This study was focused on the photocatalytic wastewater treatment technology, especially on the heterogeneous photocatalyst systems which use $TiO_2-based$ mixed oxides as a photocatalyst. The enhancement of the photocatalytic activity of $TiO_2$ was investigated as a function of added amount of other oxides to promote desired redox reaction. This thesis included the preparation and characterization of $TiO_2$ base mixed oxide photocatalysts. By introducing a metal-ion, modified redox energy level in the energy band gap can play a role to retard recombination rate by trapping some of charge carrier and to enhance interfacial charge transfer.
The mixed oxides of $Nb_2O_5$ or $Li_2O$ with $TiO_2$ prepared by a sol-gel process. While the $Nb_2O_5$/$TiO_2$ had a deleterious effect on the oxidation rate of DCA, the excess electrons due to the doping of $Nb_2O_5$ into $TiO_2$ promoted the reduction process for $Cr^{6+}$. $Li_2O$ (1wt%) with $TiO_2$ was found the most efficient photocatalyst for the DCA oxidation, resulting in photocatalytic activity of 50%.
Preparation and characterization of a nano-sized Mo/Ti and Fe/Ti mixed oxide photocatalysts were performed. Nano-sized molybdenum-doped $TiO_2$ (Mo/Ti) mixed oxide photocatalyst was prepared with the $Mo{5+}$ content varying from 0 up to 2.5 mol. TEM-electron diffraction and X-ray diffraction patterns showed that particles were in the form of anatase with the diameter of ca. 4 nm. The appreciable red shift in the UV/VIS absorption spectra was monitored at each preparation stage-aging and dialyzing. The red-shift varied with addition of Mo, in the case of 2.5% Mo/Ti the UV/VIS absorption started at around 0.22 eV less than in nano-sized pure $TiO_2$ (about 3.42 eV). EDAX verified that the amount of Mo in the prepared mixed oxides was as required, and XPS analysis revealed that Mo(V) existed in the $TiO_2$ lattice. The measured photonic efficiency increased to 0.28 with DCA at 0.5 mol% Mo, and then decreased as the Mo content increased above 0.5 mol% Mo.
Pure and doped with various amounts of the $Fe^{3+}$ (2.5 - 99.5 mol%) nano-sized particles of titanium dioxide were prepared. Considerable red shift (~0.5eV) in UV/VIS absorption spectra was observed with 10% Fe/Ti comparing with pure $TiO_2$. EDAX verified the expected amount of Fe in the prepared mixed oxides and XPS revealed that the Fe (III) exists in $TiO_2$ lattice. Specifically, the photonic efficiency increased as the Fe content increased reaching a maximum value of 0.22 at 10% Fe. At higher Fe contents, the photonic efficiency decreased.
Interestingly, the photocatalytic activity of the doped $TiO_2$ using 385nm cut-off filter is equivalent to that of pure $TiO_2$ using 320 nm cut-off filter. These results clearly demonstrated that light of wavelength at round 400 nm could be used to degrade the contaminated organic compounds.
The photocatalytic treatment of the high concentration real wastewater with Hombikat UV100 $TiO_2$, Degussa P-25, NiO and Pt-and Pd-doped ZnO were examined. The noble metal doped photocatalyst been prepared by the photodeposition method. The real wastewater of high concentration of COD(> 1,100 ppm) from a dye manufacturing industry has been treated. Initial and final CODs, TOCs and change in color have been observed to evaluate the activity of each photocatalyst under the same experimental condition. Results showed that 0.5 % Pd-doped ZnO exhibited the highest activity on the decrease in COD and TOC, and the complete color disappearance. Although color was completely bleached, the final COD of real wastewater still remained at about 450 ppm that is still much higher than the EPA standard (less than 100ppm: $COD_{Mn}$) in Korea.
