Modified Citrate Method Research Articles

Aspects of Fe-Incorporation into CaTiO3-SrTiO3 Perovskites and Their Catalytic Application for Ammonia SCO/SCR

Perovskite materials in the CaTiO3-SrTiO3 system doped with different amounts of iron (1, 2 and 5 mol.%) and various Ca/Sr ratios were prepared by the modified citrate method. Additionally, the materials with 0.05 deficiency in strontium/calcium sublattice and 5 mol.% of Fe were also synthesised. The materials were subjected to structural (XRD, XANES) and microstructural (SEM) characterisation, as well as the analysis of susceptibility to reduction/oxidation processes. The structural analysis indicates a lack of iron-containing phases; thus, an incorporation of Fe into the perovskite structure was postulated. Additionally, the oxidation state of iron in the perovskite structure changes with the dopant amount. The temperature-programmed reduction measurements showed partial reversibility of the reduction processes. For the materials with the highest iron amount, the catalytic tests in NH3-SCO and NH3-SCR reactions were carried out. The materials showed high catalytic activity and high selectivity to N2 in the NH3-SCR process; however, they were inactive in NH3-SCO.

Unravelling cobalt incorporation in Ca- and Sr-rich perovskites: How symmetry shapes the phases

The Ca-rich and Sr-rich stoichiometric and non-stoichiometric materials were obtained by the modified citrate method. After calcination at 900 °C, materials were characterized in terms of structure, microstructure and reducibility in an H2-containing atmosphere. The diffractometric data did not confirm the presence of Co-originated phases. Co K-edge and Ti K-edge were recorded and analyzed for a deeper description of the structural properties of materials, mainly the influence of non-stoichiometry on the effectiveness of Co incorporation into the perovskite systems. The X-ray absorption near edge structure study combined with temperature programmed reduction results indicated the presence of CoTiO3 and cobalt oxides phases for Ca-rich and Sr-rich. This approach also allowed to observe that non-stoichiometry in Sr-rich materials results in a lower amount of cobalt incorporated into the perovskite structure and a higher amount of Co3O4 formed. Additionally, for Ca-rich materials, XANES spectra indicated a higher amount of Co in the Ti sublattice and confirmed the higher Co2+/Co3+ ratio than in the case of Sr-rich materials.

Synthesis and characterisation of high-purity alumina achieved through a dip coating process

ABSTRACT This study focuses on the synthesis of alumina using a modified citrate method. The chelation process involved thermal activation through biphasic drops, employing the hydrated aluminium nitrate precursor and citric acid complex. The thermal behaviour of the resulting gel was investigated using DSC/TGA followed by FTIR, which revealed absorption bands at distinct temperatures leading up to the formation of white crystallised alumina powder at 1000°C. Structural analysis via XRD parameters indicated that the obtained sample is refined in the hexagonal system with the R-3C space group. SEM observations demonstrated morphology akin to an overlapping hive, highlighting significant stoichiometric purity. The semi-colloidal sol was applied in a dip-coating method to reinforce the matrix of a steel substrate, specifically E335. SEM-EDS analyses revealed the adhesion recorded as a brittle intermetallic phase. Additionally, a transformation of 22% haematite to magnetite was observed under the conditions of coating.

Improvement of ferroelectric properties via Zr doping in barium titanate nanoparticles

Barium titanate is still the prototype of a piezoelectric crystalline material that has attracted many researchers and industrial partners to use. A modified citrate method was used to create barium titanate nanoparticles BaTi1−xZrxO3. The samples were crystallized in a single-phase tetragonal structure, as revealed using X-ray powder diffraction. The crystallite size decreases with increasing Zr concentration. Fourier-transform infrared spectra showed the main absorption bands of the samples BaTi1−xZrxO3. Field emission scanning electron microscopy micrographs illustrate that the doped sample BaTi0.9Zr0.1O3 is more porous and finer than the parent. For low Zr doping concentrations (x = 0.1), the ferroelectric properties of barium titanate are improved. The conduction mechanisms in the samples are small polaron hopping and correlated barrier hopping. The Zr/Ti ratio is a crucial parameter for tailoring the ferroelectric–paraelectric phase transition.

Comparison of Precursor Preparation Routes on Final Density of Y3Fe5O12 Garnets Prepared via Reactive Sintering.

Yttrium iron garnet was obtained using four methods of synthesis. A modified citrate method and a modified citrate method with YIG (yttrium iron garnet, Y3Fe5O12) nucleation were used. In two subsequent methods, YIP (yttrium iron perovskite, YFeO3) and α-Fe2O3 obtained in the first case by the citrate method and in the second by precipitation of precursors with an ammonia solution were used as the input precursors for reaction sintering. Differential scanning calorimetry (DSC) measurements of the output powders obtained by all methods allowed to identify the effects observed during the temperature increase. Dilatometric measurements allowed to determine the changes in linear dimensions at individual stages of reaction sintering. In the case of materials obtained by the citrate method, two effects occur with the increasing temperature, the first of which corresponds to the reaction of the formation of yttrium iron perovskite (YIP), and the second is responsible for the reaction of the garnet (YIG) formation. However, in the case of heat treatment of the mixture of YIP and α-Fe2O3, we observe only the effect responsible for the solid state reaction leading to the formation of yttrium iron garnet. The obtained materials were reaction sintered at temperatures of 1300 and 1400 °C. Only in the case of material obtained from a mixture of perovskite and iron(III) oxide obtained by ammonia precipitation at temperature of 1400 °C were densities achieved higher than 98% of the theoretical density. The use of Hot Isostatic Pressing (HIP) in the case of this material allowed to eliminate the remaining porosity and to obtain full density.

Sol-Gel Synthesis And Characterization Of New Inorganic Pigments Containing Oxides of Iron, Aluminum, Strontium and Silicate

A series of inorganic pigments containing iron, aluminum, strontium, and silicate were prepared by the sol-gel method using compounds with different structures and morphologies as starting materials. The synthesis of the compounds was carried out using the modified citrate method. All powders were placed in a muffle furnace and calcined in air at 1100°C for 6 hours. Powder samples containing Fe, Sr, Al and Si oxides were obtained. The structure and chemical composition of the synthesized five inorganic pigments were investigated comparatively by Fourier transform infrared spectroscopy (FTIR), UV-visible spectroscopy (UV-vis), X-ray diffraction (XRD), scanning electron microscopy (SEM), and Energy Distribution X-ray Analysis (EDX).

The LaCo1−xVxO3 Catalyst for CO Oxidation in Rich H2 Stream

The LaCo1−xVxO3 catalysts for CO oxidation in H2 rich stream were studied. These perovskite-type oxides with variable amounts of vanadium (x = 0, 0.05 and 0.1) were synthesized by modified citrate method. It was found that the modification in LaCoO3 perovskite after vanadium addition affected directly the structure and morphology leading to decrease in the crystallite size and increase in BET surface area. XPS results confirmed quantitatively the higher presence of V4+ species at the surface of the modified perovskite. The existence of Co2+ was reported to facilitate the interplay between Co2+/Co3+ and V5+/V4+ redox couples, which can enhance the lattice oxygen mobility and create interfacial active sites of CoOx/V2O5. Tests during 20 h on stream at 250 °C showed excellent stability, high resistance to coke deposition and no sintering. Perovskite modified by addition of vanadium LaCo1−xVxO3 for preferential CO oxidation in H2-rich stream.

Morphology characterization and dielectric performances of Ce-doped barium strontium titanate powders prepared by modified citrate method

ABSTRACTCitrate acid (CA), ethylene glycol (EG), and absolute ethyl alcohol (AEA) were combined to synthesize 0∼5% mol cerium doped barium strontium titanate (Ce-BST) powders, and the morphology characterization and dielectric performances of the powders were studied. X-ray diffraction (XRD) shows that the powders have ABO3 perovskite structures with no secondary phase until ≥4% mol because AEA avoids the formation of cerium hydroxide caused by the hydrolyzation of Ce3+ ions, and Ce3+ ions enter into lattices to make lattice parameters increase. Scanning electronic microscope (SEM) reveals that Ce3+ ions affect grain growth and refine grain size with uniform grain size of about 100 nm at 2 mol%. Permittivity and loss tangent of the doped powders are decreased, where 2 mol% corresponds to the optimum dielectric properties with permittivity of 58 and loss of 0.007 at 1 GHz, which can meet the requirements in many applications like microwave devices. The relation between dielectric properties and Ce doping is also discussed.

Deposition of NiO on 3 mol% yttria-stabilized zirconia and Sr0.96Y0.04TiO3 materials by impregnation method

The main purpose of this work was the synthesis of composite materials by deposition of nickel oxide on 3 mol% yttria-stabilized zirconia (3 mol% Y2O3–97 mol% ZrO2–YSZ) and Sr0.96Y0.04TiO3 (4 mol% Y-doped strontium titanate—SYTO) supports. Yttria-stabilized zirconia and yttrium-doped strontium titanate were obtained by modified citrate method. The deposition of nickel oxide was performed by impregnation method. All materials were impregnated with proper amount of nickel(II) nitrate aqueous solution to receive the 1 and 5 mass% of nickel in the final composite. The temperature-programmed reduction (TPR) was applied to analyze the reduction mechanism of deposited NiO. Three forms of nickel oxide: non-stoichiometric NiO1+x, hexagonal and cubic are postulated on the base of the TPR profiles and analysis of bonds length in nickel oxides structures. Moreover, TPR analysis shows that support strongly affects the reduction mechanism as a consequence of various interactions between NiO and YSZ/SYTO. In the case of YSZ, the nickel oxide reveals the tendency to deposit on the support surface, while using SYTO as the support, nickel oxide exhibits significant ability to react with doped strontium titanate and what is more important nickel incorporates into Sr0.96Y0.04TiO3 structure.

COx-free hydrogen production via decomposition of ammonia over Cu–Zn-based heterogeneous catalysts and their activity/stability

Cu–Zn mixed metal oxides were synthesized with a modified citrate method, supported on an Al2O3 substrate, and tested for ammonia decomposition process to attain a high-purity hydrogen generation for fuel cells. Alumina-supported catalytic materials prepared by wet impregnation technique exhibited the highest surface copper species dispersion along with an increase in distributed acidic and basic sites. TPD studies using NH3 and CO2 revealed the presence of the Lewis and Brønsted acidity and basicity. SEM demonstrated a uniform particle distribution and morphology, regardless of Cu/Zn ratio. Cu/ZnO/Al2O3 exhibited a superior conversion activity when compared to neat Cu–Zn, which may be due to an improved Cu–Zn synergistic effect, a smaller average bimetallic nanoparticle size, and moderate acid–base characteristics. TEM micrographs confirmed the round-shaped metallic particles, ranging up to 7nm in diameter and comprising both active copper oxide phases (Cu1+ and Cu2+), further investigated by EELS spectroscopy, as well as by XPS analysis. With reactions resulting in an effective first-order turnover (rate-determining step limitation), the apparent activation energies of (50–80)±5kJmol−1 were estimated in between 450 and 600°C which is in agreement with other commercial catalysts. All fabricated bi-functional catalysts exhibited a high long-term stability and hydrogen productivity; nonetheless, comprised no critical scarce raw resources (e.g. platinum-group metals), which is appealing for emerging chemically-bonded H2 storage and release be it in relation to production only (electrolysis) or with consumption (regenerative fuel cells).

Removal of NOx and soot over Ce/Zr/K/Me (Me = Fe, Pt, Ru, Au) oxide catalysts

The potentiality of ceria/zirconia based catalysts in the simultaneous removal of particulate matter (soot) and NOx is investigated in this work, and compared with that of a model LNT Pt-K/Al2O3 sample. Ceria-zirconia (molar ratio 75/25) catalysts doped with Pt, Au, Ru or Fe (2% by weight) and containing K (7% by weight) were prepared by a modified citrate method and characterized by X-ray diffraction, surface area and pore volume measurements. The behavior of the catalysts in the soot combustion and NOx removal was separately analyzed by means of temperature programmed oxidation (TPO), isothermal combustion and isothermal NOx adsorption experiments. The results showed that all the ceria/zirconia based catalysts are more active than Pt-K/Al2O3 in soot combustion; the Ru-containing system also showed NOx storage performances comparable to Pt-K/Al2O3. Accordingly the capability of the Ru-based catalyst to accomplish the removal of NOx in the absence and in the presence of soot was further investigated by reactivity experiments and FT-IR spectroscopy to analyze both the gas phase and the catalyst surface species. The data indicate that the Ru-based system is able to simultaneously remove soot and adsorb NOx pointing out higher performances in the soot combustion as compared to the Pt-K/Al2O3 catalyst, and similar behavior in the NOx storage capacity. However the NOx reduction activity results lower than the traditional LNT Pt-based catalyst. Conversely, when the Ru-based catalyst is mixed with the LNT sample (physical mixture) a NOx reduction efficiency similar to Pt-K/Al2O3 is found.

Structural and electrocaloric properties of multiferroic-BiFeO3 doped 0.94Bi0.5Na0.5TiO3-0.06BaTiO3 solid solutions

Single-phase (1-x)(0.94Bi0.5Na0.5TiO3-0.06BaTiO3)-xBiFeO3 (BNT-BT-xBF, x = 0.05, 0.1 and 0.15) solid solutions are fabricated using a modified citrate method. The structural, ferroelectric and electrocaloric properties are significantly affected by the multiferroic BiFeO3 doping, which is found to favor the antiferroelectric (AFE) state of the solid solutions. The dynamic mechanical analysis suggests that the BiFeO3 induced ordering of oxygen octahedrons plays a dominant role in the formation of AFE incommensurate phases and leads to the enhanced negative electrocaloric effect (ECE). A maximum ECE temperature change of 1.65 °C under an applied field of 70 kV/cm is observed. It is suggested that the magnetoelectric coupling is effective in enhancing the negative ECE in BNT-BT-xBF.

Crystal structure, chromatic and near-infrared reflective properties of iron doped YMnO3 compounds as colored cool pigments

The new application of YFexMn1−xO3 (x = 0, 0.05, 0.10, 0.15 and 0.20) compounds as colored cool inorganic pigment has been developed. Nanocrystalline YFexMn1−xO3 samples with different average grain sizes have been synthesized by modified citrate method at a certain calcination temperature. The structure of these compounds changes by the increase in degree of substitution from the hexagonal to the orthorhombic perovskite. With the increase of Fe content, the color of the designed pigments changes from blue-green to pale blue and then dark blue. Most importantly the near-infrared reflectance and solar reflectance values of corresponding pigmented coatings increase from 47.6 to 60.6% and from 20.4 to 28.4% with increasing Fe content from x = 0 to 0.15, respectively, which are higher than that of commercially available roofing materials of the same color.

Catalytic combustion of methane on BaZr(1−x)MexO3 perovskites synthesised by a modified citrate method

The catalytic combustion technology still lacks of catalytic systems stable at high temperatures, such as those of the gas turbine inlet in the last turbine generation. In this paper we tried to solve this problem by fitting transition metals inside a crystal structure which is known to be very stable.Several oxides have been synthesised by a modified citrate method and investigated in the catalytic combustion of methane. Synthesised catalysts consist of a solid solutions with a perovskite structure of formula BaZr(1−x)MexO3, where Me=Rh, Pd, Mn, Ni, Ru, Pt, Co.The goal of the work has been to provide the physicochemical, structural and morphological characterisation and the activity of these catalysts containing different amounts of transition metals, by means of BET, XRD profile fitting and TPC reactivity tests, respectively.The synthesised powders showed a very limited hysteresis between the increasing and decreasing temperature curves and therefore should display no oscillations in working conditions.These systems could be a high temperature alternative to the hexa-aluminates and could avoid the homogeneous post combustion volumes typical of the hybrid and XONON combustors.

Thermal and dynamic mechanical analyses on Bi0.5Na0.5TiO3–BaTiO3 ceramics synthesized with citrate method

Bi0.5Na0.5TiO3–xBaTiO3 (BNT–xBT) nano-powders are successfully synthesized by a modified citrate method. The as-prepared BNT-BT powders and the sintered ceramics are homogeneous with a pure perovskite crystal structure. The effects of Ba2+ substitutions for (Bi0.5Na0.5)2+ in the A-sites of Bi0.5Na0.5TiO3 on its phase transformations are explored. The transformations among ferroelectric (FE), anti-ferroelectric (AFE) and paraelectric (PE) states in these ceramics are characterized using ferroelectric hysteresis tests, modulated differential scanning calorimetry and dynamic mechanical analysis. The FE-AFE transition in BNT–xBT with 0≤x≤0.15 is found to relate with a structural transformation which is a first-order phase transition. The mechanical and thermal analyses provide evidence that AFE state (0≤x≤0.15) could be associated with the incommensurate modulation of rhombohedral structures while the mechanisms of forming AFE state in BNT–xBT (x>0.15) could be different.

Preparation and Ferroelectric Properties of Lead-Free Bi<sub>0.5</sub>Na<sub>0.5</sub>TiO<sub>3</sub>-BaTiO<sub>3</sub> Ceramics Synthesized with Citrate Method

Lead-free (1-x)Bi0.5Na0.5TiO3-xBaTiO3(BNT-xBT) nano-powders synthesized by a modified citrate method and the sintered ceramics were characterized using various techniques. The rSubscript textesults indicated that all the BNT-xBT samples were homogeneous with a pure perovskite crystal structure. It is found BNT-0.1BT exhibitSubscript texts the best ferroelectric properties. BNT-0.0Subscript text6BT has excellent performance in thermo-electrical energy conversion.

Influence of Cation Dopants in the A-Site on the Crystal Structure, Microstructure, and Electrical Conductivity of Ba0.5Sr0.5Co0.8Fe0.2O3-δ as a New Solid-Oxide-Fuel-Cell Cathode

(Ba0.5Sr0.5)1-xCaxCo0.8Fe0.2O3-δ (BSCCF), (Ba0.5Sr0.5)0.8La0.2Co0.8Fe0.2O3-δ (BSLCF), and (Ba0.5Sr0.5)0.8La0.2-xCaxCo0.8Fe0.2O3-δ (BSLCCF) perovskite oxides were synthesized by the modified citrate method and characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). These oxides showed primarily a cubic phase with their dense bodies after sintering. The lattice parameter was decreased with increasing Ca2+-ion content in BSCCF. Conversely, the substitution of Ca2+ ions for La3+ ions in BSLCCF increased the lattice parameter. The maximum electrical conductivities were 96 S·cm-1 at 650 °C for the 30 mol % Ca2+-substituted BSCCF and 353 S·cm-1 at 350 °C for BSLCF. The thermal expansion coefficient (TEC) of BSCCF was almost constant regardless of the Ca2+-ion contents, whereas that of BSLCCF decreased with decreasing Ca2+-ion content.

Effect of Fe substitution on multiferroic hexagonal YMnO 3

Polycrystalline YMn 1− x Fe x O 3 ( x=0.02–0.20) powders were synthesized by means of modified citrate method. Powder X-ray diffraction gives evidence that all the samples are single phase and exhibit hexagonal structure with P6 3 cm space group as observed for YMnO 3. The solubility limit of Fe was determined as about 6 wt.%. Cell parameter values were found to increase with Fe content, since Fe 3+ and Mn 3+ have the same ionic radii. This can be attributed to the increase of the tilting of MnO 5 bipyramid and the buckling of Y atoms. In addition, 57Fe Mössbauer spectrometry provides evidence of two Fe 3+ sites attributed to two different nearest atomic neighbours. Magnetic properties reveal a paramagnetic-to-antiferromagnetic transition, a possible increase of the magnetic anisotropy, and a competition between ferromagnetic and antiferromagnetic interactions.

A novel method for preparation of barium strontium titanate nanopowders

Abstract Ba0.6Sr0.4TiO3 (BST) nanopowders have been prepared using the modified citrate method with ammonium nitrate as a combustion promoter, and the formation mechanism, phase evolution, and particle size have been investigated using TG/DTA, XRD, and SEM. It is found that the peaks of barium carbonate disappear when the precursor powders are calcined at 650 °C. The fine particles of the nanopowders calcined with the combustion promoter addition are homogeneous and well-dispersed and their narrow size distribution is about 60–90 nm. Comparatively, the particles of the powder calcined without the ammonium nitrate addition are inhomogeneous, with an evident agglomeration. The mechanism for the above results is attributed to that a reaction can generate soft and loose precursor powders by the adoption of ammonium nitrate, and hence a pure BST phase could occur at the low synthesis temperature of 650 °C.

Effect of synthesis parameters on catalytic properties of CuO-CeO2

A modified citrate method incorporating hydrothermal treatment of the precursors has been employed for the synthesis of CuO-CeO2 catalysts with varying CuO content. Catalyst characterization was done by N2 physisorption, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and H2-TPR. The activity and selectivity of the catalysts for the selective oxidation of CO in excess H2 was examined employing a simulated reformate gas mixture. Hydrothermal treatment of the citrate precursors was found to induce more efficient mixing of Cu2+ and Ce4+ ions in the precursor compound. CuO-CeO2 catalysts containing highly dispersed, XRD-invisible CuO species were synthesized with this method at even higher CuO contents than those reported previously. Interaction between CuO and CeO2 leads to structural and thermal stabilization of the catalysts, while surface segregation of Cu2+ ions takes place in all cases. CuO-CeO2 catalysts are significantly more active, more selective and less inhibited by CO2 than pure CuO. The activation temperature and the CuO content of the catalysts seem to play an important role by influencing, on one hand, the degree of interaction between CuO and CeO2 and, on the other hand, the phenomena of sintering and surface area loss.