Photocatalysis with semiconductor has recently emerged as an advanced oxidation process for environmental decontamination. It is safe and versatile, and consumes only light. It is very promising for solving the environmental problems in the most economic way. TiO2 of anatase phase has proven to be the most promising semiconductor photocatalyst for widespread environmental applications because it shows a high reactivity under ultraviolet (UV) light and it is nontoxic, stable and inert chemically. However, anatase TiO2 has a band gap of 3.2eV. This limits its application under sunlight as well as providing its high reactivity because the light with the energy larger than 3.2eV (corresponding to a wavelength of 387nm) constitutes only 3~4% of the energy of solar light reaching the earth. TiO2 of rutile phase is claimed as catalytically inactive or much less active for organic compound photodegradation although it has a smaller band gap (3.0eV) corresponding to a wavelength of 413nm. Therefore, modifications of TiO2 are needed to allow TiO2 to efficiently utilize the solar spectrum. The present dissertation aims at developments of the photocatalysts which can work under visible light.
One method used in the study was modification of rutile TiO2 by surface platinization. The experimental results indicate that Pt can act as a bridge for electrons created in rutile TiO2 to transfer to O2, which enables rutile TiO2’s photoactivity under visible light. Roles of Pt deposited on anatase and anatase-rutile TiO2 for aqueous organic photooxidation were also investigated experimentally and theoretically.
The other method used was modification of TiO2 with transition metal ions. MCM-41, a kind of mesoporous material, was used as a host for incorporation of transition metal ions (Cr, V, Fe, Cu, Mn, Co, Ni, Mo and La) and TiO2 loading. Only Cr6+ was found to be able to sensitize TiO2 for organic degradatoin under visible light. Two other mesoporous materials (MCM-48, and SBA-15) with different pore sizes and dimensionalities were also studied as supports for Cr6+ and TiO2. MCM-41 was found to be the best for visible light photocatalyst development. Cr6+ concentration in TiO2-loaded Cr-incorporated MCM-41 was parametrically investigated and the optimal atomic ratio of Si to Cr was 20. The catalyst deactivates with reation time; the deactivated catalyst can be 100% reactivated by recalcination under 450 °C. Cr leaching is minimal when the catalyst deactivates completely after reaction.
What is more, the synergistic effect between anatase and rutile titanias named in the literature was investigated with different kinds of titanias. A technique was discovered for removal of organics containing acid and/or phenolic groups under visible light by using commercial pristine TiO2.
Conclusively, it has been shown that visible light utilization for aqueous organic oxidation can be achieved by modification of TiO2 with surface platinization or Cr6+ incorporation.