Preparation Of Transition Metal Doped Mixed Crystal TiO₂ And Investigation Of Their Photocatalytic Activity

With the development of industrial course, people's life has been improved dramatically. Meantime, people face the problem of energy sources lacking and environmental pollution that cause negative influence on our survival. Lots of industrial wastewaters containing high concentration organic pollutants are discharged into river and sea without treatment, which seriously damage the health of human being. How efficiently to eliminate the toxic and hazardous substances such as organic surfactant, dyestuff, noble metal ion from industrial waste effluents has become a major concern. In order to solve this problem, people do a lot of researches and adopt kinds of methods such as physical adsorption, biological degradation, semi-conductor catalysis and gas extraction to eliminate the hazardous chemical compounds from wastewater. However, these organic pollutants can not be decomposed easily and completely by physical and biological methods, because these organic dyestuff compounds in wastewater usually contain one or several benzene molecule rings. In recent years, the advanced oxidation processes which can completely decompose these hazardous organic pollutants have drawn more and more attentions.Among the advanced oxidation processes, the TiO2 as photocatalyst is studied and applied extensively by researchers because it is chemical stable, cheap, non-toxic and catalytic activity high. However, the band-gap of TiO2 (Eg =3.2 eV) is broad, so only under ultraviolet light (λ< 387 nm) irradiation TiO2 catalyst can effectively decompose the organic pollutants in wastewaters. In general, the sunlight only contains 3 %~5 % ultraviolet light, so it will consume large numbers of energy source and need costly equipments to treat wastewaters. Recently, people perform a lot of researches to improve the photocatalytic activity of TiO2. The visible light utilization efficiency of TiO2 is enhanced by modification methods such as surface sensitization, surface organic modification, transition metal ion doping, nonmetal doping, narrow semiconductor coupling and acid or base pretreatment. In general, the photocatalytic activity of the mixed crystal TiO2 is better than that of anatase and rutile TiO2. By heat-treatment, the anatase TiO2 will transform to rutile TiO2 and then the mixed crystal TiO2 is obtained. Meanwhile, researchers found that some transition metal ion doping can also improve the photocatalytic activity of TiO2. The preparation of Fe, Co, Cr ions doped mixed crystal TiO2 is realized by sol-gel and heat-treatment methods. The prepared catalysts were characterized by XRD, TEM, TG-DTA and FT-IR. The experiments of photocatalytic degradation of dyestuff wastewaters using the transition metal doped mixed crystal TiO2 as photocatalyst under sunlight or visible light irradiation were conducted and the influence factors such as dopant content, heat-treatment temperature, heat-treatment time, catalyst addition amount, dyestuff concentration, solution pH and irradiation time were investigated. The results revealed that the prepared photocatalysts behaved high photocatalytic activity under sunlight or visible light irradiation, which can completed decomposed dyestuff molecules. Thus, the experiments explore the way of large-scale using sunlight to treat dyestuff wastewater and the method has a good application future.