Preparation of freestanding and crack-free titania–silica aerogels and their performance for gas phase,photocatalytic oxidation of VOCs

Freestanding and crack-free titania–silica aerogels with high titanium content (i.e., Ti/Si = 1) were successfully prepared by adjusting the hydrolysis of the two alkoxide precursors to a comparable rate during the sol–gel processing. Two titania–silica aerogels were prepared by ethanol and CO₂ supercritical drying methods. Well-dispersed, nanometer-sized anatase crystal domains (ca. 10 nm) were crystallized by high temperature, ethanol supercritical drying. The crystalline domains were solidly anchored to the aerogel network by Ti–O–Si bonds. Titania–silica aerogels prepared by CO₂ supercritical drying method were devoid of TiO₂ crystals. A molecular-level mixing was achieved and anatase TiO₂ was only crystallized with difficulty by high temperature calcination (1073 K). Both aerogels were mesoporous and displayed similar open pore structure that is readily accessible to reactant molecules. However, only the titania–silica aerogel with anatase TiO₂ prepared by ethanol supercritical drying was active for the gas phase, photocatalytic oxidation of volatile organic compounds (i.e., isopropanol and trichloroethylene). Catalysts prepared from Degussa P25 TiO₂ displayed lower activity under similar reaction conditions.     

Fabrication of 3D-networks of native starch and their application to produce porous inorganic oxide networks through a supercritical route

Herein an effective method for fabrication of three dimensional networks of starch is presented. In this method, the native starch was dissolved in hot water to form solution, resulting in the formation of starch gel after cooled to room temperature. Then the water in the starch gel was exchanged by ethanol, followed by drying with supercritical (SC) CO₂ extraction, the nano-networks of starch were prepared. The obtained starch aerogel was composed of starch fibers and presented porous structure. Changing the initial concentration of native starch and controlling the drying conditions could control the microstructure of the starch networks. The resulting starch networks were utilized as a directing template to prepare the hierarchically ordered TiO₂ networks. In this process, TiO₂ precursor dissolved in SC CO₂ diffused onto the surface of starch gel, and reacted with the hydroxyl groups and adsorbed water molecules on the starch surface, to form an organic/inorganic hybrid material. After calcining the composites at 500 °C, the networks composed of anatase TiO₂ nanocrystals were produced, which accurately replicated the fine structures of the starch gel and possessed higher BET surface area and total pore volume compared with those of other TiO₂ materials templated by some biomaterials or polymer gels.     

Selective oxidation of hydrogen sulfide containing excess water and ammonia over vanadia–titania aerogel catalysts

A series of V₂O₅–TiO₂ aerogel catalysts were prepared by sol–gel method with subsequent supercritical drying with CO₂. The aerogel catalysts showed much higher surface areas and total pore volumes than V₂O₅–TiO₂ xerogel and impregnated V₂O₅–TiO₂ catalysts. Two species of surface vanadium in the aerogel catalysts were identified by Raman measurements: monomeric vanadyl and polymeric vanadates. The selective oxidation of hydrogen sulfide in the presence of excess water and ammonia was studied over these catalysts. Aerogel catalysts showed very high conversion of H₂S without harmful emission of SO₂. Temperature programmed reduction (TPR), XRD and Raman analyses revealed that the high catalytic performance of the aerogel catalysts originated from their highly dispersed VO_x species and high reducibility.     

Preparation and characterization of nitrogen-doped mesoporous titania with high specific surface area

Nitrogen-doped mesoporous titania aerogel photocatalysts were prepared by supercritical drying technique with carbon dioxide (SCCO₂) and calcination the urea impregnated TiO₂ aerogel at 773 K under NH₃ or N₂ + NH₃aq gaseous atmosphere. The pore properties were investigated from nitrogen adsorption measurement at 77 K. The prepared N-doped TiO₂ aerogel had a high specific surface area (116 m²/g), a total pore volume (0.33 cm³/g) and a sharp pore radius distribution (r_peak = 4.2 nm). The doping of the nitrogen atom into the TiO₂ lattice is expected from X-ray photoelectron spectroscopy (XPS), X-ray powder diffraction (XRD), and UV–Vis spectroscopy. Nitrogen states in the lattice and crystalline structure were measured using X-ray photoelectron spectroscopy and X-ray diffractometry. The N-doped TiO₂ aerogel absorbed well into the visible region up to 600 nm.     

冬瓜基含TiO2炭气凝胶的制备、表征及其光催化性能

以冬瓜和钛酸四丁酯为原料,采用水热法和冷冻干燥法成功制备出含TiO₂炭气凝胶（WTCA）,研究了其对罗丹明B废水的光催化降解性能,并考察了水中常见无机盐离子对其光催化效果的影响。利用X射线衍射仪（XRD）、扫描电子显微镜（SEM）、透射电子显微镜（TEM）、拉曼光谱仪（Raman）和荧光光谱仪（FL）等对样品结构和性能进行了表征。XRD、Raman和EDS分析表明复合气凝胶中含有锐钛矿TiO₂和碳元素;SEM和TEM分析表明TiO₂纳米颗粒均匀地负载在具有多级孔洞结构的炭气凝胶的骨架表面;含TiO₂炭气凝胶的光催化活性明显高于纯TiO₂,当负载量达30%时,其对罗丹明B的光催化降解效率最高（降解率达96.11%）。此外,无机盐阴离子对复合气凝胶的光催化效果有一定的抑制作用（CO₃^2- > SO₄^2- > Cl^- > NO₃^-）,而无机盐阳离子Na⁺、K⁺、Ca²⁺和Mg²⁺的抑制作用不明显。     

SiO_2气凝胶超临界干燥工艺参数的优化

用溶胶-凝胶两步催化法和乙醇超临界干燥法制备了S iO2气凝胶。以密度为评价标准,设计了四因素、三水平的正交实验研究乙醇超临界干燥工艺(乙醇的用量、预加压力、超临界温度和超临界压力)。结果表明,最佳工艺条件为:乙醇400 mL,预加压8 MPa,超临界温度270℃,超临界压力12 MPa,得到样品的密度为0.091 1 g/cm3,并通过SEM、TEM、FTIR对其微观结构进行了研究。     

Synthesis of nanosized TiO2/SiO2 particles in the microemulsion and their photocatalytic activity on the decomposition of p-nitrophenol

Nanosized pure TiO₂ particles were prepared by hydrolysis of TTIP in the sodium bis(2-ethylhexyl)sulfosuccinate (AOT) reverse micelles. TiO₂/SiO₂ nanoparticles were also prepared from TEOS as a silicon source and TTIP as a titanium source. These particles were characterized by TEM, XRD, FT-IR, BET, TGA and DTA. From thermal analysis and XRD analysis, the anatase structure of pure titania appeared in the 300–600 °C calcination temperature range and the rutile structure was showed above 700 °C. However, no rutile phase was observed for the TiO₂/SiO₂ particles up to 800 °C. The crystallite size decreased and the surface area of TiO₂/SiO₂ particles monotonically increased with an increase of the silica content. From FT-IR analysis, the band for Ti–O–Si vibration was observed and the band intensity for Si–O–Si vibration increased with an increase of the silica content. The micrographs of TEM showed that the TiO₂/SiO₂ nanoparticles had a spherical and a narrow size distribution. In addition, TiO₂/SiO₂ particles showed higher photocatalytic activity than pure TiO₂ and the TiO₂/SiO₂ (90/10) particles showed the highest activity on the photocatalytic decomposition of p-nitrophenol.     

Photocatalytic conversion of CH4 and CO2 to oxygenated compounds over Cu/CdS–TiO2/SiO2 catalyst

Coupled semiconductor (CS) Cu/CdS–TiO₂/SiO₂ photocatalyst was prepared using a mutli-step impregnation method. Its optical property was characterized by UV–vis spectra. BET, XRD, Raman and IR were used to study the structure of the photocatalyst. Fine CdS was found dispersed over the surface of anatase TiO₂/SiO₂ substrate. Chemisorption and IR analysis showed methane absorbed in the molecular state interacted weakly with the surface of catalyst, and the interaction of CO₂ with CS produced various forms of absorbed CO₂ species that were primarily present in the form of formate, bidentate and linear absorption species. Photocatalytic direct conversion of CH₄ and CO₂ was performed under the operation conditions: 373 K, 1:1 of CO₂/CH₄, 1 atm, space velocity of 200 h⁻¹ and UV intensity of 20.0 mW/cm². The conversion was 1.47% for CH₄ and 0.74% for CO₂ with a selectivity of acetone up to 92.3%. The reaction mechanisms were proposed based on the experimental observations.     

Preparation of rough anatase films and the evaluation of their photocatalytic efficiencies

Sol–gel derived rough anatase films without controlled particle sizes were prepared by surfactant templating. The coating sol–gel was obtained by hydrolysis of Ti(OC₃H₇)₄ in ethanol/HNO₃ solution. The gel films, prepared by dipping glass substrates in surfactant solutions, were dried after immersion under an atmospheric pressure. The rough films of TiO₂ anatase were obtained after calcining at 500 °C. The resultant films were transparent, semitransparent or opaque and 136–402 nm thick. It was found that the TiO₂ films prepared from the sol–gel with surfactant showed a granular nanostructure, and they were composed of regular particles, for example; between 50 and 70 nm. The roughness of the films was found to depend on the surfactant concentration in the sol–gel solution and can show a roughness between 0.82 and near of 17 nm. The photocatalytic activity of the films for the degradation and mineralization of phenol, an industrial pollutant, in water and under 365 nm irradiation was improved by the surfactant modification. Kinetic analysis of degradation and mineralization of phenol in water were employed to evaluate the different TiO₂ films under the same experimental conditions. The global photonic efficiency for degradation and mineralization of phenol ξ_g, was calculated to facilitate comparison with a TiO₂ standard photocatalyst named Degussa P-25.     

A study of anatase–supported Mn oxide as catalysts for 2-propanol oxidation

Several samples of TiO₂ (anatase)-supported Mn oxide XMn–TiO₂ (with X = 0.5, 1.0, 1.5 and 2.0, the theoretical monolayer fractions) have been prepared by a wet impregnation method and characterised by conventional techniques. Mn oxide was found to be well dispersed on the support surface as Mn³⁺species, while part of it is stabilised into the TiO₂ bulk as Mn⁴⁺species. This cation seem to hinder the support sintering and to favour the anatase to rutile phase transition at higher temperatures. Mn oxides supported on TiO₂ are stable and retain medium-high surface area in the 600–900 K range. They are less active than pure Mn oxides in the isopropanol oxidation but, above 600 K, allow complete oxidation of isopropanol to CO₂ only. FT–IR studies and the GC–MS analysis of some intermediate products allow us to confirm the 2-propanol total oxidation occurs through the previously proposed 2-propoxide/acetate/formate surface pathway.     

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 (>30 kN m⁻²), large surface area (>550 m²/g), mesoporous structure (8–11 nm) 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.     

Fabrication and characterization of Ag–TiO2 multiphase nanocomposite thin films with enhanced photocatalytic activity

Ag–TiO₂ multiphase nanocomposite thin films were prepared on quartz substrates by the liquid phase deposition (LPD) method from a mixed aqueous solution of ammonium hexafluouotitanate, silver nitrate and boric acid under ambient temperature and atmosphere followed by calcination at 500 °C for 1 h. The grain growth of anatase was depressed upon Ag⁺ doping. However, silver ions not only promoted (or catalyzed) the formation of brookite phase but also reduced the phase transformation temperature of anatase to rutile. With increasing AgNO₃ concentration, the transmittance and band gap of the composite thin films decreased; however, the intensity of surface plasmon absorption (SPA) peaks increased and their peak position shifted to a longer wavelength range. When AgNO₃ concentration was higher than 0.03 M, the prepared samples consisted of anatase, brookite, rutile and metal silver nanocrystal particles, and their grain size ranges were 5–30 nm. The photocatalytic activity of the Ag–TiO₂ multiphase nanocrystal composite thin films prepared by this method exceeded that of pure TiO₂ thin films by a factor of more than 6.3 when AgNO₃ concentration was kept in the range of 0.03–0.05. This was attributed to the fact that there were many hetero-junctions, such as anatase/rutile, anatase/brookite, Ag/anatase, Ag/rutile and so on, existed in the Ag–TiO₂ multiphase nanocomposite films.     

酸碱催化剂浓度对柔性硅气凝胶性能和结构的影响

以甲基三甲氧基硅烷(MTMS)和正硅酸乙酯(TEOS)作为混合硅源,甲醇为溶剂,十六烷基三甲基溴化铵(CTAB)为表面活性剂,通过酸碱两步催化溶胶凝胶法制备醇凝胶,经超临界干燥可以制备高弹性疏水块状硅气凝胶。分别采用0.1 mol·L^-1 和0.01 mol·L^-1 的草酸作为酸催化剂,5 mol·L^-1 和10 mol·L^-1 的氨水作为碱催化剂,研究不同酸碱催化剂浓度对其网络结构的影响。发现高浓度酸和高浓度碱以及低浓度酸和低浓度碱作为催化剂合成的硅气凝胶的网络结构更加均匀,孔径分布更窄。其中,在草酸浓度为0.01 mol·L^-1、氨水浓度为5mol·L^-1 时,所得硅气凝胶密度为0.135 g·cm^-3、比表面积为807 m²·g^-1、孔隙率约为93%,凝胶最大可压缩至其起始长度的60%,压缩回弹率为100%。     

Preparation and characterization of surface bond-conjugated TiO2/SiO2 and photocatalysis for azo dyes

Surface bond-conjugated TiO₂/SiO₂ was prepared by means of the impregnation method. Based on the results of XRD, FTIR, XPS and BET measurements, the growth of titania (predominantly anatase) on the silica substrate seems to occur by anchoring of the TiO₂ phase through Ti–O–Si cross-linking bonds. The structure model of TiO₂/SiO₂ was proposed. Compared to B–TiO₂, the most efficient catalyst is 30 wt.% TiO₂/SiO₂ (Ims30), which showed three times higher photoactivity for the degradation of reactive 15 (R15). In addition, the catalyst had a higher photoactivity on a silica of smaller particle size than on the silica of larger particles. Silica gel plays the basic role of dispersion and support for power TiO₂. The isoelectric point of the catalyst was 3.0 pH units by the measurement of zeta-potential, indicating the presence of the surface acidity of the catalyst. The photodegradation and the adsorption of R15 and cationic blue X-GRL (CBX) were investigated with the change of initial aqueous pH.     

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.     

Effects of hydrothermal temperature and time on the photocatalytic activity and microstructures of bimodal mesoporous TiO2 powders

Bimodal nanocrystalline mesoporous TiO₂ powders with high photocatalytic activity were prepared by a hydrothermal method using tetrabutylorthotitanate (TiO(C₄H₉)₄, TBOT) as precursor. The as-prepared TiO₂ powders were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS) and N₂ adsorption–desorption measurements. The photocatalytic activity of the as-prepared TiO₂ powders was evaluated by the photocatalytic degradation of acetone (CH₃COCH₃) under UV-light irradiation at room temperature in air. The effects of hydrothermal temperature and time on the microstructures and photocatalytic activity of the TiO₂ powders were investigated and discussed. It was found that hydrothermal treatment enhanced the phase transformation of the TiO₂ powders from amorphous to anatase and crystallization of anatase. All TiO₂ powders after hydrothermal treatment showed bimodal pore-size distributions in the mesoporous region: one was intra-aggregated pores with maximum pore diameters of ca. 4–8 nm and the other with inter-aggregated pores with maximum pore diameters of ca. 45–50 nm. With increasing hydrothermal temperature and time, the average crystallite size and average pore size increased, in contrast, the Brunauer-Emmett-Teller (BET) specific surface areas, pore volumes and porosity steadily decreased. An optimal hydrothermal condition (180 °C for 10 h) was determined. The photocatalytic activity of the prepared TiO₂ powders under optimal hydrothermal conditions was more than three times higher than that of Degussa P25.     

Vanadia–silica aerogels. Structure and catalytic properties in selective reductionof NO by NH3

Vanadia-silica aerogels, containing 10 to 30 wt% V₂O₅, and a xerogel were prepared from vanadium(V) oxide triisopropoxide and vanadium (III) acetylacetonate (V(III)_acac) precursors using the solution-sol-gel method and different drying processes, including conventional evaporative and high-temperature and low-temperature supercritical drying. The behavior of these mixed oxides in the selective catalytic reduction of NO by NH₃ was tested and compared to that of other vanadia-silica and vanadia-titania catalysts. The structural and catalytic properties of the sol-gel derived vanadia-silica mixed oxides were found to be mainly influenced by the drying method, the vanadia content and the vanadia precursor used. For a particular vanadia content (10 wt%), low-temperature supercritical drying and evaporative drying resulted in significantly higher vanadia dispersion than high-temperature supercritical drying, which led to crystalline V₂O₅. Turnover frequencies for SCR at temperatures T < 475K were highest for low-temperature aerogels containing well-dispersed vanadium oxide species. Exposing these catalysts to higher temperatures under SCR conditions resulted in agglomeration/redispersion phenomena and at temperatures T > 550K best catalytic behavior was observed with vanadia-silica mixed oxides for which Raman spectroscopy indicated the presence of crystalline V₂O₅, as was the case for aerogels obtained by high-temperature supercritical drying and the low-temperature aerogel with the highest vanadia content (30 wt%).     

Diffusion During the Supercritical Drying of Silica Gels

Drying is the most critical elaboration step of large monolithic and crack-free silica aerogel plates. In the present work, we are studying the supercritical CO₂ drying and more precisely the first step, here called the supercritical washing step. This phase consists of replacing the liquid phase contained in the nanopores with supercritical CO₂. Within this study, this step is governed by molecular diffusion through the gels. These phenomena were investigated experimentally in order to estimate the duration of the washing step. The experimental results were then fitted with an analytical mass transfer model to identify the effective diffusion coefficient.     

纳米孔超级绝热材料气凝胶的制备与热学特性

以正硅酸四乙酯(TEOS)为硅源，通过溶胶–凝胶及超临界干燥过程制备了SiO2气凝胶. 同时，采用相对廉价的多聚硅(E–40)为硅源，以三甲基氯硅烷(TMCS)为表面修饰剂，硅油为干燥介质, 在常压条件下制备了同样具有纳米多孔结构的SiO2气凝胶. 用透射电镜、扫描电镜及孔径分布仪对其结构进行了表征，并用动态热线法对其热学特性进行了测试. 结果表明: 两种方法制备的气凝胶均是典型的纳米孔超级绝热材料，后者热导率略高但成本低许多，所以更具应用推广潜力.