δ –40 wt.% La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ , and 60 wt.% La₂NiO_4+δ –40 wt.% Ce_0.8Sm_0.2O_1.9 electrodes were prepared from fine powders on dense Ce_0.8Sm_0.2O_1.9 electrolyte substrates by screen-printing technique. Electrochemical impedance spectroscopy and chronopotentiometry techniques were employed to evaluate the electrochemical properties of the composite electrodes in comparison with the La₂NiO_4+δ electrode. For the three electrodes, main electrode processes were resolved to be charge-transfer at the electrode/electrolyte interface and oxygen exchange on the electrode surface. The contribution of the surface oxygen exchange process was detected to be dominant for the overall electrode polarization. The addition of Ce_0.8Sm_0.2O_1.9 into La₂NiO_4+δ was favorable for the charge transfer process whereas it was undesired for the surface oxygen exchange process. On comparison, adding La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ into La₂NiO_4+δ was found to benefit both the two electrode processes. The La₂NiO_4+δ -La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ composite electrode showed optimum electrochemical properties among the three electrodes. At 800 °C, the composite electrode achieved a polarization resistance of 0.20 Ω cm², an overpotential of 45 mV at a current density of 200 mA cm^?2, together with an exchange current density of ～200 mA cm^?2.     

Luminescence in NaLn(SO4)2H2O: A host lattice with high-energy vibrations

The luminescence of Ce³⁺, Sm³⁺, Eu³⁺, Gd³⁺, Tb³⁺, and Dy³⁺ in NaLn(SO₄)₂H₂O (Ln = lanthanide) is reported. Only Ce³⁺, Gd³⁺, and Tb³⁺ show efficient emission. This is explained in terms of an energy-gap law. Energy transfer is studied in several codoped compositions. The mutual transfer between Gd³⁺ ions is the only one encountered with high probability. The several transfers are discussed and where possible their rates are calculated.     

Thermal analysis of rare-earth metal(III) hexa(isothiocyanato)chromate(III) complexes with ɛ-caprolactam

Thermal decomposition of the complexes [LnL₈][Cr(NCS)₆] (Ln = La³⁺, Ce³⁺, Pr³⁺, Nd³⁺, Sm³⁺, Eu³⁺, Gd³⁺, Tb³⁺, Dy³⁺, Ho³⁺, Er³⁺, Tm³⁺, Yb³⁺, Lu³⁺; L = ɛ-C₆H₁₁NO) in air and in inert atmosphere was studied by thermogravimetry, X-ray phase analysis, IR spectroscopy, and mass spectrometry. The compositions of gaseous and solid thermolysis products were established. A reversible thermochromic effect was found on heating to 200–210°C.     

Effect of preparative methods on electrical and electrochemical performance of lanthanum strontium manganite

The electrochemical performance of La_0.8Sr_0.2MnO₃:Ce_0.8Gd_0.2O₂ composite cathode was investigated for solid oxide fuel cell applications. Sol–gel, combustion, and solid-state syntheses yielded rhombohedral La_0.8Sr_0.2MnO₃, whereas mechanochemical process gave cubic structure. X-ray diffraction results established good chemical stability of La_0.8Sr_0.2MnO₃ with Ce_0.8Gd_0.2O₂ composite cathode. Combustion synthesis was found best among all preparative methods on the basis of lowest area specific resistance 0.70 Ω cm² at 800 °C. The activation energy E _a = 1.09 ± 0.01 eV indicated absorption of O₂ and was the rate-limiting process of cathode.