Structure of hybrid colloid-polymer xerogels

Crack-free monolithic gels were prepared from different mixtures of colloidal silica with a sol solution containing tetraethoxysilane, under powerful ultrasonic agitation (sonosol). Recently, information on the structure of these gels, inferred from N2 adsorption and mercury intrusion porosimetry, was presented. In the present paper, these data were used to construct structural models of the gels using Monte Carlo calculations on the basis of random close packing (RPC) premises. In addition, the structure of gels under study was investigated by transmission and scanning electron microscopy. The material can be described as a composite in which the sonogel is the matrix and the colloid particles the reinforcing phase. For low colloid content, the colloid forms discrete clusters, and the main structural characteristic of sonogels, i.e., a network of uniformly sized particles of approximately 3-4-nm radius, remains unmodified. However, for high colloid silica content, a multimode distribution appears, the structure is discontinuous, and only colloid aggregates larger than 100 nm are observed. For medium colloid content, aggregates of approximately 50-100 nm can be seen, but the sonogel structure extends throughout the whole material. By the processing method and election of a suitable precursor concentration, it is possible to design the composite for specific purposes.     

Further Insights into the Porous Structure of TEOS Derived Silica Gels

The porous structure of TEOS derived silica gels was studied using nitrogen adsorption at 77 K. Silica gels were prepared using TEOS, H₂O and ethanol for different molar ratios. No catalyst was used in this study. Silica gels were also heat treated up to 1000°C. The nitrogen sorption isotherms were analyzed by two models: Fractal and Percolation Theories. Using the fractal analysis approach, the surface roughness of the porous structure of silica gels was determined. The surface fractal dimension depends on the hydrolysis conditions and heat treatment. The surface fractal dimension decreases with increasing H₂O/TEOS molar ratio or heating temperature. For the silica gels studied, the surface fractal dimension changed from 2.6 to 2.5 after heating the gels, and from 2.4 to 2.6 with decreasing H₂O/TEOS ratio.Using the Percolation theory, we have determined the connectivity of the porous structure of silica gels. The extent of sorption hysteresis of the nitrogen isotherms reflects the connectivity of the pore network. The mean coordination number (connectivity) Z, and the linear dimension of the network, L, have been calculated from the hysteresis of the isotherms. For the as-prepared silica gels, Z was about 8 and L close to 2. On heating the gels, Z decreases to 4 and L increases to 7, results which are in accordance with the collapse of the porous network.     

Densification of Sol-Gel Thin Films by Ultraviolet and Vacuum Ultraviolet Irradiations

Structural changes in SiO₂ and TiO₂ gel films were investigated using ultraviolet (UV) and vacuum ultraviolet (VUV) irradiations. A significant compaction with dehydration of SiO₂ gel films was induced by irradiation of photons in the range of 9–18 eV. The refractive index and the shrinkage of the irradiated SiO₂ gel films were comparable to those obtained by sintering at 1000°C. Densification of TiO₂ gel films was also observed with irradiation of 5–14 eV photons. However, effects of the irradiation on TiO₂ gel were smaller that those on SiO₂ gel. The structural changes in the gel films are attributed to electronic excitations which are induced by irradiation with photons having higher energies than the bandgap of the oxides. The photo-induced effects are presumed to depend on the optical properties and structure of the gels.     

Silica-Zirconia Coatings Produced by Dipping and EPD from Colloidal Sol–Gel Suspensions

SiO₂-ZrO₂ sols have been prepared via acid catalysis using a commercial colloidal suspension of zirconia and two silica alkoxides; tetraethoxysilane (TEOS) and methyltriethoxysilane (MTES). Suspensions with 10, 15 and 25-mol% of ZrO₂ were prepared. The stability of the suspensions was followed by rheological measurements showing that the amount of water incorporated with the colloidal suspension is the factor that limits the maximum ZrO₂ content. Coatings have been prepared by dipping using the suspensions up to 25-mol% ZrO₂ onto glass-slides at different withdrawal rates. EPD process has been used to prepare coatings onto stainless steel AISI 304 using the suspension with 25-mol% ZrO₂ at different pHs. The parameters associated with the EPD process (current density, electric field, potential and deposition time) have been evaluated. The critical thickness for a ZrO₂ addition of 25-mol% was 0.8 μm and it increased for diminishing ZrO₂ content.     

Synthesis and characterization of Fe2O3/SiO2 nanocomposites

Fe₂O₃/SiO₂ nanocomposites based on fumed silica A-300 (S_BET = 337 m²/g) with iron oxide deposits at different content were synthesized using Fe(III) acetylacetonate (Fe(acac)₃) dissolved in isopropyl alcohol or carbon tetrachloride for impregnation of the nanosilica powder at different amounts of Fe(acac)₃ then oxidized in air at 400–900 °C. Samples with Fe(acac)₃ adsorbed onto nanosilica and samples with Fe₂O₃/SiO₂ including 6–17 wt% of Fe₂O₃ were investigated using XRD, XPS, TG/DTA, TPD MS, FTIR, AFM, nitrogen adsorption, Mössbauer spectroscopy, and quantum chemistry methods. The structural characteristics and phase composition of Fe₂O₃ deposits depend on reaction conditions, solvent type, content of grafted iron oxide, and post-reaction treatments. The iron oxide deposits on A-300 (impregnated by the Fe(acac)₃ solution in isopropanol) treated at 500–600 °C include several phases characterized by different nanoparticle size distributions; however, in the case of impregnation of A-300 by the Fe(acac)₃ solution in carbon tetrachloride only α-Fe₂O₃ phase is formed in addition to amorphous Fe₂O₃. The Fe₂O₃/SiO₂ materials remain loose (similar to the A-300 matrix) at the bulk density of 0.12–0.15 g/cm³ and S_BET = 265–310 m²/g.     

Preparation of Cu/SiO2 Catalyst by Solution Exchange of Wet Silica Gel

Structural formation process of Ni/SiO₂ and Cu/SiO₂ catalysts prepared by solution exchange of wet silica gel was investigated. Microstructures of Cu/SiO₂ and Ni/SiO₂ were quite different from each other. In the case of Cu/SiO₂, Cu particles with diameter of ca. 3–5 nm dispersed homogeneously at less Cu content, and the particle size of Cu as well as pore size of silica gel support increased with increasing Cu content. In the Ni/SiO₂, the Ni particles with diameter of ca. 6–10 nm gathered densely to form aggregates in silica matrix resulting in sea-island structure, whereas the size of Ni particle slightly increased with increasing Ni content. The difference in the structure of the metal-silica composites is probably caused by the difference in interaction between silica gel network and metal ions during drying and heating processes.     

Thermodynamic considerations of the grain size dependency of material properties

Phase transitions which depend on grain size induce very interesting properties in materials such as zirconia or barium titanate. A new and rigorous thermodynamic treatment of this type of phase transition is proposed with consideration of the surface phenomena. An interpretation is given of the observed differences when the material—particularly BaTiO₃—under consideration is a fine grain powder or is a fine grain ceramic. In celebration of the 60^th birthday of Dr. Andrew K. Galwey     

Selective water sorbents for multiple applications, 3. CaCl2 solution confined in micro- and mesoporous silica gels: Pore size effect on the “solidification-melting” diagram

In this communication we present a low-temperature “solidification-melting” phase diagram for CaCl₂/H₂O solutions confined in KSK and KSM silica gels. At salt concentrations of 0–48 wt. %, the diagram has been found to lie below the diagram reported for the bulk system by 15–30°C. It shows a depression of the solution melting point due to its confinment to the pores. Several other peculiarities of melting and solidification in this system are also reported and discussed. Beside fundamental interest, the data obtained could be of importance in many commercial areas such as refrigeration, accumulation of low temperature heat, frost prevention in building materials,etc.     

Synthesis of Ag/SiO2 nanocomposite material by adsorption phase nanoreactor technique

Ag/SiO₂ nanocomposite was synthesized in a nanoreactor formed by adsorption layer on silica surface. Ag nanoparticles were prepared by the reduction of Ag ion with ethanol at alkaline condition. By using TEM and XRD, the effects of NaOH concentration, water and temperature on the appearance and grain size of Ag particles were analyzed, respectively. The adsorption curve of NaOH was measured by electrical conductivity meter. The experiment result revealed that Ag grain size decreased while increasing NaOH concentration or while increasing water in our system. Ag grain size increased with the increase of temperature. And Ag aggregated seriously when temperature is up to 60 °C. Finally, after exploring the optimum conditions of reaction, we successfully obtained the well-distributed Ag nanoparticles on surface of silica, and average grain size of Ag nanoparticles reached 5 nm.     

Highly sensitive spectrofluorimetric determination of cysteine by Cu2+-morin complex

p-Toluenesulfonylamide was immobilized on silica gel and on nm-sized silicium dioxide (SiO₂). Their adsorption efficiency toward metal ions was investigated by the batch equilibrium technique. Although silica gel and nm-SiO₂ have the same composition (silicon and oxygen), the difference in their sizes and surface structures results in distinct chemical activity and selectivity. At pH 4, the adsorption capacity of modified silica gel adsorbent was found to be 4.9, 5.0, 33.2, and 12.6 mg g⁻¹ for Cr(III), Cu(II), Pb(II) and Zn(II), respectively. However, the adsorption capacity of nm-SiO₂ adsorbent toward Cr(III) was 26.7 mg g⁻¹ under ultrasonic dispersing. The potential application of p-toluenesulfonylamide-modified silica gel for simultaneous preconcentration of trace chromium, copper, lead and zinc from two standard reference materials and two food samples was performed with satisfactory results. Correspondence: Xijun Chang, Department of Chemistry, Lanzhou University, Lanzhou 730000, P.R. China     

Photocatalytic Reduction of Greenhouse Gas CO2 to Fuel

Sun is the Earth’s ultimate and inexhaustible energy source. One of the best routes to remedy the CO₂ problem is to convert it to valuable hydrocarbons using solar energy. In this study, CO₂ was photocatalytically reduced to produce methanol, methane and ethylene in a steady-state optical-fiber reactor under artificial light and real sunlight irradiation. The photocatalyst was dip-coated on the optical fibers that enable the light to transmit and spread uniformly inside the reactor. The optical-fiber photoreactor, comprised of nearly 120 photocatalyst-coated fibers, was designed and assembled. The XRD spectra indicated the anatase phase for all photocatalysts. It is found that the methanol yield increased with UV light intensity. A maximum methanol yield of 4.12 μmole/g-cat h is obtained when 1.0 wt% Ag/TiO₂ photocatalyst was used under a light intensity of 10 W/cm². When mixed oxide, TiO₂–SiO₂, is doped with Cu and Fe metals, the resulting photocatalysts show substantial difference in hydrocarbon production as well as product selectivity. Methane and ethylene were produced on Cu–Fe loaded TiO₂–SiO₂ photocatalyst. Since dye-sensitized Cu–Fe/P25 photocatalyst can fully harvest the light energy of 400–800 nm from sunlight, its photoactivity was significantly enhanced. Finally, CO₂ photoreduction was studied by in situ IR spectroscopy and possible mechanism for the photoreaction was proposed.     

Highly Dispersed Ce x Zr1−x O2 Nano-Oxides Over Alumina, Silica and Titania Supports for Catalytic Applications

We have been exploring the utilization of supported ceria and ceria–zirconia nano-oxides for different catalytic applications. In this comprehensive investigation, a series of Ce _x Zr_1−x O₂/Al₂O₃, Ce _x Zr_1−x O₂/SiO₂ and Ce _x Zr_1−x O₂/TiO₂ composite oxide catalysts were synthesized and subjected to thermal treatments from 773 to 1073 K to examine the influence of support on thermal stability, textural properties and catalytic activity of the ceria–zirconia solid solutions. The physicochemical characterization studies were performed using X-ray diffraction (XRD), Raman spectroscopy (RS), X-ray photoelectron spectroscopy (XPS), and high-resolution transmission electron microscopy (HREM), thermogravimetry and BET surface area methods. To evaluate the catalytic properties, oxygen storage/release capacity (OSC) and CO oxidation activity measurements were carried out. The XRD analyses revealed the formation of Ce_0.75Zr_0.25O₂, Ce_0.6Zr_0.4O₂, Ce_0.16Zr_0.84O₂ and Ce_0.5Zr_0.5O₂ phases depending on the nature of support and calcination temperature employed. Raman spectroscopy measurements in corroboration with XRD results suggested enrichment of zirconium in the Ce _x Zr_1−x O₂ solid solutions with increasing calcination temperature thereby resulting in the formation of oxygen vacancies, lattice defects and oxygen ion displacement from the ideal cubic lattice positions. The HREM results indicated a well-dispersed cubic Ce _x Zr_1−x O₂ phase of the size around 5 nm over all supports at 773 K and there was no appreciable increase in the size after treatment at 1073 K. The XPS studies revealed the presence of cerium in both Ce⁴⁺ and Ce³⁺ oxidation states in different proportions depending on the nature of support and the treatment temperature applied. All characterization techniques indicated absence of pure ZrO₂ and crystalline inactive phases between Ce–Al, Ce–Si and Ce–Ti oxides. Among the three supports employed, silica was found to stabilize more effectively the nanosized Ce _x Zr_1−x O₂ oxides by retarding the sintering phenomenon during high temperature treatments, followed by alumina and titania. Interestingly, the alumina supported samples exhibited highest OSC and CO oxidation activity followed by titania and silica. Details of these findings are consolidated in this review.     

SiO2-TiO2 membranes by the sol-gel process

The use of membranes for gas separation represents an important alternative from the viewpoint of energy conservation in industrial separation processes. Polymeric Si-Ti sols prepared from titanium isopropoxide (Ti(OPrⁱ)₄) and tetraethoxysilane (TEOS) were used to deposit membranes on α-Al₂O₃ supports. Acetylacetone (2,4 pentanedione, acacH) and isoeugenol (2-methoxy-4-propenylphenol, isoH) were employed separately to chelate the Ti precursor in order to slow down the chemical reactivity, avoiding precipitation. The radial distribution functions (RDF) of the gels aging at room temperature were obtained. The xerogels were studied by Thermal Gravimetric (TGA) and Differential Thermal (DTA) Analysis in air. The Microporosity of the solids calcined at 773 K was determined by N₂-adsorption at 77 K. The membrane thickness was determined from SEM photographs. Preliminary permeance results of the supported membranes on commercial alumina support were obtained for He, N₂ and CO₂ in a single gas equipment. At 773 K the separation factors α(He/CO₂) and α(N₂/CO₂) for both membranes exceeds the theoretical Knudsen limit.     

Basic Indole Ring Enantiomer Separation on Cellulose Tris(3,5‐dimethylphenylcarbamate) Coated TiO2/SiO2 Chiral Stationary Phase

Abstract

Cellulose tris(3,5‐dimethylphenylcarbamate) (CDMPC) coated TiO₂/SiO₂ has been prepared by coating CDMPC on TiO₂/SiO₂ which consists of micrometer‐sized silica spheres as core and nanometer‐sized titania particles as surface coating. Eight basic indole ring derivative enantiomers were separated on this CDMPC coated CSP and symmetrical peaks were obtained using hexane as the mobile phase and various alcohols as modifiers. The influence of the mobile phase composition and structural variation of the solutes on the enantioseparation was investigated and discussed.     

Reactivity of some sulphur- and non-sulphur-containing amino acids towards water soluble colloidal MnO<Subscript>2</Subscript>. A kinetic study

Kinetic data for the oxidation of glutathione (reduced, GSH), cysteine, glycine and glutamic acid by colloidal manganese dioxide, (MnO₂) _n are reported. Colloidal MnO₂, oxidized glutathione to disulphide (glutathione, oxidized), was reduced to manganese (II). Glycine and glutamic acid (structural units of glutathione) are not oxidized by colloidal MnO₂, but the other structural unit, cysteine, is also oxidized by the same oxidant under similar experimental conditions. This is interpreted in terms of the rate-determining colloidal MnO₂-S bonded intermediate. The reactivity of GSH towards colloidal MnO₂ is very much higher than cysteine. Kinetics of oxidation of GSH and cysteine by colloidal MnO₂ were performed spectrophotometrically as a function of [GSH], [cysteine], colloidal [(MnO₂) _n ], [HClO₄], temperature and trapping agents sodium fluoride and manganese (II) (reduction product of colloidal MnO₂). The purpose of this work was to study the role of –NH₂, –COOH, –SH groups present in the carbon chain of the above amino acids. It was found that the reactivity of –SH group is higher than –NH₂ and –COOH groups. The mechanisms, involving a colloidal MnO₂ complex with GSH and cysteine, are proposed. The complexes decompose in a rate-determining step, leading to the formation of free radical and manganese (III), which is also an intermediate. The dimerization of radicals takes place in a subsequent fast step to yield the products.     

Bi<Subscript>3.25</Subscript>La<Subscript>0.75</Subscript>Ti<Subscript>3</Subscript>O<Subscript>12</Subscript> powders by the complex polymerization method: synthesis,characterization and morphology

Lanthanum-modified bismuth titanate (Bi_3.25La_0.75Ti₃O₁₂, BLTO) powders were prepared by the complex polymerization method. The structure and morphology of BLTO powders were investigated by X-ray diffraction and scanning electron microscopy. The complexation of citric acid with the metallic cations was detected by Fourier transformed infrared (FT-IR). The thermal analyses of obtained gels were investigated by differential thermal gravimetric (DTG). The pure and normally stoichiometric phase of BLTO powders could be obtained at relatively low temperature of 550–700 °C even if the bismuth content is not excess in the starting precursors, while the secondary phase could be detected at lower and higher calcination temperatures. The shape of the BLTO grains is similarly to platelet in Bi-layer structure and stoichiometry BLTO was detected by the analysis of energy dispersive spectrometry.     

The synthesis and characteristic study of 6FDA-6FHP-NLO polyimide/SiO2 nanohybrid materials

Hybrid nanocomposite films of silica (SiO₂) in polyimide (PI) from 4,4^′-(hexafluoroisopropylidene) diphthalic arhydride (6FDA), 2,2-Bis (3-amino-4-hydroxyphenyl) hexafluoropropane (6FHP) and nonlinear optical (NLO) molecule have been successfully fabricated by an in situ sol-gel process. The silica content in the hybrid films was varied from 0 to 22.5 wt%. These nanocomposite films exhibit fair good optical transparency. Fourier transform infrared (FTIR) spectroscopy results confirm the formation of SiO₂ particles in PI matrix. Scanning electron microscope (SEM) images show that the SiO₂ phase is well dispersed in the polymer matrix. Their glass transition behavior and thermal stability were investigated by differential scanning calorimeter (DSC) and thermal gravimetric analysis (TG).     

Preparation and Optical Properties of Submicron SiO2 Spheres Doped with YAG:Nd3+ Nanocrystallites

The sol-gel technology has been applied to obtain SiO₂ spheres of submicron size. The spheres have been doped with YAG:Nd³⁺ nanocrystallites. The nanocrystallites have been obtained from aqueous solutions of citric acid, yttrium, aluminum and neodymium chlorides. The obtained gels have been heated up to 800°C. Emission spectra as well as the excited state lifetimes have been measured at room and liquid nitrogen temperatures. The structural characterization has been performed by means of transmission and scanning electron microscopies (TEM, SEM) and powder diffraction (XRD) measurements. We have observed that the YAG:Nd³⁺ nanocrystallites demonstrate pronounced dependence of the emission intensities on the excitation power as compared to the YAG:Nd³⁺ crystallites embedded into the submicron SiO₂ spheres. The results suggest that silica spheres/YAG:Nd³⁺ composites are expected to be good hosts for microlaser systems.     

SiO2气凝胶负载的Ni催化剂在甲烷部分氧化制合成气反应中的催化性能及稳定性

以常压有机溶剂置换(A)和溶剂置换-表面改性(B)方式制备的两种SiO₂气凝胶(SiO₂-A(或B)型气凝胶,记为SiO₂-A(or B)G)为载体, 采用常规浸渍法和聚乙烯吡咯烷酮(PVP)添加浸渍法合成不同SiO₂气凝胶负载的Ni/SiO₂催化剂, 并考察其催化的甲烷部分氧化(POM)制合成气的反应性能. 结果表明, 各催化剂的初始反应性能相近, 但Ni/SiO₂-BG的POM稳定性明显较Ni/SiO₂-AG的差, 而PVP添加制备的催化剂稳定性则获明显改善, Ni/SiO₂-AG-PVP、Ni/SiO₂-BG-PVP上POM稳定性相近. 结合X射线衍射(XRD)、程序升温还原反应(H₂-TPR)、高分辨透射电镜(TEM)和Brunauer-Emmett-Teller (BET)等表征结果的分析发现: (1) SiO₂-AG表面上存在一定量的羟基, 可促进亲水性金属物种与其的相互作用, 而SiO₂-BG表面上基本为有机基团, 与亲水性金属物种几乎无作用; (2) PVP的存在可使金属物种进入亲/疏水载体孔道深处, 抑制焙烧中载体骨架的收缩和金属颗粒的生长, 进而促进金属-载体的相互作用. 这二者均能有效地提高催化剂的POM反应稳定性.