| Environmental issues are among the biggest challenges in the 21st century, with problems related to hazardous waste remediation emerged as a high international priority. As the representative of advanced oxidation technology, photocatalytic degradation of organic compounds over TiO2 has been extensively studied as a potential technology for water treatment and air purification. By using abundant solar light and molecular oxygen as driving force, a variety of organic pollutants can be degraded, and/or mineralized into CO2 over irradiated TiO2 at ambient temperature and pressure. However, the efficiency of TiO2 photocatalysis achieved so far is still low for practical application, mainly due to fast recombination of the charge carriers.This thesis focused on the intrinsic mechanism of TiO2 photocatalysis, and catalysts with high activity were designed via surface modification and the method of phase mixing. A relatively systematic investigation was accomplished about the basic principle and factors of photocatalysis. In this thesis, results were also gained valuable in both academic discipline and practical application.(1) A commercially available TiO2 was modified with hardly water soluble salts, fluorite and fluorapatite. The modified catalysts displayed a higher activity than bare TiO2 for the photocatalytic degradation of phenol and 2,4-dichlorophenol in water under UV light, with the optimum loading of about 10 wt%. Results revealed that the composite catalysts showed good affinity towards organic substrate, and facilitated the generation of hydroxyl radicals compared with bare TiO2. Recycling experiments confirmed the stability of such composites with photocatalytic activity nearly unchanged after five cycles.(2) It has been proposed that fluoride and phosphate anions specifically adsorbed onto TiO2 surface may incluence the reaction of organic photocatalysis. Our results indicate that fluoride with negligible sorption on the surface of TiO2 promoted the photocatalytic degradation of phenol at neutral pH, which could be well fitted by the double layer model. Phosphate showed good affinity towards TiO2 surface at both neutral and acidic pH, as well as the enhancement of phenol degradation. The adsorption of catechol onto TiOi surface was drastically suppressed in the presence of either fluoride or phosphate, which may be an explonation for the accelerated rate of photocatalysis. With the results of DMPO-EPR, we inferred that the generation of free'OH was enhanced in the presence of fluoride, while phosphate anchored onto TiO2 surface facilitated the sorption of organic substrates.(3) Literatures suggest that the charge transfer between anatase and rutile TiO2 particles leads to the synergestic effect in mixed phase TiO2.We have found that the existence of such effect was largely determined by the nature of TiO2 precursor and the organic substrate selected. Comparations were made by using O2 and Ag+ as electron scavenger, with phenol and dye molecules as substrate. It was inferred that the observed synergestic effect was the combined effect of oxide crystallinity and its sorption capacity toward O2. We proposed the capacity of O2 sorption of anatase was higher than that of rutile, while in the mixed phase TiO2, O2 transferd from anatase to rutile explored the masked "intrinsic" photoactivity of rutile particles.(4) Brookite TiO2 with mesoporous structure was obtained by hydrothermal method. Mixed phase TiO2 composed of brookite and rutile in different proportions were obtained by thermal treatment. Phenol photocatalytic degradation was carried out with such mixed phase TiO2 both under air and in the presence of AgN03. Results showed again that the synergestic effect was the concequence of the oxide crystallinity combined with its sorption capacity toward O2.
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