Preperation Of Modified TiO₂Photocatalysts Using Fly Ash Cenospheres As Substrates And Their Performance Under Visible Light

TiO2has been considered a profound photocatalyst due to its non-toxicity, easy reforming, long-term stability, good photocatalytic activity under UV light as well as convenience of manufacture. However, its low photoefficiency under visible light and hard recovery because of small powder size have restricted its commercial application. Thus, it is of great significance for photocatalysis development to design a recyclable TiO2photocatalyst with good performance under visible light. Surface photosensitization and noble metal deposition have been considered efficient methods to improve TiO2photocatalytic performance under visible light. It’s suggested that fly ash cenospheres (FACs) has superior properties because of their hollow structures. Applying TiO2coating on FACs has been developed to avoid the use of powder, which becomes easy to separate photocatalysts from water in a slurry system after photocatalytic reaction. In this thesis, FACs were employed as substrates for preparation of buoyant and efficiently combinative Pt-doped TiO2/FAC (Pt-TiO2/FAC) and polypyrrole-sensitized TiO2/FAC (PPy-TiO2/FAC) composites. These fresh photocatalysts were characterized by SEM, XRD, FTIR, UV-vis and XPS to obtain their surface morphology and composition, crystal and energy band structure, and element speciation. Photoactivity was evaluated experimentally by measuring degradation of methylene blue under visible light illumination. The photodegradation mechanisms were also investigated, using potassium iodide as a valance-band holes scavenger. Fluorescence spectroscopy (FL) was used to detect active species produced in reaction. The main results are displayed as follows:(1) The as received FACs were hydroxylated before they were coated by TiO2films, which could increase the TiO2coating amount from9.5wt.%to17.6wt.%.The average size of TiO2particles synthesized in this thesis was about50nm. Anatase phase emerged at the calcinations temperature of350℃, and then transformed to rutile phase after it rose to650℃.(2) Polypyrrole was successfully loaded on the surface of TiO2/FAC by the chemical in-situ polymerization method. UV-vis spectra indicated that the introduction of polypyrrole efficiently decreased the forbidden band of TiO2/FAC, which contributed to improve photocatalytic activity of photocatalysts under visible light. The first order reaction can explain the kinetics of MB photodecomposition on the catalyst surface. The results demonstrate that when the molar ratio of pyrrole to Ti is1%and the doping concentration of HC1is1mol L-1, the optimized photocatalytic activity under visible light can be achieved. The recovery test showed that PPy-TiO2/FAC photocatalyst still had photocatalytic activity after recovery three times.(3) Pt particles were deposited on the surface of TiO2/FAC by photochemical reduction method, with zero-and tetra-valence states. The deposition of Pt induced red shift of absorption edge of TiO2/FAC, which contributed to improve photocatalytic activity of photocatalysts under visible light. The photocatalytic degradation ratio can reach the peak using Pt-TiO2/FAC photocatalyt prepared at450℃with the Pt/Ti mass ratio of1.5%, under the catalysts amount of3g L"1.(4) The photocatalytic efficiency of MB improved as pH value increased in the reaction system. Effects of inorganic anions in systems were investigated on the photocatalytic decomposition of MB. The results indicated that with the concentration of HCO3-, SO42-, F, NO3" and Cl" anions below5mmol L-1, the photocatalytic efficiency in system can be enhanced, and the HCO3-is the best anion to improve this efficiency. The photocatalytic degradation ratio decreased gradually as increasing these anions dosage. However, the inhibitory effects on photocatalytic reaction is considered ignorable even the dosage up to100mmol L". FL spectra and holes-scavenged experiments exhibited that the reaction mechanism of the photodecomposition process of MB was governed by·OH radicals and holes. The recovery test showed that calcinations contributed to recover the photocatalytic activity of Pt-TiO2/FAC photocatalyst.