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.     

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.     

Surface characterization of sulfated zirconia and its catalytic activity for epoxidation reaction of castor oil

The solid acid catalysts SO₄^2?/ZrO₂ were prepared by impregnation technique at different calcination temperatures. The surface characterizations were carried out by using scanning electron microscope (SEM), Fourier transform infrared spectrometer (FTIR), X-ray diffraction (XRD), temperature programed desorption of NH₃ (NH₃-TPD), and N₂-BET. The SEM results showed that the size of the SO₄^2?/ZrO₂ was not uniform and varied from about 1 to 20?µm. The characteristic peaks in FTIR spectra were essentially the same within the calcination temperature range of 400–700?°C. The XRD results indicated that the transition temperature from amorphous to tetragonal phase was up to 500?°C. The strong acid and superacid sites of the samples could be observed by the NH₃-TPD results. The largest BET surface area was 140 m²/g, when the calcination temperature was at 500?°C, and all the pore size distributions belong to mesoporous range. The solid acid SO₄^2?/ZrO₂ was used for the epoxidation of castor oil. When the calcination temperature of SO₄^2?/ZrO₂ was 600?°C, reaction temperature 45?°C, and reaction time 8?h, the reaction effect was better with an iodine value of 33.0?±?1.6?g/100?g and an epoxy value of 2.45?±?0.11?mol/100?g.     

Weakly agglomerated α-Al2O3 nanopowders prepared by a novel spray precipitation method

In this paper, weakly agglomerated and well dispersed α-Al₂O₃ powders were synthesized by a novel spray precipitation method. It was demonstrated that the as-prepared powders exhibited better dispersity than powders from conventional precipitation due to the increased phase contact and reaction area during the precipitation process. The effects of different titration ways, calcination temperature and holding time on the morphology, phase composition and sintering behaviour of Al₂O₃ powders were systematically investigated. Weakly agglomerated and well crystallized α-Al₂O₃ powders were obtained when the as-prepared precursors were calcined at 1150?°C for 2?h in air. The average particle size of α-Al₂O₃ powders with higher sintering activity was approximately 68.6?nm, and the specific surface area was above 22.4?m² g^?1.     

Aqueous synthesis of high surface area metal oxides

By applying high throughput synthesis and characterization technologies, we have been optimizing common dry or aqueous synthetic routes for the preparation of high surface area metals and oxides, such as precipitation and modified Pechini methods. For wet combustion synthesis, we have been screening a variety of organic acids as dispersants and developed proprietary recipes for individual metals. By resorting to easily decomposable organic acids (as opposed to citric acid in the original Pechini combustion method), such as glyoxylic acid, oxalacetic acid and ketoglutaric acid, it is possible to obtain high surface area materials for many metals after careful optimization of acid/metal ratio and calcination conditions. Examples are Sn, In, Co, Ru, Ni, Fe, Mn, Y, Ce and Rare Earth oxides and their mixtures. After calcination in the temperature range of about 300–400 °C, surface areas >150 m²/g could be obtained for Er, Tm, Co, Ru, and Nb; >200 m²/g for Sn, Fe, Mn, and Y; >300 m²/g for Ce; and >400 m²/g for Ni oxide. Noteworthy are also >140 m²/g for La₂O₃, >80 m²/g for CuO, and 75 m²/g for ZnO. For V, around 40 m²/g was possible for the nearly carbon-free V₂O₅, whereas up to 90 m²/g was obtained for a 90% V–10% carbon composite (by incomplete combustion of the organic acid). Residual carbon helps in stabilizing the porous oxide against sintering. Thus, conventional aqueous routes (precipitation, Pechini) can be competitive to more elaborate and costly methods such as those using organic solvents, sol–gel, supercritical drying or template/hydrothermal synthesis. Combustion synthesis is well suited for the preparation of mixed oxides from mixed metal solutions in aqueous organic acids. Bulk porous Co and CoRu mixed oxides have been screened for liquid phase alcohol oxidations and CoRuCe oxides for CO oxidation and VOC destruction, and doped NiO has been reduced to the metal and tested for various hydrogenations.     

Preparation of monolithic silica-based aerogels with high thermal stability by ambient pressure drying

A monolithic silica-based aerogels were successfully synthesized via a sol-gel process using tetraethoxysilane and inexpensive inorganic aluminium salt Al(H₂O)₉(NO₃)₃ as precursors. The gels were dried at ambient pressure and the molar ratio of Al/Si was varied from 0 to 0.1. The structure and morphology of the aerogels were investigated by field-emission scanning electron microscopy, high-resolution transmission electron microscopy, Fourier transform infrared spectrometry, and N₂ adsorption-desorption. The microstructural images showed that a three-dimensional nanoscale structure formed in the as-prepared aerogels. The specific surface areas of the samples were in the range of 769–821?m²/g. Furthermore, when the Al/Si mole ratio was 0.1, its specific surface area and total pore volume remained at 103.5?m²/g and 0.25?cm³/g respectively after heating at 1100?°C for 2?h, exhibiting the best thermal stability of all aerogels fabricated in this study. The addition of the aluminium salt not only slowed the sintering of the silica, but also crystallization was restrained.     

Preparation of KNbO3 powders by sol-gel method using water-soluble potassium and niobium compounds as precursors

In the paper the full sol-gel process of KNbO₃ powder preparation, using KOH, Nb₂O₅ and K₂CO₃ as initial substrates as well as oxalic and citric acid as a chelating agent is presented. It was also confirmed that a mixture of compounds obtained during calcination of KOH and Nb₂O₅, mixed in adequate proportions, could be source of Nb ions in the sol-gel process. The crystal and electronic structure parameters of the manufactured powder proper for KNbO₃ compound were established using XRD, XPS and UV-VIS techniques.     

Synthesis and characterization of mesoporous anatase TiO2 nanostructures via organic acid precursor process for dye-sensitized solar cells applications

Titania (TiO₂) nanoparticles have been synthesized using organic precursor technique. The titania nanoparticles were characterized. The results indicated that the prepared titanium oxalate and citrate precursors were transformed to anatase TiO₂ phase at temperature 400 °C for 2 h. Dye-sensitized solar cells were assembled using the prepared nanocrystalline TiO₂ with large surface area. The specific surface area S_BET was 80.9 and 78.6 m²/g using oxalic and citric acids, respectively. The power efficiency was 3.5 and 2.4%. A brief discussion on the possible reasons behind the low power conversion efficiency observed for these type of solar cells was reported.     

Effects of chemical modification of petroleum cokes on the properties of the resulting activated carbon

Activated carbons have been prepared from petroleum cokes by the combination of a chemical treatment with HClO₄ or H₂O₂ and a chemical activation with KOH at a constant KOH/coke ratio of 3/1. The influence of different chemical treatments on the properties of the activated carbon precursors and final carbons activated with KOH was invested by using XRD, FTIR, and BET techniques. XRD results indicated that the value of interplanar distance d₀₀₂ increased by chemical treatment and the disappearance of the peak corresponding to 0 0 2 faces correlated to high specific surface area. FTIR studies showed that chemical modification promoted the formation of surface oxygen functionalities. Significant effects on BET surface area, pore texture and iodine adsorption capacity were evidenced. The results show that chemical modification prior to activation dramatically increased the BET surface area and total pore volume of the resulting activated carbon. Modified petroleum coke based activated carbon with chemical activation had higher specific surface area (2336 m²/g) and better iodine adsorption value (1998 mg/g).     

Mesoporous semiconducting oxides for gas sensor application

A fabrication procedure of thermally stable mesoporous SnO₂ and TiO₂ powders has been overviewed along with their gas-sensing properties. Treatment of an as-prepared composite material of a supramolecule surfactant and SnO₂, i.e. a self-assembly of the surfactant fringed with a SnO₂ thin wall, with phosphoric acid enabled us to fabricate thermally stable ordered mesoporous SnO₂ powder having a d₁₀₀ value of 3.2 nm, a crystallite size of 2.0 nm and a large specific surface area of 305 m² g⁻¹ even after calcination at 600 °C for 5 h. A thick film sensor fabricated with the ordered mesoporous SnO₂ powder exhibited higher sensing performance than that fabricated with SnO₂ powder prepared by a conventional method and therefore having a lower specific surface area. Surface modification of the conventional SnO₂ powder with a mesoporous SnO₂ layer was also found to be effective for improving the sensing properties. Mesoporous TiO₂ powder could be prepared by employing a modified sol-gel method with Ti(NO₃)₄ and polyethylene glycol having different molecular weights. Higher sensitivity was achieved with a disc-type sensor fabricated with mesoporous TiO₂ powder, in comparison with one fabricated with commercially available TiO₂ powder in the same form, but its sensing properties needed to be further modified.     

Template‐Free Synthesis of Hollow Porous Strontium Titanate Particles

Strontium titanate (SrTiO₃) is a potential photocatalyst of H₂ evolution for providing green energy. One of the main strategies for increasing the efficiency of H₂ evolution is to increase the surface area of photocatalysts. In this study, spray pyrolysis was used because of its superior flexibility in morphological control. Three typical reactants, nitric acid, acetic acid, and citric acid, were added to Sr and Ti precursors to form various morphological SrTiO₃ powders. The SrTiO₃ powders were investigated by X‐ray diffraction, scanning electron microscopy, transmission electron microscopy, focused ion beam technique, and the Brunauer–Emmett–Teller (BET) method to determine the phase composition, surface morphology, geometry, inner structure, and specific surface area, respectively. Four main morphologies of SrTiO₃ particles, namely, porous, concave porous, wrinkled spherical, and hollow porous structures, were obtained. Among these powders, SrTiO₃ particles prepared with citric acid had the highest surface area (46.9 m²/g) because of their hollow porous structure. Finally, the formation mechanisms of these four distinct SrTiO₃ morphologies are discussed.     

Synthesis,characterization, and optimization of α‐Fe2O3/silica nanocomposites using homogenous precipitation method

The purpose of this study was to systematically synthesize and characterize the high surface area 10 wt% nanocomposites of α‐Fe₂O₃ (hematite)/silica using a simple and economically effective homogenous precipitation (HP) route via Response Surface Method combined with Central Composite Design (CCD). Accordingly, the RSM‐CCD approach including 20 experiments was designed to investigate the effects of three factors including concentration of iron chloride solution, pH and calcinations temperature on the final surface area of α‐Fe₂O₃/silica nanocomposites. The optimum surface area was 373 m²/g at the condition including iron chloride concentration of 0.018 mol/L, pH=8.95, and calcination temperature of 573°C.     

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.     

Activation of coal tar pitch carbon fibres: Physical activation vs. chemical activation

Activated carbon fibres (ACF) are obtained mainly by physical activation with steam or carbon dioxide. Additionally, there are many papers dealing with chemical activation of carbon fibres, or a polymeric raw material, with several chemical agents like for example, phosphoric acid, zinc chloride, aluminium chloride,… Nevertheless, although it is well known that hydroxides are good activating agents, there are few papers about the activation of carbon fibres with KOH or NaOH. In the present work, ACF with high surface area are obtained by chemical activation with KOH and NaOH. Both chemical agents present different behaviour; thus, NaOH developed the highest value of porosity and KOH developed samples with narrower micropore size distribution. In order to compare the results with those obtained by physical activation, some ACF have been prepared using CO₂ activation. The main conclusion of this work is that by using chemical activation it is possible to obtain similar, or even higher, porosity (∼1 ml/g, ∼3000 m²/g) than by physical activation. However, chemical activation presents two important advantages: (1) a much higher yield (27-47% for chemical activation and 6% physical activation for ∼2500 m²/g activated carbon fibres) and (2) the surface of the fibres prepared by chemical activation is less damaged than by physical activation.     

Silica-coated nanocrystalline TiO2 with improved thermal stability

Deposition of a SiO₂ coating on anatase TiO₂ nanocrystals is shown to improve their thermal stability. As low as 0.5% Si was shown to preserve the small anatase crystallite size after calcination at 600?°C. Such treatment led to considerable sintering of TiO₂ nanocrystals without the silica with the average particle size growth from 9 to 50?nm, surface area decrease from 135 to 22?m²/g and partial anatase conversion to rutile. The phase composition, crystallite size, and surface area of 5%Si-TiO₂ samples were largely preserved till the temperatures as high as 800?°C whereas the anatase phase was mostly stably even after calcination at 1000?°C. The phase transformation from anatase to rutile in xerogel TiO₂ and TiO₂@SiO₂ samples apparently did not occur until the crystallites grew larger than the critical size about 50?nm. Electron-acceptor sites capable of ionizing perylene to its radical cations were observed on all samples with anatase crystalline structure. So, the silica shell deposition improves the TiO₂ thermal stability without limiting access to the surface active sites.     

Surface area variations of coal during solvent extraction

Changes in the surface area of a Wyoming sub-bituminous coal with progressive extraction have been investigated. Surface areas were determined from CO₂ adsorption isotherms at 196 K using the BET equation with 0.234 nm² for the molecular cross-sectional area of the CO₂ molecule at 196 K. Surface areas of the extracted coal varied with extraction time, yield and with the nature of the solvent. A maximum surface area of 265 m²/g was obtained from a four-hour treatment using tetralin at 350 °C as the solvent. The raw coal had a surface area of 99 m²/g.     

Synthesis of nano-sized indium oxide (In2O3) powder by a polymer solution route

Indium oxide (In₂O₃) is a n-type semiconductor with various applications in thin film coatings, on the basis of its optical properties, and in gas sensing equipment, due to its high sensitivity to various oxides such as CO_x and NO_x. In this study, a synthesis process for obtaining In₂O₃ nanoparticles is examined. The precursor used is indium nitrate hydrate (InN₃O₉·H₂O) because of its high solubility in water. By dissolving the nitrate salt in a PVA (polyvinyl alcohol) solution, the precursor is dispersed homogeneously, which reduces the agglomeration of the resulting powder. Calcination at a low temperature of 200–250 °C burns out the organic materials of the PVA with NO_x gas emission and allows the oxidation of the indium, resulting in indium oxide nanoparticles. The influence of the PVA solution characteristics and the heat treatment temperature on the powder morphology and size was analyzed by using SEM, TEM, XRD, TGA/DSC, and four point BET for a specific surface area analysis. The measured specific surface area varies from 3 m²/g to 76 m²/g depending on the calcination temperature, and the particle size of the synthesized powders is under 10 nm for the samples heat treated at 300 °C.     

Porous structure and adsorptive properties of pineapple peel based activated carbons prepared via microwave assisted KOH and K2CO3 activation

The present research explores the feasibility of microwave irradiation for preparation of high surface area activated carbon from pineapple peel (PPAC), an agricultural effluent emitted from the food can processing industries via KOH and K₂CO₃ activation. The activation process was performed at the microwave power of 600 W and irradiation time of 6 min. The equilibrium behavior of PPAC was investigated by performing batch adsorption experiments using methylene blue as adsorbate. Nonlinear adsorption isotherm models, Langmuir, Freundlich and Temkin were used to simulate the equilibrium data. KOH activated sample demonstrated a better development of pore structure, with the BET surface area, total pore volume and average pore size of 1006 m²/g, 0.59 m³/g and 23.44 Å, respectively, while the monolayer adsorption capacity of methylene blue was determined to be 462.10 mg/g. The findings support the potential use of microwave assisted KOH and K₂CO₃ activation as a promising activation technique.     

High-temperature resistant ambient pressure-dried aluminum doped silica aerogel from inorganic silicon and aluminum sources

Aluminum doped silica aerogel (ASA) exhibiting improved high-temperature resistance is usually prepared via supercritical drying from organic silicon and/or aluminum precursors, which propels the production cost significantly. Herein we demonstrate a simple and effective method to prepare highly thermal resistant ASA via the sol-gel and ambient pressure drying route by using water glass and aluminum chloride as precursors. Effects of the Al/Si molar ratio in precursor, the calcination temperature and the modifier type on the crystallinity, morphology, pore structure of ASA are investigated. Results show that the Al/Si molar ratio and the calcination temperature have significant effects on the structure and heat resistance performance of ASA at temperature of 600–1000 °C. The sample with Al/Si molar ratio of 0.15 shows the highest specific surface area of 805.9 m²/g and pore volume of 5.038 cm³/g after heated to 600 °C, and retains 179.5 m²/g and 1.295 cm³/g respectively after heated to 1000 °C. Mechanism analysis indicates that, though the actual aluminum content is extremely low (0.18%, wt%), the high-temperature resistance of ASA is greatly improved owing to the effective doping of aluminum in the lattice of SiO₂ and the corresponding electrostatic repulsion between neighboring nanoparticles induced by the replacement of Si⁴⁺ by Al³⁺ ions.     

Changes in pore properties of phenol formaldehyde-based carbon with carbonization and oxidation conditions

Porous carbon beads were prepared by carbonizing at 700 and 1000 °C under N₂ (NN-series) or CO₂ atmospheres (CO-series) and a subsequent oxidization with boiling nitric acid solution (13%, v/v) for 3 h (ANN- and ACO-series). BET surface area of both CO- and NN-series samples tends to increase with increasing carbonization temperature, but CO-series samples show higher BET surface areas (700 °C: 610 m²/g, 1000 °C: 780 m²/g) compared with those of NN-series samples (700 °C: 380 m²/g, 1000 °C: 580 m²/g). After acid oxidation, BET surface areas of NN-series samples increased from 580 to 650 m²/g, whereas those of CO-series samples decreased from 780 to 600 m²/g. On subsequent acid oxidation, acidic surface functional groups increased in both CO- and NN-series samples, but the CO-series samples tend to have much more acidic surface functional groups. Adsorption of halogenated methanes, such as tetra-, tri- and dichloromethanes, onto the samples of CO-, NN-, ACO- and ANN-series was thought to be driven initially by the basicity, but the acidic functional groups that could attract adsorbates via dipole-dipole interaction might hinder the adsorption of subsequently incoming adsorbates.