High surface area sol–gel alumina–titania nanocatalyst

Alumina–titania mixed oxide nanocatalysts with molar ratios = 1:0.5, 1:1, 1:2, 1:5 have been synthesized by adopting a hybrid sol–gel route using boehmite sol as the precursor for alumina and titanium isopropoxide as the precursor for titania. The thermal properties, XRD phase analysis, specific surface area, adsorption isotherms and pore size details along with temperature programmed desorption of ammonia are presented. A specific surface area as high as 291 m²/g is observed for 1:5 Al₂O₃/TiO₂ composition calcined at 400 °C, but the same composition when calcined at 1,000 °C, resulted in a surface area of 4 m²/g, while 1:0.5 composition shows a specific surface area of 41 m²/g at 1,000 °C. Temperature programmed desorption (of ammonia) results show more acidic nature for the titania rich mixed oxide compositions. Transmission electron microscopy of low and high titania content samples calcined at 400 °C, shows homogeneous distribution of phases in the nano range. In the mixed oxide, the particle size ranges between 10–20 nm depending on titania content. The detailed porosity data analysis contributes very much in designing alumina–titania mixed oxide nanocatalysts.     

Thermal stabilities of hexagonal and orthorhombic YbFeO3 synthesized by solvothermal method and their catalytic activities for methane combustion

Thermal stabilities of hexagonal and orthorhombic YbFeO₃ samples synthesized by the solvothermal method were investigated. The morphology and BET surface area of hexagonal YbFeO₃ did not change by calcination, while orthorhombic YbFeO₃ easily sintered, resulting in a decrease of the BET surface area. The hexagonal YbFeO₃ sample, which had a high surface area (29 m²/g) after calcination at 800 °C, had higher catalytic activity for methane combustion than the orthorhombic YbFeO₃ samples calcined at 800 °C.     

Synthesis of nitrogen-containing carbon materials for solid polymer fuel cell cathodes

The following nitrogen-containing supports with various nitrogen contents and structure and texture properties were synthesized: carbon nanofibers (N-CNFs) and amorphous microporous carbon materials (N-AMCMs). It was found that the above characteristics can be regulated by varying synthesis conditions: precursor compositions and reaction temperature and time. Mesoporous nitrogen-containing CNFs with a specific surface area of 30–350 m²/g and a pore volume of 0.10–0.83 cm³/g were formed by the catalytic decomposition of a mixture of ethylene with ammonia at 450–675°C. Microporous materials (N-AMCMs) with a specific surface area of 472–3436 m²/g and a micropore volume of 0.22–1.88 cm³/g were prepared by the carbonization of nitrogen-containing organic compounds at 700–900°C. An increase in the carbonization temperature and reaction time resulted in an increase in the specific surface area and microporosity of N-AMCMs, whereas lower temperatures of 450–550°C and reaction times of 1–3 h were optimal for the preparation of N-CNFs with a developed texture. It was found that milder synthesis conditions and higher nitrogen contents of precursors were required for obtaining high nitrogen concentrations in both N-CNFs and N-AMCMs. The synthetic method developed allowed us to prepare carbon supports with nitrogen contents to 8 wt %.     

Preparation and characterization of alumina-zirconia composite material with different acid ratios by the sol-gel method

Alumina-zirconia composite materials were produced with different acid ratios by the sol-gel method using aluminum isopropoxide and zirconium chloride. The composites were produced by changing acid/alkoxside ratio in alumina. The composite materials were calcinated at 600°C, 900°C and 1300°C. The effects of acid concentration and calcination temperature on the surface area and pore radius were determined from the nitrogen adsorption isotherm at 77 K. The density of the composites was also measured. The minimum density of produced material was recorded as 1.35 g cm⁻³ at an acid/alkoxside ratio of 0.2. The highest specific surface area and pore diameter of the lightest material are 191.86 m² g⁻¹ and 18.4 Ǻ, respectively. Although pore diameter and specific surface area are not changed at any of the experimental temperatures which were tested by decreasing acid/alkoxside ratio, the density is slightly increased. However, it was observed that the calcination temperature significantly affects the surface area and density of the material.      

Thermal study in Eu3+-doped boehmite nanofibers and luminescence properties of the corresponding Eu3+:Al2O3

Eu³⁺-doped boehmite nanofiber materials with different Eu³⁺ concentrations were synthesized without any surfactant, and followed by a series of characterizations. It was found that the boehmite nanofibers became coarser with the increase of Eu³⁺ concentration, which resulted in a gradual decrease of their specific surface areas. Moreover, the thermal stability of the boehmite nanofibers was studied by thermogravimetry–differential scanning calorimetry. All materials showed the phase transition from γ-Al₂O₃ to other forms. Yet the transition temperature was increased with the increase of Eu³⁺ concentration. The Eu³⁺-doped boehmite nanofibers with the maximum Eu³⁺ concentrations showed the best thermal stability. Photoluminescence spectra showed that the 2 mol% of doping concentration of Eu³⁺ ions in Eu³⁺:Al₂O₃ nanofiber was optimum.     

Synthesis and characterization of sol–gel alumina nanofibers

Abstract Alumina nanofibers of high aspect ratio with surface area of >300 m² g⁻¹ has been prepared successfully in bulk quantities by the sol–gel method. The synthesis parameters including the binary water–alcohol solvent system to aluminium isopropoxide ratio, pH, type of solvent and aging temperature affect the uniformity and formation of nanofibers. It is proposed that alumina nanofibers were formed by the curling of the nanosheets upon condensation after the hydrolysis. The phase evolution of alumina nanofibers from pseudoboehmite to α phase has been shown by XRD and FTIR. ²⁷Al NMR investigations show that the Al atoms are six and four coordinated. The morphology of the alumina nanofibers does not change much as the calcination temperature was increased. In addition, the average pore size increases and the BET surface area decreases as a function of calcination temperature. The thermal behavior of alumina nanofibers was investigated by TGA. Graphical Abstract      

Influence of aluminium precursor on physico-chemical properties of aluminium hydroxides and oxides Part II. Al(ClO4)3·9H2O

Aluminium hydroxide was precipitated during a hydrolysis of aluminium perchlorate in ammonia medium. The materials were studied with the following methods: thermal analysis, IR spectroscopy, X-ray diffraction, low-temperature nitrogen adsorption and adsorption–desorption of benzene vapours. Freshly precipitated boehmite had a high value of S_BET=211 m² g^–1 determined from nitrogen adsorption, good sorption capacity for benzene vapours, developed mesoporous structure and hydrophobic character. After prolonged refluxing at elevated temperature its crystallinity increased which was accompanied by an increase of specific surface determined from nitrogen adsorption up to 262m²g^–1 , decrease of sorption capacity for benzene vapours and stronger hydrophobic character. The calcinations of all boehmites at temperature up to 1200°C resulted in formation of à-Al₂O₃ via transition form of γ-, δ- and θ-Al₂O₃. The samples of aluminium oxides obtained after calcination at 550 and 900°C were characterised with high values of specific surface area of 205–220 and 138–153 m² g^–1 , respectively. The SBET values calculated for the oxide samples derived from aged hydroxides and calcined at 1200°C are higher than for the analogous sample prepared without the ageing step. It was concluded that the process of ageing at elevated temperature developed thermal stability of aluminium oxides.     

Anionic surfactant‐aided preparation of high surface area and high thermal stability ceria/zirconia‐mixed oxide from cerium and zirconium glycolates via sol–gel process and its reduction property

This work is focused on the ceria zirconia mixed oxide prepared through a surfactant‐introduced synthesis method. High surface area nanoparticle mesoporous ceria/zirconia‐mixed oxide was successfully synthesized and characterized using various techniques. High surface area mesoporous fluorite‐structured CeO₂? ZrO₂ was obtained from the elimination of surfactants upon calcination. A surface area in excess of 205.6 m²/g was obtained after calcination at 500 °C, and dropped to 75.96 m²/g by heating at 900 °C. Temperature‐programming reduction (TPR) results showed that the lowest reduction temperature was obtained from the sample containing 40% zirconia content. Copyright © 2008 John Wiley & Sons, Ltd.     

Formation and characterization of magnetic barium ferrite hollow fibers with high specific surface area via sol–gel process

The magnetic barium ferrite (BaFe₁₂O₁₉) hollow fibers with a high specific surface area about 22–38 m² g^?1, diameters around 1 μm and a ratio of the hollow diameter to the fiber diameter estimated about 1/2–2/3 have been prepared by the gel-precursor transformation process. The precursor and resulting ferrite hollow fibers were analyzed by thermo-gravimetric and differential scanning calorimetry, infrared spectroscopy, scanning electron microscopy and X-ray diffraction. The specific surface area was measured by the Brunauer–Emmett–Teller method. The gel formed at pH 5.5 has a good spinnability. A pure barium ferrite phase is formed after calcined at 750 °C for 2 h and fabricated of nanograins about 38 nm with a hexagonal plate-like morphology, which are increased to about 72 nm with the calcination temperature increased up to 1050 °C. The barium ferrite hollow fibers obtained at 750 °C for 2 h have a specific surface area 38.1 m² g^?1 and average pore size 6.5 nm and then the specific surface area and average pore size show a reduction tendency with the calcination temperature increasing from 750 to 1050 °C owing to the particle growth and fiber densification. These barium ferrite hollow fibers exhibit typical hard-magnetic materials characteristics and the formation mechanism for hollow structures is discussed.     

焙烧温度对Ni/CaO-Al2O3结构及其催化重整性能的影响

采用共沉淀法制备了一系列具有类水滑石结构前驱体的Ni/CaO-Al₂O₃复合催化剂，考察了制备过程中焙烧温度对复合催化剂结构及性能的影响。结果表明，焙烧温度可调控活性组分Ni与载体之间的相互作用力，进而调变复合催化剂的比表面积、活性组分Ni的颗粒粒径。当焙烧温度为700 ℃时，Ni与载体之间相互作用力适宜，复合催化剂具有最大的比表面积（21.42 m²/g）和最小的Ni颗粒粒径（19.51 nm）；该复合催化剂在CO₂吸附强化CH₄/H₂O重整制氢过程中可得到98.31%的H₂浓度和94.87%的CH₄转化率，循环10次后，H₂浓度仍能保持在97.35%以上。这是因为大的比表面积为反应提供了更多的活性位点，利于CO₂吸附过程的强化，而小的Ni颗粒粒径提高了复合催化剂的抗烧结能力。     

Thermal evolution of mesoporous titania prepared by CO2 supercritical extraction

Porous anatase is attractive because of its notable photo-electronic properties. Titania wet gel prepared by hydrolysis of Ti-alkoxide was immersed in the flow of supercritical CO₂ at 60°C and the solvent was extracted (aerogel). Mesoporous TiO₂ consisting of anatase nano-particles, about 5 nm in diameter, have been obtained. Thermal evolution of the microstructure of the aerogel was evaluated by TGA-DTA, N₂ adsorption, TEM and XRD, and discussed in comparison with that of the corresponding xerogel. The diffraction peaks of anatase were found for the as-extracted gel while the xerogel dried at 90°C was amorphous. After calcination at 600°C, the average pore size of the aerogel, about 20 nm in diameter, was 4 times larger than that of the xerogel, and the pore volume, about 0.35 cm³ g⁻¹, and the specific surface area, about 60 m² g⁻¹, were 2 times larger than those of the xerogel. XRD peaks of rutile have been found after calcination at 600°C. The particle sizes of anatase and rutile are about 13 and 25 nm in diameter, respectively. The surface morphology of TiO₂ nano-particles has been discussed in terms of their surface fractal dimensions estimated from the N₂ gas adsorption isotherms.     

Influence of aluminium precursor on physico-chemical properties of aluminium hydroxides and oxides

The paper concerns aluminium hydroxides precipitated during hydrolysis of aluminium acetate in ammonia medium, as well as aluminium oxides obtained through their calcination at 550, 900 or 1200°C for 2 h. The following techniques were used for analysing of obtained materials: thermal analysis, IR spectroscopy, X-ray diffraction, low-temperature nitrogen adsorption, adsorption-desorption of benzene vapours and scanning electron microscopy. Freshly precipitated boehmite/pseudoboehmite had high value of S _BET, very good sorption capacity for benzene vapours, developed mesoporous structure and hydrophilic character. After prolonged refluxing at elevated temperature its crystallinity increased which was accompanied by a decrease of specific surface determined from nitrogen adsorption, decrease of sorption capacity for benzene vapours and weakening of the hydrophilic character. Calcination of all hydroxides at the temperature up to 1200°C resulted in the formation of α-Al₂O₃ via transition forms of γ-, δ-and θ-Al₂O₃. The samples of aluminium oxides obtained after calcination at 550 and 900°C were characterised with high values of specific surface area and displayed quite high heat resistance, probably due to a specific morphology of starting hydroxides. The process of ageing at elevated temperature developed thermal stability of aluminium oxides.     

Mesoporous MgO: Synthesis,physico-chemical,and catalytic properties

Mesoporous MgO was obtained via the hydrothermal synthesis using both ionogenic and non-ionogenic surfactants as structure-directing templates. The materials prepared were characterized by SEM, BET-N₂, XRD, and TG-DTA techniques. MgO particles are spherical 20-μm aggregates of primary oxide particles well shaped as rectangular parallelepipeds. Magnesium oxide samples have the specific surface area of 290–400 m²/g and pore sizes of 3.3–4.1 nm. Their mesoporous structure remained unchanged after calcination up to 350°C. Catalytic activity of mesoporous MgO was studied in acetone condensation reaction.     

Synthesis of nanosize europium oxide particles by using W/O microemulsion

Europium hydroxide particles with an average diameter of 10 run and a BET surface area of 127 m2 /g have been prepared by controlled precipitation in the polyoxyethylene octylphenol (Triton X-100) (hex-anol)/cyclohexane/water microemulsion system. After calcination in air at 750℃, the obtained europium hydroxide particles were readily converted to the nanosize Eu2O3 particles with an average size of 30 nm and a high BET surface area of 36.5 m2/g.     

Structural evolution and magnetic properties of SrFe<Subscript>12</Subscript>O<Subscript>19</Subscript> nanofibers by electrospinning

The SrFe₁₂O₁₉/poly (vinyl pyrrolidone) (PVP) composite fiber precursors were prepared by the sol-gel assisted electrospinning with ferric nitrate, strontium nitrate and PVP as starting reagents. Subsequently, the M-type strontium ferrite (SrFe₁₂O₁₉) nanofibers were derived from calcination of these precursors at 750–1,000 °C.The composite precursors and strontium ferrite nanofibers were characterized by Fourier transform infrared spectroscopy, X-ray diffraction, scanning electron microscopy and vibrating sample magnetometer. The structural evolution process of strontium ferrite consists of the thermal decomposition and M-type strontium ferrite formation. After calcined at 750 °C for 2 h the single M-type strontium ferrite phase is formed by reactions of iron oxide and strontium oxide produced during the precursor decomposition process. The nanofiber morphology, diameter, crystallite size and grain morphology are mainly influenced by the calcination temperature and holding time. The SrFe₁₂O₁₉ nanofibers characterized with diameters of around 100 nm and a necklace-like structure obtained at 900 °C for 2 h, which is fabricated by nanosized particles about 60 nm with the plate-like morphology elongated in the preferred direction perpendicular to the c-axis, show the optimized magnetic property with saturation magnetization 59 A m² kg⁻¹ and coercivity 521 kA m⁻¹. It is found that the single domain critical size for these M-type strontium ferrite nanofibers is around 60 nm.     

Effect of heat treatment on particle growth and magnetic properties of electrospun Sr<Subscript>0.8</Subscript>La<Subscript>0.2</Subscript>Zn<Subscript>0.2</Subscript>Fe<Subscript>11.8</Subscript>O<Subscript>19</Subscript> nanofibers

Sr_0.8La_0.2Zn_0.2Fe_11.8O₁₉/poly(vinyl pyrrolidone) (PVP) composite fiber precursors were prepared by the sol–gel assisted electrospinning. Subsequently, the M-type ferrite Sr_0.8La_0.2Zn_0.2Fe_11.8O₁₉ nanofibers with diameters about 120 nm were obtained by calcination of these precursors at different heat treatment conditions. The precursor and resultant Sr_0.8La_0.2Zn_0.2Fe_11.8O₁₉ nanofibers were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, energy-dispersive X-ray spectrometer and vibrating sample magnetometer. With the calcination temperature increased up to 1,000 °C for 2 h or the holding time prolonged to 12 h at 900 °C, the Sr_0.8La_0.2Zn_0.2Fe_11.8O₁₉ particles gradually grow into a hexagonal elongated plate-like morphology due to the dimensional control along the nanofiber length. These elongated plate-like particles will be linked one by one to form the nanofiber with a necklace-like morphology. The magnetic properties of the Sr_0.8La_0.2Zn_0.2Fe_11.8O₁₉ nanofibers are closely related to grain sizes, impurities and defects in the ferrite, which are influenced by the calcination temperature, holding time and heating rate. After calcined at 900 °C for 12 h with a heating rate of 3 °C/min, the optimized magnetic properties are achieved with the specific saturation magnetization 75.0 A m² kg⁻¹ and coercivity 426.3 kA m⁻¹ for the Sr_0.8La_0.2Zn_0.2Fe_11.8O₁₉ nanofibers.     

Synthesis of ultrafine yttrium aluminum garnet using sol-gel technology

Mesoporous yttrium aluminum garnet Y₃Al₅O₁₂ powders were prepared using sol-gel technology proceeding from solutions of metal alkoxoacetylacetonates. Xerogel microstructure was studied by SEM, and the fact of mesopores being formed was established. The temperature range within which Y₃Al₅O₁₂ crystallizes in a dynamic mode from the xerogel was determined to be 850?C950°C using an SDT Q600 TGA/DTA/DSC analyzer. A 1-h isothermal treatment of the xerogel was shown to reduce the garnet phase formation temperature to 800°C. At lower temperatures (400, 450 or 500°C), even long-term (6-h) calcination yielded X-ray amorphous powders with developed surfaces (specific surface areas were 230?C350 m²/g). Powder particle coarsening was studied upon sintering for 2 and 4 h at 1000, 1200, and 1400°C.     

Aerosol-assisted synthesis of mesoporous titania nanoparticles with high surface area and controllable phase composition

Mesoporous titania nanoparticles (denoted as MTN) with high surface area (e.g., 252 m² g⁻¹) were prepared using tetrapropyl orthotitanate (TPOT) as a titania precursor and 10–20 nm or 20–30 nm silica colloids as templates. Co-assembly of TPOT and silica colloids in an aerosol-assisted process and immediate calcination at 450 °C resulted in anatase/silica composite nanoparticles. Subsequent removal of the silica colloids from the composite by NaOH solution created mesopores in the TiO₂ nanoparticles with pore size corresponding to that of silica colloids. Effects of silica colloids’ contents on MTN porosity and crystallites’ growth at a higher calcination temperature (e.g., 1000 °C) were investigated. Silica colloids suppressed the growth of TiO₂ crystallites during calcination at a higher calcination temperature and controllable contents of the silica colloids in precursor solution resulted in various atomic ratios of anatase to rutile in the calcinated materials. The mesostructure and crystalline structure of these titania materials were characterized by transmission electron microscope (TEM), scanning electron microscope (SEM), X-ray diffraction (XRD), differential thermal analysis (DTA)-thermo-gravimetric analysis (TGA), and N₂ sorption.     

Mesoporous Carbon Nanofibers Embedded with MoS2 Nanocrystals for Extraordinary Li‐Ion Storage

MoS₂ nanocrystals embedded in mesoporous carbon nanofibers are synthesized through an electrospinning process followed by calcination. The resultant nanofibers are 100–150 nm in diameter and constructed from MoS₂ nanocrystals with a lateral diameter of around 7 nm with specific surface areas of 135.9 m² g^?1. The MoS₂@C nanofibers are treated at 450 °C in H₂ and comparison samples annealed at 800 °C in N₂. The heat treatments are designed to achieve good crystallinity and desired mesoporous microstructure, resulting in enhanced electrochemical performance. The small amount of oxygen in the nanofibers annealed in H₂ contributes to obtaining a lower internal resistance, and thus, improving the conductivity. The results show that the nanofibers obtained at 450 °C in H₂ deliver an extraordinary capacity of 1022 mA h g^?1 and improved cyclic stability, with only 2.3 % capacity loss after 165 cycles at a current density of 100 mA g^?1, as well as an outstanding rate capability. The greatly improved kinetics and cycling stability of the mesoporous MoS₂@C nanofibers can be attributed to the crosslinked conductive carbon nanofibers, the large specific surface area, the good crystallinity of MoS₂, and the robust mesoporous microstructure. The resulting nanofiber electrodes, with short mass‐ and charge‐transport pathways, improved electrical conductivity, and large contact area exposed to electrolyte, permitting fast diffusional flux of Li ions, explains the improved kinetics of the interfacial charge‐transfer reaction and the diffusivity of the MoS₂@C mesoporous nanofibers. It is believed that the integration of MoS₂ nanocrystals and mesoporous carbon nanofibers may have a synergistic effect, giving a promising anode, and widening the applicability range into high performance and mass production in the Li‐ion battery market.     

Characteristics of granular boehmite and its ability to adsorb phosphate from aqueous solution

In this study, we investigated the surface properties of granulated boehmite with vinyl acetate (G-BE20) and measured the amount of phosphate it adsorbed and the effect of contact time and solution pH on the adsorption process. The specific surface area (144.9?m²/g) and the number of surface hydroxyl groups (0.88?mmol/g) of G-BE20 were smaller than those of virgin boehmite (BE), which gave a specific surface area and number of surface hydroxyl groups of 297.0?m²/g and 1.08?mmol/g, respectively. The amount of phosphate adsorbed increased with the temperature. The isotherm model of Langmuir was used to fit experimental adsorption equilibrium data for phosphate adsorption onto G-BE20. The calculated thermodynamic parameters show the spontaneous and endothermic nature of the adsorption process. The equilibrium adsorption onto G-BE20 was reached within 16?h and the amount of phosphate adsorbed was 8.4?mg/g. The kinetic mechanism of phosphate uptake was evaluated with two different models: the Largergren pseudo first- and pseudo second-order models. The data obtained showed a better fit to the pseudo second-order model (0.991) than to the pseudo first-order model (0.967), as indicated by the r values. The rate constants for the adsorption of phosphate onto G-BE20 were calculated as 0.481?1/h and 0.029?g/mg?h. The adsorption of phosphate onto G-BE20 was the maximum in the pH range 3.0-4.0.