Photocatalytic Degradation Of Dye Pollutants In The Presence Of TiO₂ Based Photocatalysts Prepared By Stearic Acid Gel Method

With the development of industry, the limited water sources are being contaminated increasingly. The contamination of water is directly threatening human being's health and restricting the economic development seriously. So it is very important for us to take some effective measures to eliminate all the dangerous and harmful substances in water. In recent years, the photocatalytic technique is gaining importance in the area of wastewater treatment.The photocatalytic technique can decompose various dangerous organic and inorganic compounds in water using solar energy without causing second pollution. Semiconductor photocatalysts have attracted considerable attention of the workers in environmental science due to their some significant advantages such as lower consumption, mild reaction condition, simple operation and decrease of second pollution in the photodegradation of organic pollutants. When the semiconductor particle is illuminated by light having higher energy than its band gap energy, an electron is excited from the valence band to conduction band to reduce the molecule adsorbed on the surface of particles, and at the same time, the hole in valence band can act as an oxidant. The photocatalytic characteristics of semiconductor particles have proved that the photocatalytic activity of semiconductor particles is inversely related to the particle size, when the particle size is decreased to nanosize level, its photocatalytic activity is significantly enhanced due to volume effect and quantum size effect. TiO₂ as a semiconductor is the most promising green catalyst due to its higher photodegradation ability in decomposing the organic pollutants into CO2, H2O and some mineral acids as well as its safety, low cost and so on.In the first chapter, the concept and development of photocatalysis are introduced. At the end of this chapter, we pointed out the significance of our work and summarized the important results obtained.In the second chapter, TiO₂ nanocrystals were prepared using stearic acid gel method and characterized by DTA-TGA, XRD, TEM, SEM, BET, UV-vis, SPS and XPS. The results obtained was compared with that of TiO₂ prepared by sol-gel method and P25, TiO₂ prepared by stearic acid gel method exhibited bigger surface area than the sample prepared by sol-gel method. When the nanocrystals were used for the photodegradation of methyl orange in water under UV irradiation, it was found that TiO₂ prepared by stearic acid gel method under 450℃exhibited higher photoactivity than TiO₂ nanocrystals prepared by sol-gel method.However, from the point of view of the utilization efficiency, there are some shortcomings as follow for TiO₂:â‘ it can only utilize 1³% sunlight energy because of its narrow band gap;â‘¡Its photocatalytic activity is lower due to the higher combination rate of photoinduced electron and hole.In the third chapter, metal ions doped TiO₂ prepared by stearic acid gel method was used for the photodegradation of methyl orange in water under UV irradiation, it was found that the photoactivity of 0.1at.%Zn-doped TiO₂ was the highest, even higher than international reference material, P25 titania. The methyl orange was completely degraded after 30 min. Mass spectra of the solutions collected at different intervals during the degradation of 0.1% Zn-TiO₂ confirmed that the dye was completely degraded into CO2 and H2O without introducing new impurity.In the fourth chapter, in the search for efficient photocatalysts working under visible light, we have investigated the effect of metal ions (Bi/Co, Fe/Co) codoping on the photocatalytic activity of TiO₂ prepared by stearic acid gel method. The results showed that Fe(0.1%)/Co(0.4%) codoped TiO₂ had the highest photoactivity among all as-prepared samples under visible light, indicating that the photoactivity relates to the cooperative effect of the two dopants.Porous TiO₂ has larger surface area than TiO₂ nanocrystals, which results more active centers and more photoexcited charge carriers. Many researches attempt to prepare porous TiO₂ using template through a self-assembled route of surfactants. But this route has some shortages, the porous structures tend to collapse when being calcined, thus decreasing the surface area of the material. And this method is hard to handle. In the fifth chapter, we try another route to prepared porous TiO₂. The samples were prepared by stearic acid gel method or sol-gel method with the addition of tetraethyl orthosilicate, and then HF treatment, the porous TiO₂ was obtained. The synthesized samples were characterized by BET; the results showed the porous TiO₂ has larger surface area than nanocrystalline TiO₂. It was also found that the porous TiO₂ prepared by sol-gel method exhibited the best photodegradation effect for MeO. The methyl orange was completely degraded after 20 min.