| With the rapid development of industry, agriculture and urbanization, the issue of water environmental pollution has been increasingly prominent. The treatment of wastewater, especially removal of organic pollutants from the wastewater, has become a major challenge. Among various wastewater treatment methods, heterogeneous photocatalysis of titania (TiO2) is a promising technology to remove recalcitrant toxic organic pollutants from wastewater. However, in engineering applications for water purification, catalyst recovery remains a big challenge due to the costly and inefficient separation process for TiO2 nanoparticles. Moreover, TiO2 powders frequently cause secondary pollution. Efforts have been directed toward immobilization of TiO2 nanoparticles onto various materials, such as glass, ceramic foam, plastic and resins. However, immobilization will result in dramatic reduction of surface area and inhibited mass transfer efficiency, leading to low photocatalytic performance. Furthermore, the immobilized TiO2 may also be detached from the supporting matrix over long periods of operation, causing attenuation of photocatalytic performance and occurrence of secondary pollution.To solve these problems, monolithic TiO2 aerogels have been considered as alternatives for water treatment. Aerogels are designated as dried gels with low density and high relative pore volume. Moreover, they are a class of mesoporous materials assembled by fine particles with network structure and gaseous dispersion media. TiO2 aerogels combine the photocatalytic activity, nontoxicity, good chemical stability of TiO2 with the low density, high surface area and high porosity of aerogels. Especially, the monolithic structure can be easily separated from aqueous solution. In this thesis work, centimeter sized titania-based monolitic aerogels were prepared with hierarchical porosity in the micrometer and nanometer range under ambient pressure condition. Scanning electron microscope (SEM), Energy Dispersive Spectrometer (EDS), X-ray diffraction (XRD), Fourier Transform Infrared Spectroscopy (FT-IR), characterization the crystalline phase, microstructure, apparent groups, UV-visible Diffuse reflectance (UV-Vis DRS) and N2 adsorption-desorption isotherms were employed to characterize the synthesized materials. The monolithic aerogels adsorption behavior and catalytic properties as well as the mechanism and impact factors were studied and the synthesized materials tested in treating recalcitrant toxic organic pollutants in wastewater.This dissertation includes four parts as follows.1. Titania monolith photocatalyst synthesized using template method has bi-continuous inner structure with hierarchical porosity in the micrometer and nanometer range, and its crystalline structure was anatase. Photocatalytic evaluation results using MO and MB as a model showed that the titania monolith has high degradation efficiency and almost no change in photocatalytic activity after reuse of four cycles. The porous titania monolith can be easily recovered from solution for recycling use, eliminating the difficult process in separating and recycling catalyst, thus improving the process economics. The titania monolith with high and robust photocatalytic performance therefore possesses high potential to be widely used in photocatalytic processes.2. Titania-silica monolithic aerogels (TSMA) have been prepared by a sol-gel method via ambient pressure drying using tetraethoxysilane (TEOS) and tetraisopropyl titanate (TPT) as precursors. TSMA were investigated in detail by N2 adsorption-desorption techniques, SEM, XRD, FT-IR and UV-vis DRS. The results showed that the as obtained aerogels with a porous structure possessed a surface area of 357.86 m2/g, average pore size 10.01 nm, and pore volume 0.922 cm3/g. SEM result showed the prepared TSMA have interconnected macropores in micrometer scale and skeleton pores in nanoscale. EDS results showed that silica and titania were evenly distributed in the obtained materials and the weight ratio of Si:Ti is nearly 4:1. XRD results showed that the prepared TSMA is anatase. However, the crystalline intensity of titania sharply decreased because of the presence of silica. FTIR spectra showed the existence of Si-O-Ti in the aerogels. To investigate photocatalytic performance of the TSMA, photocatalytic degradation of MB and paranitrophenol (PNP) in water by UV illumination was conducted. Experimental results showed the prepared TSMA has good catalytic performance. Moreover, the TSMA remained intact and had good photocatalytic stability after four cyclic experiments.3. The performance of a new type photocatalytic reactor was constructed with annular titania-silica monolithic aerogels as catalyst and a 16 W UV lamp in the center. Photocatalytic degradation of glyphosate and 2-chloro-4-nitrophenol in water was chosen as a test reaction to evaluate the performance of the new type photocatalytic reactor. The results of the reaction indicate that the new reactor has good catalytic performance. And since the annular titania-silica monolithic aerogels are up to several tens of centimeters in size, the new type photocatalytic reactor has advantages as compared with traditional powder suspension.4. Hierarchically porous monolithic titania (TiO2) aerogels with highly crystalline walls and well-defined macropores and mesopores were prepared by the sol-gel process combined ambient pressure drying, the pore size can be tailored by adjusting the HAC/Ti ratio or calcination temperature. Monolithic TiO2 aerogels can be obtained by increasing the amount of HAC(HAC/Ti=20). The monolithic TiO2 aerogels are mainly composed of many nanorods clusters, and has a hierarchical porous structure with 9.15 nm mesopores and 10-20 μm macropores. The crystalline anatase phase was obtained after heat treatment at 500 ℃. Heat treatment did not destroy the monolithic shape and macrostructure of titania aerogels, and still maintained a surface area as high as 70 m2 /g. The photocatalytic activities of monolithic TiO2 aerogels have been studied using methylene blue (MB) as the model pollutant. Catalyst calcined at 500 ℃ exhibited high performance for methylene blue decolorization, which may have been due to the combination of better crystallinity and the continued presence of mesopores in the monolithic TiO2 aerogels. In addition, it was found that ·O2- and·OH radicals are the main reactive species for the degradation of MB under full spectrum light irradiation.To evaluate the photocatalytic stability of the monolithic TiO2 aerogels, cyclic experiments are conducted. After four cycles of experiments, the photocatalytic activity of catalyst showed almost no change. Moreover, the structure of the catalyst remained intact after the cyclic experiments.AgI nanopartics have been immobilized on the TiO2 monolithic aerogels by a simple impregnating-precipitation method. The AgI-TiO2 monolithic aerogels exhibited a high and stable visible photocatalytic activity for MB degradation, which is attributed to the suitable energy band match of Agl and TiO2, leading to the efficient transfer of photo-generated electrons. Under visible light irradiation (λ>420 nm), almost 80% of the 10 mg/L MB was degraded after 60 min irradiation. |
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