| As a member of advanced oxidation technology, semiconductor photocatalytic technology becomes hot topic in today’s environment and energy fields. By using the clean and renewable solar energy, photocatalytic technology can occur under milder reaction conditions, and completely mineralize the organic pollutants, without causing secondary pollution. The common photocatalysts are generally non-toxic, low prices, which have great application prospects in environmental field. Since1980’s, photocatalysis in dealing with contaminants (water phase and gas phase) achieved full development, has become the hotspot in research. Semiconductor photocatalysts, such as TiO2and Fe2O3, have the advantages of high activity, low cost, environmentally friendly, and good stability, and the research in application and principles cause widespread attention.Around this theme, from the basic principles of photocatalysis, we prepared the the photocatalyst of high activity and stability by Fe2O3dispersed on the alumina support. And by studying the intrinsic photocatalytic activity of anatase TiO2, we found that the thermally treated TiO2has more surface defects than the hydrothermally treated one, which would facilitate charge separation, and consequently accelerate phenol degradation at the solid-liquid interface. Furthermore, by studying the brookite TiO2, we confirm that TiO2has the same "intrinsic" photocatalytic activity, regardless of the solid structures in the form of anatase, rutile and brookite. By modifying rutile TiO2with CeO2, we found that the positive effect of CeO2was observed with rutile TiO2, but not with anatase or P25TiO2. This thesis is mainly divided into seven chapters, the main research contents and results are as follows:(1) Silica supported hematite (Fe2O3/silica) that is more active but less stable than the supported hematite for organic photodegradation in aqueous solution has been reported. In this work, we report on alumina supported hematite (Fe2O3/alumina) with significantly improved activity and stability. The catalysts were prepared by mixing alumina with a pre-made colloidal iron oxide at various loading (0-100wt%), followed by sintering at different temperatures (200-900℃). Solid characterization with X-ray diffraction and N2adsorption showed that hematite particles were small in size, and large in surface area, as compared with the unsupported hematite prepared in parallel. The catalyst activity was evaluated with anionic Orange Ⅱ as a model substrate, and the reaction was carried out in aerated aqueous suspension under light irradiation at wavelengths longer than320nm. As the Fe2O3loading on alumina or the catalyst sintering temperature increased, the apparent rate constant of dye degradation increased, and then decreased. The maximum rate of dye degradation was obtained with25wt%Fe2O3/alumina, sintered at400℃. Moreover, five consecutive experiments for dye photodegradation showed that Fe2O3/alumina was much more stable than Fe2O3/silica, due to alumina that has a positively charged surface and thus facilitates the dissolved iron species back onto iron oxide. The higher activity of Fe2O3/alumina than Fe2O3/silica and bare hematite is ascribed to the combined effect between the reduced particle size of hematite and the enhanced surface adsorption of dye on the catalyst.(2) Photocatalytic activity of anatase TiO2that increases with the increase of its synthesis temperature has been widely reported, but the reason for that remains incompletely understood. In this work, the positive effect of synthesis temperature, presumably due to the growth of particle size, has been examined. Three series of anatase samples with various particle sizes were prepared from the hydrolysis of TiOSO4in water at150℃, followed by calcination in air. The particle size of TiO2, estimated by X-ray diffraction, and/or by N2adsorption, increased with the increases of the hydrothermal time, calcination time, and calcination temperature, respectively. For phenol photodegradation in aerated aqueous suspension, three series of the samples showed different correlation between the activity and particle size of TiO2. However, for phenol photodegradation in a N2-purged aqueous suspension, these catalysts with the same amount of Ag+adsorbed on the oxide surface showed activities all in proportion to the particle size of TiO2, whereas at given particle size, the thermally treated TiO2was much more active than the hydrothermally treated one. The observed effect of particle size is discussed in terms of the solid crystallinity, surface area, exposed facets, surface hydroxyl groups and light absorption, but they only correlate with the trend in the number of surface defects, as revealed by photoluminescence spectroscopy. Moreover, at given particle size, the thermally treated TiO2has more surface defects than the hydrothermally treated one, which would facilitate charge separation, and consequently accelerate phenol degradation at the solid-liquid interface.(3) It has been reported that with the same amount of electron scavenger on the catalyst surfaces, anatase and rutile actually have a similar "intrinsic" photocatalytic activity at a given sintering temperature (Ts), for organic degradation or water oxidation. But for brookite TiO2, its "intrinsic" photocatalytic has not been studied. In this work, the brookite was synthesized by using a hydrothermal method. The characterizations reveal that the brookite is in pure phase. In the aerated aqueous suspension of TiO2, the initial rate of phenol photodegradation, per surface area of the catalyst, brookite shows higher photocatalytic activity than anatase. By using Ag+as the electron scavenger, we found that at the same amount of electron acceptor adsorbed on the catalyst surfaces, anatase, brookite and rutile actually have a similar photocatalytic activity at a given Ts, for organic oxidation. And for Cr(VI) photoreduction process, the similar result is found. Moreover, there is a synergistic effect between brookite and rutile particles for organic degradation in aerated aqueous suspension, and the higher activity is caused by the process of O2on brookite diffusing onto the rutile sites nearby.(4) Modification of anatase TiO2with CeO2resulting into enhancement in the photocatalytic activity for organic degradation has been widely reported, but the role of CeO2remains unclear. In this work, the biphase oxide has been prepared by mixing individual oxides together, without changes in the physical properties of TiO2itself. For phenol degradation in aqueous suspension under UV light, the positive effect of CeO2was observed with rutile TiO2, but not with anatase or P25TiO2. As the synthesis temperatures of rutile and CeO2increased, the activity enhancement of the mixed oxide increased and decreased, respectively. However, with the same amount of Ag+adsorbed on each catalyst for phenol degradation under N2, the CeO2-mixed TiO2was always less photoactive than parent TiO2(anatase, rutile and P25). Several factors were considered, including the photogeneration of H2O2, and the conduction band edge potentials measured with the oxides. It is proposed that nonstoichiometric CeO2produced at low temperature has ability to store and release oxygen to TiO2nearby, consequently exploring the masked photocatalytic activity of rutile for phenol degradation in aqueous suspension. |
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