Study On The Ultrasonic/microwave Assisted Preparation Of TiO₂ And Its Photocatalytic Performance

Heterogeneous photocatalysis by semiconductor catalyst as one of the advanced oxidation processes (AOPs) is receiving increasing attention. Due to its high oxidation potential of the species, such as hydroxyl radials (·OH) generated in AOPs, refractory pollutants can be efficiently decomposed into H2O and CO2. Photocatalysis has been researched in the mineralization of organic pollutants in wastewater and air. Among various oxide semiconductor photocatalysts, titanium dioxide (TiO2), especially its anatase phase, is one of the most promising photocatalysts because of its high photocatalytic activity, high chemical stability, low cost and non-toxicity in the purification of air and wastewater. The broad band gap and low quantum efficiency limit its extensive use in environment purification, so the development of high efficiency TiO2 photocatalyst is still the hot-pot in this field.At present, modification of preparation methods is thought one of promising ways. However, less research had been reported in ultrasonic and microwave assisted synthesis of nano-TiO2 photocatalyst. So it will have far-reaching reality meaning to study above problems for application of nano-TiO2 photocatalyst.In this dissertation, the research progress about improving the quantum-efficiency of semiconductor photocatalyst has been summarized. Aiming at the problems in research and practical application of TiO2, such as easy agglomeration and bad dispersibility, a series of nanosized TiO2 photocatalysts was successfully prepared by ultrasonic and microwave assisted method. The effects of crystal phase and structure on activity were explored. On this basis, the sono-photocatalytic degradation efficiency and mechanism of chlorobenzene (CB) and reactive brilliant red X-3B simulated wastewater were studied. The detailed works are as follows:(1) The ultrasonic assisted preparation process of TiO2 photocatalyst was optimized using Ti(OC4H9)4 as precursor. Using the photodegradation rate of reactive brilliant red X-3B simulated wastewater as an indicator, the effects of the dosage of Ti(OC4H9)4, water, ethanol (C2H5OH), acetic acid (CH3COOH) and ultrasonic reaction time on the photocatalytic activity of TiO2 were investigated by using experimental methods of the L16(45) orthogonal design. The results of orthogonal design showed that the important sequence of the factor affecting TiO2 photocatalysis was dosage of Ti(OC4H9)4>dosage of acetic acid > ultrasonic reaction time >dosage of water>dosage of ethanol.The as-prepared TiO2 was characterized by XRD and TEM. The results showed that the TiO2 particles were of good single dispersion and had bi-phase contained anatase as the main phase and rutile phase, with about 24.0nm diameter.(2) The transparent Fe-doped nano-TiO2 films on the surface of glass were prepared by the ultrasonic assisted method. Photocatalytic activity of the films in p-nitrochlorobenzene (p-CNB) simulated wastewater was studied. The Fe-TiO2 films are characterized by XRD and SEM. The results indicated that the Fe3+ doped TiO2 film with an average diameter of 13.0nm contained anatase as the main phase and rutile phase after calcination at 500℃for 2h, and the films are uniform and have no cracking on the surface. The results showed that Fe-doping obviously influenced the photocatalytic activity, the mass fraction of 6.0% Fe2O3 for the Fe3+/doped films as catalyst can increase degradation rate of p-CNB by 2.1 times on comparison with that of undoped nano-TiO2.(3) TiO2 were prepared by microwave assisted hydrolysis of Ti (SO4)2. The effects of pH value of Ti(SO4)2 solution, calcination temperature and Ti(SO4)2 concentration on the photocatalyst particle size, chemical properties, crystal form and transition temperature from anatase to rutile were examined. The catalyst calcined at 700℃has optimal photocatalytic performance. The photocatalytic degradation percentages of p-CNB reached 89.3% at 90min. When the temperature increased to 800℃, only anatase phase presented. The presence of SO42- restrains the transformation of anatase to rutile, strengthens the ability against sintering.(4) Nano-TiO2 powders were prepared with titanium sulphate as initial materials by microwave assisted precipitation method. The optimum synthesis condition for nano-TiO2 were obtained by the experiment: the concentrations of Ti(SO4)2 was 0.2mol/L, the pH value of the reaction end point was 9.0, the calcination temperature was 600℃. The as- synthesized TiO2 was characterized by means of XRD and TEM. The results indicated that the TiO2 samples possessed small size, good dispersity, and had a high pure crystalline phase etc. Photocatalytic properties of nano-TiO2 were researched by degradating the p-CNB simulated wastewater. The results showed that degradation rate of p-CNB reached 87.1% in 90min. Takes urea as the instance, nano-TiO2 powders had been successfully prepared by microwave assisted hydrolyzation-deposition, in which titanium sulfate solution was used as the precursor, urea and sodium dodecylbenzene sulfonate (SDBS) were utilized as precipitator and surfactant respectively. The prepared TiO2 samples were mainly consisted of anatase TiO2 with a uniform size of 20.8nm through the analysis of XRD and SEM. When the content of surfactant is 1.0wt %, the size of nano-TiO2 can be controlled effectively because of the coated effect of SDBS.(5) Fe3+-doped TiO2 photocatalysts with good quality, which crystallite sizes were 10.4nm could be readily prepared by the method of microwave assisted converse precipitation and characterized by XRD and TEM. It is found that Fe3+ doping restrains crystal size increasing. Fe3+ ions enter into the crystal lattice of TiO2 to lead to matrix distortion and lattice expansion.TiO2 photocatalysts have been also evaluated by the photocatalytic degradation of p-CNB in simulated wastewater. The results showed that the photocatalytic activity of the Fe3+-doped TiO2 photocatalyst was much higher than that of undoped TiO2, and the optimum value of Fe3+ doped amount should be at 0.5wt %, the enhanced photocatalytic activity might be attributed to an increase in the charge separation efficiency. The experimental results of the photocatalytic degrading p-CNB indicated that the degradation rate of p-CNB was 95.1% by the UV rays irradiation in 90min, and the degradation rate of p-CNB was 75.2% on the photocatalyst used repeatedly 4 times.(6) Sonolysis and photocatalysis of organic pollutions are all of advanced oxidation technologies following the same free radical mechanism, on the other hand, ultrasonic cavitation can greatly improve the mass transfer between photocatalyst and liquid. Therefore, a synergetic effect may be expected by coupling these two technologies. The effects of dosage of catalyst, pH value, the distance from light resource to reactor, H2O2 dosage and initial reactive brilliant red X-3B concentration on sono-photocatalytic degradation were examined. The optimal conditions of the degradation of X-3B were determined by the uniform experiment design as follow: the mass concentration of TiO2 added is 0.73g/L, the initial pH value of the simulated wastewater is 3.5, and the dosage of H2O2 is 0.33g/L. Kinetic equation of sono-photocatalytic degradation of X-3B was established and verified. The results showed that the sono-photocatalytic degradation reactions of low-concentration X-3B was the first-order kinetics model, and the degradation reactions were in accordance with Langmuir-Hinshelwood equation well, and the reaction rate constant and adsorption equilibrium constant in the equation were determined by means of initial concentration method. In order to investigate the mineralization activity of X-3B under sono-photocatalytic system, UV-Vis and GC-MS analysis were used to investigate the degradation mechanism. It is proved by combining reference conclusions that and a possible sono-photocatalytic degradation mechanism was inferred.(7) The synergistic effects between ultrasound and photocatalytic degradation processes are significant. The sono-photocatalytic degradation of chlorobenzene (CB) simulated wastewater was investigated using the TiO2 prepared by microwave assisted method as photocatalyst. The effects of the dosage of TiO2, the initial concentration of CB, the dosage of H2O2 and the pH value of the simulated wastewater on the degradation of CB were tested. The results indicated that the CB underwent a fast degradation. In the process, the pH value of the simulated wastewater had little effect on the photodegradation rate of CB, while the dosage of H2O2 and the initial concentration of CB affected photodegradation rate greatly. The degradation rate of CB simulated wastewater is over 91.2% when the mass concentration of TiO2 is 300mg/L, CB concentration is 110.6 mg/L and the reaction time is 90min. Based on the results of the experiment. The mechanism of sono-photocatalytic degradation for chlorobenzene simulated wastewater was also discussed.(8) HCB has extremely low solubility in water. In this work, the photocatalytic degradation of HCB was examined with a preadsorption method for the first time. HCB was first preadsorbed on the TiO2 particles (HCB/TiO2) and then suspended in an aqueous solution. Almost all the HCB molecules were adsorbed on the surface of TiO2 during the whole reaction process due to its extremely low solubility in water and strong adsorption on TiO2. This is a good way to investigate the photodegradation of insoluble organic compounds. The influencing factors such as surface coverage of TiO2, dosage of HCB/TiO2, pH value, dosage of H2O2 and concentration of Fe3+ were studied. The pH value of the dispersion and the surface coverage had little effect on the photooxidation rate of HCB catalyzed by TiO2, while the dosage of HCB/TiO2, addition of Fe3+ and H2O2 affected the photocatalytic degradation rate significantly. The dechlorination rate of HCB surpassed 94.8% within 90min under the experimental conditions when surface coverage for TiO2 is 4×10-5 mol/g and the dosage of oxidant H2O2 is 16 mmol/L.