Titania Silica Aerogels Research Articles

Investigation of compound state of SiO2-TiO2 aerogel synthesized through controlled sol-gel reaction

In this work, a compound state of silica-titania aerogel was investigated through sol-gel reaction for hybridization of silica and titania with increasing reactivity by using high surface area and charge imbalances. By minimizing the sol-gel reaction rate difference and preparing a homogeneous and stable sol, a compound state of silica-titania aerogel could be obtained, without separation onto single metal oxide aerogel. The benefits of using the sol-gel synthesis method for synthesizing silica-titania compound aerogels are over a limit of substitution solubility and get binary metal oxide compound material. Diffraction and spectroscopic analyses showed that the heterogeneous linkage resulted from interaction among silicon, titanium, and oxygen ions. The coordination environment changes by the formation of an Si-O-Ti linkage following interaction among ions without phase separation on SiO2 and TiO2 aerogels. Thus, the atomically controlled sol-gel reaction can be used to design new metal-oxide-based compound aerogels and immiscible oxide materials.

Versatile Thermal-Solidifying Direct-Write Assembly towards Heat-Resistant 3D-Printed Ceramic Aerogels for Thermal Insulation.

Ceramic aerogels have great potential in the areas of thermal insulation, catalysis, filtration, environmental remediation, energy storage, etc. However, the conventional shaping and post-processing of ceramic aerogels are plagued by their brittleness due to the inefficient neck connection of oxide ceramic nanoparticles. Here a versatile thermal-solidifying direct-ink-writing has been proposed for fabricating heat-resistant ceramic aerogels. The versatility lies in the good compatibility and designability of ceramic inks, which makes it possible to print silica aerogels, alumina-silica aerogels, and titania-silica aerogels. 3D-printed ceramic aerogels show excellent high-temperature stability up to 1000 °C in air (linear shrinkage less than 5%) when compared to conventional silica aerogels. This improved heat resistance is attributed to the existence of a refractory fumed silica phase, which restrains the microstructure destruction of ceramic aerogels in high-temperature environments. Benefiting from low density (0.21 g cm-3 ), high surface area (284 m2 g-1 ), and well-distributed mesopores, 3D-printed ceramic aerogels possess a low thermal conductivity (30.87 mW m-1 K-1 ) and are considered as ideal thermal insulators. The combination of ceramic aerogels with 3D printing technology would open up new opportunities to tailor the geometry of porous materials for specific applications.

Functionalization of cotton fabric with titania silica aerogel using rice husk as precursor for enhanced UV shielding

In the present study titania silica aerogel (Ti 1-6 SA) was prepared using rice husk as silica source via sol–gel method and the optimized for higher UV shielding behavior based on titania concentration. The optimized sample (TiSA) has been surface modified using 3-glycidoxypropyltriethoxysilane. Further, the surface modified titania silica aerogel (FTiSi) was further coated over cotton fabric through pad-dry-cure method and studied. The FTiSi coated cotton fabrics were studied for their UV shielding property, thermal and other basic fabric properties in addition to micrograph analysis. It is found that 5 wt% FTiSA coated cotton fabric delivers UPF value of 16.6. These results show that FTiSA coated cotton fabric possesses improved UV shielding when compared to neat cotton fabric. Thus, the low-cost silica aerogel with minimum quantity of titania developed in the present work can be utilized as functional material for industrial textiles to shield UV radiations.

Towards thermally stable aerogel photocatalysts: TiCl4-based sol-gel routes for the design of nanostructured silica-titania aerogel with high photocatalytic activity and outstanding thermal stability

Despite their excellent textural/structural properties, usage of aerogels in photocatalytic applications is critically limited by the necessity of high temperature annealing procedures to improve crystallinity, which lead to drastically decrease in surface area and the undesirable phase transformations. In this study, we report two novel straightforward TiCl4-based sol-gel routes for synthesis of SiO2/TiO2 nanocomposite and SiO2@TiO2 core-shell aerogels with exceptional photocatalytic performance even after annealing at temperatures as high as 1000 °C. While core-shell aerogels prepared by thermo-induced deposition method show higher and unprecedented thermal stability, epoxide-assisted gelation method offers greater control over phase composition, allowing preparation of highly photoactive anatase/rutile bicrystalline silica-titania nanocomposite aerogels. The improved structural properties achieved result in an increase of photocatalytic activity of silica-titania aerogels after high temperature annealing at 1000 °C. Such unusually high thermal stabilities outperform those shown by previously reported titania and silica-titania materials, representing significant advance on the design of thermally stable photocatalysts for applications that require the catalyst to undergo high temperature processing or endure extreme operating conditions.

Photocatalytic performance of highly amorphous titania–silica aerogels with mesopores: The adverse effect of the in situ adsorption of some organic substrates during photodegradation

Titania–silica composite aerogels with 16–29% Ti-content by the mass were synthesized by the sol–gel method from different Ti-precursors, and calcined at 500°C. These aerogels are highly amorphous as no crystalline TiO2 phase can be detected in them by X-ray diffraction methods, and show the dominating presence of either mesopores or macropores. The incorporation of Ti into the silica structure is shown by the appearance of characteristic IR transitions of SiOTi vibrations. The characteristic band-gap energies of the different aerogels are estimated to be between 3.6 and 3.9eV from UV reflection spectra. Band-gap energy decreases with decreasing pore-size. When suspended in solution, even these highly amorphous aerogels accelerate the photodegradation of salicylic acid and methylene blue compared to simple photolysis. Kinetic experiments were conducted under illumination, and also in the dark to study the adsorption of the substrates onto the suspended aerogels. We assume that the fast in situ adsorption of the organic substrates mask the suspended aerogel particles from UV photons, which reduces the rate of photocatalysis. We managed to mathematically separate the parallel processes of photocatalysis and adsorption, and develop a simple kinetic model to describe the reaction system.

Silica-titania composite aerogel photocatalysts by chemical liquid deposition of titania onto nanoporous silica scaffolds.

Silica-titania composite aerogels were synthesized by chemical liquid deposition of titania onto nanoporous silica scaffolds. This novel deposition process was based on chemisorption of partially hydrolyzed titanium alkoxides from solution onto silica nanoparticle surfaces and subsequent hydrolysis and condensation to afford titania nanoparticles on the silica surface. The titania is homogeneously distributed in the silica-titania composite aerogels, and the titania content can be effectively controlled by regulating the deposition cycles. The resultant composite aerogel with 15 deposition cycles possessed a high specific surface area (SSA) of 425 m(2)/g, a small particle size of 5-14 nm, and a large pore volume and pore size of 2.41 cm(3)/g and 18.1 nm, respectively, after heat treatment at 600 °C and showed high photocatalytic activity in the photodegradation of methylene blue under UV-light irradiation. Its photocatalytic activity highly depends on the deposition cycles and heat treatment. The combination of small particle size, high SSA, and enhanced crystallinity after heat treatment at 600 °C contributes to the excellent photocatalytic property of the silica-titania composite aerogel. The higher SSAs compared to those of the reported titania aerogels (<200 m(2)/g at 600 °C) at high temperatures combined with the simple method makes the silica-titania aerogels promising candidates as photocatalysts.

Anatase–silica composite aerogels: a nanoparticle-based approach

Herein we present the synthesis of anatase–silica aerogels based on the controlled gelation of preformed nanoparticle mixtures. The monolithic aerogels with macroscopic dimensions show large specific surface areas, and high and uniform porosities. The major advantage of such a particle-based approach is the great flexibility in pre-defining the compositional and structural features of the final aerogels before the gelation process by fine-tuning the properties of the titania and silica building blocks (e.g., size, composition and crystallinity) and their relative ratio in the dispersion. Specific surface functionalization enables control over the interaction between the nanoparticles and thus over their distribution in the aerogel. Positively charged titania nanoparticles are co-assembled with negatively charged Stoeber particles, resulting in a binary aerogel with a crystalline anatase and amorphous silica framework directly after supercritical drying without any calcination step. Titania–silica aerogels combine the photocatalytic activity of the anatase nanoparticles with the extensive silica chemistry available for silica surface functionalization.

Effect of Acid Treatment on Silica-Titania Aerogel as Oxidative-Acidic Bifunctional Catalyst

Acid treatment using H2SO4, HCl and H3PO4 has been carried out in an attempt to improve catalytic performance of silica-titania aerogel. X-ray diffraction results showed the amorphous structure of the aerogels remained after the acid impregnation and calcinations steps. Hammert analysis revealed these acid modified silica-titania aerogels were superacids with pKa &lt; -14.52. Different Ti species was observed in the samples upon the acid treatment. As compared to silica-titania aerogel, Lewis acidity increased remarkably in HCl treated sample without formation of any Brønsted acid site. Meanwhile, H2SO4 and H3PO4 treated samples possessed both Lewis and Brønsted acid sites. The catalytic performance of these samples was evaluated through a consecutive transformation of 1-octene to 1,2-octanediol through the formation of 1,2-epoxyoctane using aqueous hydrogen peroxide as oxidant.

Synthesis and Characterization of Acid Modified Silica-Titania Aerogel as Oxidative-Acidic Bifunctional Catalyst

Acid tr eatment using H2SO4, HCl and H3PO4 has been carried ou t in an attempt to improve catalytic performance of silica-titania aerogel. X-ray diffraction results s howed the amorphous structure of the aerogels remained after the acid impregnation and calcinations steps. Hammert analysis revealed these acid modified silica-titania aerogels were superacids with pKa < -14.52. Different Ti species was observed in the samples upon the acid treatment. As compared to silica-titania aerogel, Lewis acidity increased remarkably in HCl treated sample without formation of any Bronsted acid site. Meanwhile, H2SO4 and H3PO4 treate d samples possessed both Lewis and Bronsted acid sites. The catalytic performance of these samples was evaluated through a consecutive transformation of 1-octene to 1,2-octanediol through the formation of 1,2-epoxyoctane using aqueous hydrogen peroxide as oxidant.

Enhancement of Brønsted Acidity in Sulfate-Vanadium Treated Silica-Titania Aerogel as Oxidative-Acidic Bifunctional Catalyst

Enhancement of Brønsted acidity in silica-titania aerogel was achieved by loading both sulfate and vanadium onto the material. This phenomena could be due to formation of V(OSi)2OH-O-SO3- in the resulted sample. In contrast, no noticeable Brønsted acidity was detected in vanadium loaded silica-titania aerogel (V/ST). UV-vis DR analysis results showed that the amount of hydrated tetrahedral titanium species, which acts as oxidative site, increased significantly upon sulfation and vanadium loading on the surface of the aerogel. However, presence of vanadium in the samples tended to transform framework tetrahedral titanium to extra framework octahedral titanium structure. The catalytic performance of the samples were evaluated in consecutive transformation of 1-octene to 1,2-octanediol through formation of 1,2-epoxyoctane using aqueous H2O2.

Visible Light Enabled V and Cr Doped Titania-Silica Aerogel Photocatalyst

A series of high surface area (~700 m2/g) nanostructured coupled semiconductor photocatalysts (NCSP) based on transition metals (M = Cr, V) doped titania-silica aerogel of various molar ratios of Ti:Si (1:25, 1:33 and 1:50) were synthesized via direct synthesis and wet impregnation methods. All samples reveal well-dispersed TiO2 with three different types of Ti coordination namely isolated Ti, non-isolated Ti and octahedrally coordinated Ti species. Cr and V (5 wt%)-titania silica aerogel under visible light irradiation reduced the bandgap energy of T25 to 2.9 eV. The photocatalytic activity of NCSP is 3 fold more active than photocatalyst supported on amorphous silica. Cr (5 wt%)-titania silica aerogel photocatalyst gives the highest conversion of styrene under both UV and visible light irradiation (55% and 47% respectively).

Silica–titania aerogel monoliths with large pore volume and surface area by ambient pressure drying

Ambient pressure drying has been carried out for the synthesis of silica–titania aerogel monoliths. The prepared aerogels show densities in the range 0.34–0.38 g/cm3. The surface area and pore volume of these mixed oxide aerogels are comparable to those of the supercritically dried ones. The surface area for 5wt% titania aerogel has been found to be as high as 685 m2/g with a pore volume of 2.34 cm3/g and the 10wt% titania aerogel has a surface area of 620 m2/g with a pore volume of 2.36 cm3/g. Some gels were also made hydrophobic by a surface treatment with methyltrimethoxysilane and trimethylchlorosilane. The surface modified aerogels possess high surface areas in the range of 540–640 m2/g, and are thermally stable in terms of retaining hydrophobicity up to a temperature of 520 °C. The pore size distribution of the aerogels clearly indicates the preservation of the aerogel structure. High Resolution Transmission Electron microscopy has been employed to characterise the aerogels and Fourier Transform infrared spectroscopy to study the effect of titania addition to silica and the surface modification. X-ray diffraction patterns were recorded to verify the molecular homogeneity of the aerogel.

Physical Properties and Bifunctional Catalytic Performance of Phosphate–Vanadium Impregnated Silica–Titania Aerogel

The excellent catalytic performance of phos- phate-vanadium impregnated silica-titania aerogel as a bifunctional oxidative and acidic catalyst was evaluated in consecutive transformation of 1-octene to 1,2-octanediol through formation of 1,2-epoxyoctane using aqueous H2O2. Phosphate group and vanadium were impregnated onto silica-titania aerogel via wet impregnation method at room temperature, followed by a high calcination temperature. Significant increase of hydrated, tetrahedrally coordinated Ti species was found in phosphate-vanadium impregnated silica-titania sample. Pyridine adsorption study revealed that the combination of phosphate group and vanadium is essential for generation of Bronsted acidic sites. Phos- phate-vanadium impregnated silica-titania catalyst pro- duced 1,2-octanediol (275 lmol) with selectivity of 65%. In contrast, no noticeable diol was produced when silica- titania aerogel and phosphate treated silica-titania sample were used.

Reactivity and antimicrobial properties of nanostructured titanium dioxide

The photo-induced reactivity of three nanostructured titanium dioxides including TiO 2 nanoparticles (i.e., nano-TiO 2), titania–silica aerogel and nanotextured TiO 2 film, were investigated for gas phase photocatalytic oxidation of trichloroethylene and inactivation of Bacillus subtilis and Escherichia coli microbial cells. A correlation was observed between photoreactivity for TCE conversion and bactericidal activity within TiO 2 of similar nanostructures and composition (i.e., elemental and phase). This indicates that the photogenerated radical species from UV-irradiated TiO 2 responsible for TCE photo-oxidation also play a major role in the inactivation of microbial cells. The superb bactericidal activities of titania–silica aerogels (i.e., up to 6log reductions in viable B. subtilis) and nanotextured TiO 2 film (3.4log reduction) compared to commercial Degussa P25 TiO 2 (0.64log reduction) and optimum nano-TiO 2 (1.3log reduction) far exceeds the observed activity enhancement for TCE photo-oxidation. This implies that nanoscale chemical and structural environment also contribute towards the bactericidal activities.

An investigation of the performance of catalytic aerogel filters

Gas permeable, photoactive and crack-free titania–silica aerogels of high titanium content (i.e., up to Ti/Si=1) were prepared by two-steps acid–base catalyzed method involving an acid-catalyzed prehydrolysis of silicon alkoxide followed by a base-catalyzed hydrolysis/condensation reactions with a chelated titania precursor. The prepared titania–silica aerogels displayed good mechanical strength (>30kNm−2), large surface area (>550m2/g), mesoporous structure (8–11nm) and good gas permeation. The porous aerogels trap and filter airborne particulates and the titania–silica aerogel have a fair performance for aerosol (65%) and bioaerosol (94%) filtrations. The photoactive anatase nano-TiO2 crystallized within the aerogel displays an order of magnitude higher reaction rate for UVA photooxidation of trichloroethylene compared to commercial Degussa P25 TiO2. The bactericidal activity of the titania–silica aerogel for Bacillus subtilis cells under UVA was also six orders of magnitude better.

Sulfated silica–titania aerogel as a bifunctional oxidative and acidic catalyst in the synthesis of diols

Sulfated silica–titania aerogel (SO 4/ST) catalyst was prepared by direct synthesis and modified with H 2SO 4 by wet impregnation method. Brønsted acid sites were generated after sulfation of the silica–titania aerogel with a pKa of −14.52. Consecutive transformation of 1-octene, catalyzed by sulfated silica–titania aerogel using aqueous hydrogen peroxide as oxidant shows high activity towards 1,2-octanediol through the formation of 1,2-epoxyoctane. The formation of 1,2-octanediol is enabled by the co-existence of sulfated Ti species with Brønsted acid sites in the SO 4/ST system. Sulfated silica-titania aerogel catalyst produced 1,2-octanediol (326 μmol) with selectivity of 63%. In contrast, no diol was produced when silica–titania aerogel was used.

Preparation and application of nanoglued binary titania–silica aerogel

Nanoglued binary titania (TiO 2)–silica (SiO 2) aerogel, as a novel type of photocatalyst, has been synthesized on glass substrates. Using an about-to-gel SiO 2 sol as nanoglue, anatase TiO 2 aerogel was immobilized into a three-dimensional mesoporous network of the SiO 2. Factorial designs were employed to optimize both TiO 2 aerogel and binary TiO 2–SiO 2 aerogel synthesis. Characterization of the as-prepared TiO 2 and binary samples by surface area, porosity, and surface chemical composition showed that the photocatalysts were high-surface-area nanoporous materials, with a Ti 4+ valency. The binary aerogel exhibited high photocatalytic activity for the degradation of methylene blue (MB) under simulated solar light; the reaction followed the pseudo first-order Langmuir–Hinshelwood (L–H) kinetic model. Fluorescence spectroscopy revealed that the hydroxyl ( OH) radical was formed during the illumination of the binary TiO 2–SiO 2 aerogel in a solution of probe molecules, which corroborates the probable mechanism of hydroxyl radical oxidation of contaminants in photocatalytic reactions.

Synthesis of freestanding silica and titania-silica aerogels with ordered and disordered mesopores

Freestanding blocks of silica and titania-silica aerogels were prepared by the sol-gel method. It is possible to prepare crack-free, titania-silica aerogels with high titanium content by a careful control of the synthesis conditions. Prehydrolysis, complexation and polymer addition were used to adjust the hydrolysis and condensation rates of the silicon and titanium alkoxide precursors. Photoactive anatase TiO2 nanocrystals with a large surface area (i.e., up to 300m2g−1) were crystallized from the gel network by the high-temperature ethanol supercritical drying, and the resulting aerogel blocks were gas permeable and display a transition-regime diffusion behavior. Pore and volume shrinkages were observed in samples prepared by ethanol supercritical drying when the titanium content was increased resulting in a lower flux. Adding Pluronic P123 creates ordered mesopore domains and produces large pore aerogels even at high titanium contents. The photocatalytic oxidation reaction of trichloroethylene was performed by flowing the reactant gas mixture through the UV-irradiated aerogel blocks with excellent results.

Photocatalytic degradation of methanol on titania and titania–silica aerogels prepared by non-alkoxide sol–gel route

Titania and titania–silica aerogels were prepared by alkoxide or non-alkoxide sol–gel route and subsequent supercritical drying with carbon dioxide at low temperature. The resulting aerogels having high surface area and mesoporosity were used as photocatalysts for gas phase methanol degradation reaction. Photocatalytic degradation reactions were carried out on titania and titania–silica aerogels, and commercial Degussa P-25 titania. The photocatalytic activities of titania and titania–silica aerogels were higher than that of the P-25. While the conversion of methanol degradation over the P-25 catalyst was only 50–60%, that for the titania aerogel was observed to be above 98% due to the higher specific surface area and the well developed mesoporous structure. In spite of lower titania contents, much higher surface area and high dispersion of titania of titania–silica aerogel gave rise to the high photocatalytic activity in comparison to those of titania aerogels. Moreover, titania–silica aerogel was also used for the photodegradation and adsorption hybrid system. It was observed that the high removal efficiency for methanol was caused by the combination of higher catalytic activity and adsorption capacity.

The influence of preparation method on the physicochemical properties of titania–silica aerogels: Part two

The influence of preparation method on the physicochemical properties of titania–silica aerogels: Part two