High Surface Area CuMn2O4 Prepared by Silica-Aquagel Confined co-precipitation. Characterization and Testing in Steam Reforming of Methanol (SRM)

High surface area CuMn₂O₄ and mixtures of CuO and Mn₂O₃ were prepared via a novel template technique based on the coagulation—precipitation processes that occur when the metallic cations of salts dissolved in a silica aquagel medium are forced to precipitate by basic reagents. The oxides prepared by this method were tested as catalysts for the methanol steam reforming reaction at 250 °C and a very high space velocity (WHSV = 27.2 h⁻¹). CuMn₂O₄ with a high surface area (300 m²/g) and a tangled structure that apparently resembles the internal surface of the amorphous silica gel was obtained. The catalytic activity of this material is similar to that of the most active catalyst reported in the literature, with the advantage that its stability is enhanced. Contrary to what has been reported for low surface area particles, the physical mixture of copper oxide and manganese oxide nanoparticles prepared by the confined co-precipitation technique displayed a much lower catalytic activity than the spinel.     

Structural and transport properties of polydimethylsiloxane based polyurethane/silica particles mixed matrix membranes for gas separation

Mixed matrix membranes of synthesized polyurethane (PU) based on toluene diisocyanate (TDI), polydimethylsiloxane (PDMS) and polytetramethylene glycol (PTMG) with polyvinyl alcohol based polar silica particles were prepared by solution casting technique. The homogeneity and thermal properties of the prepared PDMS-PU/silica membranes were characterized using scanning electron microscope (SEM), differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA). The SEM micrographs confirmed the distribution of silica particles in the polymer matrix without agglomerations. Gas permeation properties of membranes with different silica contents were studied for pure CO₂, CH₄, O₂, He and N₂ gases. The obtained results indicated the permeability of the condensable and polar CO₂ gas was enhanced whereas permeability of other gases decreased upon increasing the silica content of the mixed matrix membranes. The permeability of CO₂ and its selectivity over N₂ was increased from 68.4 Barrer and 22 in pure PDMS-PU to 96.7 Barrer and 64.4 in the mixed matrix membranes containing 10 wt% of the silica particles.     

Synthesis of magnetic mesoporous silica composites via a modified Stöber approach

In this work, magnetic Fe₃O₄@mesoporous silica composites were synthesized by a microemulsion (oil-in-water/ethanol) approach which was applied with a modified Stöber reaction. Cetyl trimethyl ammonium bromide was employed as the surfactant, nano-Fe₃O₄ particles were dispersed in microemulsion. Tetraethyl orthosilicate (TEOS) formed oil drops, and ammonia solution facilitated the hydrolysis polymerization of TEOS. The diameters of the magnetic Fe₃O₄@mesoporous silica composites can be tuned within the range 120–380 nm by varying the ratio of ethanol/water and the amount of nano-Fe₃O₄ particles. Brunauer–Emmett–Teller surface areas of magnetic Fe₃O₄@mesoporous silica composites were determined to be within the range 490–759 m²/g and their pore sizes were around 2.3 nm as it was determined by Barrett–Joyner–Halenda method. Furthermore, the encapsulation of poorly water-soluble drugs within magnetic Fe₃O₄@mesoporous silica composites was investigated using protoporphyrin IX. Magnetic Fe₃O₄@mesoporous silica composites showed a drug loading within 22–68 mg/g, which can be an excellent drug delivery platform for photodynamic therapy.     

Ion beam-induced shaping of Ni nanoparticles embedded in a silica matrix: from spherical to prolate shape

Present work reports the elongation of spherical Ni nanoparticles (NPs) parallel to each other, due to bombardment with 120 MeV Au⁺⁹ ions at a fluence of 5 × 10¹³ ions/cm². The Ni NPs embedded in silica matrix have been prepared by atom beam sputtering technique and subsequent annealing. The elongation of Ni NPs due to interaction with Au⁺⁹ ions as investigated by cross-sectional transmission electron microscopy (TEM) shows a strong dependence on initial Ni particle size and is explained on the basis of thermal spike model. Irradiation induces a change from single crystalline nature of spherical particles to polycrystalline nature of elongated particles. Magnetization measurements indicate that changes in coercivity (H _c) and remanence ratio (M _r/M _s) are stronger in the ion beam direction due to the preferential easy axis of elongated particles in the beam direction.     

The effect of octamethylcyclotetrasiloxane (D4) addition on the structure and properties of film‐forming polyacrylate/silica core–shell composite particles

The film‐forming polyacrylate/silica core–shell nanocomposite particles with octamethylcyclotetrasiloxane (D₄) were successfully synthesized via aqueous emulsion polymerization in the presence of a glycerol‐functionalized nano silica sol. The ring‐opening polymerization of D₄ and the reaction with the glycerol‐functionalized nano silica particles before emulsion polymerization was the key procedure in this process. Transmission electron microscopy results showed that more nano silica particles tended to coat on the polyacrylate particles surface after the nano silica sols were modified with D₄. The silica aggregation efficiency was increased from 90.9 to 98.6% when the amount of D₄ used in the system was varied from 0 to 8.0 wt %. The transparency of the nanocomposite films was not compromised after D₄ was incorporated into the system. The films of the nanocomposite particles with or without D₄ both exhibited superior abrasive resistance. Furthermore, the water resistance and hydrophobicity of the films of these particles with D₄ were also improved significantly. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42003.     

Preparation of small Au-Pd particles on silica

Small Au-Pd particles 2.0 to 4.5 nm in diameter were prepared by ion exchange of [Au(en)₂]³⁺ and [Pd(NH₃)₄]²⁺ with surface protons of silica gel to form a precursor, which was subsequently reduced in dihydrogen. The Mössbauer effect of ¹⁹⁷Au yielded direct evidence of alloy formation and suggested that alloy particles were of a uniform composition close to the composition of the whole alloy sample. Further, selective chemisorption and X-ray line broadening showed that the surface composition of the alloy particles was similar to the overall composition.     

Adsorption behaviour of functional grafting particles based on polyethyleneimine for chromate anions

Polyethyleneimine (PEI) was first grafted on the surfaces of micro-sized silica gel particles in the manner of the coupling graft, forming the grafting particles PEI/SiO₂. Afterward, for the grafted PEI, two polymer reactions, tertiary amination reaction and quaterisation, were allowed to be carried out in turn, and a kind of functional composite particles, QPEI/SiO₂, was obtained. QPEI/SiO₂ particles were used as solid adsorbent in the removal of chromate anions from aqueous solution. The static adsorption experiments (batch method) were performed, the effects of various factors on the adsorption capacity of QPEI/SiO₂ were examined, and the dynamic desorption experiments were also carried out. The experimental results show that QPEI/SiO₂ particles have strong adsorption ability for CrO₄²⁻ ions by right of electrostatic interaction, and the saturated adsorption amount actually reach up to 0.14 g/g. The isothermal adsorption behaviour is fitted to Langmuir model. The adsorption ability of QPEI/SiO₂ particles for CrO₄²⁻ ions is affected greatly by the quaterisation degree of the grafted PEI macromolecules. The QPEI/SiO₂ particles with higher quaterisation degrees have greater adsorption capacities. The adsorption ability of QPEI/SiO₂ particles for CrO₄²⁻ ions is nearly independent of pH values of the medium. QPEI/SiO₂ particles have excellent eluting and regenerating property as a mixed solution of NaOH and NaCl is used as eluent.     

Tailor-made morphologies for Pd/SiO2 catalysts through sol–gel process with various silylated ligands

With the aim of obtaining highly dispersed, immobilized and accessible catalytically active palladium nanoparticles in a silica support with a tailored morphology, a sol–gel cogelation method is developed. Pd/SiO₂ catalysts are synthesized in ethanol by concomitant hydrolysis and condensation, i.e. cogelation, of a Pd²⁺[L–X–Si(OR)₃]_n complex with Si(OC₂H₅)₄ (TEOS) as network-forming reagent followed by calcinations in air and by reductions of the resulting xerogel catalysts in hydrogen. By varying the nature of L, X and R in the Pd²⁺ complex, the purpose of the present work is to determine the influence of the nature of the L–X–Si(OR)₃ ligand on textural properties of silica and on palladium dispersion in Pd/SiO₂ catalysts.By changing the molecular weight of L–X– and thus its bulkiness of L and X, it is observed that with bulkier silylated ligand, the textural properties of resulting gels are strongly modified from micro- and macroporous materials to very highly micro- and mesoporous materials. In the latter ones, the size of silica particles is smaller and their void fraction is very high. In such a case, palladium particles are not trapped inside silica particles and are then able to migrate and to sinter. The Pd/SiO₂ xerogel catalysts were evaluated in the hydrodechlorination of 1,2-dichloroethane.     

Production and Characterization of Pd/SiO2 Catalyst Nanoparticles from a Continuous MOCVS/MOCVD Aerosol Process at Atmospheric Pressure

A continuous 2-step aerosol process is described for the generation of SiO₂ supported palladium (Pd/SiO ₂ ) catalyst particles from metal-organic (MO) precursors. In a first flow reactor, submicron SiO ₂ support particles are generated by chemical vapor synthesis (CVS) from TEOS [tetraethyl(ortho)silicate]. These silica particles are then coated with palladium in a second flow reactor by chemical vapor deposition (CVD) from ( η ³ -allyl)( η ⁵ -cyclopentadienyl)palladium [Pd(allyl)Cp].

The sublimation and decomposition behavior of both metal-organic precursors was measured by thermo-gravimetric analysis (TGA) and FTIR; the vapor concentration of Pd(allyl)Cp was determined for the range of process conditions used.

Each process step was characterized both with regard to aerosol properties as well as morphology and composition of individual particles. This was done with a variety of on-line and off-line techniques including electrical mobility analysis, TEM, energy-dispersive X-ray analysis, and physisorption methods like BET. The coating thickness was also measured on line by a high-resolution single-stage impactor (SS-LPI) technique.

It is shown that the continuous CVS process can be set to generate constant concentrations and sizes of silica support particles with a specific surface area of 350 m ² g ^?1 , which are carbon free and non-porous. The silica particles can be restructured to spheres if desired. The continuous palladium CVD process was able to generate variable and defined coatings of narrowly distributed Pd nanoparticles with mean sizes between 0.75 and about 3.5 nm. The on-line measurements by SS-LPI showed equivalent coating thicknesses from 0.3 nm up to 3 nm, which were stable over several hours.     

Low-temperature synthesis of cements from rice hull ash

Rice hull is an agricultural by-product containing about 20% of silica. Usually, this material is burned at the rice fields generating small silica particles, which may cause respiratory and environmental damage. This work describes the use of rice hull ash as a raw material to prepare Ca₂SiO₄-related cements, which is a component of commercial Portland cement. Rice hull was heated at 600 °C rendering silica with a surface area of 21 m² g⁻¹. This material was mixed with CaO and BaCl₂·2H₂O in several proportions, added stoichiometricaly in order to keep a ratio (Ca+Ba)/Si=2. The solids were mixed with water 1:20 (w/w) and sonicated for 60 min. The suspensions were dried and heated at several temperatures (from 500 to 1100 °C). The resulting solids were analyzed by FT-IR spectroscopy and X-ray diffraction. Cements with structure similar to that of β-Ca₂SiO₄ were obtained at temperatures as low as 700 °C, according to the composition.     

Synthesis,morphological and thermomechanical characterization of light weight silica foam via reaction generated thermo-foaming process

Low-cost thermo-foaming technique involving sucrose dehydration reaction with H₂SO₄ has been used for ceramic foam processing. The heat and gas generated during the reaction induces a thermo-foaming effect in the sucrose-ceramic mixture. This converts the sucrose-ceramic slurry into a carbonaceous porous ceramic-scaffold. The converted carbon is seen to agglutinate with the ceramic particles. During sintering, they are removed subsequently, thereby enlarging the pores. Influence of solids-loading, sintering temperature and H₂SO₄ concentration on the foaming behaviour, pore morphology and thermo-mechanical properties are studied. The fabricated silica foams are found to have porosity within the range of 70 %? 90 % and compressive strength of 0.8–2.8 MPa. The extreme porosity of the silica foams and their intercrystallite pores within the struts results in the low thermal conductivity (0.0943 Wm^?1K^?1) of the specimens. The fabricated foam is seen as a promising material for insulation applications like catalyst supports, filters and bio-scaffolds.     

Surface functionalized colloidal silica particles from an inverse microemulsion sol gel process

Colloidal silica particles are prepared via a sol gel technique carried out in an inverse microemulsion of water in a toluene solution of tetraethoxysilane (TEOS), stabilized by either an anionic surfactant AOT or isopropanol. Functionalized material was obtained using a functional coupling agent (RO)₃Si(CH₂)₃ X, X being a functional group such as methacryloyl, thiol, vinyl, amino group, or a chlorine atom. Functionalization can be carried out either directly via the direct copolycondensation of TEOS and the coupling agent, or in a two-step process involving a core-shell polycondensation of the coupling agent onto preformed silica particles. Kinetic studies of the copolycondensation are carried out using either²⁹Si NMR analysis or liquid chromatography. They show that the consumption of TEOS is more rapid than that of the coupling agent. The materials are characterized both chemically (elemental analysis, FTIR,¹³C and²⁹Si NMR CPMAS analysis), and by their particle size. The silica functionalized with a polymerizable methacryloyl group is encapsulated by a polymer layer in an inverse emulsion polymerization of acrylic acid. After inversion of the emulsion in water, the resulting material is covered with a layer of hydrophobic polymer in a conventional emulsion polymerization.     

Preparation of PDMS—Silica Nanocomposite Membranes with Silane Coupling for Recovering Ethanol by Pervaporation

In this article, the composite polydimethylsiloxane (PDMS) membranes supported by cellulose-acetate (CA) microfiltration membrane were successfully prepared by adding nano-fumed silica particles modified with a silane coupling reagent, NH₂-C₃H₆-Si(OC₂H₅)₃. The effects of silica content, feed concentration, and feed temperature on the pervaporation performances of the nano-composite PDMS membranes were investigated for recovering ethanol from aqueous solution by pervaporation. It was found that adding the modified silica particles significantly improved the pervaporation performances of the composite membranes. When the silica content in the membrane was 5 wt%, for a 5 wt% ethanol/water mixture at 40°C, the permeation flux of the membrane maintained about 200 g · m^?2 · h^?1 and separation factor reached the maximum value of 19.     

Continuous synthesis of Zn2SiO4:Mn fine particles in supercritical water at temperatures of 400-500 °C and pressures of 30-35 MPa

Sub-micron sized Zn₂SiO₄:Mn²⁺ phosphors particles were continuously synthesized in supercritical water with a flow reactor. Colloidal silica or sodium silicate was used as the Si source. Zn and Mn sources were chosen from their nitrates, sulfates, and acetates. The syntheses were carried out at temperatures from 400 to 500 °C, at pressures from 30 to 35 MPa, at NaOH concentrations from 0.014 to 0.025 M, and for residence times from 0.025 to 0.18 s. Sodium silicate formed α- and β-Zn₂SiO₄:Mn²⁺ phases regardless of the Zn and Mn sources, while colloidal silica formed phases dependent on the type of Zn and Mn sources used in addition to the use of alkali. As the reaction temperature increased, the crystallinity of α-Zn₂SiO₄:Mn²⁺ phase increased and the Mn substitution into the Zn sites of the α-Zn₂SiO₄ phase decreased. Of the conditions studied, the most highly crystalline α-Zn₂SiO₄:Mn²⁺ was produced at a temperature of 400 °C, a pressure of 30 MPa, a NaOH concentration of 0.14 M, and a residence time of 0.13 s with Zn and Mn sulfates and colloidal silica as starting materials. The α-Zn₂SiO₄:Mn²⁺ fine particles synthesized were round in shape, had an average diameter of 268 nm, and exhibited a green-emission with a peak wavelength of 524 nm.     

Erosion Behavior of Gelcast Fused Silica Ceramic Composites

In this paper, the characteristics of solid particle erosion on fused silica ceramics are investigated. Gelcasting, a near net shape forming process, is adopted for the fabrication of ceramics. Three types of ceramics with a combination of pure fused silica, fused silica+5 wt% silicon nitride (Si₃N₄)?+?1 wt% boron nitride (BN) and fused silica+5 wt% silicon nitride (Si₃N₄) +1 wt% alumina (Al₂O₃) are prepared at a constant 52 vol% solid loading, 10 wt% monomer content and 10:1 monomer ratio. Different impingement angles (30^o, 45^o, 60^o and 90^o) and three impact velocities (86 m/s, 101 m/s and 148 m/s) were chosen to examine the behavior of erosion on gelcasted ceramics using SiO₂ particles as erodent. The maximum rate of erosion is obtained at normal impingement angle (90^o), which shows the brittle nature of ceramics. The impact velocity and angle of impingement have an appreciable effect on erosion rate. Resistance to erosive wear is found to have improved with the inclusion of reinforcements in the fused silica ceramics. The erosion rates of different ceramics are compared. Ceramic composite with a combination fused silica+5 wt% Si₃N₄+ 1 wt% BN shows the highest resistance to wear. The surface roughness and morphology of the eroded surfaces have also been studied.

     

Oxidative Coupling of Poly (2-Vinylthiophene) Chains by FeCl3

Summary Poly(2-vinylthiophene) (PVT) can be cross linked with FeCl₃ either in dichloromethane solution or when immobilized on silica particles. Extend of cross linking is strongly dependent on the FeCI₃/PVT ratio used for the reaction as shown by GPC plots and gravimetric analyses of the formed insoluble polymer fraction. A stoichiometric FeCl₃/PVT ratio of about four is necessary to achieve complete conversion, because consecutive formations of thiophene H⁺FeCl₄ ^- and bisthiophene H⁺FeCl₄ ^- sigma complexes take place. PVT can be also cross linked when adsorbed on silica using the same procedure. The cross-linking is proved by IR and ¹³C CPiMAS NMR spectroscopy.     

Coating Titania Aerosol Particles with ZrO2, Al2O3/ZrO2 and SiO2/ZrO2 in a Gas-Phase Process

The formation of ZrO₂, Al₂O₃/ZrO₂, and SiO₂/ZrO₂ coatings on TiO₂ particles by a continuous gas-phase process was studied. Titania particles were formed by the reaction of TiCl₄ vapor with O₂ in a hot-wall tubular reactor at 1300°C and were mixed with ZrCl₄ and AlCl₃ or SiCl₄ vapors near the end of the reactor. The ZrCl₄/TiCl₄ molar ratio was varied from 6.7 x 10^-4 to 5 x 10^-3 while the AlCl₃/TiCl₄and SiCl₄/TiCl₄ molar ratios varied in the ranges 8 x 10^-3-8 x 10^-2 and 2 x 10^-2-8 x 10^-2, respectively. Discrete tetragonal ZrO₂ nanoparticles of average diameter 20 nm were formed on the surfaces of the titania particles at a surface concentration that increased with the ZrCl₄ gas-phase concentration. The sequential introduction of AlCl₃ and ZrCl₄ vapors resulted in composite coatings. These consisted of dense, coherent, amorphous, and smooth Al₂O₃ layers approximately 10 nm thick, on top of which ZrO₂ nanocrystalline particles were dispersed in a similar pattern as in the absence of Al₂O₃. Concentration depth profiles of these powders were obtained by Auger Electron Spectroscopy and supported the TEM observations. Particles coated by sequentially introducing SiCl₄ and ZrCl₄ vapors had amorphous rough SiO₂ coatings, 5-10 nm thick, and ZrO₂ particles sporadically dispersed on their surfaces. Increasing the SiCl₄/ZrCl₄ molar ratio from 5 to 19 increased the thickness and roughness of the silica layers and resulted in encapsulation of the ZrO₂particles inside the silica coatings. The different coating morphologies obtained were attributed to different coating formation mechanisms for the metal oxides used in this study.     

Preparation of Fischer–Tropsch cobalt catalysts supported on carbon nanofibers and silica using homogeneous deposition-precipitation

Homogeneous deposition-precipitation on either a silica or carbon nanofiber (CNF) support of cobalt from basic solution using ammonia evaporation was studied and compared with conventional deposition from an acidic solution using urea hydrolysis. In the low-pH experiment, the interaction between precipitate and silica was too high; cobalt hydrosilicates were formed requiring a reduction temperature of 600 °C, resulting in low cobalt dispersion. Lower interaction in experiments performed in a basic environment yielded a well-dispersed Co₃O₄ phase on silica, and after reduction at only 500 °C, a catalyst with 13-nm cobalt particles was obtained. On CNF from an acidic solution, cobalt hydroxy carbonate precipitated and displayed a low interaction with the support resulting after reduction at 350 °C in a catalyst with 25-nm particles. From basic solution we obtained high dispersion of cobalt on the CNF, probably related to the greater ion adsorption. After drying, Co₃O₄ crystallites were obtained that, after reduction at 350 °C, resulted in a catalyst with 8-nm Co particles. Samples prepared in the high-pH experiment had 2–4 times higher cobalt-specific activity in the Fischer–Tropsch reaction than their low-pH counterparts. CNF support materials combined with the high-pH deposition-precipitation technique hold considerable potential for cobalt-based Fischer–Tropsch catalysis.     

Viscoelastic behavior of poly(butyl methacrylate) densely grafted on silica nanoparticles

A series of poly(butyl methacrylate)s (PBMAs) with various molar masses (33 000–270 000 g mol^?1), which were densely grafted on fumed silica nanoparticles (PBMA–SiO₂), were synthesized by surface‐initiated atom transfer radical polymerization. The dynamic viscoelastic behavior of PBMA–SiO₂ was systematically investigated in the solid and molten states with oscillatory strains, and compared to that of a conventional nanocomposite (PBMA/SiO₂). The storage moduli of PBMA–SiO₂ and PBMA/SiO₂ are equivalent in the solid state, whereas the storage modulus of PBMA–SiO₂ is lower than that of PBMA/SiO₂ in the molten state, especially at high silica loading. This is because the formation of a network structure composed of the silica nanoparticles in PBMA–SiO₂ is strongly suppressed by the polymer brushes on the particles. In contrast, even at low silica loading, the PBMA–SiO₂ system exhibits a gel‐like behavior resulting from a steric repulsion between the composite particles, because all of the tethered polymers behave as bound polymers. Copyright © 2011 Society of Chemical Industry     

Impedance spectroscopy of siloxane‐containing polyazomethines blended with SiO2

Two siloxane‐containing polyazomethines (PAZx) blended with SiO₂ were investigated. SiO₂ was obtained by sol‐gel method. The size of obtained SiO₂ particles was about 408 nm as was confirmed by SEM technique. For the blended with silica polymers absorption UV‐vis properties were tested and compare with unblended ones. Electrical behavior of the two kind devices indium tin oxide (ITO)/PAZx : SiO₂/Al and ITO/PEDOT : PSS/PAZx : SiO₂/Al were tested by impedance spectroscopy in dark and under illumination (halogen lamp, 100 mW/cm²) in the frequency range of 1 Hz to 1 MHz with maximum voltage value of 20 mV. For all measured devices, Nyquist plots were presented. PEDOT : PSS interlayer improved electrical properties of made prototype polymeric solar cells. Blending PAZx with silica increased conductivity from 10^–15 to 10^–8 S/cm. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013