The effect of the calcination temperature of LaFeO3 precursors on the properties and catalytic activity of perovskite in methane oxidation

In the synthesis of perovskite-type LaFeO₃ oxides iron and lanthanum nitrates were used as a precursors. The nitrates were dissolved in water, evaporated, crushed and calcined in temperature range of 650–850?°C. The obtained perovskites were applied as an active layer on monolithic catalysts for the oxidation of methane. The increase in the calcination temperature of the perovskite precursors from 650° to 850°C results in a reduction in the surface area of the powders from 10.1 to 4.2?m²/g. XRD studies revealed that calcination at 800–850?°C caused the formation of an almost homogeneous LaFeO₃ perovskite phase. A decrease in the La/Fe surface ratio from 12 to 5.2 with the rise in calcination temperature from 650° to 800°C was detected by XPS. EDX results confirmed that at 750–850?°C, the La/Fe ratio in the perovskite layer is close to the stoichiometric and amount to 1.01–1.03. The highest activity in methane oxidation was achieved when the LaFeO₃ perovskite was calcined at 700?°C. A further slight increase in the activity was noticed after H₂ treatment. As the calcination temperature of the perovskites is increased, the catalyst activity decreases due to a reduction in the specific surface area, despite the more complete LaFeO₃ perovskite phase formation.     

LaFeO3 ceramics as selective oxygen sensors at mild temperature

In this study, an investigation about the oxygen sensing properties of lanthanum orthoferrite (LaFeO₃) ceramics is reported. LaFeO₃ nanoparticles were synthesized by using tartaric sol-gel route and annealed in air at different temperatures (500, 700 and 900 °C). The samples have been characterized by using thermal analysis (TA), BET surface area and porosity, Fourier-transform infrared spectroscopy (FTIR), Raman spectroscopy, X-ray diffraction (XRD), and scanning electron microscopy (SEM). Results of sensing tests indicate that LaFeO₃ nanoparticles exhibit good response to oxygen at mild temperatures (300–450 °C). The effect of annealing temperature on gas sensing performance was investigated, demonstrating that LaFeO₃ ceramics obtained after annealing at 500 °C display better characteristics with respect to others. The oxygen sensor developed shows also high stability in humid environment and excellent selectivity to oxygen over other interfering gases such as CO, NO₂, CO₂, H₂ and ethanol.     

Structural Investigations on the Compositional Anomalies in Lanthanum Zirconate System Synthesized by Coprecipitation Method

The study set out to investigate the compositional inconsistency in lanthanum zirconate system revealed the presence of nonstoichiometry in lanthanum zirconate powders when synthesized by coprecipitation route. X‐ray diffraction (XRD) and high‐resolution transmission electron microscopy (HRTEM) investigations confirmed the depletion of La³⁺ ions in the system. Analysis using Vegard's law showed the La/Zr mole ratio in the sample to be around 0.45. An extra step of ultrasonication, introduced during the washing stage followed by the coprecipitation reaction, ensured the formation of stoichiometric La₂Zr₂O₇. Noteworthy is also the difference between crystal sizes in the samples prepared by with and without ultrasonication step. This difference has been explained in light of the formation of individual nuclei and their scope of growth within the precipitate core. The differential scanning calorimetry (DSC) analyses revealed that optimum pH for the synthesis of La₂Zr₂O₇ is about 11. The ultrasonication step was pivotal in assuring consistency in mixing and composition for the lanthanum zirconate powders.     

Effect of precursors on the morphology and surface area of LaFeO3

Surface area and morphology of materials play an important role on their gas sensing performance because of the varying number and nature of adsorption sites. Current work reports a comparative study of LaFeO₃ synthesized by the facile hydrothermal method using two precursors; citric acid and KOH. The microstructure observed through FESEM and TEM showed different morphologies for the two precursors and calcination time (2?h & 6?h). Prior to calcination, higher surface area (50.54?m²/g) was obtained for LaFeO₃ prepared using KOH as compared to that for LaFeO₃ using citric acid (3.21?m²/g). Surface area increased from 3.21 to 7.06?m²/g for citric acid and decreased from 50.54 to 11.42?m²/g for KOH as calcination takes place for 6?h. Needle-shaped morphology of p-type LaFeO₃ with high surface area (50.54?m²/g) for KOH would provide large active sites which would enhance sensitivity towards gases. Hence, LaFeO₃ samples prepared using KOH with and without calcination are expected to give better performance for gas sensing than LaFeO₃ samples synthesized using citric acid.     

Characterization of perovskite LaFeO3 synthesized by microwave plasma method for photocatalytic applications

Perovskite LaFeO₃ nanoparticles were successfully synthesized by microwave plasma method combined with high temperature calcination at 700–1000?°C. The influences of calcination temperature on morphology, crystalline structure, purity and the atomic compositions of samples were studied. The photocatalytic performance of LaFeO₃ was evaluated though the photodegradation of Rhodamine B (RhB) under visible light. In this research, the orthorhombic LaFeO₃ nanoparticles showed band gaps in the range of 2.15–2.30?eV. The particle size increased with increasing in the calcination temperature, leading to the decreasing in the surface area. The LaFeO₃ sample calcined at 900?°C showed the highest photodegradation of 77.8% and the apparent rate constant of 0.0077?min^?1 within 180?min because of the narrower of band gap and the higher crystalline degree and oxygen adsorption.     

LaFeO<Subscript>3</Subscript> Perovskite as New and Performant Catalyst for the Wet Peroxide Oxidation of Organic Pollutants in Ambient Conditions

The purpose of the present study was to evaluate the ability of LaFeO₃ perovskites prepared using the “self-combustion method” to behave as efficient and stable catalysts in phenol oxidation using H₂O₂ in ambient conditions. Depending on the synthesis conditions, strong differences in the structural, textural and reducibility characteristics of the perovskites, as well as in phenol conversion and TOC abatement level were observed. High TOC abatement (76%) and very low iron leaching (0.27 wt%) were obtained at 40 °C in the presence of LaFeO₃ nanoparticles exhibiting the lowest crystal domain size.     

Studies of physicochemical and surface properties of alumina modified with rare earth oxides

Results are presented of investigations of the porous structure of the Al₂O₃-La₂O₃ systems containing up to 10 wt% of the La modifying oxide prepared by the coprecipitation method. Pore size distribution was determined on the basis of low temperature nitrogen adsorption measurements and mercury intrusion porosimetry. The changes in pore size distribution of different forms of Al₂O₃-La₂O₃ are discussed in relation to the concentration of the lanthanum oxide.     

Effect of mechanical activation of coprecipitated precursor on synthesis of La2Zr2O7

A lanthanum zirconate (La₂Zr₂O₇, LZ) precursor has been prepared by coprecipitation of the relevant hydroxides from nitrate solution. The effect of the mechanical activation of the precursor on its thermal decomposition and LZ formation in the temperature range of 700–1100 °C has been investigated. After mechanical activation, the precursor releases volatile components (H₂O and CO₂) at lower temperatures. Mechanical activation accelerates the LZ crystallisation and results in the formation of phase-pure LZ without admixtures of unreacted zirconia and lanthana. The LZ powder prepared from the mechanically activated precursor is characterised by a larger BET surface area, in comparison to that synthesised from the as-prepared precursor under the same calcination conditions.     

A green and economical route to the precursor for the synthesis of single crystal LiNi0.5Co0.2Mn0.3O2

Single crystal LiNi_1-x-yCo_xMn_yO₂(NCM) cathode materials are typically synthesized using spherical polycrystalline hydroxides which are often prepared via coprecipitation reactions. However, the spherical morphology of polycrystalline hydroxides is not essential for the precursor in the synthesis of single crystal NCM, and also the coprecipitation process is not environmentally friendly and cost-effective enough. Herein a new process based on room-temperature solid-state metathesis reactions is developed to prepare the precursor for the synthesis of single crystal LiNi_0.5Co_0.2Mn_0.3O₂(NCM523). The whole process is free of any undesirable chemicals, and the resulting nanosize precursor can facilitate the synthesis of micron-level single crystal NCM523 at relatively lower sintering temperatures with less lithium excess. Moreover, the obtained single crystal NCM523 can exhibit comparable reversible capacities as compared with that synthesized from the coprecipitated spherical polycrystalline hydroxides. This work demonstrates a green and economical route to the precursor for the synthesis of single crystal NCM.     

Comparative Study of Powders Based on the ZrO<Subscript>2</Subscript>–Y<Subscript>2</Subscript>O<Subscript>3</Subscript>–СeO<Subscript>2</Subscript> System Obtained by Various Liquid Phase Methods of Synthesis

Using the liquid-phase methods of synthesis—the coprecipitation of hydroxides and the hydrothermal method—mesoporous xerogels are obtained based on the ZrО₂–Y₂О₃–CeО₂ system with 5–8-nm particles and powders (after the heat treatment of xerogels at 600°C) with a coherent scattering region (CSR) size of 9 to 10 nm and S_sp = 96–156 m²/g. After calcination at 1400°C, the powders are transformed into tetragonal solid solutions with a CSR size of 65 nm in the synthesis by the coprecipitation method, and they are transformed into solid solutions with a CSR size of 84 nm with a high degree of tetragonality of c/a = 1.438–1.431 in the synthesis by the hydrothermal method.     

Effects of PVA content on the synthesis of LaFeO3 via sol-gel route

LaFeO₃ were synthesized via a sol-gel route based on polyvinyl alcohol (PVA). Differential scanning calorimetry (DSC), Thermogravimetric (TG), Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), Raman spectroscopy and field emission scanning electron microscopy (FESEM) techniques were used to characterize precursors and derived oxide powders. The effect of the ratios of positively charged valences to hydroxyl groups of PVA (Mⁿ⁺/-OH) on the formation of LaFeO₃ was investigated. XRD analysis showed that single-phase and well-crystallized LaFeO₃ was obtained from the Mⁿ⁺/-OH = 4:1 molar ratio precursor at 700 °C. For the precursor with Mⁿ⁺/-OH = 2:1, nanocrystalline LaFeO₃ with average particle size of ∼50 nm was formed directly in the charring procedure. With increase of PVA content to Mⁿ⁺/-OH = 1:1, phase pure LaFeO₃ was obtained at 500 °C.     

Facile Preparation of LaFeO<Subscript>3</Subscript>/rGO Nanocomposites with Enhanced Visible Light Photocatalytic Activity

The present work demonstrates a facile route for preparing LaFeO₃/rGO nanocomposites comprising of metal oxide nanoparticles and graphene. Structural, morphology, optical and photocatalytic studies of the samples were characterized using powder X-ray diffraction (XRD), FT-IR, Raman, high resolution scanning electron microscopy (HRSEM), high resolution transmission electron microscope (HRTEM), atomic force microscopy (AFM), thermogravimetry (TGA), X-ray photoelectron spectroscopy, UV–visible and photocatalytic. LaFeO₃/rGO nanocomposites believed as an effective photocatalyst for the degradation of methyl orange (MO) dye under visible light irradiation. The inclusion of carbon enhances the light absorption of LaFeO₃, resulting in the enhanced photocatalytic activity of the nanocomposite. The degradation of MO dye under visible light source was completely achieved using LaFeO₃/rGO as a catalyst.     

Spray-flame synthesis of La(Fe,Co)O3 nano-perovskites from metal nitrates

Nano-sized perovskites were synthesized in a spray flame from nitrate precursors dissolved in ethanol and in ethanol/2-ethylhexanoic acid (2-EHA) mixtures. Experiments with ethanol led to a broad particle-size distribution and to the formation of undesired phases such as La₂CoO₄, La₂O₃, and Co₃O₄. The addition of 2-EHA can initiate micro explosions of the burning droplets and has been systematically investigated toward the formation of single-phase, high-surface-area LaCoO₃ and LaFeO₃ with a narrow size distribution. To investigate the effect of 2-EHA, temperature-dependent changes of the chemical composition of the precursor solutions were analyzed with ATR-FTIR between 23 and 70°C. In all cases, the formation of esters was identified while in the solutions containing iron, additional formation of carboxylates was observed. The synthesized materials were characterized by BET SSA, XRD, SAED and EDX-TEM and their catalytic activity was analyzed, reaching 50% CO conversion at temperatures below 160 and 300°C for LaCoO₃ and LaFeO₃, respectively.     

Nanoparticles of iron and vanadium oxides supported on iron substituted LDHs: Synthesis, textural characterization and their catalytic behavior in ethylbenzene dehydrogenation

Nanostructured catalysts derived from nanoparticles of iron or vanadium oxides supported on the matrices of iron substituted hydrotalcite-like anionic clays (layered double hydroxides, LDHs) have been obtained and tested in the process of ethylbenzene dehydrogenation to styrene. A simple synthesis method based on the LDHs “memory effect” has been used to prepare the new oxides-anionic clay structures. TEM analysis shows that on the typical FeLDH particles (average size equal to 75 nm) smaller nanoparticles are supported; their average size is equal to 7 and 11 nm for Fe/FeLDH and V/FeLDH respectively. XPS analysis indicates the presence of Fe₂O₃ and V₂O₃ on the surface of the supported LDHs. N₂ adsorption at 77 K reveals that the supported anionic clays have less accentuated mesoporous properties in comparison to the parent FeLDH matrix. The catalytic behavior of the samples is a function of the nature of the supported nanoparticles.     

A simple and efficient preparation of LaFeO3 nanopowders by glycine-nitrate process: Effect of glycine concentration

LaFeO₃ perovskites have been prepared by the glycine-nitrate process (GNP) at various glycine-to-nitrate molar ratios. The perovskites have been systematically characterized by X-ray diffraction, BET surface area, scanning electron microscopy, transmission electron microscopy and temperature-programmed reduction to study the effect of glycine concentration on various properties of LaFeO₃. The X-ray diffraction patterns of the as-prepared and calcined samples show the formation of orthorhombic phase without any impurities. The BET specific surface areas of various perovskites increased with an increase in glycine-to-nitrate ratio (GNR) of 2.0 but were nearly constant at higher ratios. The scanning electron microscopy indicates that the prepared material is flake-like at GNRs ≤1.5 and exists as agglomerated particles at GNRs ≥2.0. The particle size of the as-prepared samples was in the range of 30-130 nm depending on the GNR and the calcined samples exhibited particle size in the range of 60-160 nm. The samples that were prepared at GNR < 1.5 did not show any peaks in temperature-programmed reduction, but the samples prepared at a GNR of 2.0 and above showed the reduction of Feⁿ⁺.     

Synthesis,characterization, and performance assessment of a perovskite-type nano photocatalyst for degradation of thiamethoxam

It is essential to remove thiamethoxam (THMX) from surface water as it negatively impacts the ecology and neurological systems of insects. Hence, a study was conducted to degrade THMX with different compositions of LaFeO₃/g-C₃N₄ (LFCN) composites. Several methods, including X-ray powder diffraction (XRD), ultraviolet–visible spectroscopy (UV–Vis), X-ray photoelectron spectroscopy (XPS), Brunauer–Emmett–Teller (BET), and field emission scanning electron microscopy (FESEM), were applied to characterize the synthesized photocatalyst. The influence of many parameters on degradation, for example, THMX concentration, dosage of catalyst, and pH were studied experimentally. The degradation was highest under both UV-C and sunlight for the synthesized catalyst, 1% LaFeO₃/g-C₃N₄ (LFCN1), in comparison to graphitic carbon nitride (g-CN), and bare lanthanum ferrite (LF). The degradation was around 95% for LFCN1 under UV-C light having an intensity of 15 W/m² whereas degradation was 71.8% with LFCN1 photocatalyst under sunlight at neutral pH in 120 min of reaction time. The increased activity of LFCN1 was attributable to an improved surface area and a lower band gap. The study of reaction kinetics indicated second-order behaviour. Additionally, a probable mechanism for degradation was put forth.     

Facile preparation and characterization of lanthanum oxide powders by the calcination of lanthanum carbonate hydrate in microwave field

Micron-sized lanthanum oxide powders are prepared by the calcination of lanthanum carbonate hydrate in microwave field. The decomposition process of lanthanum carbonate hydrate was analyzed by TG-DSC and indicates the reaction undergoes three stages, resulting in the generation of lanthanum oxide at 770 °C. For microwave assisted calcination, XRD patterns demonstrate that hexagonal La₂O₃ structure is initially formed after calcination at 650 °C for 2 h, and FT-IR analyses confirm the decomposition of precursor is complete after calcination at 750 °C for 2 h. SEM investigations reveal that 800 °C is the optimal calcination temperature to generate La₂O₃ powders with uniform morphologies. In comparison, conventionally calcination experiments are carried out in electrical furnace. Both XRD and FT-IR analyses are in consistence with TG-DSC, which indicate the temperature required for fully decomposition of lanthanum carbonate hydrate by conventional heating is higher than that of microwave heating. SEM images present irregular morphologies and wide particle size distribution of conventionally prepared samples. All the techniques are utilized to prove the feasibility of decomposing La₂(CO₃)₃ to generate La₂O₃ in microwave field and highlight the advantages of microwave heating.     

Structure formation and degradation of partially stabilized zirconium dioxide

The process of structure formation of partially stabilized zirconium dioxide powders during the heat treatment of precipitated zirconium and yttrium hydroxides and structure degradation in time depending on hydroxide precipitation method (mutual and sequential precipitation) has been investigated. During calcination, the t→m-ZrO₂ transition takes place in sequentially precipitated hydroxides, and the m→t-ZrO₂ transition takes place in coprecipitated hydroxides. It was noted that soft readily destroyable aggregates are formed during the heat treatment of sequentially precipitated hydroxides. This makes it possible to produce fine ZrO₂ powders (d=50–100 nm) avoiding disaggregation and/or milling. It has been found that the zirconium dioxide degradation process, which is due to the t→m-ZrO₂ transformation, decreases with increasing amount of Y₂O₃ and cubic modification of ZrO₂ in heat treatment products and in the case of coprecipitation of hydroxides     

The effects of Mg2+ and Ba2+ dopants on the microstructure and magnetic properties of doubly-doped LaFeO3 perovskite catalytic nanocrystals

Using citrate sol-gel method, we prepared La_0.85Mg_0.15-xBa_xFeO₃(x?=?0.02–0.12) samples. X-ray powder diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), and vibrating sample magnetometer (VSM) were used to investigate whether the microstructure, composition, and magnetic properties of LaFeO₃ were affected by the following parameters: calcination temperature, calcination time, and the doping concentration of Mg²⁺ ion and Ba²⁺ ions. The XRD spectra showed that a trace of impurity phase (MgFe₂O₄) is visible when the content of Mg²⁺ ions was higher; however, there was no change in the orthorhombic perovskite structure of LaFeO₃ even at higher doping concentrations. The space group was still Pnma. No other phase was generated in the sample subjected to low-temperature calcination. Furthermore, FT-IR spectra confirmed the presence of some functional groups in the sample. Then, SEM showed that the size distribution of the particles is uniform in the sample, and the grain boundary is also clear. Finally, VSM measurements proved that the significant changes were produced in the magnetic properties of samples when they were doubly doped with Mg²⁺ and Ba²⁺ ions. Moreover, calcination temperature has a great influence on the magnetic properties of the samples.     

A pigment based on coprecipitated chromium(III) and copper(II) hydroxides

The results of experiments on coprecipitation of copper and chromium hydroxides are presented. It is established that the final product of coprecipitation is an individual hydroxide compound that transforms when heated into Cu₂Cr₂O₄ and CuCr₂O₄ spinels which preserve their stability up to 1200°C. The pigment can be used for decorating porcelain and ceramic tile.Translated from Steklo i Keramika, No. 7, pp. 21 – 23 , July, 1996.