Preparation Of Mesoporous TiO₂ And Study Of Photocatalytic Reduction Cr（Ⅵ） Under Simulated Sunlight

With the development of chemical industry,the problem of water environmental pollution caused by heavy metal ions becauses worse and worse.Therefore,the safe,energy-saving and efficient treatment of chromium-containing wastewater had become a research hotspot and difficulty.Compared with adsorption,ion exchange and electrolysis,TiO₂has more advantages of low cost,stable structure and performance,and good environmental compatibility.TiO₂had been concerned as a photocatalyst in recent years,but the common TiO₂had the disadvantages of large band gap and harsh excitation conditions,which leads to its limited application.As a result,it was necessary to prepare a photocatalyst based on TiO₂to broaden the light response range,so as to effectively treat chromium-containing wastewater.Mesoporous TiO₂ photocatalysts with high catalytic activity were prepared using CTAB as template by sol-gel method in this study.The effects of different sol pH,CTAB addition,calcination temperature and calcination time on the photocatalytic reduction Cr（Ⅵ）properties of the prepared mesoporous were compared and investigated.Surface morphology,crystal form,specific surface area,functional groups,photoresponse range and elemental composition of the materials were characterized by SEM,XRD,BET,FT-IR,UV-Vis and XPS.The effects of different influencing factors（initial pH value of solution,dosage of mesoporous TiO₂,initial concentration of Cr（Ⅵ）,reaction atmosphere,sacrificial agent and inorganic salt ions）on the reaction rate of mesoporous TiO₂photocatalytic reduction Cr（Ⅵ）under simulated sunlight were investigated,the repeatability of use of mesoporous photocatalysts was explored.Finally,the adsorption and reaction kinetics model of the photocatalytic reduction reaction of Cr（Ⅵ）by mesoporous TiO₂under simulated sunlight was established,the apparent reaction rate constants pH,the dosage of mesoporous TiO₂and the initial concentration of Cr（Ⅵ）in different solutions were calculated.The results showed that the optimum preparation conditions of mesoporous TiO₂photocatalyst were as follows:sol-gel pH was 3.00,CTAB addition was 0.20 g,calcination temperature was 450.00℃,calcination time was 120.00 min.The preparation conditions of mesoporous TiO₂photocatalyst according to the response surface curve optimization method set up by single factor experiment were as followed:sol-gel pH was 3.12,CTAB addition was 0.18 g,calcination temperature was 474.54℃,calcination time was 126.47 min.The prepared mesoporous TiO₂was anatase type with high crystallinity and a spherical shape with an average particle size of about 10-20 nm,forming a dense grape-like structure between the particles.The mesoporous TiO₂was a typical LangmuirⅣtype with a specific surface area of80.920 m²/g,a pore volume of 0.23 cc/g,and an average pore size of 57.4 A.The band gap was about 2.75 e V,which was smaller than the pure TiO₂sample,and the band gap of mesoporous TiO₂after photocatalytic reaction was larger than that of the prepared mesoporous TiO₂.The O1s spectra after peak separation showed that oxygen vacancies appear in the mesoporous TiO₂after photocatalytic reaction,and the Ti2p spectra showed that there were oxygen vacancies in the prepared catalysts Ti³⁺,which was one of the reasons for the narrowing of the band gap.The optimum reaction conditions for photocatalytic reduction Cr（Ⅵ）of mesoporous TiO₂under simulated sunlight were as follows:initial pH value of reaction solution was 3,dosage of mesoporous TiO₂was 2 g/L,Cr（Ⅵ）initial concentration was 1 mg/L.When entering N₂or adding Na₂SO₃,the photocatalytic reaction can be promoted,and entering O₂or adding Na₂S₂O₈will inhibit the photocatalytic reaction.The photocatalytic reduction efficiency of mesoporous TiO₂to Cr（Ⅵ）can be improved by adding a certain amount of hole sacrificial agent in the reaction system,and the effect was the best when 0.1 M was added.Under acidic conditions,the pH of the reaction system increases gradually with the reaction,but the pH of the reaction system did not change under neutral and alkaline conditions.With the reuse of mesoporous TiO₂photocatalysts,the removal rate of Cr（Ⅵ）by mesoporous TiO₂gradually increased at 20 min of reaction.The results of calculating the apparent total reaction rate constant of the initial pH and the initial concentration of Cr（Ⅵ）in the photocatalytic reduction reaction solution by mesoporous TiO₂under simulated sunlignt were as follows:k₁（28）1.7703C^-0_H（10）.⁰¹³⁸⁷n^-0.22602C₀⁰.¹²⁰⁶（2<pH≤4,0≤n≤2）k₁（28）1.9651C_H⁰._（10）⁰⁰²⁸³n^-0.22602C₀⁰.¹²⁰⁶（4<pH≤9,0≤n≤2）k₃（28）1.5806C^-0_H（10）.⁰¹³⁸⁷n^0.09119C₀⁰.¹²⁰⁶（2<pH≤4,2<n<5）k₄（28）1.5818C_H⁰._（10）⁰⁰²⁸³n^0.09119C₀⁰.¹²⁰⁶（4<pH≤9,2<n<5）...