Amorphous Citrate Process Research Articles

Magnetic properties of mixed valence La2/3Sr1/3Mn1 − x T x O3 (T = Fe,Cr) manganites obtained by Pechini method

Polycrystalline manganites of composition La2/3Sr1/3Mn1 − x T x O3 (T = Cr3 + or Fe3 + , x = 0.0–0.10) were obtained by the Pechini method. Their magnetic properties exhibited a marked dependence on Fe/Cr content, with significant reduction of the magnetic moment per formula unit and their Curie temperatures. In addition, the magnetocaloric effect, determined by isothermal magnetization measurements, displayed a decreasing tendency with increasing Fe/Cr concentration. Results were interpreted on the basis of a deleterious effect on the double exchange interaction provoked by the presence of Fe3 + and Cr3 + ions within the crystal structure. Polycrystalline mixed valence manganites of composition La2/3Sr1/3Mn1 − x T x O3 (T= Cr3 + or Fe3 + , x = 0.0–0.10) were obtained by the amorphous citrate process known as Pechini method. Their magnetic properties (Curie temperature and spontaneous magnetization) exhibited a marked dependence with Fe or Cr content, together with magnetic entropy variations of up to 2.0 J/kgK.

Catalytic application of CeO2–CaO nanocomposite oxide synthesized using amorphous citrate process toward the aqueous phase one pot synthesis of 2-amino-2-chromenes

A series of CeO2–CaO nanocomposite oxides were synthesized by the amorphous citrate method and characterized by TGA, XRD, UV–Vis-DRS, SEM and TEM experimental techniques. The XRD analysis indicated the presence of a substitutional type solid solution for ceria rich composite oxides whereas for the calcium oxide rich composite oxides, the presence of crystalline CaO was observed along with the solid solution phase. Fourier line profile analysis of the broadened X-ray diffraction profiles revealed the presence of small nanocrystallites of the mixed oxides with size in the range of 5–25nm. UV–Vis-DRS study of the composite oxide showed well dispersion of the ceria in the CaO matrix. The characteristic reflections from the low coordinated surface Ce4+ ions were observed in the UV study along with a blue shift in the absorption edge for samples containing up to 20mol% ceria. The direct band gaps calculated from the UV spectra were found to be in the range of 3.45–3.66eV. The composite oxide materials were found to be porous and spongy in nature observed from the SEM analysis. TEM study of the CeO2(20%)–CaO material indicated the presence of well dispersed mixed oxide nanoparticles in a continuous CaO matrix. The Ce–Ca–O nanocomposite oxides were used as an environmentally benign catalyst for the multicomponent one pot synthesis of 2-amino-2-chromenes by multicomponent condensation of aryl aldehydes, α-naphthol and malononitrile in aqueous media. The nanocomposite oxides were found to be highly efficient for the synthesis of structurally diverse aminochromenes with excellent yield and purity.

Preparation of zirconium tungstate (ZrW 2O 8) by the amorphous citrate process

Citrate gels were prepared from aqueous solutions of zirconium dinitrate oxide, ammonium metatungstate and citric acid in the range of pH 1.1–10. The citric acid to total metal molar ratio was varied from 0.66 to 1.33. The thermal decomposition of these precursor gels and the oxide phase evolution were studied by thermal analysis (DTA, TG, EGA-MS) and X-ray powder diffraction (XRD), respectively. Complex stability appeared to be largely insensitive to pH variations, whereas consistency and thermal behaviour of the gels were found to be influenced by pH and molar ratio of the starting solutions. Under optimized thermolysis conditions, phase-pure trigonal ZrW 2O 8 was obtained which was converted to cubic ZrW 2O 8 by up-quenching to 1125 °C for a very short time (<1 min). The cubic ZrW 2O 8 has particle sizes in the lower micron to submicron range and shows a negative linear coefficient of thermal expansion CTE = −10.2 × 10 −6 K −1 (30–120 °C). The polymorphism of ZrW 2O 8 is discussed with respect to its preparation by the amorphous citrate process and other soft chemistry routes.

Solution combustion synthesis of nano particle La0.9Sr0.1MnO3 powder by a unique oxidant-fuel combination and its characterization

Strontium-doped Lanthanum manganite (Li1−xSrxMnO3) is the most used cathode material in modern Solid Oxide Fuel Cells (SOFC). A large quantity of LSM is required for making MW scale SOFC units. Many methods of preparation has been reported by various researchers. These include amorphous citrate process, freeze drying, spray pyrolysis, sol-gel, glycine nitrate method and self propagating high temperature synthesis etc. In this paper, a solution combustion synthesis method has been described using a unique combination of oxidant and fuel, nitrate-acetate stoichiometrically, which not only produces nano particle LSM powder but is safe even in large scale production due to gradual combustion without causing any fire hazard or explosion condition which is usually found in most of the solution combustion synthesis reactions. The powder obtained in this method has been characterized by XRD, SEM, particle size analysis, surface area analysis etc. The results have been found to be quite good and suitable for SOFC application. This powder has also been used in forming tubular structure as a part of preliminary experiments to make solid oxide fuel small cell.

A Novel Approach for Synthesis of a Nanocrystalline Yttria Stabilized Zirconia Powder via Polymeric Precursor Routes

A nanocrystalline powder of 5 mol% yttria stabilized zirconia (YSZ) is prepared by polymeric precursor routes. Three different routes, viz. (i) amorphous citrate process, (ii) Pechini process and (iii) polyethylene glycol (PEG) assisted process are examined. In these processes, parameters such as metal cations to fuel ratio or components of fuel are varied and the effects analyzed. A novel experimental procedure for controlled combustion based on the thermal analysis has been adopted. All the three processes yield nanocrystalline powders with average crystallite size lying in the range of 9 to 18 nm. Densities as high as 99% of theoretical density (TD) could be achieved by sintering the compacts of powders at a relatively low temperature of 1400°C for 2 h.

Novel approach for the bulk synthesis of nanocrystalline yttria doped thoria powders via polymeric precursor routes

Three different polymeric precursor routes viz., (i) amorphous citrate process, (ii) Pechini process and (iii) polyethylene glycol assisted process were compared for synthesising nanocrystalline powders of 7.5 mol% yttria doped thoria (YDT). In each of the processes, parameters such as metal-to-fuel ratio or composition of fuel were varied and the effects were analysed. TG/DTA studies were conducted to identify the ignition temperatures of the precursors. Also, a novel experimental procedure with controlled combustion was devised for the preparation of powders based on the thermal analysis data. All the processes result in phase pure and nanocrystalline powders. The average crystallite size of the powders ranged between 9 and 18 nm. The powder samples were analysed for their carbon content and studied for their sinterability. Densities as high as 99% th.d. could be achieved by sintering the compacts of powders obtained from (i) amorphous citrate process with CA/M ratios 2.0 and 3.0, (ii) Pechini processes (independent of fuel composition) and (iii) PEG-assisted process using PEG 4000 at a relatively low temperature of 1500 °C for 2 h.

Hydrogen transfer reaction of cyclohexanone with 2-propanol catalysed by CeO2-ZnO materials: Promoting effect of ceria

Ce-Zn-O mixed oxides were prepared by amorphous citrate process and decomposition of the corresponding acetate precursors. The resulting materials were characterised by TGA, XRD, UV-Vis-DRS, EPR, SEM and surface area measure- ments. XRD and DRS results indicated fine dispersion of the ceria component in the ZnO matrix. EPR results clearly indicate the presence of oxygen vacancy and defect centres in the composite oxide. Addition of CeO2 to ZnO produced mixed oxides of high surface area compared to the pure ZnO. Hydrogen transfer reaction was carried out on these catalytic materials to investigate the effect of rare earth oxide on the activity of ZnO. Addition of ceria into zinc oxide was found to increase the catalytic activity for hydrogen transfer reaction . The catalytic activity also depended on the method of preparation. Citrate process results in uniformly dispersed mixed oxide with higher catalytic activity.

Promoting effect of CeO2 on cyclohexanol conversion over CeO2-ZnO mixed oxide materials prepared by amorphous citrate process

CeO2-ZnO materials were prepared by amorphous citrate process and characterized by TGA, XRD, UV-DRS and surface area measurements. TGA showed that the citrate precursors decompose in the range 350–550°C producing CeO2-containing catalytic materials. XRD and DRS results indicated the formation of well-dispersed interstitialZn xCe4+ 1−2x Ce3+ 2x O2 solid solution on ZnO matrix. Addition of CeO2 to ZnO produced high surface area mixed oxide materials in citrate method. Cyclohexanol conversion reaction was carried out on these catalytic materials to investigate the effect of rare earth oxide on the activity and selectivity. It was found that CeO2 promotes the activity of ZnO without affecting the selectivity to cyclohexanone significantly. The factors such as reaction temperature and WHSV have turned out to be important for cyclohexanol conversion over CeO2-containing ZnO catalyst materials.

Preparation of B-Site Ordered Complex Perovskite SrYb 0.5 Nb 0.5 O 3 by Amorphous Citrate Process

Amorphous citrate process has been used to synthesize the complex perovskite SrYb 0.5 Nb 0.5 O 3 (SYN) powder. Citrate-nitrate gels were produced at room temperature by first reacting NbCl 5 with aqueous solution of NH 4 OH to produce Nb(OH) 5 and then mixing the filtered Nb(OH) 5 with strontium nitrate and yitterbium nitrate in citric acid aqueous solution. The precursor ash was produced by auto ignition of the dehydrated precursor gel at 200°C. Finely crystallized SYN powder could be obtained by thermal decomposition of the citrate precursors ash at temperature as low as 1000°C. X-ray diffraction, Infra-red transmittance spectroscopy and electron microscopy have been used to characterize the precursors and annealed materials synthesized by citrate process. The citrate process is very promising and simple for preparing complex perovskite SrYb 0.5 Nb 0.5 O 3 .

Preparation of B-Site Ordered Complex Perovskite SrYb 0.5 Nb 0.5 O 3 by Amorphous Citrate Process

Amorphous citrate process has been used to synthesize the complex perovskite SrYb 0.5 Nb 0.5 O 3 (SYN) powder. Citrate-nitrate gels were produced at room temperature by first reacting NbCl 5 with aqueous solution of NH 4 OH to produce Nb(OH) 5 and then mixing the filtered Nb(OH) 5 with strontium nitrate and yitterbium nitrate in citric acid aqueous solution. The precursor ash was produced by auto ignition of the dehydrated precursor gel at 200°C. Finely crystallized SYN powder could be obtained by thermal decomposition of the citrate precursors ash at temperature as low as 1000°C. X-ray diffraction, Infra-red transmittance spectroscopy and electron microscopy have been used to characterize the precursors and annealed materials synthesized by citrate process. The citrate process is very promising and simple for preparing complex perovskite SrYb 0.5 Nb 0.5 O 3 .

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

Perovskite type La 1 − x Sr x MnO 3 ( 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 La 1 − x Sr x MnO 3 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 Mn 4+ is probably the most important parameter, but excess of oxygen and atomic surface composition should also be taken into account.

Preparation of lithium aluminate via polymeric precursor routes

Gamma lithium aluminate is a potential candidate for tritium breeding material. Various preparation routes have been proposed including solid state reaction methods and wet chemical methods such as sol–gel process. But it is still needed to develop reliable and economic preparation methods that can be used for large production. In this paper, we examined three polymeric precursor methods – amorphous citrate process, Pechini process, and polyvinyl alcohol-assisted process – by studying thermal behavior of the precursor powders by X-ray diffractometry, differential thermal analysis, Infrared spectrometry, and BET surface area analysis. All these three processes produced phase pure gamma lithium aluminate powders at a relatively low temperature. The phase purity and processing temperature were comparable to the alkoxide derived sol–gel method and yet these polymeric precursor methods did not require high quality starting materials such as alkoxides.

Catalytic combustion of methane over perovskites

Perovskite-type oxides of the series La 1− x A x MnO 3 (A Sr, Eu and Ce) were prepared by the amorphous citrate process, leading to high surface area catalysts (up to 45 m 2 g −1). They were tested in a flow reactor for the total combustion of methane. Complete conversion was obtained over all of the catalysts between 500 and 600°C and catalyst performance did not change significantly after 100 h on-stream. Specific activity was found to decrease monotonically with increasing the temperature of the O 2 TPD desorption peak maximum. The rate of methane combustion was low below 500°C, then grew very fast, showing that two kinds of oxygen are active in these catalysts: an adsorbed oxygen species, that reacts at low temperature, and a lattice oxygen species, that becomes available at high temperature, boosting the catalytic activity.

Vapor-Phase Dehydrocoupling of Methanol to Methyl Formate over CuAl2O4

CuO–Al2O3systems with different Cu/Al ratios were prepared by the amorphous citrate process, and their catalytic activities for the dehydrocoupling of methanol to methyl formate were examined. In the CuO–Al2O3systems, CuAl2O4samples calcined at temperatures below 1000°C were partially reduced during the reaction at 310°C and decreased the selectivity to methyl formate (MF) with increasing Cu(0) content. Cu(0) species catalyzed the dehydrocoupling reaction without decomposition of methyl formate at temperatures below 210°C, whereas Cu(0) decomposed methyl formate into methanol and CO above 210°C. On the other hand, CuAl2O4calcined at 1100°C showed the highest activity of the formation of methyl formate at a reaction temperature of 310°C. Since the CuAl2O4sample was not reduced even at 310°C, Cu(II) species in the CuAl2O4were found to be effective sites for the dehydrocoupling at temperatures above 250°C. In addition, Cu(I) species in CuAlO2was also found to be active with keeping high MF selectivity up to 290°C.

Preparation and characterization of epitaxial La 0.65Sr 0.35MnO z thin films grown by pulse laser deposition

The amorphous citrate process was used to synthesize powder of conducting and ferromagnetic oxide La 0.65Sr 0.35MnO z ( z = 3) and thin films of it were prepared by pulse laser deposition (PLD). The epitaxial nature of the films had been established by X-ray diffraction (XRD), and Rutherford backscattering spectrum(RBS). After anneal at 800 °C in O 2 atmosphere, the thin films exhibit metallic and magnetic properties. Ferromagnetic resonant spectra reveal that there are nonuniform regions of distinct magnetization in-plane of the films, which are related to the random distribution of lanthanum in the film, as confirmed by the RBS spectrum. The magnetization measured by a vibrating sample magnetometer is different from that obtained from the ferromagnetic resonance spectrum.

Characterization of perovskite-type GdFeO3 powders prepared by amorphous citrate process

The purposes of this study were to obtain GdFeO 3 powders by calcining their precursor gels which were prepared from Gd 2 O 3 and Fe 2 O 3 by the amorphous citrate process, and to characterize the powders in comparison with those prepared by the solid-state reaction (SSR) of Gd 2 O 3 and Fe 2 O 3 powders. Furthermore, it was attempted to estimate precisely crystallite sizes and lattice constants of GdFeO 3 powders by x-ray diffraction line profile analyses

アモルファスクエン酸錯体法によるペロブスカイト型ＬａＦｅＯ３粉末の調製

アモルファスクエン酸錯体法によるペロブスカイト型ＬａＦｅＯ３粉末の調製

Preparation of ternary Cu/Co/Al catalysts by the amorphous citrate process II. The effect of the decomposition-calcination atmosphere

The effect of the decomposition-calcination procedure of Cu/Co/Al citrate precursors on the characteristics of the resulting mixed oxides and on the catalytic activity and selectivity for synthesis gas conversion to methanol and higher alcohols was studied. Cu/Co/Al amorphous precursors were prepared by means of the citric complexing method. Four different decomposition-calcination procedures were employed: I, air-air 500°C; II, N 2-N 2 500°C; III, N 2 500°C-air 500°C, and IV, N 2 280°C-air 500°C. The physicochemical properties of the mixed oxides (major surface species, porous structure, binary phases) were influenced by the decomposition-calcination procedure employed. The mixed oxides obtained through procedure II had low surface areas (8 m 2 g −1), large particles of Cu and Co in metallic state, and high amounts of residual carbon. As a consequence, the catalytic activity was low and the selectivity to hydrocarbons was enhanced up to 70 wt%. Procedure III eliminated the residual carbon and regenerated metal oxides phases. However, the surface area was still low (14 m 2 g −1) and a Cu-rich layer was formed at the sample surface, thereby causing an inhomogeneous distribution of the metal ions. These catalysts exhibited poor higher alcohol selectivities. The mixed oxides obtained through procedures I and IV exhibited similar physicochemical properties and catalytic activities. XRD and TPR characterization identified the formation of CuO and Co 3O 4 oxides and Co(Cu)Al 2O 4 aluminates. Surface species were mostly CuO and Co 3O 4. The distribution of the metals was uniform and the surface area values were relatively high (60–65 m 2 g −1). These catalysts exhibited higher catalytic activity and were selective for the formation of alcohols, producing 52–54 wt% of total alcohols and 23–25 wt% of ethanol and higher alcohols. The catalytic results suggest that the homogeneous distribution of the metallic elements is crucial to higher alcohol synthesis.

Preparation of Garnet-Type Gd&lt;sub&gt;3&lt;/sub&gt;Al&lt;sub&gt;5&lt;/sub&gt;O&lt;sub&gt;12&lt;/sub&gt; Powders by an Amorphous Citrate Process

Preparation of Garnet-Type Gd&lt;sub&gt;3&lt;/sub&gt;Al&lt;sub&gt;5&lt;/sub&gt;O&lt;sub&gt;12&lt;/sub&gt; Powders by an Amorphous Citrate Process

Preparation of YBa&lt;sub&gt;2&lt;/sub&gt;Cu&lt;sub&gt;3&lt;/sub&gt;O&lt;sub&gt;7-x&lt;/sub&gt; Superconducting Powders by the Amorphous Citrate Process

Preparation of YBa&lt;sub&gt;2&lt;/sub&gt;Cu&lt;sub&gt;3&lt;/sub&gt;O&lt;sub&gt;7-x&lt;/sub&gt; Superconducting Powders by the Amorphous Citrate Process