Synthesis and characterization of nanocrystalline zinc aluminate spinel powder by sol–gel method

Single-phase nanocrystalline zinc aluminate (ZnAl₂O₄) spinel powder has been synthesized by the sol–gel method. Zinc aluminate nanoparticles were formed at 600 °C, which is at much lower temperature than by solid state reactions. Formation of ZnAl₂O₄ and their particle size depend on the calcination temperature. Calcination temperature also affects the specific surface area and pore volume. The nanocrystalline zinc aluminate was characterized by powder X-ray diffraction, FT-IR spectroscopy, thermal gravimetric analysis, diffuse reflectance spectroscopy, surface area measurements, field emission scanning electron microscopy coupled with energy dispersive X-ray analysis and transmission electron microscopy. Catalytic reactivity of nanocrystalline zinc aluminate was tested for the reduction of 4-nitrophenol to 4-aminophenol using NaBH₄.     

Preparation of nanocrystalline SrTiO3 powder by sol–gel combustion method

In this research a sol–gel combustion route has been presented to synthesize strontium titanate (SrTiO₃:ST) nanocrystalline, using citric acid as fuel. The synthesis procedure was optimized by systematically varying the molar ratios of total metal nitrate to citric acid (MN:CA) from 1:1 to 1:3. The effect was investigated through XRD, SEM and TEM analysis. Analysis of XRD spectrum shows the complete of SrTiO₃ nanocrystalline, however, a minor phase of SrCO₃ impurity was found. Hence, an acid treatment process, with 1 mol/l HNO₃ solution and deionized water, was applied to remove the impurity. The results show that the appropriate condition to prepare the single phase nanocrystalline SrTiO₃ powders is MN:CA molar ratio of 1:3, coupled with an acid treatment process and at the lower calcination temperature of 500 °C. The particle size of powders was in nanometer ranges. The average crystallite size calculated from FWHM was about 23 nm. Morphology of powders was identified by SEM analysis. However, TEM estimated the average particle size about 7.5 nm after applying an acid treatment technique at 600 °C.     

Synthesis and characterization of nanocrystalline zirconia powder by simple sol–gel method with glucose and fructose as organic additives

The present study has devised the sol–gel method using glucose and fructose as two organic additives so as to synthesize zirconia nanoparticles. The presence of these organic additives has produced some positive effect on the phase transition from tetragonal to monoclinic and played an important role in the morphology and crystallite size of the nanoparticles. Fourier transform infrared (FT-IR) spectra have shown Zr–O–Zr bond. Crystal phase and crystallite size have been determined by X-ray Diffraction (XRD) analysis. Besides, the morphology of the samples has been investigated by field emission scanning electron microscope (FE-SEM) and transmission electron microscope (TEM). The optical properties of the samples have been analyzed using photoluminescence (PL) spectroscopy, too. All the analyses consistently have shown fairly uniform nanoparticles with small size, containing both tetragonal and monoclinic phases with crystallite size between 10 and 30 nm.     

Hydration of ethene over W–P mixed metal oxide catalysts

W–P mixed metal oxide catalysts are active and selective for the gas-phase hydration of ethene to ethanol. The activity and selectivity of this catalytic reaction depend on the W/P atomic ratio. However, ethene conversion slightly decreases at higher W/(W + P) atomic ratio. The selectivity for ethanol increases with the W/P atomic ratio and reaches the highest value (92%) at W_0.81P_0.19O_x. The W_0.81P_0.19O_x catalyst is less active than the conventional H₃PO₄/SiO₂ catalyst, but the activity is maintained for a long time without the supply of any catalyst components. The reaction temperature does not affect substantially the rate of ethene hydration over the W_0.81P_0.19O_x catalyst. The H₂O/ethene molar ratio of 0.4 is the most appropriate for both reaction rate and selectivity. The active species of W–P mixed metal oxide are amorphous. But there is Keggin structure of W–P oxide species (PW₁₂O₄₀ ³⁻) in the presence of steam. And the species are the active sites for the hydration of ethene, confirmed by in situ Raman spectroscopy. This revised version was published online in July 2006 with corrections to the Cover Date.     

Synthesis of mesoporous SiO2–ZrO2 mixed oxides by sol–gel method

A series of SiO₂–ZrO₂ mixed oxides were prepared by sol–gel method in the presence of directing agent, with variable amounts of ZrO₂ between pure silica and pure zirconia, with the aim to obtain catalytic materials suitable as solid acid catalysts. SiO₂–ZrO₂ mixed oxides differ from the two pure starting oxides. While SiO₂ has a low OH density without peculiar acid character, the introduction of increasing amounts of Zr increases the density of the acid sites in the materials. Furthermore both SiO₂/ZrO₂ molar ratio and drying procedure are able to influence the physico-chemical characteristics (textural properties, acid sites distribution, etc.) of these mixed oxides.     

Synthesis of monodisperse nanocrystalline 8 mol-% yttria doped ceria powder by oleate complex route

Abstract

Preparation of monodisperse nanocrystalline yttria doped ceria powder by the oleate complex route has been reported. Y(III) and Ce(III) oleate complexes have been prepared by the reaction between YCl₃, Ce(NO₃)₃ and sodium oleate at the interface of hexane rich and water rich conjugate layers of water-ethanol-hexane ternary liquid system. Cubic yttria doped ceria crystallises when the waxy solid containing Y(III) oleate and Ce(III) oleate complexes was heat treated at 400°C. The powder after planetary ball milling contains monodisperse near spherical particles of 0·2?μm. These particles contain monodisperse nanocrystallites of size <10?nm. The yttria doped ceria powder pellets were sintered to >98% theoretical density at 1450°C. The sintered ceramic showed an ionic conductivity of 0·0623?S?cm^?1 at 800°C and activation energy of 1·0?eV.     

Styrene epoxidation over gold supported on different transition metal oxides prepared by homogeneous deposition–precipitation

Epoxidation of styrene by anhydrous tert-butyl hydroperoxide over a number of transition metal oxide (viz. TiO₂, Cr₂O₃, MnO₂, Fe₂O₃, Co₃O₄, NiO, CuO, ZnO, Y₂O₃, ZrO₂, La₂O₃ and U₃O₈) supported nano-size gold catalysts, prepared by the homogeneous deposition–precipitation method, has been investigated. The supported gold catalysts (except Au/MnO₂ and Au/U₃O₈) showed good styrene conversion activity and selectivity for styrene oxide in the epoxidation. The Au loading, Au particle size and performance in the epoxidation of the supported gold catalysts are found to be strongly influenced by the transition metal oxide support used in the catalyst. The Au/TiO₂ and Au/CuO are promising catalysts for the selective epoxidation.     

Alumina–copper joining by the sintered metal powder process

Sintered metal powder process (SMPP) is one of the high technology methods in ceramic–metal joining domain. The present study examines the effect of temperature and time of metalized layer sintering on the thickness and homogeneity of the joining layer, the leakage rate in alumina–copper joining zone, and also identifies the different phases formed during sintering. The samples were characterized by optical microscopy (OM), scanning electron microscopy (SEM) and energy dispersion spectroscopy (EDS). Microstructure studies indicate that sintering the metalized layer with a holding time of 90 min at the temperature of 1530 °C, and with an applied layer thickness of 50 μm with proper plating and brazing stages lead to a completely homogeneous joining zone with an adequate thickness (about 33 μm). The results of leak tests on alumina–copper specimen in this condition was less than 10^?9 Pa l s^?1.     

Microstructure evaluation of sintered combustion-derived fine powder NiO–YSZ

Citrate–nitrate combustion synthesis was used for the preparation of NiO–YSZ. The main advantage of the preparation method used was reflected in the fact that after the synthesis both phases NiO and YSZ were randomly distributed on a nanometre level. The prepared NiO–YSZ powder composites were shaped, sintered and reduced to Ni–YSZ and subsequently submitted to microstructure investigations. Relative sintered densities higher than 90% were obtained at sintering temperatures as low as 1200 °C. A sintering temperature 1200 °C was also recognized as the preparation temperature that provided the smallest Ni grains in the final Ni–YSZ cermet with an average Ni-particle diameter as low as 0.27 μm.     

Structural and magnetic properties of nanocrystalline zinc-doped metal ferrites (metal=Ni; Mn; Cu) prepared by sol–gel combustion method

In this work, nanocrystalline M–Zn ferrites (M=Ni; Mn; Cu) with compositions of M_1?xZn_xFe₂O₄ (x=0.0, 0.2 and 0.4) were synthesized from metal nitrate precursors by rapid the sol–gel combustion method using diethanolamine (DEA) as the fuel. As-synthesized powders were calcined at 1000 °C for 4 h. The crystal structures and morphologies of these compounds were characterized by X-ray diffraction (XRD) and field-emission scanning electron microscopy (FE-SEM), respectively. The chemical interaction of ferrite powders was investigated by Fourier transform infrared spectroscopy (FTIR). The magnetic properties of after-calcined nanoparticles were measured at room temperature using a vibrating sample magnetometer (VSM). The single phase spinel cubic structure formation is confirmed by XRD and FTIR results. Meanwhile FE-SEM micrographs show the appearance of both undoped and Zn-doped ferrite ceramic samples. In addition, the VSM analyses indicate that the Zn content has a significant influence on the magnetic properties such as saturation magnetization (M_s) and coercivity (H_c).     

Facile Synthesis of ZnO Nanorods by Microwave Irradiation of Zinc–Hydrazine Hydrate Complex

ZnO nanorods have been successfully synthesized by a simple microwave-assisted solution phase approach. Hydrazine hydrate has been used as a mineralizer instead of sodium hydroxide. XRD and FESEM have been used to characterize the product. The FESEM images show that the diameter of the nanorods fall in the range of about 25–75 nm and length in the range of 500–1,500 nm with an aspect ratio of about 20–50. UV–VIS and photoluminescence spectra of the nanorods in solution have been taken to study their optical properties. A mechanism for microwave synthesis of the ZnO nanorods using hydrazine hydrate precursor has also been proposed.     

Characterization of spherical silica–titania mixed oxide particles synthesized by using a dry process

Silica–titania mixed oxides have excellent properties, such as a low thermal expansion coefficient and a refractive index that can be adjusted by changing the Ti content. However, when the Ti content increases, silica and titania phases in silica–titania mixed oxides can separate. This phase separation leads to the precipitation of the titania component as rutile or anatase crystals. When silica–titania mixed oxides undergo phase separation, their properties become unstable; for example, the refractive index of the particles becomes non-uniform. Therefore, it is preferable to synthesize silica–titania mixed oxides in an amorphous state without causing phase separation. Based on our previous studies on particle size control in silica synthesis, we employed a dry process using organosilicon compounds to synthesize silica–titania mixed oxides. In this study, spherical amorphous silica–titania mixed oxide particles were obtained via flame synthesis using organosilicon and organotitanium compounds. The purpose of this study was to characterize the obtained powder and explore the possibility of controlling particle size during synthesis. By studying the dry process synthesis of spherical silica–titania mixed oxide particles, we confirmed the relationships: between the Si/Ti molar ratio and the obtained crystal structure and between the adiabatic flame temperature and the particle size.     

Adsorptive removal of Congo red dye from wastewater by mixed iron oxide–alumina nanocomposites

Nanocomposites composed of mixed iron and aluminium oxide (Fe₂O₃–Al₂O₃), have been synthesized by hydrothermal method, and further used as adsorbent for the adsorptive decolorization of Congo red dye from an aqueous system. The as-prepared nanomaterials were sintered at 500 °C and 1000 °C, to obtain pure Fe₂O₃–Al₂O₃ mixed composites. The XRD studies confirmed the formation of pure and crystalline FeOOH–AlOOH (as-prepared), γ-Fe₂O₃–Al₂O₃ at 500 °C and α-Fe₂O₃–Al₂O₃ phases at 1000 °C. The morphology and size of the obtained nanocomposites were characterized by SEM and TEM. Effects of pH, contact time, initial concentration of adsorbate have been studied. The optimum pH for maximum removal of Congo red in all the three phases of nanocomposites was found to be 7. The maximum removal capacity was 498 mg/g for γ-Fe₂O₃–Al₂O₃ phases. Among the three different adsorbents, γ-Fe₂O₃–Al₂O₃ shows complete removal within 15 min of contact time.     

Platinization of sunlight active Ti–W mixed oxide photocatalysts

The photocatalytic effects of the platinization on sunlight-active nanosize Ti–W mixed oxide materials with anatase structure were investigated. Platinization was shown to promote the photocatalytic mineralization of toluene, leading to an enhanced reaction rate of ca. 2.3 with respect to the parent Ti–W mixed oxide. This reaction rate increase is significantly higher than that obtained using ultraviolet excitation for titania-based oxides in the degradation of toluene and seems intimately related to the presence of PtO bonds located at the platinum–anatase interface. The chemical/physical bases of such behavior are discussed on the light of a structural/electronic characterization of the solids with the help of X-ray diffraction and Raman/UV–visible spectroscopy, along with additional in situ experiments run under sunlight excitation using infrared and electron paramagnetic resonance spectroscopy.     

Oxidation of isobutane over Mo–V–Sb mixed oxide catalyst

The catalytic performances of Mo–V–Sb mixed oxide catalysts have been studied in the selective oxidation of isobutane into methacrolein. V–Sb mixed oxide showed the activity for oxidative dehydrogenation of isobutane to isobutene. The selectivity to methacrolein increased by the addition of molybdenum species to the V–Sb mixed oxide catalyst. In a series of Mo–V–Sb oxide catalysts, Mo₁V₁Sb₁₀O_x exhibited the highest selectivity to methacrolein at 440°C. The structure analyses by XRD, laser Raman spectroscopy and XPS showed the coexistence of highly dispersed molybdenum suboxide, VSbO₄ and -Sb₂O₄ phases in the Mo₁V₁Sb₁₀O_x. The high catalytic activity of Mo₁V₁Sb₁₀O_x can be explained by the bifunctional mechanism of highly dispersed molybdenum suboxide and VSbO₄ phases. It is likely that the oxidative dehydrogenation of isobutane proceeds on the VSbO₄ phase followed by the oxidation of isobutene into methacrolein on the molybdenum suboxide phase.     

Synthesis of nanocrystalline zirconia powders for TZP ceramics by a nitrate–citrate combustion route

A nitrate–citrate combustion route to synthesise nanocrystalline yttria-doped zirconia powders for tetragonal zirconia polycrystal (TZP) ceramics is presented. This route is based on the gelling of nitrate solutions by the addition of citric acid and ammonium hydroxide, followed by an intense combustion process due to an exothermic redox reaction between nitrate and citrate ions. X-ray diffraction characterisation of powders showed the stabilisation of the tetragonal phase at room temperature because of their small crystallite size (about 10 nm). Dense ceramic samples prepared by uniaxial pressing and sintering in air were also studied.     

Synthesis of silica nanoparticles from Vietnamese rice husk by sol–gel method

Silica powder at nanoscale was obtained by heat treatment of Vietnamese rice husk following the sol–gel method. The rice husk ash (RHA) is synthesized using rice husk which was thermally treated at optimal condition at 600°C for 4 h. The silica from RHA was extracted using sodium hydroxide solution to produce a sodium silicate solution and then precipitated by adding H₂SO₄ at pH = 4 in the mixture of water/butanol with cationic presence. In order to identify the optimal condition for producing the homogenous silica nanoparticles, the effects of surfactant surface coverage, aging temperature, and aging time were investigated. By analysis of X-ray diffraction, scanning electron microscopy, and transmission electron microscopy, the silica product obtained was amorphous and the uniformity of the nanosized sample was observed at an average size of 3 nm, and the BET result showed that the highest specific surface of the sample was about 340 m²/g. The results obtained in the mentioned method prove that the rice husk from agricultural wastes can be used for the production of silica nanoparticles.     

Synthesis and characterization of boehmitic alumina coated graphite by sol–gel method

Non-wettability property makes graphite a good protecting material against the molten metal and/or slag. Properties like high oxidation potential between 600 and 1200 °C and non-wettability with water at room temperatures limits the usage of graphite in castable refractory applications. In this study, sol–gel method, which is a relatively cheaper process, was used. Boehmitic sol was obtained by hydrolyzing and peptiziting the alkoxide AIP (aluminum isopropoxide) used as alumina source. Then natural flake graphite was mixed with the boehmitic solution for coating of graphite. At 120 °C boehmitic sol coated graphite was dried and gelled. Then heat threaded at 550 °C for γ-Al₂O₃ transformation of boehmite. Products that obtained from the studies were characterized with FTIR and XRD tests. Alumina coated graphite samples were made by repeating the same steps and TG analysis were made to investigate the oxidation behaviour of the samples. Finally, SEM–EDS analyses were carried out to investigate the microscopic properties of the alumina coated graphite powders.     

Synthesis of graphene oxide-intercalated α-hydroxides by metathesis and their decomposition to graphene/metal oxide composites

Graphene oxide-intercalated α-metal hydroxides were prepared using layers from the delaminated colloidal dispersions of cetyltrimethylammonium-intercalated graphene oxide and dodecylsulfate-intercalated α-hydroxide of nickel/cobalt as precursors. The reaction of the two dispersions leads to de-intercalation of the interlayer ions from both the layered solids and the intercalation of the negatively charged graphene oxide sheets between the positively charged layers of the α-hydroxide. Thermal decomposition of the intercalated solids yields graphene/nanocrystalline metal oxide composites. Electron microscopy analysis of the composites indicates that the nanoparticles are intercalated between graphene layers.