Study On The Synthesis Of TiO₂ Based Photocatalysts And Their Photocatalytic Reduction Of Cr（Ⅵ）

Photocatalysis technology has the advantages of economy,environmental protection and efficiency,and can be used to to solve the serious energy and environmental problems in the world today.In order to realize its practical industrial application,high performance photocatalyst is essential.TiO2 is probably the most promising photocatalyst for industrial applications due to its good photocatalytic activity for many reactions under natural sunlight,excellent chemical and photochemical stability,cheapness and greenness.However.TiO2 only responds to ultraviolet light because of its wide band gap,so its utilization of sunlight is very low.In addition,it also has a high recombination rate of photogenerated carriers.Therefore,it is necessary to develop a TiO2-based photocatalyst with visible light activity and low recombination rate of photogenerated charges.In this thesis,in order to develop efficient TiO2based photocatalysts for the photocatalytic reduction of Cr(VI),structural control was adopted to improve the photocatalytic activity of TiO2 under xenon lamp irradiation,and combination with hydrochar(HC)was employed to introduce the high visible light photocatalytic activity,which is not possessed by pure TiO2.The details are as follows:1.The effects of acidity and basicity of the solution on the structure and photocatalytic activity of hydrothermally synthesized of TiO2 were studied.It was found that the acidity and basicity of the solution affected the phase,morphology and size of the product.Anatase TiO2 nanoparticles(Diameter=8-12 nm),anatase TiO2 nanoparticles(Diameter=10-14 nm)and rutile TiO2 nanorods(Diameter=17-26 nm)were obtained in deionized water,1.818 mol/L HAc and 1.818 mol/L HCl aqueous solution,respectively,but pure TiO2 could not be obtained in 1.818 mol/L NaOH aqueous solution.Under the irradiation of Xe lamp,the photocatalytic activities of the four samples were evaluated by photocatalytic reduction of Cr(VI)in water.The anatase TiO2(H2O-TiO2)synthesized in deionized water showed the best photocatalytic activity.The results of transmission electron microscope,N2 adsorption-desorption isotherms,infrared and UV-vis diffuse reflectance spectra,transient photocurrent and photoluminescence spectra show that the smallest particle size,the largest specific surface area,the most surface hydroxyl groups and the highest photogenerated carrier separation efficiency may be the reasons for the maximum photocatalytic activity of H2O-TiO2.However,the best H2O-TiO2 is almost inactive for photocatalytic reduction of Cr(Ⅵ)under visible light,possibly because it is only sensitive to UV light.2.In order to improve the photocatalytic activity of TiO2 under visible light irradiation,we tried to combine TiO2 with hydrochar(HC)by hydrothermal method.In this chapter,glucose and tetrabutyl titanate were put into deionized water,and a series of HC/TiO2 nanocomposites with different compositions were synthesized by one-pot hydrothermal method.Their photocatalytic reduction properties of Cr(Ⅵ)under xenon lamp and visible light were studied.Compared with pure TiO2,HC/TiO2 binary composites exhibited significantly improved activity in the photocatalytic reduction of Cr(Ⅵ).When the mass percentage of glucose to tetrabutyl titanate was 14,the photocatalytic activity of HC/TiO2-2 was the highest.and the photocatalytic reduction rate of Cr(Ⅵ)was about 17 times that of pure TiO2.The results of N2 adsorption-desorption isotherm,instantaneous photocurrent,electrochemical impedance(EIS)and photoluminescence spectroscopy(PL)tests show that the excellent photocatalytic performance of HC/TiO2 binary composites is due to the formation of type Ⅱheterojunction between HC and TiO2,which enables the photogenerated carrier to have a high separation efficiency.The highest separation rate of photogenerated electron-hole(e--h+)pairs is the key factor for the highest photocatalytic Cr(Ⅵ)reduction activity of HC/TiO2-2.In addition,the best sample HC/TiO2-2 showed great potential for practical application in the photocatalytic treatment of diluted refrigerant and black chromium plating solution.3.In order to further explore the photocatalytic potential of HC/TiO2 composites,this chapter further studied the effects of carbon source,hydrothermal synthesis temperature and the amount of optimal carbon source on the photocatalytic Cr(Ⅵ)reduction performance of HC/TiO2 nanocomposites.It was found that the photocatalytic activity of the HC/TiO2 composite(g-160)synthesized with fructose as the carbon source was the highest when the synthesis temperature was 160℃.When the synthesis temperature was 180℃,the photocatalytic activity of the HC/TiO2 composite(z-180)synthesized with sucrose as the carbon source was the highest.However,the HC/TiO2 composite(d-200)synthesized at 200℃with starch as the carbon source has the highest photocatalytic activity among all the HC/TiO2 binary composites synthesized using different temperatures and carbon sources.In addition,when the synthesis temperature was 200℃ and the mass percentage of the optimal carbon source(starch)to tetrabutyltitanate was 15,the photocatalytic activity of the prepared dHC/TiO2-2 was the highest.The photocatalytic Cr(Ⅵ)reduction rate over d-HC/TiO2-2 was about 23 times that over pure TiO2.According to the analysis results of UV-vis diffuse reflection spectra,it can be concluded that their strong visible light absorption capacity is an important factor for the high visible light photocatalytic activity of HC/TiO2 composites.The results of transient photocurrent,electrochemical impedance(EIS)and photoluminescence spectra(PL)indicated that the highest photogenerated carrier separation efficiency may account for the highest photocatalytic activity of d-HC/TiO2-2.Based on the above experimental results and band structure analysis,the type II heterojunction was adopted to explain the possible mechanism of photogenerated carrier transfer and separation in dHC/TiO2 nanocomposites.