Effect of pH in a sol–gel synthesis on the physicochemical properties of Pd–alumina three-way catalyst

The effect of pH during sol–gel synthesis on the structural and physicochemical properties of a Pd–Al₂O₃ three-way catalyst (TWC) prepared by the sol–gel method was investigated by using BET, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) and solid state ²⁷Al MAS NMR. The Pd–Al₂O₃ catalyst prepared at pH=10 (Pd–Al₂O₃–B) showed a high activity in three-way catalytic reaction, a high dispersion of Pd, and large surface area and pore volume. A basic condition (pH=10) in the sol–gel process was essential for the preparation of highly dispersed palladium clusters on alumina gel. The formation of highly stable palladium oxide species in Pd–Al₂O₃–B that were not completely reduced at 423 K was ascribed to the strong interaction between Pd and oxygen in alumina texture, resulting in the formation of –Al–O–Pd bond.     

Fischer–Tropsch synthesis over Co–SiO2 catalysts prepared by the sol–gel method

Fischer–Tropsch synthesis was carried out in slurry phase over uniformly dispersed Co–SiO₂ catalysts prepared by the sol–gel method. When 0.01–1 wt.% of noble metals were added to the Co–SiO₂ catalysts, a high and stable catalytic activity was obtained over 60 h of the reaction at 503 K and 1 MPa. The addition of noble metals increased the reducibility of surface Co on the catalysts, without changing the particle size of Co metal significantly. High dispersion of metallic Co species stabilized on SiO₂ was responsible for stable activity. The uniform pore size of the catalysts was enlarged by varying the preparation conditions and by adding organic compounds such as N,N-dimethylformamide and formamide. Increased pore size resulted in decrease in CO conversion and selectivity for CO₂, a byproduct, and an increase in the olefin/paraffin ratio of the products. By modifying the surface of wide pore silica with Co–SiO₂ prepared by the sol–gel method, a bimodal pore structured catalyst was prepared. The bimodal catalyst showed high catalytic performance with reducing the amount of the expensive sol–gel Co–SiO₂.     

Syntheses and optical properties of hybrid materials containing azobenzene groups via sol–gel process for reversible optical storage

We focused on the synthesis and optical properties of new organic–inorganic hybrid materials containing azobenzene groups for rewritable optical recording media. Hybrid material (SGUR19‐n's) design has the structure of azobenzene group with urethane bonds in the silica network. The structures of the synthesized monomers and precursors were confirmed by FTIR and ¹H NMR spectrophotometer. Also, we reported on the effects of the direction of the polarized beam, the intensity of the induced beam, and the structures of SGUR19‐n's materials on the diffraction efficiency of SGUR19‐n's films. It was revealed that SGUR19‐n's films prepared by the sol–gel process have adequate writing–erasing–rewriting properties when used as a reversible optical storage material. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 4811–4818, 2006     

Periodic mesoporous hybrid monolith with hierarchical macro–mesopores

We describe the synthesis of hybrid monolith with hierarchically macro–mesoporous structure by a sol–gel process from the mixture of 1,4-bis(triethoxysilyl)benzene (BTEB) and tetraethoxysilane (TEOS) using poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) (P123) as structure directing agent under acid medium. The ordered mesoporous structure is embedded in the skeleton of the well-defined co-continuous macropore of the hybrid monolith. The influence of aging condition, acidic concentration, P123/Si ratio and TMB/P123 ratio on the macro- and meso-structure of the resultant monolith was investigated in details. The three-successive aging processes are necessary for the mesostructural order of the monolith to survive the surfactant removing process. The well-defined co-continuous macropore could be formed with the aid of TMB, which probably delays the phase separation rate during the sol–gel process through enhancing the interaction between the hydrophobic BTEB with P123 surfactant micelles. The macro–mesoporous hybrid monolith with phenylene bridged in the mesoporous framework may have potential application as a novel kind of stationary phase for high performance liquid chromatography (HPLC).     

Transparent UV curable antistatic hybrid coatings on polycarbonate prepared by the sol–gel method

UV curable, hard, and transparent organic–inorganic hybrid coatings with covalent links between the inorganic and the organic networks were prepared by the sol–gel method. These hybrid coating materials were synthesised using a commercially available, acrylate end-capped polyurethane oligomeric resin, hexanedioldiacrylate (HDDA) as a reactive solvent, 3-(trimethoxysilyl)propoxymethacrylate (MPTMS) as a coupling agent between the organic and inorganic phase, and a metal alkoxide, tetraethylorthosilicate (TEOS). The materials were applied onto polycarbonate sheets and UV cured, followed by a thermal treatment to give a transparent coating with a good adhesion and abrasion resistance. The high transmission and the thermogravimetric behaviour indicate the presence of a nanoscale hybrid composition. In a taber abrasion test, uncoated polycarbonate sheets exhibit a 48% decrease in light transmittance at 633 nm after 300 wear cycles, whereas the hybrid coating system containing 10 wt% silica shows only 10% decrease in light transmittance. For obtaining antistatic coatings, an intrinsically conductive polymer (ICP) was added to the optimised coating formulation. It is shown that the surface resistivity of the organic–inorganic hybrid coating can be reduced from 10¹⁶ to 10⁶ Ω for a high concentration of ICP in the coating formulation.     

Surface sol–gel modification of mesoporous silica molecular sieve SBA-15 with TiO2 in supercritical CO2

Mesoporous silica molecular sieve SBA-15 has been grafted with monolayer and doublelayer of titanium dioxide via surface sol–gel process in supercritical carbon dioxide (SC CO₂). The materials were characterized by nitrogen adsorption, X-ray photoelectron, X-ray diffraction (XRD), and Raman spectroscopy. The results suggest that the incorporated titanium dioxide is present mainly as anatase phase. The pore diameter decreases with the modification cycle number, and the titanium dioxide exists as a 5 Å thick film chemically anchored on the mesopores of SBA-15.     

Preparation, characterization and catalytic properties of Co–Nb2O5–SiO2 catalysts

Co–Nb₂O₅–SiO₂ catalysts were prepared using three different sol–gel procedures: (i) the colloidal sol–gel method using NbCl₅ and SiCl₄ as precursors; (ii) the polymeric sol–gel method using niobium ethoxide and tetraethyl-orthosilicate (TEOS); (iii) an intermediate procedure between the colloidal and polymeric sol–gel method in which the precursors were those utilized in the CSG but dissolved in a mixture of anhydrous ethanol and CCl₄. In all procedures, the elimination of the solvent carried out between 80 and 110°C was followed by a reduction in hydrogen flow (30 ml min⁻¹) at 773 K. Following these procedures, samples containing 10 wt.% Co and 15 wt.% niobium oxide (expressed as Nb₂O₅) were obtained. The characterization of the catalysts was performed using various techniques: N₂ adsorption and desorption curves at 77 K, NH₃- and H₂-chemisorption, TPO, XPS, XRD, and solid state ¹H MAS-NMR. Hydrogenolysis of butane was evaluated. The low reaction rates are assigned to the effect of the metal size, whereas the isobutane selectivity as well as the relatively high stability is due to the acidity of the support.     

Synthesis, characterization and catalytic properties of NiMo/Al2O3–MCM-41 catalyst for dibenzothiophene hydrodesulfurization

Ni–Mo/Al₂O₃–MCM-41 supported catalysts have been investigated for modification of MCM-41 by using sol–gel alumina incorporation method. Different catalysts were synthesized with variation of Si/Al molar ratios of 10, 50, 100 and 200. High specific surface area ordered meso-porous solid (MCM-41) was synthesized by using organic template method. In order to modify the low acidity of silica solid, the surface of MCM-41 was modified by incorporation of alumina. The surface acidity of solids modified significantly with variation of alumina content in the supports. The sol–gel method of alumina incorporation was used, which does not modify extensively the pore characteristics of MCM-41 material during the preparation of Al₂O₃–MCM-41. The X-ray diffraction intensities indicated that alumina as well as MCM-41 were present in the synthesized supports. Additionally, the hydrothermal stability of the Al₂O₃–MCM-41 materials was maintained up to 873 K using sever conditions like 100% water vapor stream. The catalytic activity of the catalysts was tested in the hydrodesulfurization (HDS) of dibenzothiophene (DBT). Selectivity was oriented mainly to the production of biphenyl (BP) and for high Si/Al ratios toward cyclohexylbenzene (CHB) and showed a higher conversion and better selectivity to hydrogenation (cyclohexylbenzene).     

Sol–gel-derived mesoporous γ-alumina granules

Synthesis of stable boehmite sols and structural properties of the sol–gel-derived γ-alumina using alkoxides, commercial boehmite powders, and aluminum chloride were studied for cost-effective preparation of high quality γ-alumina granules. Stable boehmite sols could be prepared from aluminum butoxide, aluminum isopropoxide, and commercial boehmite powders but not from aluminum chloride. The stable sol-derived γ-alumina has large surface area and nanopore size with uniform pore size distributions. Mesoporous spherical γ-alumina granules were prepared by an oil-drop process from aluminum butoxide, aluminum isopropoxide and a commercial boehmite powder under specific conditions. The sol–gel-derived γ-alumina granules are better than the commercial γ-alumina granules in terms of the pore structure and mechanical strength. The successful synthesis of γ-alumina granules by the sol–gel method from the commercial boehmite powder provides a basis for large-scale, cost-effective production of the high quality γ-alumina granules for use as sorbents or catalysts in industrial processes.     

Electromagnetic shielder compatible ZnO transparent conducting oxides hybridized with various sizes of Ag metal nanoparticles

In this paper, the effect of different sizes of Ag-nanoparticles dispersed in ZnO matrix using sol–gel method has been focused. Low-temperature crystallized ZnO thin films was achieved by sol–gel process, using zinc acetate dihydrate and 2-methoxyethanol as starting precursor and solvent, respectively. Various sizes of Ag-nanoparticles could be prepared by the spontaneous reduction method with changing the preparation temperatures and mole concentrations of Ag 2-ethylhexanoate in dimethyl sulfoxide solvent. The crystallographic structure of the Ag–ZnO hybrid film was analyzed by X-ray diffraction. Ag-nanoparticle size and optical property of Ag–ZnO hybrid films were measured by UV–vis spectrophotometer.     

Inorganic/organic hybrid coatings for aircraft aluminum alloy substrates

A series of water-based stable sol–gel systems have been developed. Various functional groups including amino, epoxy, vinyl, and allyl groups can be incorporated into the sol–gel network to interact with organic polymer resins. The solid content of these sol–gel-based coating formulations varies from 2.5 to 45%. The sol–gel coating of alumina–silica networks derived from low solid content solutions (2.5%) has been developed and evaluated to replace the current conversion coating pretreatment process. Sol–gel coatings derived from the high solid content solutions (17–45%) have shown excellent mechanic strength, good adhesion, and provide corrosion protection of the aluminum substrate when cured at elevated temperatures. Sol–gel/epoxy resin hybrid coatings have been formulated and studied. The hybrid coating showed enhanced mechanical strength such as hardness and abrasion resistance. When cured at elevated temperatures (80°C), all of the hybrid coatings studied passed wet adhesion testing. Some of the hybrid coatings pass wet adhesion testing when cured at room temperature. However, water-sensitivity remains for most of the room temperature cured hybrid coatings.     

Nanostructured sol–gel derived conversion coatings based on epoxy- and amino-silanes

Inorganic/organic hybrid conversion surface coatings for long-term protection of aluminum alloys against atmospheric corrosion have been developed based on a unique self-assembled nanophase particle (SNAP) coating process. Nano-particles with peripheral epoxy functional groups are pre-formed in an aqueous sol–gel process and then assembled and crosslinked upon application on the substrate surface. Mono-, di-, and tri-functional amino-silanes have been used as crosslinking agents. Corrosion resistance properties of these hybrid nanocomposite coatings studied by a variety of electrochemical testing methods including electrochemical impedance spectroscopy, scanning vibrating electrode technique, and potentiodynamic scan method, indicate excellent barrier protection performance of the coatings. For comparison, coatings crosslinked with amino-silanes offer significant improvement in coating performance over the previously described SNAP formulations with a conventional amine crosslinker—diethylenetriamine.     

The preparation and characterization of pyroelectric hybrid films containing C.I. Acid Red 29 (chromotrope 2R)

Pyroelectric hybrid materials were prepared from 4,5-dihydroxy-3-(phenyldiazenyl)naphthalene-2,7-disulfonic acid (C.I. Acid Red 29) and Nb or Ta alkoxides via a sol–gel process. The hybrid materials were analyzed and characterized using infrared spectrometry, atom force microscopy and thermal gravimetry differential thermal analysis. The dye was covalently linked to the inorganic network and organic–inorganic hybrid films had excellent pyroelectric character (average pyroelectric coefficient 1.79 × 10⁻⁶ C cm⁻² K⁻¹) and high thermal stability; no phase separation was observed in the films.     

Synthesis and characterization of urea bridged hybrid periodic mesoporous organosilica materials

Urea bridged organic–inorganic hybrid mesoporous SiO₂ materials (U-BSQMs) were synthesized through a sol–gel procedure by co-condensation of bis(triethoxysilyl propyl) urea (BSPU) under basic conditions using cetyltrimethylammonium bromide (CTAB) as organic template. X-ray diffraction (XRD) and transmission electron microscopy (TEM) confirmed the mesoporous structure of the sample. Fourier-transform infrared spectroscopy (FT-IR), solid state CP-MAS NMR spectroscopy of ²⁹Si (²⁹Si CP-MAS NMR) and ¹³C (¹³C CP NMR) indicated that most of the Si–C bonds are unbroken during the synthesis process. The N₂ adsorption–desorption results revealed that these hybrid mesoporous SiO₂ materials have bimodal distribution of pores with pore diameters of 2.4 and 3.8 nm, respectively. Thermogravimetric analysis (TG) demonstrated that about 16% Si–C bonds have been broke during the synthesis progress. This kind of material is expected to find possible application in ion supporting, drug delivery and catalysis.     

Passivation of metal alloys using sol–gel-derived materials — a review

The sol–gel method is being investigated as an environmentally compliant alternative for chromate-based conversion coatings currently in use. An overview of recent advances in the use of sol–gel derived coatings for improved corrosion resistance of aluminum and steel metal surfaces is given.     

Ternary evaluation of UV-curable seed oil inorganic/organic hybrid coatings using experimental design

Inorganic/organic hybrid coatings were prepared using epoxidized linseed oil with combinations of the two sol–gel precursors (titanium(IV) isopropoxide, tetraethyl orthosilicate), and a telechelic silicate based on a modified oligomeric caprolactone. The coatings were UV-cured with sulfonium initiators which concomitantly cured the epoxy functional organic phase and the sol–gel inorganic phase to form a co-continuous inorganic/organic system. A ternary experimental design was employed to elucidate the influence of inorganic modifier on the mechanical properties of the inorganic/organic hybrid coatings. Small angle X-ray scattering (SAXS) was used to evaluate radius of gyration of the metal-oxo-cluster. Various coating properties, such as hardness, impact resistance, adhesion, solvent resistance, and surface energy were investigated as a function of sol–gel precursors. Inorganic/organic hybrid coatings containing both tetraethyl orthosilicate and the modified caprolactone resulted in improved hardness and solvent resistance with no loss of impact strength. The inclusion of titanium(IV) isopropoxide in to the systems resulted in a systematic reduction in the coatings properties. This was attributed to inhibition of the organic crosslinking process as a consequence of absorption of ultraviolet light by the titanium-oxo-clusters.     

Effects of chemical additives on high-field electromechanical properties of PMN–PT–BT ceramics

A chemical additive method using sol–gel reactions was used to modify the composition and resultant properties of a commercially available 0.96(0.91Pb(Mg_1/3Nb_2/3)O₃–0.09PbTiO₃)–0.04 BaTiO₃ (PMN–PT–BT) ceramic. Without an additional ball-mixing process, several combinations of minor additives such as Fe, Ba, Sr, Zn, and Ti were incorporated by the chemical method. Weak- and high-field characteristics including dielectric properties, induced strain and polarization, and associated hystereses were evaluated for the samples sintered at 1200 °C for 4 h. All properties were found to depend on the chemical additives and temperature. Especially, the temperature dependence of high-field characteristics revealed different behavior from that reported for conventionally prepared samples. For example, the samples containing 0.5 wt.% SrO, 0.5 wt.% ZnO, and 0.5 wt.% TiO₂ did not exhibit a transition to piezoelectric behavior at the temperature expected from the dielectric measurements. Overall, the coating process has been successfully used to modify, and in some cases, enhance the high-field characteristics of PMN-based ceramics for electromechanical uses.     

Sol–gel derived organic/inorganic hybrid gels based on poly(2-hydroxyethyl aspartamide) and silica

The sol–gel derived polymer/silicate hybrid materials have attracted considerable attention in recent years. The incorporation of silicate phase into polymeric materials may constitute an important tool to either enhance mechanical properties or provide more biocompatibility to the resulting hybrids. PHEA, α,β-poly(N-2-hydroxyethyl-dl-aspartamide), is a class of poly(amino acid)s that has been widely studied as a biodegradable functional polymer with potential biomedical and pharmaceutical applications. Hydrogels from PHEA are formed easily by a chemical or physical crosslinking reaction but the resulting gels are mechanically weak and less thermally stable. In this study, hybrid materials were prepared based on PHEA and silicate. A sol–gel process was employed using TEOS and modified PHEA to introduce inorganic silicate phase within the polymer gel matrix. FT-IR and NMR were used to analyze the chemical structure of the PHEA derivatives. In addition, the morphology, thermal and swelling properties of the hybrid gels were examined.     

Silica based organic–inorganic hybrid nanocomposite coatings for corrosion protection

Silica based organic–inorganic hybrid nanocomposite coatings have been developed for corrosion protection of 1050 aluminum alloys by dip coatings technique. The hybrid sols were prepared by hydrolysis and condensation of 3-glycidoxypropyl-trimethoxysilane (GPTMS) and tetramethoxy silane (TMOS) in the presence of an acidic catalyst and bisphenol A (BPA) as cross-linking agent. Such prepared hybrid coatings were found to be relatively dense, uniform and defect free. Structural characterization of the hybrid coatings were performed using optical microscopy, scanning electron microscopy (SEM) and attenuated total reflectance-infrared (ATR-IR) spectroscopy. Corrosion resistance properties of the hybrid sol–gel coatings were studied by potentiodynamic scanning (PDS) and salt spray testing methods. The results indicate excellent barrier protection performance of the coatings. In addition, the effect of molar ratio of GPTMS–BPA (silane content) on corrosion resistance of the coatings was investigated. The PDS results demonstrated that the corrosion resistance of hybrid coatings improved by decreasing of silane content.     

Comparative photocatalytic behavior of Ta catalysts prepared by d.c.-sputtering, sol–gel and grafting in acetone degradation

Ta-doped photocatalysts were prepared using three different techniques: reactive d.c.-magnetron sputtering, sol–gel and grafting of tantalum on MCM-41 and TiO₂. The composition of the catalysts prepared by reactive d.c.-magnetron sputtering consisted in tantalum and titania, while that of sol–gel and grafted catalysts of Ta–titania–silica mixed oxides. The characterization of these catalysts was carried out using the adsorption-desorption isotherms of N₂ at 77 K, X-ray diffraction (XRD) patterns, small-angle X-ray scattering (SAXS), atomic force microscopy (AFM), XPS, TEM, and ²⁹Si- and ¹⁸¹Ta-CP/MAS NMR spectra. The behavior of these photocatalysts was checked in acetone degradation. The photocatalytic tests indicate that, depending on the preparation conditions and tantalum content, an enhancement of the activity occurs as compared with pure titania. Sol–gel and Ta-grafted MCM and TiO₂ catalysts exhibited a rather poor activity, which was correlated with the lack of crystallinity of titania.