Preparation Of Modified Nano-TiO₂ Photocatalyst And Its Application In Treatment Of Dye Wastewater

Nano-TiO2 as an advanced oxidation technology has become a research hotspot in environmental, chemistry and material field, because of its high photocatalytic activity, chemical stability, low cost, and nontoxicity in wastewater treatment, air purification, etc. However, the practical application of TiO2 is hampered by the fact that it absorbs only the very small ultraviolet part (3-4%) of solar light due to its wide band-gap of 3.2 eV, and moreover there exists problems of separation and recycles. Through a variety of modification methods to improve the photocatalytic activity, enrich the separation means and increase the utilization of sunlight is the important step to make this technology large-scale application. Our long term in this dissertation is to initiate the research of doping modification, support technology and the photodegradation of dye pollutants in water, and explore the optimal preparation techniques and degradation conditions, by which we look forward to building up the research of photocatalytic technology.TFAC as a magnetically separable photocatalyst has been successfully prepared by dip-coating technique and sol-gel method. Firstly, soft magnetic ferrite of MnxZn1-xFe2O4 was adsorbed onto AC (FAC). Afterward, anatase titania was deposited onto the FAC using n-Ti(OBu)4 as titania precursor followed by calcining in N2 atmosphere. The results showed that TFAC catalysts calcined at 450°C for 2 h exhibited the highest activity for degradation of methyl orange (MO) and the best mass proportions of TiO2/MnxZn1-xFe2O4/AC was 8∶1∶4. These photocatalysts exhibited enhanced photocatalytic activity compared to Degussa P25 and pure TiO2. The composite photocatalysts could be easily separated from the bulk solution by a magnetic field and also could be easily redispersed into aqueous solution after removing the external magnetic field. Furthermore, the photocatalysts could maintain high photocatalytic activity after five cycles, and the degradation rate of MO was still close to 90%. The prepared composites were characterized by XRD,FT-IR,SEM,BET etc. The crystal phase of TiO2 was anatase after deposited onto AC, the AC could inhibit the grain size and decrease aggregation of TiO2. Meanwhile, there was a synergetic effect between AC and TiO2 and formed a new Ti-O-C bond. The interaction of AC and TiO2 could enhance the photoactivity of TFAC.The adsorption and photocatalytic properties of TiO2/MnxZn1-xFe2O4/AC were studied. The influence of operational variables, such as initial MO concentration, pH values, hydrogen peroxide concentration on UV photodegradation of MO was investigated. The results showed that the decolorization of MO was inversely proportional to the initial MO concentration. The worst decolorization of MO was obtianed at the pH value near the isoelectric point, acidic or alkaline conditions were beneficial to the decolorization of MO. In order to promote the efficiency of MO decolorization, there was an optimal concentration of H2O2, which was 2mmol/L. The effect of different co-existing ions on the adsorption and decolorization of MO was distinct. The decolorization and degradation of MO was analyzed according to the the TOC decrease, inorganic ion production from the dye degradation, such as SO42– and NH4+, change of UV-Vis spectra and HPLC. With the reaction proceeded, decolorization rate was increased ,but TOC removal was slower. The characteristic absorption peak of MO disappeared completely and there was not any new peak appeared. The HPLC study indicated that there were small amounts of intermediates. The transformation of -SO3– and -N=N- in the degradation of MO was inferred from differences between the theoretical value and the detected values of SO42– and the total N. The release and decolorization was the initial step, with the nitrogen of azo group transformed predominantly to N2. The Langmuir and Freundlich isotherm adsorption model was used to simulate the adsorption kinetics of MO and phenol on TiO2/MnxZn1-xFe2O4/AC. Then their model parameters were calculated and compared to study the differences of their adsoption properties. The results show that TiO2/MnxZn1-xFe2O4/AC has excellent adsorption property, and has a higher adsorption capacity to MO than phenol. The photocatalytic degradation of MO follows L-H kinetic model.The composite photocatalyst of P/TiO2/MWCNTs was prepared by hydrothermal method using n-Ti(OBu)4 as starting material, hypophosphorous acid (H3PO2) as phosphorous precursor and MWCNTs as supporter. The results showed that P-TiO2 particle size was smaller than pure TiO2 and the specific surface area was also increased. The P-TiO2 particles were uniformly coated onto the surface of MWCNTs. After trace amount of P doping into TiO2, doped phosphorus was present as the pentavalent-oxidation state, which could replace a part of Ti4+ in the crystal lattice of anatase. Because P5+ can accept photoelectron as the electron trap center, its doping reduces the recombination rate of photogenerated charge carriers and increases the photon efficiency. At the same time, there was a synergetic effect between MWCNTs and P-TiO2. MWCNTs were eminent electronic acceptors that could orderly export excited electron (e–) from conduction band of TiO2 and fleetly reduce electronic accumulation on TiO2 nanoparticles. So the recombination of electron/hole (e–/h+) pairs could be effectively decreased. The fluorescence intensity of composite catalysts reduced obviously, indicating the reduction of the recombination of electron/hole (e–/h+) pairs. The composite catalysts not only extended the adsorption edge to visible light region, but also exhibited high photoactivity under visible light and UV irradiation. Among them, the CPT005 with a mass ratio of CNTs to P-TiO2 of 5% had the best photoactivity. Therefore, adding a suitable amount of phosphorus and MWCNTs could greatly improve the photocatalytic activity of TiO2 and expand the light response scope. The composite photocatalysts were also prepared by sol-gel method for contrast study. a mass ratio of CNTs to P-TiO2 in the nanocomposite catalyst was considered at 5:100A novel structure of semiconductor Bi2O3 hollow microsphere was prepared by hydrothermal method with cetyltrimenthylammonium bromide (CTAB) as a template and urea as precipitant, and characterized by XRD,FT-IR and SEM technologies. Photocatalytic degradation of methylene blue under visible light was carried out using Bi2O3 as the photocatalyst. It was found that Bi2O3 hollow microsphere was obtained through theral decomposition of bismuth subcarbonate at 260°C for 24 h and the best mol proportions of Bi3+/CO(NH2)2 was 1∶4.5. Bi2O3 exhibited good photoactivity under visible light for degradation of low concentration methylene blue solution. The photoactivity of Bi2O3 could be enhanced under acidic environment or by adding centain amounts of oxidant H2O2.