Modified Citrate Route Research Articles

Tunable magnetocaloric effect in La0.7Ca0.3MnO3 nanoparticles

We study ferromagnetic to paramagnetic transition using Banerjee criterion in colossal magnetoresistive La[Formula: see text]Ca[Formula: see text]MnO3 nanoparticles of different sizes (20, 26 and 32 nm). These particles are chemically prepared by a modified citrate route. Particle sizes are estimated by X-ray diffraction (XRD). Sample morphology and size distribution are examined by a transmission electron microscope. Order of transition is determined by measuring magnetic field dependence of magnetization near Curie temperature T[Formula: see text]. Our results reveal second-order magnetic transition in all three different-sized particles in contrast to the first-order transition reported in bulk samples. This is further supported by temperature dependences of zero-field-cooled (ZFC) and field-cooled (FC) magnetizations. FC curves show gradual transition for all three samples without any saturation region as is the case of bulk. T[Formula: see text] is extracted from ZFC curves and is found to be 240 K for 20 nm particles and 258 K for both 26 and 32 nm particles. Magnetocaloric effect as a function of particle size also confirms the second-order magnetic transition in all three samples. The relative cooling power at 20 kOe is found to be decreasing with decreasing particle size.

Synthesis and Characterization of Alkaline-Earth Metal (Ca, Sr, and Ba) Doped Nanodimensional LaMnO3 Rare-Earth Manganites

The substitution of divalent cations of alkaline-earth elements in nanodimensional structures of rare-earth manganites produces advanced materials with potential electrical and magnetic functionalities. A systematic investigation of La0.65A0.35MnO3 (A = Ca, Sr, Ba) materials synthesized with a modified citrate route adopting ethanol dehydration has been undertaken. The structural and morphological analyses are carried out by using x-ray diffraction and scanning electron microscopy, respectively. Resistivity measurements are performed in variation with temperature to study the electrical transport properties which are found to vary with the size of the A-site cationic radius. Room temperature magnetic measurements are carried out to investigate the type of magnetic phase present in materials. The stability of the magnetic phase and coercivity are found to be dependent on the size of nanocrystallites.

Pd-integrated perovskites for TWC applications: Synthesis, microstructure and N2O-selectivity

Two versions of Pd-doped perovskite based catalysts (Pd impregnated onto or integrated into perovskite: Pd–LaFe0.65Co0.35O3 and LaFe0.65Co0.3Pd0.05O3) were synthesized by applying a modified citrate route and analyzed by XRD, TEM and XPS. The catalytic properties of the perovskites were measured for NO reduction reactions occurring in three way catalysis, including lean, stoichiometric and rich conditions. The N2O concentrations were measured and compared to the N2O formation of a commercial three way catalyst. The perovskite-based catalysts produced significantly less N2O than the TWC – ca. 75% under lean conditions at the temperatures of maximum N2O-formation and ca. 60% less N2O under stoichiometric conditions. Under rich conditions the Pd–LaFe0.65Co0.35O3 catalyst produced ca. 58% less N2O than the TWC, while no N2O was detected at all on the Pd-integrated perovskite LaFe0.65Co0.3Pd0.05O3 under these conditions.

Effect of Fe/Co-ratio on the phase composition of Pd-integrated perovskites and its H2-SCR of NOx performance

La-based perovskites doped with palladium were synthesized by a modified citrate route and calcined at high temperature. The Fe/Co-ratio at the B-site of the perovskite (LaFe0.95−xCoxPd0.05O3, x=0.475, 0.4, 0.3) was intentionally varied in order to produce a single phase catalyst and to correlate the phase(s) variations with its catalytic performance during the H2-SCR of NOx. A mixture of crystal phases containing orthorhombic and cubic perovskites, cobalt oxide and strongly bonded palladium oxide were observed in the sample with Fe/Co=1. Single orthorhombic perovskite phase with traces of iron oxide and weak bonded palladium oxide was obtained with Fe/Co≈2.2. The perovskite with Fe/Co=1 displayed better NOx-reductions and N2-selectivities than the perovskite with Fe/Co=2.2 ratio during dry H2-SCR of NOx. The same was observed with H2O+CO2 and in the presence of CO in the feed. The observed catalytic behaviour of both perovskites is discussed in association with the state of palladium in each investigated composition since the Pd-free perovskite displayed an extremely low NOx-reduction (below 4%).

Pd-Integrated Perovskite as Effective Catalyst for Selective Catalytic Reduction of NOx by Propene

Perovskite based Pd catalysts were prepared by a modified citrate route and analyzed with SEM, XRD, TEM and XPS techniques regarding the state of palladium. Integration of Pd into the perovskite lattice was compared with impregnation onto the support. Clear indications of Pd-substitution in the La-based perovskites were found by XPS and SEM. The integrated Pd-ions diffuse out of the perovskite lattice under reductive conditions forming metallic palladium nano-particles (less than 15 nm size) while the Pd particles obtained via the impregnation route were in the order of 80 nm. In the selective catalytic reduction of NO by propene, up to 35% NO conversion at 250 °C were obtained at a very low W/F of 0.015 g s mL−1, with decreasing tendency at increasing oxygen content. Differences between impregnated and Pd-integrated catalysts were obvious only at high O2 content (5 vol.%) where the Pd-integrated catalyst exhibited a lower tendency to oxidize the propene reductant.

Meta-stability and microstructure of the LaFe 0.65Co 0.3Pd 0.05O 3 perovskite compound prepared by a modified citrate route

In this paper we report the synthesis and analysis of lanthanum based perovskites substituted with Pd by employing a modified citrate route. Crystallization and phase sequences of the LaFe 0.65Co 0.3− x Pd x O 3 ( x = 0, 0.05) perovskite(s) are analyzed by means of powder XRD and Rietveld refinement. Regarding the crystal phase development at the LaFe 0.65Co 0.3Pd 0.05O 3 perovskite a meta-stable structure can be postulated at temperatures up to 700 °C. Since no PdO species were detected up to the calcination temperature of 700 °C we can conclude that Palladium ions are successfully incorporated into the B-site of the crystal structure of the perovskite. Reduction of the LaFe 0.65Co 0.3Pd 0.05O 3 perovskite at 600 °C under 5 vol.% H 2 in N 2 which was precalcined at 700 °C causes segregation of palladium to form metallic particles with sizes between 10 and 15 nm. Reversible Pd ions reincorporation into the crystal lattice of the LaFe 0.65Co 0.3Pd 0.05O 3-700 °C perovskite can be concluded since no PdO was detected after re-oxidation at 600 °C.

Magnetic studies on Sr 0.8Ce 0.1Fe 0.7Co 0.3O 3− δ perovskite

The A-site deficient perovskite oxide Sr 0.8Ce 0.1Fe 0.7Co 0.3O 2.66 was prepared using a modified citrate route with the oxygen content determined by TGA analysis and iodometric titration. Physical characterisation of this compound was carried out using electrical conductivity, magnetisation and Mössbauer studies. From the magnetic characterisation of this compound the existence of two different ordered magnetic states below 100 K was deduced, the overall behaviour being antiferromagnetic. The combined results of chemical analysis, Mössbauer spectroscopy and magnetisation allowed the determination of the oxidation states of the Co and Fe ions.

Characterization of perovskite powders for cathode and oxygen membranes made by different synthesis routes

Strontium lanthanum manganite (LSM) and lanthanum ferrite (LSF) perovskite cathode and oxygen membrane materials were synthesized using different techniques: spray pyrolysis, a modified citrate route, oxalate and carbonate co-precipitations. The use of Ca, a cheaper substituent on the A-site, was explored along to the substitution of La by Pr. The differently sourced powders were characterized by TG/DTA, XRD, ICP, TEM, XPS, PSD and BET. The co-precipitation of La, Ca and Fe was also possible using the cyanide route. This complexation method allowed the precipitation of a crystalline phase as evidenced by XRD. Among all methods, the cyanide and carbonate co-precipitation allowed the lowest perovskite phase transformation for LSF and LCF, followed by the nitrate (i.e. ‘ spray pyrolysis’). These phase transformation differences affected much the particle size distribution and the surface areas of these materials, the carbonate and the cyanide routes giving rise to very fine powders in the nm range. XPS and TEM analyses indicated uneven composition distributions. These different powder characteristics are expected to affect the catalytic and electrochemical properties of these materials.

LiMn 2− xCr xO 4 spinel prepared by a modified citrate route with combustion

A series of LiMn 2− x Cr x O 4 ( x=0, 0.01, 0.05, 0.1, 0.2) has been synthesized by a modified citrate route with combustion. The influence of the substitution of Cr for Mn on the electrochemical performances was investigated and compared. The results show that Cr substitution in spinel LiMn 2O 4 can effectively depress the capacity fading over 4 V and improve the cycleability.

Preparation and performance of spinel LiMn 2O 4 by a citrate route with combustion

Nano- and submicrometer LiMn 2O 4 spinel particles were successfully synthesized by a modified citrate route with spontaneous combustion at 250 °C followed by a calcination process. Citric acid was used as the chelating and combustion-assistant agent. The synthesized LiMn 2O 4 showed a high electrochemical performance compared with commercial LiMn 2O 4 materials. The initial discharge specific capacity reached 130 mAh/g. The effects of the processing factors and element substitution on the microstructure and electrochemical performance of LiMn 2O 4 were also examined.

Structural and electrochemical properties of the Sr 0.8Ce 0.1Fe 0.7Co 0.3O 3− δ perovskite as cathode material for ITSOFCs

Powders of the perovskite of nominal composition Sr 0.8Ce 0.1Fe 0.7Co 0.3O 3− δ (SCFC) were prepared by a modified citrate route in order to determine its structural and microstructural characteristics. In this study, the interpretation of the X-ray diffraction has been performed based on a diagonal perovskite. However, electron diffraction demonstrates that the microstructure is rather more complex than that deduced from the average cell √2 a p×2 a p×√2 a p. Two reflections in the selected area electron diffraction may be interpreted as first-order satellites, of different basic reflections, arising from an incommensurate modulation along the b* direction ( b≈2.4 a p Å). Symmetrical electrodes of Sr 0.8Ce 0.1Fe 0.7Co 0.3O 3− δ powders were deposited on Ce 0.9Gd 0.1O 1.95 (CGO) ceramic pellets, and the electrochemical properties of the interfaces between the porous cathodes (SCFC) and the electrolyte (CGO) have been investigated at intermediate temperatures (500–766 °C) using AC impedance spectroscopy. SCFC electrodes exhibited an area-specific resistivity (ASR) at 700 °C of 0.86 Ω cm 2. The behaviour of the perovskite cathodes was interpreted in terms of available oxygen ion kinetic data.

Harmonic analysis of AC magnetic susceptibility of YBCO/Ag produced by chemical amorphous citrate route

The dependence of AC magnetic susceptibility on temperature and magnetic field amplitude of YBCO/Ag superconductors produced by a modified citrate route is studied. Superconducting powders have been prepared with nine different heat treatments, sintered and characterized structurally and magnetically. X-rays analysis, scanning electron microscopy and scanning Auger microscopy show the compositional purity and homogeneity, the size of the grains, and the way silver is dispersed in the sample. AC susceptibility is interpreted in terms of a critical-state model providing a quantitative estimate of the critical current density and qualitative insight on the flux dynamic inside the samples.

99/03197 Physical and electrochemical characterization of nanocrystalline LiMn 2O 4 prepared by a modified citrate route

99/03197 Physical and electrochemical characterization of nanocrystalline LiMn 2O 4 prepared by a modified citrate route

Physical and electrochemical characterization of nanocrystalline LiMn2O4 prepared by a modified citrate route

Abstract Nanocrystalline LiMn 2 O 4 spinel oxides have been synthesized successfully by using a modified citrate route with ethanol dehydration of a Li–Mn–citrate complex solution. The effects of sintering temperature and atmosphere on the physicochemical properties of the LiMn 2 O 4 powders have been carefully examined by means of X-ray diffraction, electron microscopy, BET measurement, and X-ray absorption spectroscopy. It becomes clear that an oxidative atmosphere induces a partial agglomeration of particles at high temperature, but gives rise to an effective decomposition of the citrate precursors at low temperature. It is also found that a lowering of the sintering temperature enhances the average oxidation state of manganese with an increase in surface defects. The electrochemical property of the present LiMn 2 O 4 nanocrystals has also been measured to compare with that of lithium manganate prepared by a solid-state reaction, which confirms the effectiveness of the present modified citrate route. The evolution of geometrical and electronic structures during the charging process has also been investigated with X-ray absorption spectroscopy, and reveals shortening of the (Mn–O) bond distances induced by lithium de-intercalation.