Citrate Route Research Articles

Correlation between microstructure and electrical conductivity in composite electrolytes containing Gd-doped ceria and Gd-doped barium cerate

Composite electrolytes with nominal compositions, Ce 0.8Gd 0.2O 1.9 + xBaO ( x = 0.2 and 0.3), have been synthesized through the citrate route. Formation of two phases, namely Gd-doped ceria and Gd-doped barium cerate, has been confirmed through XRD and SEM studies. The impedance spectra show three distinct semi-circles, all originating from the composite electrolytes. In the temperature range 175–350 °C, the activation energies for the conductivity values extracted from the high frequency and intermediate frequency parts of the impedance spectra remains the same, irrespective of compositional and micro-structural variation. On the other hand, the activation energies for the conductivity values associated with the low frequency impedance spectra show a significant change with micro-structural variation. Solid oxide fuel cells constructed using these composite electrolytes exhibit a higher open circuit voltage compared to those based on single phase 20 mol% Gd-doped ceria.

Preparation and characterization of CuFe2O4 bulk catalysts

This work is devoted to the investigation of the influence of the preparation process on the physical–chemical properties of a copper spinel applied as catalyst for hydrocarbon (HC) oxidation. Samples of CuFe2O4 mixed oxide belonging to the inverted spinel type structure have been obtained by high temperature solid state reaction and by wet chemical synthesis methods, applying nitrate and citrate precursor procedures, within the thermal range from 150 to 700°C.The bulk catalysts were characterized by XRD, TPR, FTIR, SEM-EDX, TEM and Mössbauer spectroscopy. Propane combustion was performed as a reaction test.In this work it is demonstrated that the calcination temperature and chemical synthesis affect the crystal properties and cation distribution in the spinel structure, microstructure, surface area and reducibility; which are among the most relevant physical chemical properties for the catalytic activity. The materials obtained by wet chemical procedure, through nitrate and citrate routes, are better suited for propane combustion. This feature is assigned to the microstructure, to the presence of nanometric size particles and also to the cation distributions in the spinel sublattices of these materials.

Phase formation, crystal structures and magnetic properties of perovskite-type phases in the system La 2Co 1+ z(Mg xTi 1− x) 1− zO 6

Perovskite-type cobaltates in the system La 2Co 1+ z (Mg x Ti 1− x ) 1− z O 6 were studied for z=0≤ x≤0.6 and 0≤ x<0.9, using X-ray and neutron powder diffraction, electron diffraction (ED), magnetic susceptibility measurements and X-ray absorption near-edge structure (XANES) spectroscopy. The samples were synthesised using the citrate route in air at 1350 °C. The space group symmetry of the structure changes from P2 1/ n via Pbnm to R3¯ c with both increasing Mg content and increasing Co content. The La 2Co(Mg x Ti 1− x )O 6 ( z=0) compounds show anti-ferromagnetic couplings of the magnetic moments for the Co below 15 K for x=0, 0.1 and 0.2. XANES spectra show for the compositions 0≤ x≤0.5 a linear decrease in the L 3/( L 3+ L 2) Co– L 2,3 edge branching ratio with x, in agreement with a decrease of the average Co ion spin-state, from a high-spin to a lower-spin-state, with decreasing nominal Co 2+ ion content.

Improvement of the photostability for YVO4:Bi3+,Eu3+ nanoparticles synthesized by the citrate route

YVO4:Bi3+,Eu3+ nanophosphor synthesized by the citrate route shows the photobleaching behavior, i.e., the decrease in photoluminescence intensity under the continuous irradiation of excitation light. This photobleaching property is remarkably suppressed by the washing post-treatment combined with centrifugation and the hydrothermal post-treatment. A relation between the fraction of photobleach and the amount of citrate ions coordinating to metallic ions at the surface of nanoparticles verifies that the photobleaching behavior of YVO4:Bi3+,Eu3+ nanophosphor originates from the redox reaction between the citrate ions and V5+ in YVO4 host crystal. The long-term light fastness test reveals that the hydrothermally treated YVO4:Bi3+,Eu3+ nanophosphor has sufficient photostability for practical use over 15years outside.

Water Oxidation by λ-MnO2: Catalysis by the Cubical Mn4O4 Subcluster Obtained by Delithiation of Spinel LiMn2O4

Nanocrystalline spinel LiMn(2)O(4) has been prepared and treatment of LiMn(2)O(4) with dilute nitric acid solution resulted in the delithiation of the framework, while maintaining the spinel structure, lambda-MnO(2). LiMn(2)O(4) is not a catalyst for water oxidation. Upon removal of the lithium, the cubical Mn(4)O(4) cores become active sites for oxidizing water to molecular oxygen, which was investigated with the photochemical [Ru(2+)(2,2'-bpy)(3)]/persulfate system at pH 5.8. The nanosize lambda-MnO(2) obtained from the nanocrystalline LiMn(2)O(4), which was synthesized by the citrate route, shows a significantly higher water oxidation catalytic activity (Turnover Frequency: 3 x 10(-5) mol O(2)/s/mol Mn) than that obtained via solid state reaction with micrometer and irregular particle sizes (Turnover Frequency: 5 x 10(-6) mol O(2)/s/mol Mn).

Study on structural and electrical properties of layered perovskite-type oxide-ion conductor

Samples of BINAVOX (Bi 2Na x V 1− x O 5.5−2 x ), in the composition range 0 ≤ x ≤ 0.20 were synthesized by sol–gel citrate route. Thermal decomposition and phase structure were characterized using FT-IR, Powder X-ray diffraction and simultaneous thermogravimetric and differential thermal analysis. It has been found that BINAVOX xerogels are completely calcinated above 550 °C for 3 h of thermal treatments. Furthermore, the highly conducting γ-phase was stabilized for compositions with x ≥ 0.15. AC impedance spectroscopy revealed that the diffusion of oxide-ion vacancies is thermally activated in both grain interiors and boundaries. In general, the relaxation time of grain boundaries contribution is two orders of magnitude higher than that of grain interiors. The dielectric permittivity showed a general decay with the increase in sodium substitution. Importantly, the composition dependence of electrical conductivity exhibited a maximum for x = 0.13 at lower temperatures ( σ 300 ∼ 1.34 × 10 −5 S cm −1), which is attributed to the reduction in the activation energy of conduction.

P(O 2)-stability of LaFe 1− xNi xO 3− δ solid solutions at 1100 °C

LaFe 1− x Ni x O 3− δ ( x=0.1−1.0) perovskites were synthesized via citrate route. The p(O 2)-stability of the perovskite phases LaFe 1− x Ni x O 3− δ has been evaluated at 1100 °C based on the results of XRD analysis of powder samples annealed at various p(O 2) and quenched to room temperature. The isothermal LaFeO 3− δ –“LaNiO 3− δ ” cross-section of the phase diagram of the La–Fe–Ni–O system has been proposed in the range of oxygen partial pressure −15<log p(O 2)/atm≤0.68. The unit cell parameters of orthorhombic perovskites O-LaFe 1− x Ni x O 3− δ increase with decrease in p(O 2) at fixed composition x. This behavior is explained on the basis of size factor. The decomposition temperatures of rhombohedral phases R-LaFe 1− x Ni x O 3− δ for x=0.7, 0.8, 0.9 and 1.0 in air were determined as 1137, 1086, 1060 and 995 °C, respectively.

Layered Aurivillius compound: Synthesis, characterization and electrical properties

In the present study, sol–gel citrate route was utilized to synthesize BIALVOX (Bi 2Al x V 1− x O 5.5− x− δ ) in the composition range 0 ≤ x ≤ 0.20. Powder X-ray diffraction and simultaneous thermogravimetric and differential thermal analyses confirmed that the calcination of BIALVOX xerogels is fully completed at around 500 °C after 3 h of thermal treatment. It has been found that the stabilization of highly conducting γ-phase takes place for x ≥ 0.10. AC impedance spectroscopic investigation suggested that the charge accumulation at grain boundaries is more pronounced as compared to that at grain interiors. However, the maximum electrical conductivity (7.73 × 10 −5 S cm −1) was noticed for BIALVOX.13 at 300 °C. The temperature dependence of dielectric permittivity was also reported.

Oxidative dehydrogenation of butane over substoichiometric magnesium vanadate catalysts prepared by citrate route

Substoichiometric magnesium vanadate catalysts, (Mg–V i with i = 1, 2, and 3) with different V/Mg atomic ratios and stoichiometric (Mg 3V 2O 8, Mg 2V 2O 7 and MgV 2O 6), were prepared by citric acid technique. Characterization showed that substoichiometric samples correspond to well-dispersed VO 4 units in the MgO matrix. Moreover, taking into account the Raman data, the coexistence of ortho, pyro and meta vanadate phases cannot be discarded. The Mg 3V 2O 8 sample was more active for the oxidative dehydrogenation of n-butane, but less selective to C 4 alkenes than the Mg 2V 2O 7 sample. Stoichiometric samples produce mainly 1-butene, whereas for substoichiometric samples butadiene was the major reaction product for reaction temperatures higher than 748 K. The easily reducible VO 4 species, and eventually the coexistence of different vanadates phases, appear to be responsible for the catalytic behaviour displayed by the substoichiometric catalysts prepared by citrate technique.

Acrylamide assisted polymeric citrate route for the synthesis of nanocrystalline ZrO 2 powder

Nanocrystalline ZrO 2 powders have been prepared using acrylamide assisted polymeric citrate combustion (autoignition) route. Process parameters (i.e., total metal ions to citric acid ratio) were varied for the formation of polymeric resin in order to obtain the final product with desired properties (organic free, smaller crystallite size, etc.). The effect of three different citric acid amounts in the formation of nanocrystalline ZrO 2 powder was investigated using FTIR, XRD, TG/DTA and SEM-EDS, respectively, to identify the structural coordination, phase, thermal behavior and microstructure of the polymeric intermediates as well as the final ZrO 2 powders. Organic free ZrO 2 powder with two different phases of metastable t-ZrO 2 and m-ZrO 2 were prepared by calcining the polymeric intermediates at 600 °C. The lowest crystallite was found to be 18 and 16 nm, respectively, for the t-ZrO 2 and m-ZrO 2 phases prepared with total metal ions to citric acid ratio of 1:3.

Preparation of gallium oxynitride powder and its nanofibers by the nitridation of a gallium oxide precursor doped with nickel or cobalt obtained via the citrate route

Acicular crystals were grown in gallium oxynitride powder prepared by ammonia nitridation of amorphous gallium oxide precursors containing less than 5 at% of either Ni or Co, via the citrate route. The crystals were several tens of nanometres wide, several micrometres long, and grown in the temperature range 750 to 850 degrees C in a flow of ammonia of less than 200 mL min(-1). The crystal structure of the gallium oxynitride was a highly disordered 2H wurtzite-type with some 3C zinc blende-type stacking faults. The crystals grew in their basal plane changing their aspect ratio with the supplying method of small amounts of Ni or Co and an amount of residual carbon. The acicular crystals were grown by the catalytic behavior of Ni or Co to enhance one-dimensional growth in the hexagonal c-plane.

Synthesis, characterization, and electrical conductivity of new Aurivillius-type oxide-ion conductor

A new oxide-ion conductor of Aurivillius family with a general formula Bi2AlxV1 − xO5.5 − x − δ; 0 ≤ x ≤ 0.20 (BIALVOX) was synthesized by the sol-gel citrate route. Powder X-ray diffraction and simultaneous thermogravimetric and differential thermal analyses confirmed that the calcination of BIALVOX xerogels is fully completed at around 500°C after three hours of thermal treatment. It has been found that the β-orthorhombic phase is stabilized with compositions x ≤ 0.07, whereas the stabilization of the γ′-phase takes place for x ≥ 0.10. AC impedance spectroscopic investigation suggested that the charge accumulation at grain boundaries is thermally activated process. However, the maximum electrical conductivity (7.73 × 10−5 S cm−1) noticed for BIALVOX.13 at 300°C was attributed to the maximum vacancy concentration in the equatorial planes, responsible for the ion diffusion through the structure. This has been further evidenced by the temperature dependence of dielectric permittivity.

Indium and gallium oxynitrides prepared in the presence of Zn 2+ by ammonolysis of the oxide precursors obtained via the citrate route

Ammonia nitridation of indium and gallium oxide precursors obtained through a soft solution route led to their oxynitrides [In 0.97□ 0.03][N 0.92O 0.08] at 660 °C and [Ga 0.89□ 0.11][N 0.66O 0.34] at 850 °C, respectively, where □ refers to a In or Ga vacancy. Cation vacancies in their wurtzite-type lattice were eliminated in similar preparations with the co-presence of Zn 2+ by forming complete solid solutions of (InN) 1− x (ZnO) x and (GaN) 1− y (ZnO) y . The optical absorption edge shape was found to be relatively steep at the solid solution limits of x ≈ 0.23 and y ≈ 0.33 compared to the case without zinc.

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%).

Synthesis, Structure and Properties of Niobium Aluminum Oxynitride and Tantalum Based Compound Prepared through Citrate Route

The oxynitrides with composition A1-xBx(OyNz) (A: Nb and Ta, B: Al) were studied in preparation through citrate route and NH3 nitridation. In the case of Nb, the product of x = 0.5 prepared at 1000 °C was a new compound of a = 0.435 nm with rock salt structure. In this structure, Al and Nb atoms were distributed randomly in the cation sites. Anion sites were also randomly occupied by oxide and nitride ions with some amount of vacancy. The chemical composition was represented as Nb0.56Al0.44O0.38N0.37□0.25 from Rietveld refinement and oxygen/ nitrogen measurement. In the case of Ta, monoclinic Ta3N5 crystallized with a small amount of Ta4N5 impurity at x = 0.5. Aluminum compound was co-present as amorphous impurity.

Preparation of monoclinic BiVO 4 thin film by citrate route for photocatalytic application under visible light

A monoclinic BiVO 4 ( mBiVO 4) film composed of nanoparticles with a size of 100 nm was prepared via a citrate-precursor chemical route. The BiVO 4 film was characterized by X-ray diffraction, scanning electron microscopy and absorption spectra. It was found that the mBiVO 4 film showed a photocatalytic activity of Acid Orange 7 (AO7) degradation in aqueous solution under visible-light irradiation ( λ > 400 nm). The degradation efficiency was 78.9% in 3 h and basically remained constant in 10 repeated batch runs. The results demonstrated that the mBiVO 4 film had a high photocatalytic activity under visible light and good durability in repeated use.

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.

Influence of preparation methods of LaCoO3 on the catalytic performances in the decomposition of N2O

This study reports the potential interest of LaCoO3 in the catalytic decomposition of N2O from nitric acid plants. Typically, the exhaust gas contains NO, water and O2 which usually induce strong inhibiting effects depending on the surface properties of the solids particularly the surface mobility of oxygen from LaCoO3. Different preparation methods have been implemented, involving citrate route, reactive grinding and the use of templates, which lead to different structural and textural properties examined by X-ray diffraction, transmission electron microscopy and N2 physisorption. EDX analysis and XPS measurements also revealed that different surface composition may alter subsequent interactions between the surface and the reactants and related catalytic performances. LaCoO3 prepared by reactive grinding was found to be the most active catalyst due to a high specific surface area but the presence of Fe and Zn impurities inherent to the preparation method were suggested to interfere on the catalytic performances.

Properties of Ca 1− xHo xMnO 3 perovskite-type electrodes

In this work, Ca 1− x Ho x MnO 3 ( x = 0, 0.1 and 0.2) perovskite oxide pelleted electrodes were prepared from the respective powders obtained by the citrate route method at 1173 K. The electrodes exhibit particle size that decreases with the holmium content in the oxide. All the samples reveal semiconductor behaviour and the presence of holmium induces a marked decrease in the electrical resistivity. The results can be well attributed to the changes in the Mn 4+/Mn 3+ ratio. Electrodes were characterized by cyclic voltammetry and chronopotentiometry. Cyclic voltammetric studies indicate a similar behaviour of the electrodes, irrespective of their composition. Two pairs of peaks were identified and associated, one to the Mn 4+/Mn 3+ redox couple and the other to the Mn 7+/Mn 4+ and Mn 6+/Mn 4+ redox couples. The voltammetric data provide evidence that the electrodes roughness factor increases with the introduction of Ho-ions in the oxide structure, what is consistent with the crystallite size obtained by X-ray diffraction (XRD) and the morphology observed by scanning electron microscopy (SEM). The Ho-substituted electrodes present higher current density when compared with CaMnO 3 electrodes what can be attributed both to higher electrical conductivity and smaller particle size. The chronopotentiometric studies have shown that the discharge occurs by different mechanisms for the oxide electrodes with and without Ho.

Preparation of gallium oxynitride in the presence of iron through a citrate route

Gallium oxynitrides were prepared in the copresence of various amounts of Fe 3+ by ammonia nitridation of oxide precursors. Their crystallinity reduced in a compositional range up to 3 mol% and improved again above 4 mol% with an appearance of a nitride impurity. The product with 2 mol% of Fe 3+ showed the maximum oxygen content of 21 mol% and minimum nitrogen content of 24 mol%. X-ray absorption spectroscopy, Mössbauer and magnetic measurements suggested a small amount of Fe 3O 4-like clusters in the oxynitride products. Density-functional calculations supported that the Fe-O-clustered system is more preferable than the statistical distribution of iron in the hexagonal gallium oxynitride structure.