Modified Mesoporous TiO₂ With Improved Photocatalytic Properities

Photocatalysis has been widely studied because it has the ability to solve the energy crisis and environmental pollution. TiO2 is one of the most promising photocatalysts due to its non-toxicity, chemical stability and rich abundance. Many stategies have been developed to improve the photocatalytic activity of TiO2. In this thesis, mesoporous TiO2 was synthesized using a modified sol-gel process. Their photocatalytic activity in photodegradation of dyes and formaldelyde under UV light was investigated. In addition, the doping of aromatic carbon or Cu ions have been proved to be an efficient way to improve their photocatalytic performance. This dissertation consists of the following contents.(1) Mesoporous TiO2 was synthesized with different Ti precursor. The photoactivity on methylene blue degradation and formaldehyde decomposition over these samples were investigated. For the two typical reactions, it was found that the photocatalytic procedures were not the same. On photodegradation of methylene blue, the photocatalyst using TiCl4 as Ti precursor and hydrochloric acid as pH regulators showed the best activity. However, formaldehyde decomposition proceeded much faster over samples using TiCl4 and tetra butyl titanate ester as co-titanium precursor. The effect of calcinations temperature on photocatalytic efficiency of methylene blue degradation and formaldehyde decomposition was also different. The best temperature for TiO2 was 450℃in the case of methylene blue degradation but 350℃for formaldehyde. It is concluded that the main factor influences the decomposition rate was different. As for methylene blue, it was the cooperation of crystallization and specific surface area. As for formaldehyde, specific surface area played the main role. Additionally, the influence of different ion doping was different. Some metal ions, such as Ag+, Bi3+may enhance the photocatalytic activity, while others, such as Ga2+may decrease the activity.(2) Benzene siloxane doped mesoporous TiO2 (denote as BS-TiO2) was synthesized using a modified sol-gel process (AcHE method).The influence of doping concentration was investigated. Photocatalytic tests on the photodegradation of methylene blue showed that the activity of BS-TiO2 was greatly improved compared to pure TiO2. The best doping concentration was 1%.The prepared samples were characterized with XRD, TEM, BET, TG, solid NMR and EPR. The results showed that all catalysts were anatase. The crystallization was inhibited due to the incorporation of benzene siloxane, which made better sustain mesoporous structure and specific surface area. Solid NMR results show that the bond of carbon and silicon was broken when calcined at 350℃and aromatic carbon speices exist in the pore of TiO2 which is consistent with the result of TG. The reasons of the improved activity were due to longer electronic-hole life because of the existing of aromatic carbon speices. EPR proved that more hydroxyl radicals were produced for BS-TiO2 as the result of slowed recombination rate of photoinduced electron and hole. The PL also showed that the recombination of photoinduced electron and hole was inhibited after doping.(3) Cu doped mesoporous TiO2 was synthesized using AcHE method. Its photoactivity of hydrogen generation from formaldehyde solution under anaerobic conditions was investigated. The results showed that the activity of mesoporous TiO2 was greatly improved with the best doping concentration of 1%and the best calcination temperature of 350℃. Meanwhile, the carbon dioxide was also evolved as the by-product. It was found that the molar ratio of the amount of hydrogen and carbon dioxide was 2:1 with low doping concentration Cu%≤2%) and changed to 1:1 with increased doping concentration (Cu%≥5%). In addition, elevating calcination temperature led to the similar trend. When the amount of formaldehyde was reduced to 330μmol,500μmol hydrogen were produced, which proved that water must participat in the photocatalytic reaction. the XPS results showed that Cu(I) was extensively exsited with low doping concentration Cu%≤2%) and began to convert into Cu(Ⅱ) with increasing the doping concentration (Cu%≥5%). In addition, high calcination temperature also led to the formation of Cu (Ⅱ). We have shown that Cu(Ⅰ) favors the H abstraction from water but Cu (Ⅱ) doesn't. DFT theoretical calculations proved that high doping concentration or high calcinations temperature make the band gap of catalyst too small to decompose water.