Direct synthesis of CeO2/SiO2 mesostructured composite materials via sol–gel process

A series of CeO₂/SiO₂ mesostructured composite materials was synthesized by sol–gel process using Pluronic P123 as template, tetraethylorthosilicate as silica source and hexahydrated cerium nitrate as precursor under acid condition. The as-synthesized materials with Ce/Si molar ratio ranging from 0.03 to 0.3 were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), laser Raman spectroscopy (LRS), and N₂ adsorption. Characterization revealed that all samples possess ordered hexagonal mesoporous structure similar to SBA-15 and possess high surface area, large pore volume and uniform pore size. The fact that cerium species are present as highly dispersed CeO₂ nanocrystals in hexagonal matrix was confirmed by XRD combined with high-resolution TEM and selected area electron diffraction (SAED) analysis. Introduction of ceria to silica matrix can cause a distortion of hexagonal ordering structure and decrease pore diameter and increase the wall thickness of mesopores. Moreover, it can be found that this sol–gel route is a feasible, effective and simple method for templating synthesis of CeO₂/SiO₂ composite materials.     

Surface hydrophilic–hydrophobic property on transparent mesoporous silica thin films containing chromium oxide single-site photocatalyst

Using a sol–gel/spin-coating method with organic template, the transparent mesoporous silica thin films containing chromium oxide moieties can be prepared on quartz plate. The spectroscopic characterization has revealed that these thin films contain isolated and tetrahedrally coordinated chromium oxide moieties (single-site photocatalyst) in their frameworks. Even before UV light irradiation these Cr-containing mesoporous silica (CrMS) thin films have demonstrated a hydrophilic surface property more strongly than Cr-containing non-porous silica films and mesoporous silica thin films without chromium oxide. Furthermore, the Cr-containing mesoporous silica thin films showed the super-hydrophilic property not only under UV light irradiation but also even under visible light irradiation from a fluorescent lamp. In the presence of ethylene gas, the polyethylene was successfully formed on the surface of the CrMS thin film under UV light irradiation. After the formation of polyethylene the surface property of the CrMS thin film was converted into hydrophobic while keeping its transparency. The isolated and tetrahedrally coordinated chromium oxide moieties are responsible for these photo-induced surface reactions.     

Synthesis and characterization of mesoporous materials: Silica–zirconia and silica–titania

An experimental strategy was developed to obtain mesoporous SiO₂–ZrO₂ and SiO₂–TiO₂ mixed oxides by a sol–gel method, treating the gels hydrothermally. The solids were characterized by nitrogen physisorption, pyridine thermodesorption, ²⁹Si nuclear magnetic resonance, SEM and X-ray diffraction. The effects of ZrO₂ content, the generated pressure in the synthesis vessel and further modification of this type of procedure on the solids properties were studied. It was found that SiO₂–ZrO₂ and SiO₂–TiO₂ mixed oxides dried at atmospheric pressure developed type I isotherms. On the other hand, for the SiO₂–ZrO₂ and SiO₂–TiO₂ mixed oxides that were treated under pressure in the autoclave (at high SiO₂ content) the porosity was improved and mesoporous materials exhibiting type IV adsorption isotherms. Specific surface area and pore size distribution were a function of ZrO₂ and TiO₂ content. The materials exhibited narrow pore size distributions with pore diameters in the region of mesopores at about 4 nm and high surface areas, the highest being 481 m²/g for the 10 wt% ZrO₂ Si–Zr material. Differences in acidity as determined by pyridine thermodesorption were observed to depend on the synthesis parameters and ZrO₂ and TiO₂ concentration.     

Preparation and characterization of TiO2–SiO2 nano-composite thin films

TiO₂ nanocrystalline particles dispersed in SiO₂ have been prepared by the sol-gel method using titanium- and silicon-alkoxides as precursors. Nano-composite thin films were formed on the glass substrates by dip-coating technique and heat treated at temperatures up to 500 °C for 1 h. The size of the TiO₂ nanocrystalline particles in the TiO₂–SiO₂ solution ranged from 5 to 8 nm. The crystalline structure of TiO₂ powders was identified as the anatase phase. As the content of SiO₂ increased, the anatase phase tended to be stabilized to higher temperature. TEM results revealed the presence of spherical TiO₂ particles dispersed in a disk-shaped glassy matrix. Photocatalytic activity of the TiO₂–SiO₂ (1:1) thin films showed decomposition of 95% of methylene blue solution in 2 h and a contact angle of 10°. The photocatalytic decomposition of methylene blue increased and the contact angle decreased with the content of TiO₂ phase. TiO₂–SiO₂ with the molar ratio of 1:1 showed a reasonable combination of adhesion, film strength, and the photocatalytic activity.     

Carbon nanotubes/titanium dioxide (CNTs/TiO2) nanocomposites prepared by conventional and novel surfactant wrapping sol–gel methods exhibiting enhanced photocatalytic activity

In this work, a conventional sol–gel method was used to prepare CNTs/TiO₂ nanocomposites with different carbon loading in the range up to 20% CNTs/TiO₂ by weight. The bare CNTs (multi-walled), and the composites were characterized by a range of analytical techniques including TEM, XRD, BET and TGA–DSC. The results show the successful covering of the CNTs with discrete clusters of TiO₂ and bare CNTs surfaces which after annealing at 500 °C led to mesoporous crystalline TiO₂ (anatase) clusters. The photocatalytic activities of the nanocomposites were monitored from the results of the photodegradation of methylene blue (MB). The optimum CNTs/TiO₂ ratio in the composites prepared by conventional sol–gel method was found to be in the range from 1.5% to 5% by weight under the experimental conditions investigated. The maximum increase in activity was found to be 12.8% compared to the pure TiO₂ sample.In contrast, the synthesis of CNTs/TiO₂ nanocomposites by a novel surfactant wrapping sol–gel method [B. Gao, C. Peng, G.Z. Chen, G. Li Puma, Appl. Catal. B: Environ. 85 (2008) 17.] led to a uniform and well-defined nanometer-scale titania layer on individual CNTs. The nanocomposites were found to enhance the initial oxidation rate of methylene blue by onefold compared to the pure TiO₂ sample. This larger degree of rate enhancement is attributed to the supporting role of the CNTs and surface properties prepared by this novel modified sol–gel method.     

Preparation and characterization of sol–gel Al2O3/Ni–P composite coatings on carbon steel

Ceramic films have been applied to improve the resistance against high temperature oxidation of carbon steels. Alumina film was prepared on carbon steel surface by a dip coating technique. Electroless Ni–P plating film has been pre-deposited as an intermediate layer to improve the adherence of the film to carbon steel substrate. The oxidation kinetics of coated sample was investigated by measuring weight gain at 800 °C for 100 h. The surface and cross-section morphology of samples before and after oxidation were characterized by scanning electron microscopy (SEM). The composition and element distribution at the interface of the coated samples were analyzed by energy dispersive spectroscopy (EDS) and EMAX.The results show that the composite coating is uniform. The alumina coating adhesion strength to the substrate is up to 20 ± 2 N in scratch test because the alumina film presents interdiffusion of nickel and aluminum during heat treatment. The oxidation resistance test indicates higher oxidation resistance of as-coated carbon steel comparing to uncoated ones.     

Synthesis and characterization of mesoporous Ta2O5–TiO2 photocatalysts for water splitting

Two series of Ta₂O₅–TiO₂ photocatalysts (Ta:Ti = 4:1, 1:1 and 1:4) were prepared by sol–gel technique applying triblock copolymer of Pluronic P123 and were tested in platinized form (0.3 wt.%) in photodecomposition of water under ultraviolet and visible light (λ > 300 nm). It was found the mesoporous character of tantalum containing catalysts with relatively high surface area (100–130 m² g⁻¹) of these samples. However, higher concentration of TiO₂ in mixed oxides leads to the destruction of mesoporous character of synthesized photocatalysts. All samples were characterized with thermogravimetry, XRD, N₂ physisorption, DR-UV–vis and FTIR spectroscopy. The mixed oxides of Ta₂O₅–TiO₂ system showed much lower band-gap than pure Ta₂O₅ and relatively high activity in platinized state in photocatalytic hydrogen generation under visible. Doping of pure oxides and mixed systems with sulfur resulted in lowering of the band-gap values below 3 eV and much better activity in H₂ evolution reaction. Non-platinized photocatalysts showed activity in liquid phase cyclohexene photooxidation at 305 K.     

Oxidation of NO in gas phase by Au–TiO2 photocatalysts prepared by the sol–gel method

This work describes an innovative nanosemiconductor system, based on Au–TiO₂ for UV photo-assisted oxidation of nitrogen monoxide (NO). The synthesis of these materials was carried out by the sol–gel method. Titanium(IV) isopropoxide and HAuCl₄ were the precursors of the photocatalyst, which was prepared in acid conditions. The catalysts were characterized by the following techniques: BET, XRD, UV–vis and dark-field TEM. The evaluation of the photocatalytic activity was performed in situ using an FTIR spectrometer with high sensitivity and a UV spectrometer (365 nm) after 60 min at atmospheric pressure and room temperature. The NO + O₂ mixture concentration was 150 ppm. The photocatalytic conversion of nitrogen monoxide (NO) was studied by FTIR, which reached 85% in 60 min. The semiconductor type materials exhibited an enhanced photoactivity when compared with our reference TiO₂.     

Preparation and characterization of mesostructured polymer-functionalized sol–gel-derived thin films

The present study attempts to incorporate methacrylate-based polymers into ordered lamellar organic/inorganic nanocomposite films composed of alternating SiO₂/polymer layers. The films are prepared by dip-coating from a solution containing the monomers and silica precursors, thus leading to composite lamellar mesostructured materials through evaporation-induced self-assembly (EISA). A polymerizable coupling agent is added to covalently link the polymers to the silica matrix. The final polymer/SiO₂ hybrid material is obtained by a separate free-radical polymerization step, initiated by UV exposure or thermal treatment. Using trimethoxy(7-octen-1-yl)silane as a coupling agent, a procedure was established that preserved the mesostructure and maintained the swelling properties of the polymers, while acrylate-based coupling agents lead to a significant distortion of the film mesostructure. Structure and composition of the films were studied by X-ray diffraction, NMR and IR.     

On the structure-sensitivity of 2-butanol dehydrogenation over Cu/SiO2 cogelled xerogel catalysts

Cu/SiO₂ cogelled xerogel catalysts were synthesized by cogelation of tetraethoxysilane (TEOS) and chelates of Cu with 3-(2-aminoethylamino)-propyltrimethoxysilane (EDAS). The resulting catalysts are composed of metallic crystallites with a diameter of about 3 nm located inside microporous silica particles and larger metallic particles with a diameter of 8–30 nm located outside silica network. Cu/SiO₂ catalysts were tested for 2-butanol dehydrogenation. This reaction over Cu/SiO₂ cogelled xerogel catalysts is a structure-sensitive reaction: large metallic particles located outside silica particles are responsible for the dehydrogenation reaction, while small metallic particles located inside silica particles do not participate to the reaction.     

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 of templated mesoporous silica membranes on macroporous α-alumina supports via direct coating of thixotropic polymeric sols

A simple method for depositing mesoporous silica films directly on macroporous α-alumina supports is reported. A polymeric silica sol was prepared by hydrolysis of tetraethylorthosilicate in acid propanol in presence of hexadecyltrimethylammonium bromide as template. The rheology of the sol was changed by a modified urea-based thixotropic agent, which was used in a concentration range between 2% and 10% on volume. The dynamic viscosity of the sol was measured as a function of shear rate. The rheology modifier was found to increase the viscosity of the sol and confer at the same time a thixotropic behavior to it. Long range order in the unmodified and modified materials was analyzed by XRD on powders after calcination at 500 °C. It was found that the pore size increased with increasing concentration of additive, while the degree of order decreased until eventually, at high concentrations, the ordered porous structure collapsed completely. The gradual pore size increase at lower concentration is thought to be caused by swelling of the surfactant micelles by the rheology modifier.The modified sols were deposited via dip-coating on α-alumina disks. The morphology of the silica top layers was characterized by SEM. Pore size distributions and permeances of the prepared membranes were assessed by permporometry and single gas permeation measurements, respectively. The membranes had narrow pore size distributions, with an average diameter of about 2 nm. Permeabilities of H₂, He, CH₄ and O₂ were compared. Hydrogen permeances up to 6 × 10⁻⁷ mol s⁻¹ m⁻² Pa⁻¹ were measured at 473 K. The gas flux across the membrane follows a Knudsen-type mechanism, as shown by the permselectivities.     

Achievement of thick mesoporous TiO2 crystalline films by one-step dip-coating approach

A serial of surfactant-templated mesoporous TiO₂ films with the thickness over several micrometers have been successfully synthesized by one-step dip-coating and subsequent evaporation induced self-assembly method. Three different pre-condensed TiO₂ sols in the presence of surfactant (Pluronic F127) micelles with high viscosities were employed as the precursors for dip-coating. By treating the films in liquid paraffin as “shape protector” at certain high temperature for sufficient time, thick mesoporous films can be kept crack-free after calcinations. By employing the size-controlled titanium-oxo clusters in the sols as building blocks for self-assembly, the final obtained films represent tunable mesostructures. The mesoscopic characteristics of the films, such as Brunauer–Emmett–Teller surface areas, pore size distributions and pore wall crystallizations, have been comparatively studied. The results demonstrate that such tunable mesoscopic characteristics are greatly dependent on the structural and shape parameters of the initial formed inorganic clusters.     

Mesoporous silica fibers prepared by electroblowing of a poly(methyl methacrylate)/tetraethoxysilane mixture in N,N-dimethylformamide

Electroblowing and sol–gel reaction were combined to prepare mesoporous silica fibers. Poly(methyl methacrylate) (PMMA), a simple commercial polymer with weak hydrogen bonding to silica, was demonstrated to be valuable in improving the electrospinnability and as a porogenic agent. Compared with that in electrospinning, the jet stream in electroblowing was more stable and the resultant fibers were more uniform. The electroblown fibers were characterized by infrared spectroscopy, thermogravimetric analysis, scanning electron microscopy, transmission electron microscopy and nitrogen adsorption and desorption isotherms. The phase separation behavior and mechanism for the formation of the amorphous mesoporous structure were discussed. Although there was no covalent bonding between PMMA and silica, macrophase separation was completely prevented in the electroblown fibers and the pore size in the calcined silica fibers ranged from 10 to 20 nm. However, the previously reported electrospun silica fibers, in which surfactants or polymers with strong hydrogen or covalent bonding to silanol groups were used as structure directing agents, had average pore sizes below 10 nm. The present study offers a facile method for the preparation of highly mesoporous silica fibers with large mesopores.     

A mesoporous silica functionalized by a covalently bound naphthalene-based receptor for selective optical detection of fluoride ion in water

The naphthalene urea fluorescent receptor 1 immobilized mesoporous silica (FMS) and the fluorescent receptor 1 immobilized silica particles (FSP) were prepared via sol–gel reaction. The sensing ability of FMS and FSP was studied by addition of the anions F^–, Cl^–, Br^–, I^– and to water suspensions of the assayed solid. Of all the anions tested, addition of fluoride ion to a suspension of FMS resulted in the largest decrease in the fluorescence intensity of naphthalene of FMS. Thus, fluoride ions bind with four urea N–H protons of receptor 1 onto FMS. On the other hand, addition of Cl^–, I^–, Br^– and did not reduce the fluorescence of FMS. In case of FSP, the sensitivity for fluoride ion was 8 times lower than that of FMS due to immobilization of smaller amounts of receptor 1 in silica particles. The result implies that mesoporous silica with its large surface area is useful as a supporting material.     

Cs-modified iron nanoparticles encapsulated in microporous and mesoporous SiO2 for COx-free H2 production via ammonia decomposition

The stable core–shell Fe@SiO₂ catalysts reported in this paper are highly efficient for the generation of CO_x-free H₂ through ammonia decomposition. By tuning the porosity of SiO₂ shells (using C₁₈TMS agent) and with the introduction of an appropriate amount of Cs dopant (via pre-deposition as well as post-impregnation), the diffusion efficiency of the catalysts and the surface property of Fe cores can be modified for better performance. The Fe@SiO₂ structures function as microcapsular-like reactors during ammonia decomposition. Naked nanoparticles of metallic iron tend to aggregate into bulk particles spontaneously. The role of the stable SiO₂ shells is to prevent the enwrapped core particles from aggregation at high reaction temperatures.     

The effects of La2O3 on the structural properties of La2O3–Al2O3 prepared by the sol–gel method and on the catalytic performance of Pt/La2O3–Al2O3 towards steam reforming and partial oxidation of methane

The effect of La₂O₃ content on the structural properties and catalytic behavior of Pt/xLa₂O₃–Al₂O₃ catalysts in steam reforming of methane and partial oxidation of methane was investigated. There was a decrease in the density of Pt sites with the increase of La₂O₃ loadings according to Fourier transform infrared spectroscopy of adsorbed CO and to dehydrogenation of cyclohexane results. However, transmission electron microscopy data indicates an opposite trend. This apparent disagreement could be due to the partial coverage of Pt sites by LaO_x species. CH₄ turnover rates and specific rates of steam reforming of methane increased for higher La₂O₃ loadings. The Pt/Al₂O₃ catalyst was strongly deactivated during partial oxidation of methane, while La₂O₃-containing catalysts exhibited higher stability. The increase of activity observed during the reactions was ascribed to the ability of the [LaPt_xO]Pt⁰-like species to promote the gasification of coke. This cleaning mechanism led to higher accessibility of the active sites to CH₄.     

Direct growth of Cu3(BTC)2(H2O)3 · xH2O thin films on modified QCM-gold electrodes – Water sorption isotherms

The preparation of a thin film of the metal–organic framework Cu₃(BTC)₂(H₂O)₃ · xH₂O (HKUST-1) on the gold electrode of a quartz-crystal microbalance (QCM) was achieved by direct growth on a 11-mercaptoundecanol self-assembled monolayer (SAM). The formation of the SAM on the gold substrate was proven via reflection–absorption infrared-spectroscopy. The HKUST-1 thin film was characterized by X-ray diffraction, Raman-spectroscopy, and scanning-electron microscopy. Water vapor sorption measurements allow us to directly characterize the sorption properties of the thin film grown on the electrode of the QCM-device.     

Preparation of TiO2 nanofilm via sol–gel process and its photocatalytic activity for degradation of methyl orange

The preparation of TiO₂ nanofilm was conducted on common glass via the sol–gel process. Glacial acetic acid and diethanolamine were used as inhibitors to prepare acidic and alkaline TiO₂ sol, respectively. XRD, SEM, and EDS characterization showed that the film prepared from acidic TiO₂ sol had a narrow particle size distribution of 15–30 nm and relatively poor particle crystallization while in the case of the film from alkaline TiO₂ sol the nanoparticles were in a wide range of 10–80 nm and well crystallized. The photolysis evaluation through MO degradation revealed that the film from acidic sol possessed apparently better photocatalytic activity than that from alkaline sol. Heat treatment with longer time led to a 50% increase of the photocatalytic activity for the film.