| The semiconductor-mediated photocatalytic reduction technology is a promising means in treating Cr(VI) wastewater. The development of stable, high efficient, economical and nontoxic photocatalysts is essential for the industrial applications of photocatalysis technology in large-scale Cr(VI) wastewater treatment. Although SnO2, CeO2 and ZrO2 have many advantages as photocatalyst materials, such as stable, economical and nontoxic, they response mainly to UV light because of their wide bandgaps (the bandgaps of SnO2, CeO2 and ZrO2 are 3.8,3.2,5.0 eV, respectively). UV light accounts for only about 5%of the sunlight energy. This greatly affects the photocatalytic efficiency of these wide bandgap photocatalysts and restricts their practical applications. The modification of wide bandgap semiconductors to extend their ranges of visible response is an effective means of developing new and efficient visible light photocatalysts. This thesis focused on the synthesis and evaluation of visible-light-active N-modified SnO2, CeO2 and ZrO2 nanoparticles for photocatalytic reduction of aqueous Cr(VI). The main works completed were summarized as follows:1. A series of tetragonal phase N-doped SnO2 nanocrystals (such as SnO2-HNO3-1, SnO2-HNO3-2 and SnO2-HNO3-3) were sythesized via the solvothermal reactions of SnCl4-5H2O, different amounts of HNO3 and absolute ethanol at 180 ℃ for 12 h. The structure, composition and optical properties of the as-synthesized products were characterized by XRD, TEM, XPS, FT-IR and UV-vis. The photocatalytic activities of SnO2-HNO3-1, SnO2-HNO3-2 and SnO2-HNO3-3 were tested for the reduction of Cr(VI) in K2Cr2O7 aqueous solution under visible light (λ> 420 nm) irradiation, and compared with that of SnO2-NH3 (which denoted the product synthesized when 65-68 mass% HNO3 was replaced by 25-28 mass%NH3·H2O). The photocatalytic results demonstrated that:(i) all the SnO2-HNO3 products exhibited photocatalytic activity with SnO2-HNO3-3 being the most efficient one, whereas SnO2-NH3 exhibited no photocatalytic activity in the reduction of aqueous Cr(VI) under visible-light (λ> 420 nm) irradiation; (ii) the dosage of SnO2-HNO3 and the initial concentration of Cr(VI) aqueous solution had great effects on the photocatalytic reduction rate of Cr(VI); and (iii) Cr(VI) was reduced to Cr(III) after the SnO2-HNO3-mediated photocatalytic reactions.2. Cubic phase N-doped CeO2 nanocrystals (CeO2-HNO3) were sythesized via the solvothermal reactions of Ce(NO3)3·6H2O, HNO3 and toluene at 180 ℃ for 24 h. The structure, composition and optical properties of the as-synthesized products were characterized by XRD, TEM, HRTEM, XPS, Raman, and UV-vis. Besides, their photocatalytic activity was tested by the reduction of Cr(VI) in K2Cr2O7 aqueous solution under visible light (λ> 420 nm) irradiation. The photocatalytic results demonstrated that:(i) CeO2-HNO3 exhibited much higher photocatalytic activity than CeO2-NH3 (which denoted the product prepared when 25-28 mass% NH3-H2O replaced 65-68 mass%HNO3); (ii) while the other conditions were the same, the photocatalytic efficiency of CeO2-HNO3 increased with the increase of the catalyst dosage (100-500 mg) or the decrease of the initial concentration of Cr(VI) aqueous solution; (iii) the coexisting Cu2+, Fe3+, PO43-, CH3COO- and C2O42- had significant effects on the efficiency of Cr(VI) reduction, whereas Na+, Mg2+, Cl- and SO42- had almost no effects on the efficiency of Cr(VI) reduction; and (iv) Cr(VI) was reduced to Cr(III) after the CeO2-HNO3-mediated photocatalytic reactions.3. A series of cubic phase N-doped ZrO2 nanocrystals were sythesized via the solvothermal reactions of ZrCl4, HNO3 and absolute ethanol at 180℃ for 12-24 h. The structure, composition and optical properties of the as-synthesized products were characterized by XRD, TEM, HRTEM, XPS and UV-vis. Besides, their photocatalytic properties were tested by the reduction of Cr(VI) in K2Cr2O7 aqueous solution under visible light (λ> 420 nm) irradiation, and compared with that of ZrO2-NH3 (which denoted the product synthesized when 65-68 mass%HNO3 was replaced by 25-28 mass%NH3·H2O). The photocatalytic results demonstrated that:(i) the ZrO2 nanocrystals synthesized with HNO3 as the nitrogen source exhibited high photocatalytic activity, whereas the ZrO2 nanocrystals synthesized with NH3-H2O as the nitrogen source exhibited no photocatalytic activity; (ii) the dosage of ZrO2-HNO3-12, the initial pH and the initial concentration of Cr(VI) aqueous solution had great effects on the photocatalytic reduction rate of Cr(VI); (iii) the reaction time used in synthesizing ZrO2-HNO3 affected its photocatalytic activity; and (iv) Cr(VI) was reduced to Cr(III) after the ZrO2-HNO3-12-mediated photocatalytic reactions. |
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