Titania Aerogels Research Articles

Titania aerogels: Preparation and photocatalytic tests

Titania aerogels are suggested as promising photocatalysts [1]. In the present study aerogels were synthesised by sol‐gel method combined with supercritical drying. Tetraisopropyl orthotitanate was used as a precursor, anhydrous methanol and isopropanol as solvents. Three aerogels were prepared using different ways of synthesis. Volumes and surface areas of micro‐ and mesopores of each aerogel were determined. XRD and SEM analyses were carried out. For comparison analyses were also performed for TiO2 P25 Degussa. Aerogels′ BET surface areas ranged from 73 to 96 m2g-1. They indicate the crystalline structure of anatase. Finally photocatalytic tests were performed using water solution of p‐chlorophenol and 4‐hydroxybenzoic acid. Experiments were carried out in the Solar box simulating sun irradiation and on location. Aerogel Z513 prepared using 30% of precursor (tetraisopropyl orthotitanate) in isopropanol as a solvent indicates the best photocatalytic activity towards p‐chlorophenol while Z516 prepared using 60% of precursor in methanol towards 4‐hydroxybenzoic acid.

Electrochemically induced surface modification of titanols in a `nanoglued' titania aerogel–silica aerogel composite film

Titania aerogel, thermally processed into its anatase form, has been bound as submicrometer particles to an electrode with silica sol. The resulting composite gel is supercritically dried to create a titania–silica composite aerogel film on the conducting substrate. Titania aerogel particles present in the mesoporous film are electrochemically addressable. In organic electrolyte containing isopropanol and added water, the electrochemical response negative of the flatband potential of TiO 2 indicates that electrogenerated reactants (hydroxides and isopropoxides) induce condenzation reactions with surface titanols (Ti–OH). The data suggest that titania aerogels can be electrochemically modified in a controllable way with a wide variety of compounds that have reducible –OH moieties.

Effects of supercritical impregnation conditions on the properties of silica–titania aerogels

The preparation of silica–titania aerogel by supercritical impregnation of titanium isopropoxide (TP) or modified with acetylacetone (TPAA) using flow type apparatus was performed. The influence of the physical state of TP/CO 2 TPAA/CO 2 mixture was investigated. Homogeneous and rapid impregnation on monolithic silica alcogel was achieved using supercritical TP/CO 2 or TPAA/CO 2 mixtures at the single phase condition (353 K, 20 MPa). Crystallization of impregnated titania using this system was attained by increasing temperature during the depressurization process after impregnation.

The preparation of Ti(C,N,O) nanoparticles using binary carbonaceous titania aerogel

The Ti(C, N, O) nanoparticles have been prepared by using binary carbonaceous titania aerogel as precursor of carbothermal reduction reaction. The binary aerogel precursor, composed of binary carbon–titania nanoparticles with particle sizes in the range 1–8 nm, is obtained by a sol–gel supercritical fluid drying method. This method provides a homogeneous distribution of titania within carbon and high contact area between the reactants, leading to the formation of loose agglomerated Ti(C, N, O) nanoparticles at a lower carbothermal reduction temperature. The BET surface areas of binary carbonaceous–titania aerogel and its carbothermal reduction products at various temperatures are investigated. The Ti(C, N, O) formed over a range of 1000–1300°C in flowing argon are also characterized by TEM, EDX spectrum, XRD and chemical analyses. The results show that the carbothermal reduction temperature is important for the crystallite growth of Ti(C, N, O) powders.

Adsorption and photocatalytic decomposition of benzene using silica-titania and titania aerogels: Effect of supercritical drying

Silica-titania and titania aerogels were prepared by supercritical drying using different solvents such as low temperature CO2 (353 K), high temperature CO2 (553 K), ethanol (553 K) and ethanol with zeolite (553 K) and their efficiencies for the removal of benzene from a synthetic air mixture were investigated. The aerogels obtained showed both large capacities for benzene adsorption and high photocatalytic activity for its decomposition in the adsorbed state. The degree of benzene removal by silica-titania aerogel seemed to depend on the crystallinity of the titania and was in the order low temperature CO2 < high temperature CO2 < ethanol = ethanol with zeolite. The amount of CO2 liberated by the photocatalytic decomposition of benzene also followed the same sequence. Titania aerogels showed the greatest efficiency in the decomposition of benzene, while the amount of CO2 evolved was lower than those of the silica-titania systems investigated. The adsorption capacity of benzene basically depended on the silica matrix and was not greatly influenced by the difference in the crystallinity of titania.

Preparation of titania-impregnated silica aerogels and their application to removal of benzene in air

Silica–titania aerogels were prepared by impregnating titanium tetraisopropoxide modified with acetylacetone into a silica alcogel followed by supercritical drying. The effects of the impregnation conditions, the supercritical drying medium (ethanol, 2-propanol or CO2) and the calcination temperature on the homogeneity of the aerogel and the nature of the impregnated titania were investigated. Aerogels prepared in this way had large specific surface areas and maximum titanium contents of ca. 32 at%. Ethanol and CO2 dried samples were homogeneous, but 2-propanol dried samples were inhomogeneous because of the extraction of impregnated titania. Ethanol and 2-propanol dried samples contained anatase particles of <10 nm agglomerated into clusters of 100–300 nm size, whereas CO2 dried samples contained amorphous titania. Their application to the removal of benzene from air was attempted. Ethanol dried samples possessed excellent benzene adsorption capacities and the adsorbate could be decomposed to CO2 by photocatalytic reaction.

Vanadia grafted on TiO 2–SiO 2, TiO 2 and SiO 2 aerogels : Structural properties and catalytic behaviour in selective reduction of NO by NH 3

Vanadia has been grafted on high surface area titania–silica, titania and silica aerogels prepared using the sol–gel method and ensuing drying by extraction with supercritical CO 2. The aerogels exhibited favourable textural properties (high surface area combined with dominant mesoporosity) and the capability to stabilise vanadia in a well-dispersed form up to high loadings. Titania aerogel with a BET-surface area of 165 m 2 g −1 proved to be the most suitable support for vanadia affording excellent activity for the selective catalytic reduction (SCR) of NO by NH 3 at low temperature (400 K). With titania–silica aerogels, X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy indicated a preferential deposition of vanadia on Ti sites at low vanadia loading. SCR activities were found to depend mainly on the vanadia loading and the support material. High loadings of dispersed vanadia (6–10 μmol m −2) were favourable on all aerogel supports. Vanadia grafted on titania aerogel was superior in activity compared to vanadia deposited on the other aerogels. V 2O 5–SiO 2 as well as V 2O 5–TiO 2–SiO 2 catalysts showed significant deactivation at temperatures ca. >500 K. Deactivation of vanadia deposited on TiO 2–SiO 2 could be avoided at loadings ca. >6 μmol m −2. Deactivated catalysts showed a decrease in both Brönsted as well as Lewis sites, as revealed by desorption studies after exposure to SCR conditions.

Vanadia–titania aerogel deNO x catalysts

V 2O 5/TiO 2 aerogels were made by a two-step sol-gel process characterized by surface area measurements, X-ray diffraction, Raman and infrared spectroscopies and high resolution electron microscopy. Catalytic performance in the selective reduction of NO by propane was found to be significantly better than those of their homologues prepared by a conventional method.

A Comparative Study of Surface Fractality between Polymeric and Particulate Titania Aerogels

The surface fractality was studied for polymeric and particulate titania aerogels prepared via CO2supercritical drying of titania lyogels. The surface fractal dimensions of these mesoporous aerogels were determined from their nitrogen adsorption isotherms by means of the Frenkel–Halsey–Hill method and the thermodynamic method. The results from these distinctly different methods have been found to be similar; the fractal surfaces of particulate aerogels are slightly more irregular than those of polymeric aerogels. The difference in the surface morphology between the two types of aerogels, observed with scanning electron microscopy, can be attributed to the difference between the proposed mechanisms for formation of polymeric and particulate lyogels.

Fractal Analysis of Polymeric and Particulate Titania Aerogels by Adsorption

Polymeric and particulate titania (TiO2) aerogels with high surface areas were prepared from sol−gel derived TiO2 lyogels via freeze-drying as well as CO2 supercritical drying. The surface fractal dimensions of these aerogels were evaluated with two methods based on physisorption, one by Avnir and Pfeifer, and the other, the modified form of the Frenkel−Halsey−Hill (FHH) equation. The two approaches yielded almost identical results. The surface fractal dimensions measured are approximately 2.7 and 2.8 for polymeric and particulate TiO2 aerogels, respectively, thereby indicating that the fractal surfaces of particulate aerogels are slightly more irregular than those of polymeric aerogels.

High-Performance Aerogel DeNOx Catalyst: Catalytic Behavior and Kinetic Modeling

The kinetics and the parametric sensitivity of the selective catalytic reduction of NO by NH3 were investigated for a high surface area vanadia−titania aerogel. The catalyst showed high activity at low temperatures and the selectivity to N2 was higher than 99% for all experimental conditions investigated. The addition of up to 2% H2O to the dry feed notably reduced the rate of NO conversion. The effect leveled off for higher H2O concentrations. In the absence of O2 in the feed the activity collapsed, independently whether water was present or not. Small amounts of oxygen resulted in a sharp increase of the reaction rate, but for oxygen concentrations exceeding 4%, the increase diminished. A high tolerance with respect to sulfur dioxide was observed. The estimated activation energy amounted to 60 ± 1 kJ/mol. The microkinetic analysis resulted in a model based on an Eley−Rideal mechanism which describes the kinetic behavior over a wide experimental range.

Preparation and Characterization of Silica–Titania and Silica–Alumina Aerogels

Silica–titania and silica–alumina aerogels were prepared by the two-step hydrolysis of metal alkoxides, the sol–gel polymerization and the supercritical drying with carbon dioxide. As a result of characterization by nitrogen adsorption, the aerogels were mesoporous materials with high surface areas and had few micropores. The experimental results suggested that the surface area, the mesopore volume, and the pore radius of the aerogel depended on the titanium (Ti) or aluminum (Al) content. Adsorption isotherms of ethane, ethylene, propane, and propylene were measured on the aerogels prepared. It was found that the adsorption capacities for these hydrocarbons were changed by mixing Ti or Al to the silica aerogel. In the ethane–ethylene or propane–propylene system, the adsorption selectivity of ethylene or propylene on the silica–titania aerogel was lower than that on the silica aerogel. On the other hand, the selectivity on the silica–alumina aerogel was higher than that on the silica aerogel. Hence, the silica–alumina aerogel was useful for separating ethylene from ethane or propylene from propane.

Effect of ageing conditions on the pore structural properties of titania aerogels

Effect of ageing conditions on the pore structural properties of titania aerogels

Catalysts derived from polymer-stabilised colloidal platinum: Effects of support and calcination on the catalytic behaviour in hydrogenation

Platinum colloids prepared by reduction of H 2PtCl 6 with methanol in the presence of polyvinylpyrrolidone have been examined in the sol state and following immobilisation on oxide supports. Both the sol and the supported catalysts contained platinum metal particles of 3–4 nm number-mean diameter, as determined by TEM. High initial activities for hydrogenation of the medium-sized alkenes trans-stilbene and methylcinnamate were recorded. The platinum particles were compared for particle size distribution and catalytic activity in the unsupported (sol) state and in supported states, both before and after heat treatment in oxygen-argon at 300°C (calcination). Highest activities were measured for the sol state and for the calcined supported catalysts. It is suggested that this behaviour is related to the influence of support and polymer on the accessibility of the metal surface for the reactants. Marked activity differences between a platinum-colloid catalyst immobilised on a titania aerogel and a binary Pt-titania aerogel, directly prepared by a one-step sol-gel route with ensuing supercritical drying, are discussed.

Titania aerogels prepared by low-temperature supercritical drying. Influence of extraction conditions

Titania aerogels with meso- to macroporosity and high specific surface area were prepared by varying the conditions of semicontinuous extraction of methanolic titania gels with CO2. The conditions varied were extraction temperature, extraction duration, and CO2 in liquid or supercritical state. The resulting titania aerogels were characterised by means of nitrogen physisorption, X-ray diffraction, thermal analysis and transmission electron microscopy. All uncalcined aerogels contained significant amounts of organic residues (12–14 wt% elemental carbon), and remained X-ray amorphous during calcination in air up to 673 K. Thermoanalytical studies showed that crystallization generally occurred in the range 730–745 K. The variation of the extraction temperature at either constant density or pressure of CO2, the use of either liquid or supercritical CO2, and the duration of extraction greatly influenced surface area, pore size distribution, and pore volume. The highest specific surface area (623 m2 g−1) and nitrogen pore volume (4.0 cm3 g−1) were obtained, if the density of supercritical CO2 corresponded to that of methanol at the lowest temperature applied (313 K). The studies indicate that textural properties can be varied over a wide range by choosing appropriate extraction conditions.

Vanadia supported on titania aerogels morphological properties and catalytic behaviour in the selective reduction of nitric oxide by ammonia

Vanadia on titania-aerogel catalysts with a wide range of surface areas (s bet 70–190 m 2 g −1) and vanadia loadings (2.5–17μmol m −2) have been prepared and tested for the selective catalytic reduction of nitric oxide with ammonia (SCR). The high surface area of the titania aerogels allows the immobilisation of larger amounts of active vanadia species per gram of support, which results in higher overall SCR activities, compared to catalysts based on conventionally prepared titania. Transmission electron microscopy. X-ray diffraction measurements and the SCR activities indicate total dispersion of the vanadia in the VO x/TiO 2-aerogel samples up to a theoretical monolayer of V 2O 5. Raman spectroscopy showed that monomeric vanadyl species and two-dimensional polyvanadate layers were present on the titania surface. Increase of the vanadia loading favoured the formation of polyvanadate species. The intrinsic SCR activity depends on the vanadia (surface) concentration, whereas the morphology and acidity of the titania (anatase) support seem to have little influence. Temperature-programmed desorption after exposure to SCR feed gas (SCR-TPD) of both the titania supports and the VO x/TiO 2 catalysts, indicated that the acidity of the titania aerogel supports depends on the crystallite size, as smaller crystallites ( d c < 10nm) result in increased acidity. Upon deposition of less than 6 μmol m −2 (V) on the TiO 2-aerogels, the adsorption capacity of the samples decreases and centres are formed that bind ammonia more strongly. These catalysts show low SCR activity. Further vanadia immobilisation restores the total amount of adsorbed species and leads to the formation of adsorption centres with moderate acidity, which results in up to ten times higher SCR activity. The occurrence of oxidation products of adsorbed ammonia with high vanadia loadings reflects the enhanced oxidation capability of these catalysts at elevated temperatures.

In Situ Diffuse Reflectance FTIR Study of the Selective Catalytic Reduction of NO by NH3 over Vanadia-Titania Aerogels

Selective catalytic reduction of NO by NH3 (SCR) on high surface area (BET > 180 m2 g−1 vanadia-titania aerogels with 5, 10, and 20 wt% of vanadia, has been investigated by diffuse reflectance FTIR spectroscopy coupled with mass spectrometry under reaction conditions and by temperature-programmed desorption of preadsorbed NH3 and NO. The SCR activity of these catalysts increases with the vanadia loading. This behaviour is reflected by the temperatures necessary to reach 50% NO conversion under standard conditions, which decreased in the sequence 5% V2O5, 545 K; 10% V2O5, 470 K; 20% V2O5, 440 K. A similarly prepared high surface area (182 m2 g−1) titania aerogel does not exhibit significant NO conversion in this temperature range. On pure titania aerogel, mainly Lewis-bound ammonia is observed subsequent to NH3 adsorption at ambient temperature. In contrast, it is shown that Brønsted-bound ammonia, characterized by bands at 1660 and 1414 cm−1, is involved in the SCR reaction. The SCR activities correlate with the fraction of Brønsted-bound ammonia which significantly increases with the vanadia loading of the aerogel samples.

Effect of drying temperature on the physical properties of titania aerogels

Drying temperature has been shown to be a key parameter in affecting the textural and structural properties of calcined titania aerogels, prepared by the sol–gel method using titanium n-butoxide and subsequent supercritical drying with carbon dioxide.

ChemInform Abstract: Synthesis and Characterization of Titania Aerogels.

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Synthesis and characterization of titania aerogels

Synthesis and characterization of titania aerogels