Pore Structure Of Silica Research Articles

Changes in the pore structure of silica in chemical modification with organic compounds

Changes in the pore structure of silica adsorbents in their chemical modification with organic compounds of various kinds by gas-and liquid-phase processes, as well as those under conditions of high pressure of the reagents, were studied.

Preparation of silica-coated Pt metal nanoparticles using microemulsion and their catalytic performance

Pt–SiO2 catalysts were prepared by using water-in-oil type microemulsion. The catalysts were prepared by hydrolysis of tetraethylorthosilicate in the microemulsion containing Pt species, followed by calcination in air and reduction with H2. This preparation method for Pt–SiO2 provided Pt metal nanoparticles with a diameter of ca. 6 nm covered uniformly with silica layers, while a conventional impregnation method formed Pt metal particles supported on the silica surface. The Pt catalysts coated with silica (coat–Pt) showed different catalytic performance for the competitive oxidation of mixed hydrocarbons from Pt catalysts supported on silica (imp–Pt), i.e. in the competitive oxidation of methane and iso-butane, coat–Pt catalysts preferentially oxidized methane, while iso-butane was oxidized selectively over imp–Pt catalysts. The results of the adsorption of Ar on coat-Pt showed that silica in coat-Pt had porous structures of pore diameters < 1 nm. Because the porous structure of silica, which wrapped Pt metal particles, controlled the diffusion rate of reactant molecules, coat-Pt showed a specific catalytic performance for competitive oxidation.

Novel Amorphous Functional Materials for Trapping Nitrosamines

Novel amorphous functional materials are designed to modify amorphous silica with alumina. They are first presented as efficient adsorbents to trap both volatile nitrosamines and bulky tobacco specific nitrosamines in the environment. Selective adsorption, temperature programmed surface reaction (TPSR), and FTIR methods are employed to study the impact of alumina modification on the ability of silica to adsorb and catalytically degrade nitrosamines. Due to the special interaction between the N-NO group of nitrosamines and the aluminum ion in the composite, nitrosamines can be very easily trapped by the composite. Moreover, this cost-efficient material first shows a remarkable adsorptive capability and catalytic activity in reduction of NNN (N-nitrosonornicotine) in the liquid phase. The new concept of designing a multifunctional trap for carcinogenic pollutants, which combines the amorphous silica's pore structure with the specific adsorbing/catalyzing features of metal ions, proves feasible.

Influence of Humidity on Electrical Characteristics of Self-Assembled Porous Silica Low-k Films

The influence of humidity on the dielectric constant and leakage current of self-assembled porous silica films which have two-dimensional hexagonal periodic porous structures was investigated quantitatively by proposing a new water adsorption model. The amount of adsorption was calculated by the modified Rayleigh model, where molecules are assumed to be adsorbed on the inner surface of cylindrical porous silica structures and form a dispersal concentric double-layer dielectric cylinder system. The amount of calculated by the proposed model was consistent with the measured dielectric constant and thermogravimetry data. It suggests that inner-surface coverage of the cylindrical porous silica wall with a hydrophobic group is the most effective way to suppress water adsorption. Hexamethyldisilazane as a surface coverage molecule was introduced to the periodic cylindrical porous silica film and the leakage current was suppressed by a factor of even below 0.5% relative humidity, resulting in the improvement of time-dependent dielectric breakdown lifetime by a factor of 30.

Fractal dimensions of silica gels generated using reactive molecular dynamics simulations

We have used molecular dynamics simulations based on a three-body potential with charge transfer to generate nanoporous silica aerogels. Care was taken to reproduce the sol-gel condensation reaction that forms the gel backbone as realistically as possible and to thereby produce credible gel structures. The self-similarity of aerogel structures was investigated by evaluating their fractal dimension from geometric correlations. For comparison, we have also generated porous silica glasses by rupturing dense silica and computed their fractal dimension. The fractal dimension of the porous silica structures was found to be process dependent. Finally, we have determined that the effect of supercritical drying on the fractal nature of condensed silica gels is not appreciable.

Templating of Multiple Ligand Metal Ion Complexation Sites in 8-Hydroxyquinoline-Modified Silica Sol−Gel Materials Investigated by in Situ Raman Spectroscopy

Metal ion templating in a sol-gel synthesis is used to develop multiligand 8-hydroxyquinoline binding sites in porous silica structures. The acid-base equilibria and the metal ion binding equilibria and stoichiometry of these materials are investigated by in situ Raman spectroscopy. This technique is capable of resolving spectral responses of the free ligand and its acid-base forms along the monomeric and dimeric ligand complexes with Cu2+. The proton-transfer equilibrium constants and first ligand binding equilibrium constant to Cu2+ for the metal ion-templated silica are equivalent to surface-immobilized 8HQ on silica gel. The second ligand binding constant to Cu2+, however, is comparable to the first ligand binding constant, which differs from free-solution behavior, where an order of magnitude smaller value is expected. The free energy available for binding the second ligand within the templated material is comparable to the first ligand, probably due to the nearly optimal location of the second ligand for binding, based on the templating that is done during this synthesis. The metal ion concentration responses of sol-gels prepared with varying amounts of metal ion during the syntheses were also tested, and the results indicate control over the fraction of templated binding sites.

Self-assembly of supermicro- and meso-porous silica and silica/gold nanoparticles using double-chained surfactants

Porous silica nanosized particles were prepared using tetramethylorthosilicate and two double carbon chained templates, n-didecyldimethylammonium bromide and n-didodecyldimethylammonium bromide. Samples were prepared using a simple modification of the Stöber method for synthesis of colloidal silica for template to silica ratios of 0.2 and 0.1 under homogeneous cosolvent conditions. Silica/gold nanocomposites were prepared similarly except for the addition of a gold colloid suspension prior to self-assembly. All samples were analyzed using X-ray diffraction, transmission electron microscopy and liquid nitrogen adsorption. The diameters of the pores were measured using three methods: volumetric-XRD; Gurvitsch; and BJH. The diameters of the pores ranged from 1.9 to 2.5 nm using the Volumetric-XRD method with pore wall diameters from 0.64 to 1.1 nm. The presence of supermicro- and meso-pores were confirmed using comparison t-plots. Both the volumetric-XRD and Gurvitsch methods were consistent with the t-plot analysis, whereas the BJH method underestimated pore diameters significantly. Samples had pore volumes in excess of 0.37 cm g −1 at STP and surface areas greater than 700 m 2 g −1. The symmetry of the silica pore structure was found to vary depending on both the template to silica ratio and the addition of a gold tag. The diameter of nanoparticles ranged from 58 to 590 nm and the position of the gold seed in each particle was consistent with a three-stage aggregative growth model.

Sol-gel coatings to improve processing of aluminium matrix SiC reinforced composite materials

This paper analyses the influence of the inner structure of a SiO2 coating deposited on SiC particles by a sol-gel route on reactivity and wettability between SiC particles and molten aluminium during composite manufacturing. Different heat treatments were applied to the sol-gel coatings to change the porous structure of the amorphous silica. The coatings subjected to the mildest heat treatment were more reactive with molten aluminium, enriching the Si content of the matrix composite, and hence inhibiting direct reaction of the SiC particles and the formation of aluminium carbide. Thermal analysis showed that Si enrichment altered the solidification behavior of the aluminium matrix and its resultant microstructure, influencing the manufacturing conditions and final properties of the fabricated composites.

Synthesis of porous silica structures with hollow interiors by templating nanosized calcium carbonate

Shape-controlled porous silica structures with hollow interiors were synthesized by using CaCO3 nanoparticles with the corresponding morphologies and sizes as the inorganic templates. Through the hydrolysis of sodium silicate, a continuous coating of SiO2 was deposited over the surface of CaCO3 cores to form core–shell structures. Hollow silica structures were finally obtained after calcining process and dissolving CaCO3 in HCl dilute solution. The as-prepared hollow products were characterized with TEM, SEM, EDS, BET, and FT-IR.

Solid supported micellar catalysis: some syntheses and characterisations

A synthetic method for chemically tethering organic based surfactant molecules and fluorous surfactant molecules onto a high surface and pore volume silica has been developed. It is shown that heterogenisation of a simple water-soluble ions (i.e. molybdate ion and Co 2+) can be accomplished onto these composite materials. They display an excellent activity and selectivity for oxidation of organic molecules in bulk organic solvent or in supercritical CO 2. Characterisations indicate that catalysis occurs within the surface supported surfactant assemblies (chemically tethered reversed micelles) in the porous silica structure.

Silica-based monoliths for rapid peptide screening by capillary liquid chromatography hyphenated with electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry.

Capillary liquid chromatography based on particulate and monolithic stationary phases was used to screen complex peptide libraries by fast gradient elution coupled on-line to electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI-FTICRMS). A slightly modified commercial electrospray interface consisting of a fused-silica transfer capillary and low dead volume stainless steel union at which the electrospray voltage was grounded enabled the effluent of all the capillary columns to be directly sprayed into the mass spectrometer. Stable electrospray conditions were generated over a wide range of mobile phase compositions, alleviating the need for a tapered end of the spray capillary, pneumatic assistance or preheated nebulizer gas. Since the identification of complex samples containing numerous isobaric substances is facilitated by chromatographic separation prior to mass spectrometry, stationary phase materials have been employed which offer a fast, efficient elution and, due to the complexity of samples, a high loading capacity. Silica-based monolithic capillary columns combine these three characteristics in a unique manner due to a tailored adjustment of both macro- and mesopore sizes in the highly porous silica structure. As we demonstrate by a comparative study of the silica-based monolithic and packed capillaries for LC/MS analysis of complex peptide libraries, silica monoliths show superior performance over packed beds of small-diameter particles with respect to analysis time and separation efficiency. Libraries with more than 1000 different peptides could be screened in less than 20 min.

Formation Mechanism of Spherical Mesoporous Silica Prepared Using Dodecylamine.

Formation mechanism of spherical mesoporous silica particles prepared using dodecylamine (DDA) as a template was investigated by measuring surface tension, conductivity, pH and concentration of silicic acid in the mixed solutions. Porous structure of spherical mesoporous silica was found for irregular structure by means of X-ray diffraction and scanning electron microscopy. The fomation of DDA micell in DDA-H2O-C2H5OH system was confirmed by the presence of inflection points for tension and conductivity curves at about 0.5 mmol/l of DDA concentration. When tetraethyl orthsilicate was added in the solution, the conductivity raised suddenly, stayed the same for a few minutes, and decreased steadily while the concentration of silicic acid decreased largely. In addition, the value of pH shifted from about 11 to 9.8 at the same time. From these results, the formation mechanism of spherical mesoporous silica is considered to be as follows; 1st step: cationization(DDAH+) of DDA; 2nd step: formation of DDA H+ micell; 3rd step: hydrolysis of TEOS and polymerization of silicic acid; 4th step: complexation of DDAH+ and polysilicic acid; 5th step: nucleation and growth of the mesoporous silica; and 6th step: reconfiguration and growth of the mesoporous silica particles.

Preparation of nanoporous silica using copolymer template

Silica solids with different nanoscaled pore diameters were formed from tertraethoxysiliane (TEOS) and copolymer gel of hydroxyl ethyl methacrylate (HEMA) and ethylene glycol dimethacrylate (EGDMA) where the latter worked as the cross-linker. The effects of molar ratio [HEMA]/[EGDMA] of copolymers on the porous structure of silica formed in the template gel were discussed. The properties and structure of gel and porous silica were characterized by Fourier transform infrared spectrometer (FT-IR), thermogravimetric analysis (TGA), scanning electron microscope (SEM) and transmission electron microscope (TEM). The results indicated that the pore diameter of silica could be varied by the [HEMA]/[EGDMA] molar ratio which directly affects the templated structure of copolymer gel. The larger pore diameter was obtained with the increased molar ratio, which corresponds to the decrease in the cross-link density. FT-IR and TGA suggested that the silica moiety is bonded to the network of the copolymer gel due to the gel/silica affinity based on the hydrogen bonding.

The structure of porous silica–polysiloxane hybrid materials

Porous organic–inorganic hybrid materials are prepared by means of a sol–gel process where tetraethoxysilane (TEOS) is hydrolyzed in a water–tetrahydrofuran (THF) mixture containing the reactive polar polysiloxane PS124. A porous material consisting of silica and PS124 domains is obtained after evaporation of the solvents. Such materials can be deposited as thin layers at the surface of electrodes and various transducers for the design of chemical sensors. A very important property of the materials is the accessibility to the organic domains where the active species are immobilized. Thus, the key parameters of the structure have been investigated by means of different techniques, solid state NMR, electron microscopy, nitrogen adsorption and small angle scattering. Solid state 29Si NMR shows the high condensation of the silica and the reaction of the PS124 during the sol–gel process. The porous volume mainly consists of macropores in interstices between large aggregated particles (0.3–1 μm). The size distribution of the pores displays only few micropores and no mesopores. The values of the specific areas of the three interfaces separating silica and the external medium, the PS124 polymer and the external medium and silica and PS124 have been determined by means of an original method combining nitrogen adsorption, X-ray and neutron small angle scattering measurements. The silica–exterior interfacial area is vanishingly small and the PS124–exterior interfacial area is much larger than that of the silica–PS124 interface. This means complete wetting of the silica by PS124 and a rough outer surface of the PS124 domains.

Directed growth of multiwalled carbon nanotubes from ordered porous silica structures

Ordered meso- and macroporous silica films can direct the growth of multiwalled carbon nanotubes from perpendicular to parallel to the substrate surface through control of the distribution of catalysts.

Fractal structure and specific surface of nanoporous silica

The nanoscale structure of porous silica is investigated using the Molecular Dynamics (MD) simulation method. Porous silica structures are produced by Coulomb driven atom aggregation and clustering at fixed low density. The structures are then relaxed under atmospheric pressure to leave overall densities of around 0.7 g/ml. The fractal dimension and specific surface of the samples are measured and found to be in agreement with experiment.

Structured materials syntheses in a self-assembled surfactant mesophase

A self-assembled surfactant mesophase containing AOT/lecithin/isooctane/water has been exploited as a template for the syntheses of nanostructured materials. The system is a gel sustaining equal volumes of an aqueous phase and an organic hydrocarbon phase. We have explored the synthesis of extended ceramic and polymer structures in this gel phase. Iron nanoparticles synthesized in the microaqueous phase have chain-like structures, while porous silica structures are obtained using precursors soluble in the organic microphase. The retention of catalytic activity of an oxidative enzyme in the microaqueous phase allows synthesis of structured polymers at the oil–water interface.

Effect of addition of polymeric species with ether moieties on porous structure of silica prepared by sol-gel method

The authors have investigated the effect of the addition of polymeric species (polyethyleneglycol, polypropyleneglycol and polytetramethyleneoxide) containing ether moieties in their main chain on the silica porous structure yielded via the sol-gel method. The obtained silica porous structures were characterized by nitrogen adsorption/desorption and SEM. Introducing those polymeric species apparently enhanced the pore formation. The enhancement is more noticeable for higher concentrations of the polymeric species, distributed from the microporous to mesoporous regions. The pore diameter at which the pore enhancement is induced by adding the polymeric species reduces with the fraction of ether moiety. Adding the polymeric species prior to the onset of the hydrolysis of tetraethylorthosilicate (TEOS) was necessary for the enhancement in the mesoporous region to takes place. The pore enhancement was more outstanding when HCL was used as the catalyst for the hydrolysis of the alkoxide than NH3. The mutual affinity between the added polymeric and silica species is considered as an important factor of the pore generation in the micro- and meso-porous regions.

MD simulation of nanoporous silica

The microscopic structure of nanoporous silica is investigated using the Molecular Dynamics simulation method. Porous silica structures are produced by simulated atom aggregation and clustering. The structures are then relaxed at atmospheric pressure. We find that on these length scales the simulated porous silicas have a minimum fractal dimension of 2.6 and density 0.7 g/ml.

Structure of Chemical Vapor Deposition Titania/Silica Gel

The structure of porous silica gel/titania synthesized using chemical vapor deposition (CVD) of titania via repeated reactions of TiCl4 with the surface and subsequent hydrolysis of residual Ti–Cl bonds at different temperatures was investigated by means of low-temperature nitrogen adsorption–desorption, X-ray diffraction (XRD), IR spectroscopy, and theoretical methods. A globular model of porous solids with corpuscular structure was applied to estimate the porosity parameters of titania/silica gel adsorbents. The utilization of this model is useful, for example, to predict conditions for synthesis of titania/silica with a specified structure. Analysis of pore parameters and fractal dimension suggests that the porosity and fractality of samples decrease with increasing amount of TiO2 covering the silica gel surface in a nonuniform layer, which represents small particles embedded in pores and larger particles formed at the outer surface of silica globules. Theoretical simulation shows that the Si–O–Ti linkages between the cover and the substrate can be easily hydrolyzed, which is in agreement with the IR data corresponding to the absence of a band at 950 cm−1 (characteristic of Si–O–Ti bridges) independent of the concentration of CVD-titania.