| Mesoporous TiO2 has been drawn more attention because of its abundant porous structure, large specific surface area and more cataystic active sites. However, the separation and recovery of suspended small nanostructure TiO2 particles from treated water is very difficult to operate in the practical application process. Usually, small TiO2 particles are immobilized onto a carrier to solve the problem of TiO2 particles separation from liquid phase and avoiding the loss of active component. Unfortunately, a significant loss of photocatalytic activity was generally reported due to greatly reducing surface area after immobilization, shadowing and the dispersion effect. The incorporation of magnetic carrier into nanostructure TiO2 provides an alternative way to solve this problem because of its small diameter. Compared to conventional supported TiO2 photocatalysts, the submicron sizes of the magnetic photocatalyst provide a larger surface area, which subsequently enables higher efficiency in photocatalytic degradations. However, there are two major problems in employing TiO2 coated magnetic materials as a photocatalyst. The first obstacle is a photo-dissolution phenomenon, which may affect the stabilization of magnetic photocatalyst due to the electronic interaction happening between TiO2 coating and the magnetic core. To overcome this problem, an inertial layer (SiO2 or Al2O3) between the TiO2 and magnetic materials was proposed. The second obstacle is the low photocatalystic activity due to low specific area arising from heat treatment of sintering, which is to increase the crystallinity of TiO2 particles. Many researches have tried to enhance the photocatalystic activity by loading the noble metal such as Ag and Pt, and the specific area of shell layer of magnic catalyst and so on. To improve the activity of magnetic photocatalyst is still a hot problem.In the dissertation, Mesoporous magnetic photocatalyst was proposed to enhance the specific surface area and more cataystic active sites, which can increase the reactant adsorption and disperse in the porous structure.The spinel structure nickel ferrete with superparamagnetism nature was firstly prepared by using hydrothermal method, and then its surface was coated with the silica inert layer via the sol-gel method. Finally, TiO2/SiO2/NiFe2O4 composite photocatalysts was prepared by using solvothermal method. Taking nitrobenzene as a simulating pollutant, the photocatalysis avtivities and stabilization of the prepaired mesoporous magnetic photocatalyst has been appreciated.The preparied magnetic TiO2/SiO2/NiFe2O4 composite photocatalysts by wet-chamical method was characterized by N2 adsorption-desorption, X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM) and vibrating sample magnetometer (VSM). TiO2/SiO2/NiFe2O4 composite photocatalysts is spherical partical with shell/core structure, well distribution and superparamagnetism. The sample had an average size of about 50 nm in diameter with abundant mesopore structured anatase TiO2. Under the optimum experimental condition, the BET value is about 145m2/g, average pore size is 5.28 nm, and pore volume is 0.16 cm3/g.The photocatalytic activity of TiO2/SiO2/NiFe2O4 particles was also investigated by the photocatalytic degradation coupled with H2O2 oxidation of nitrobenzene in aqueous solution. Experimental results showed that the sample with 18.8% TiO2 coating amount has the best photocatalytic performance.The TOC removal could amount to 90% during 300 min, the rate of degradation of TiO2/SiO2/NiFe2O4 composite catalyst was 1.75 times of the P25. The TiO2/SiO2/NiFe2O4 composite catalyst could be recovered completely within 30s. The repetitive experiments have been also carried out in the photodegradation solution, which indicated the photocatalyst has good recovered ability and stabilization.Based on the analyzing of the degradation products including fragment ions and intermediate products with GC-MS, the possible degradation pathways of nitrobenzene were also suggested. The possible reaction routes could be divided into two pathways:One is hydroxyl radical attacked phenyl ring to form phenolic compounds, then the ring was opened, forming into various aliphatic compounds; Another is hydroxyl radical attacked directly the nitro-proup and phenol and its derivatives were generated while the nitro-proup was released by the radical addition-elimination. In the subsequent degradation processes, the two pathways have experimenced similar reaction being mineralized to inorganic compounds such as carbon dioxide and water. The probable photodegradation pathways were proposed and discussed as follows.The possible formation mechanism of mesopore shell/core structured photocatalyst TiO2/SiO2/NiFe2O4 has also been discussued as follows. |
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