Synthesis and magnetic properties of monodisperse CoFe2O4 nanoparticles coated by SiO2

The monodisperse CoFe₂O₄ nanoparticles were synthesized by a modified chemical coprecipitation method. Coating SiO₂ on the surface of the CoFe₂O₄ nanoparticles was carried out to keep single domain particles non-interacting with cubic magnetocrystalline anisotropy. The Curie temperatures (T_c) of the monodisperse CoFe₂O₄ nanoparticles can be accurately measured because the SiO₂ shells prevented the aggregation and growth of nanoparticles at high temperature. The magnetic properties of the CoFe₂O₄@SiO₂ nanoparticles with core-shell structure in a wide temperature range (300～950?K) were investigated. It is remarkable that the coercive field (H_c) of CoFe₂O₄ nanoparticles increased from about 760?Oe to 1806?Oe after being coated with SiO₂, which increased by 137.6% compared to the uncoated samples at 300?K. The saturation magnetization (M_s) of the CoFe₂O₄@SiO₂ nanoparticles is 34.59?emu/g, which is about 52% of the naked CoFe₂O₄ nanoparticles value (66.51?emu/g) at 300?K. The hysteresis loops of the CoFe₂O₄@SiO₂ nanoparticles showed an orderly magnetic behavior at high temperature, such as the M_s, remanence magnetization (M_r) and H_c decreased as temperature increasing, being equal to zero near T_c. This is a good indication that the CoFe₂O₄@SiO₂ nanoparticles are suitable for a wide variety of technological applications at high temperature.     

Direct synthesis of acetic acid from CH4 and CO2 by a step-wise route over Pd/SiO2 and Rh/SiO2 catalysts

The theoretical and experimental feasibility of direct conversion of CH₄ and CO₂ to acetic acid by an isothermal step-wise route over Pd/SiO₂ and Rh/SiO₂ catalysts was investigated. The methyl radical formation from CH₄ dissociation and CO₂ inserting into the intermediate are regarded as two limiting steps. Preliminary experimental results have shown that the following step-wise route can circumvent the thermodynamic limitation of this direct synthesis at low temperatures. Pd catalysts are more active than Rh catalysts at 170 °C and 200 °C, while formic acid is only produced on Pd catalysts. The optimum contact time of CH₄ and CO₂ with catalysts is 1 min under the experimental conditions. And there is no apparent deactivation resulting from carbon deposition for catalysts during the successive reaction cycles.     

TiCl4/SiO2负载型催化剂用于丁二烯聚合 Ⅰ .催化剂制备条件对聚合的影响

用球磨法制备的SiO2负载TiCl4催化剂进行丁二烯溶液聚合,考察了催化剂制备条件对催化效率的影响,分析了聚合物的结构。结果表明球磨48h,负载钛质量分数4％的催化剂具有较好的催化效率;SiO2在负载TiCl4前用适量Al(i-Bu)3处理,或将球磨法制备的TiCl4/SiO2催化剂与适量Al(i-Bu)2Cl在100℃下反应1h,均能大幅度提高催化剂的活性;在40～50℃下,聚合按溶液淤浆聚合形式进行,产物为低顺式聚丁二烯。     

Hierarchical Cd4SiS6/SiO2 Heterostructure Nanowire Arrays

Novel hierarchical Cd₄SiS₆/SiO₂ based heterostructure nanowire arrays were fabricated on silicon substrates by a one-step thermal evaporation of CdS powder. The as-grown products were characterized using scanning electron microscopy, X-ray diffraction, and transmission electron microscopy. Studies reveal that a typical hierarchical Cd₄SiS₆/SiO₂ heterostructure nanowire is composed of a single crystalline Cd₄SiS₆ nanowire core sheathed with amorphous SiO₂ sheath. Furthermore, secondary nanostructures of SiO₂ nanowires are highly dense grown on the primary Cd₄SiS₆ core-SiO₂ sheath nanowires and formed hierarchical Cd₄SiS₆/SiO₂ based heterostructure nanowire arrays which stand vertically on silicon substrates. The possible growth mechanism of hierarchical Cd₄SiS₆/SiO₂ heterostructure nanowire arrays is proposed. The optical properties of hierarchical Cd₄SiS₆/SiO₂ heterostructure nanowire arrays are investigated using Raman and Photoluminescence spectroscopy.     

Synthesis of nanocrystalline SnO2 powder from SnCl4 by mechanochemical processing

The rapid synthesis of nanocrystalline SnO₂ powder using a mechanochemical reaction of SnCl₄ (instead of the widely used tin (II) compounds) with (NH₄)₂CO₃ and the subsequent annealing of the product in air and under an H₂O/NH₃ atmosphere has been investigated using X-ray powder diffraction, TG and TEM. The reaction was complete within 5 min. Additional milling of the product at a higher milling intensity for 120 min led to the crystallisation of tetragonal SnO₂. The NH₄Cl salt matrix was removed by annealing at 300 °C. The average crystallite size of tetragonal SnO₂ was in the range of 2-48 nm and it can be controlled by variation heating temperatures and annealing atmospheres in the range of 300-700 °C.     

SiO2载体对Cu/SiO2催化剂的影响

分别以硅溶胶、气相SiO2、介孔SBA-15分子筛为载体,采用共沉淀法制备了Cu/SiO2催化剂,含活性组分Cu质量分数为25%。运用BET、XRD、H2-TPR、NH3-TPD等对催化剂进行表征。结果表明:以硅溶胶和气相SiO2为载体制得催化剂的Cu物种分散性较差,以SBA-15为载体制得催化剂的Cu物种以高度分散的形式存在,且具有较大的比表面积,较低的还原温度和较大的酸量。     

高岭土原位合成高硅铝比小晶粒NaY分子筛

以不同温度焙烧的苏州高岭土为原料,采用原位晶化法合成高硅铝比小晶粒NaY分子筛,考察晶种胶添加量、陈化温度、晶化温度和m(高土)∶m(偏土)对晶化过程和产物性质的影响。结果表明,m(高土)∶m(偏土)=1时,在晶种胶添加质量分数15%、陈化温度90℃和晶化温度100℃条件下,以普通高岭土为原料原位合成NaY样品的相对结晶度约为80%,骨架硅铝比(SiO2与Al2O3物质的量比)为6.4,平均粒径约500nm。调整原料中m(高土)∶m(偏土)可以控制原位晶化样品中的分子筛含量。以细化高岭土为原料合成的NaY分子筛(平均约445nm)粒径小于普通高岭土合成的样品。骨架硅铝比高于6.0的原位晶化样品的骨架坍塌温度高于950℃,具有很高的结构稳定性。     

CoCr_2O_4/SiO_2催化剂上二氯甲烷催化氧化

采用浸渍法制备了不同CoCr_2O_4负载量x CoCr_2O_4/SiO_2催化剂(x=5%、10%、20%和30%),考察其对二氯甲烷催化燃烧性能的影响。结果表明,催化剂的整体活性顺序为:30CoCr_2O_4/SiO_220CoCr_2O_4/SiO_210CoCr_2O_4/SiO_25CoCr_2O_4/SiO_2,但按照活性组分CoCr_2O_4质量归一化后本征活性顺序为:10CoCr_2O_4/SiO_2≈5CoCr_2O_4/SiO_220CoCr_2O_4/SiO_230CoCr_2O_4/SiO_2。表征结果发现催化剂本征活性与可还原性能和表面酸性存在密切关系。10CoCr_2O_4/SiO_2和5CoCr_2O_4/SiO_2具有较高的表面酸性和耗氢量,因此具有较高的本征活性。     

Structural colors from TiO2/SiO2 multilayer flakes prepared by sol-gel process

Preparation of TiO₂/SiO₂ multilayer flakes and their application to decorative powders were investigated. In contrast to conventional products prepared through the multicoating of core platelets, the coreless TiO₂/SiO₂ multilayer flakes were prepared by detaching multilayer films from their substrates. These flakes exhibited structural colors, when the optical path length of both the TiO₂ and SiO₂ layers are adjusted to be one fourth of the wavelength of visible light. A multicoating of more than five layers resulted in the propagation of cracks, which prevented the preparation of thick flakes. Paint films fabricated using the multilayer flakes and acrylic resins showed reflectance spectra that were comparable with those obtained for multicoatings on substrates.     

Role of SiO2 in synthesis of SiO2-supported CeO2 composites

Pure and SiO₂-supported CeO₂ samples were prepared by Ce(NO₃)₃ decomposition, precipitation, and sol–gel methods in an attempt to study the role of SiO₂ in the synthesis of these materials. During synthesis process, SiO₂ support uniformly adsorbed cerium ions in aqueous solution, preventing nucleation and crystal growth of CeO₂ during the subsequent water evaporation and calcination steps. Uniform adsorption and inhibition were enhanced by NH₄⁺ and, to a larger extent, C₅H₇O₅COO^-. Despite the dispersion of cerium ions on SiO₂ reduced the temperature at which CeO₂ was formed, crystal size and crystallinity of CeO₂ in composites were significantly lower than that of pure CeO₂ sample prepared by the same synthesis method and at the same temperature. Composites were quite stable upon increasing the temperature from 400 to 800?°C. Visible light absorption, reduction, and photocatalytic activity characteristics of CeO₂ were improved upon dispersion on SiO₂. This work can help synthetize supported oxides with high activity and thermal stability.     

TiCl4/SiO2负载型催化剂用于丁二烯聚合Ⅱ.聚合条件及聚合动力学

使用以球磨法制备的SiO2负载TiCl4催化剂进行丁二烯溶液聚合,考察了聚合条件对催化效率的影响以及在20～60℃下的聚合动力学.确定了适宜的聚合条件以加氢汽油为溶剂,Al/Ti摩尔比为20,Ti/Bd摩尔比为2×10-4,50℃聚合10h以上.溶液聚合动力学研究结果表明,本体系在低转化率下为稳态聚合,其稳态聚合速率方程为Rp=kpf[Ti]0[Bd],聚合的表观活化能为11.4kJ/mol.     

Synthesis and characterization of nanocrystalline merwinite (Ca3Mg(SiO4)2) via sol-gel method

In this research, the synthesis of nanocrystalline merwinite (2SiO₂-3CaO-MgO) bioactive ceramic was carried out by the sol-gel method. After crushing, obtained sol-gel derived bioceramic powder pressed uniaxially to produce cylindrical-like pellets, followed by sintering at 1300 °C. Via immersion in simulated body fluid (SBF) for various time intervals, the formation of apatite was characterized. Scanning electron microscopy (SEM), X-ray diffraction analysis (XRD), and Fourier transform infrared spectroscopy (FT-IR) studies were conducted both before and after immersion in SBF. The crystallization temperature of the merwinite was determined by thermal analysis. Attained results confirmed formation of apatite layer within the first day of soaking. Accordingly it can be concluded that merwinite is bioactive and might be used for preparation of implantable biomaterials.     

SnCl4催化合成环己烯

以四氯化锡为催化剂,对环己醇脱水制备环己烯的反应进行了研究.探讨其最佳反应条件为催化剂用量为环己醇质量的13.3%,油浴温度170～190℃,反应时间1h.收率86.4%.     

直接由SiO2低温合成四配位硅衍生物及其结构表征

研究了直接从无定型二氧化硅出发,与乙二醇、氢氧化钾反应,低温合成高反应活性的五配位硅钾化合物,然后再与氯化苄(C6H5CH2C1)反应制备四配位硅化合物,并对合成的物质作了红外光谱、核磁共振(1H、13C)、元素分析的结构表征.     

Synthesis of C2H4 by partial oxidation of CH4 over LiCl/NiO

Alkali halide added transition metal oxides produced ethylene selectively in oxidative coupling of methane. The role of alkali halides has been investigated for LiCl-added NiO (LiCl/NiO). In the absence of LiCl the reaction over NiO produced only carbon oxides (CO₂ + CO). However, addition of LiCl drastically improved the yield of C₂ compounds (C₂H₆ + C₂H₄). One of the roles of LiCl is to inhibit the catalytic activity of the host NiO for deep oxidation of CH₄. The reaction catalyzed by the LiCl/NiO proceeds stepwise from CH₄ to C₂H₄ through C₂H₆ (2CH₄ → C₂H₆ → C₂H₄). The study on the oxidation of C₂H₆ over the LiCl/NiO showed that the oxidative dehydrogenation of C₂H₆ to C₂H₄ occurs very selectively, which is the main reason why partial oxidation of CH₄ over LiCl/NiO gives C₂H₄ quite selectively. The other role of LiCl is to prevent the host oxide (NiO) from being reduced by CH₄. The catalyst model under working conditions was suggested to be the NiO covered with molten LiCl. XPS studies suggested that the catalytically active species on the LiCl/NiO is a surface compound oxide which has higher valent nickel cations (Ni^(2+δ)+ or Ni³⁺). The catalyst was deactivated at the temperatures>973 K due to vaporization of LiCl and consumption of chlorine during reaction. The kinetic and CH₄---CD₄ exchange studies suggested that the rate-determining step of the reaction is the abstraction of H from the vibrationally excited methane by the molecular oxygen adsorbed on the surface compound oxide.     

Eu2O3/SiO2 nanocomposites obtained by extraction pyrolysis

The conditions of the formation of nanosized Eu₂O₃/SiO₂ composites were studied. Amorphous silicon dioxide was coated with europium oxide by extraction pyrolysis. The resulting samples were studied by X-ray phase analysis and atomic force microscopy. The Eu₂O₃/SiO₂ composites were tested as catalysts of CO conversion.     

Properties of SiC,Si3N4 and SiO2 ceramic powders produced by vibration ball milling

SiC, Si₃N₄ and SiO₂ powders were ground by a vibration ball mill, the pot and balls of which were made of Si₃N₄ ceramics, in a purified methanol medium. The mean particle diameters of SiC and SiO₂ powders were ca. 0.1 μm and thus ultrafine particles were manufactured. The increasing rate of specific surface area by fine grinding was largest on SiO₂ powders, and the rate decreased gradually with an increase in grinding time. The specific surface area of Si₃N₄ powders increased proportionally with an increase in grinding time. The grindability was obtained in the following order: SiO₂ > SiC > Si₃N₄. The ground SiC powders showed a large lattice strain and little fragmentation of crystallites. The strain and the crystallite size of ground Si₃N₄ powders were moderate.     

Synthesis of LiFePO4/carbon composite from nano-FePO4 by a novel stearic acid assisted rheological phase method

LiFePO₄/carbon composite was synthesized at 600 °C for 4 h in an Ar atmosphere by a stearic acid assisted rheological phase method using amorphous nano-FePO₄ as the iron source. XRD, SEM and TEM observations show that the LiFePO₄/carbon composite has good crystallinity, ultrafine and well-dispersed particles of 60–150 nm size and in situ carbon coated on the surface of LiFePO₄ crystallites. The synthesized LiFePO₄/carbon composite shows a high discharge capacity of 160 mAh g⁻¹ and 155 mAh g⁻¹ at rates of 0.5 C and 1 C, respectively. Even at a high current density of 30 C, the material still presents a discharge capacity of 93 mAh g⁻¹ and exhibits an excellent cycling performance.     

Synthesis and reaction mechanism of Ti3SiC2 ternary compound by carbothermal reduction of TiO2 and SiO2 powder mixtures

The present study aims to investigate synthesis of Ti₃SiC₂ from TiO₂ and SiO₂ powder mixtures by carbothermal reduction method. Equilibrium TiO₂–SiO₂–C ternary phase diagram was used to predict the conditions for the formation of Ti₃SiC₂ at 1800 K under Ar atmosphere. A reactant mixture with a TiO₂:SiO₂ molar ratio of 1.5 and a C content of 68.75 mol% (26.86 wt%) was initially selected among the thermodynamically favorable reactant compositions for the experimental studies. Two different C sources, graphite flakes and pyrolytic C coating, were used to synthesize Ti₃SiC₂ at 1800 K under Ar atmosphere. When graphite flakes were used, the products contained a trace amount of Ti₃SiC₂ phase along with major TiC and minor SiC phases. Whereas, pyrolytic C coating on the oxide particles resulted in the products with much higher Ti₃SiC₂ contents owing to the close contact between the reactants. Optimal C concentration for the C coated oxide mixtures with a TiO₂:SiO₂ molar ratio of 1.5 was determined to be 30.05 wt% under the experimental conditions studied. Ti₃SiC₂ content of the products obtained from this reactant was observed to increase with reaction time to 31 wt% at 75 min beyond which it gradually decreased. XRD studies indicated that the product with the highest ternary carbide content also contained TiC and a trace amount of SiC. SEM-EDS analyses showed that this sample essentially consisted of spherical fine TiC particles and Ti₃SiC₂ nanolaminates. Equilibrium thermodynamic analysis of the TiO₂–SiO₂–C system suggested that the reaction of solid Ti₂O₃ with SiO and CO gases may play a dominant role in the formation of Ti₃SiC₂.     

The doping concentration dependent tunable yellow luminescence of Sr5(PO4)2(SiO4):Eu

Color tunable yellow-emitting phosphors of Sr_5−5xEu_5x(PO₄)₂SiO₄ (x = 0.05-0.15) were prepared by conventional solid-state reaction method. The X-ray powder diffraction patterns, the photoluminescence excitation and emission spectra were measured. The main excitation bands of the phosphors locate at a broad band extending from 300 to 500 nm, which can match the emission of ultraviolet- and blue-emitting diode chips. The tunable luminescence color was realized by the changing Eu²⁺ doping in Sr₅(PO₄)₂SiO₄. The structure and luminescence properties were investigated. Sr_5−5x(PO₄)₂SiO₄:Eu_5x displays two typical luminescence centers, which originate from two different Sr²⁺ (Eu²⁺) sites in the host. The site-occupation, the luminescence intensity and energy transfer between the Eu²⁺ ions occupying two different crystallographic Sr²⁺ sites were discussed on the base of the luminescence spectra and crystal structure. This is helpful to improve this phosphor for a potential application as a white light emitting diode phosphor.