| Photocatalysis is a green water-treatment technique, and photocatalyst is a key and core in this technique. Out of many semiconductor photocatalysts, TiO2is commonly selected to decompose refractory organic matters as well as micro-pollution water. 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.This thesis aims to design a magnetically recyclable TiO2photocatalyst with hollow structure, which has good performance under visible light. We focused on the preparation of four photocatalysts, including hollow TiO2(HT), Pt-deposited hollow TiO2(HPT), magnetically-recyclable hollow TiO2(TSC) as well as magnetically-recyclable Pt-deposited hollow TiO2(PTSC). We deposited noble metals on TiO2to increase absorbance of visible light because of Schottky barrier. Magnetic separation provides a very convenient approach for removing and recycling magnetic particles using external magnetic fields. The incorporation of magnetic components into TiO2nanoparticle-based catalysts enhances the separation and recovery of photocatalysts. However, direct combination of magnetic cores and TiO2layers increased electro-hole recombination rate, which would reduce photocatalytic activity of photocatalyst. A cut-off layer was proposed to circumvent this problem in this thesis. In addition, all the as-synthesized photocatalysts were fabricated with hollow structures. The magnetic core was introduced into the cavity to prepare magnetically recyclable photocatalysts, which can decrease the contact area of magnetic particles and TiO2, and thus improve photocatalytic performance of photocatalysts.For preparation of the HT photocatalyst, we employed carbon spheres as templates. These templates were synthesized by the method of hydro-thermal synthesis using glucose as precursor. The effects of hydro-thermal time on particle size and dispersion character were discussed in details. Poly(diallyldimethylammonium chloride)(PDDA) and poly(sodium4-styrenesulfonate)(PSS) were used to activate carbon surface; it becomes easier to deposit TiO2layer on these activated carbon spheres. HT photocatalysts can be successfully prepared after calcinations by which the carbon templates were removed and the TiO2crystal phase formed. The average diameter of HT spheres is about600nm with smooth100-nm-(average) thick of TiO2shells. TiO2crystal in HT is composed mainly of anatase phase. Photocatalysis experiments were provided using high-pressure mercury lamp as simulated UV light. The photocatalytic activity of HT spheres was determined by degradation of methylene blue (MB) and methyl orange (MO). Reaction mechanism of the photodecomposition process of MB was first governed by·OH radicals, and then by valence band holes, whereas, holes played a major role in the whole photodegradation process of MO. Compared with solid TiO2, HT exhibited enhanced performance in photocatalytic degradation of both MB and MO. Based on the successful synthesis of HT, well-defined HPT spheres were prepared through the photochemical reduction process. The average size of these deposited Pt particles is about3nm. The photocatalytic activity of HPT was evaluated by photocatalytic decomposition of MB at ambient temperature under visible-light irradiation. The results showed that the HPT photocatalyst with0.75%of Pt/TiO2mass ratio exhibited the optimal photocatalytic ability. The pseudo-first-order rate constant for0.75%HPT (k=0.013min-1) is five times that for P25(k=0.0025min-1) and three times that for HT (k=0.048min-1). EPR results indicated that·OH is the main active species in reaction.In addition, TSC was also fabricated in this thesis. In this photocatalyst, the magnetic CoFe2O4was as core, and the TiO2was as the active shell. A SiO2interlayer between the CoFe2O4core and TiO2shell was used to both weaken the adverse influence of the magnetic core on photocatalysis and improve crystallinity of anatase phase TiO2(TSC is65.30%, TC without interlayer is58.18%, HT is58.92%). Furthermore, hollow structure of photocatalysts can also weaken the adverse influence of CoFe2O4. Photoactivity was evaluated by measuring degradation of organic dyes, including MB, CV, RhB, MO, AO7and CR under visible light illumination. Compared with P25(kMB=0.101min-1), HT (kMB=0.089min-1), the solid TiO2(kMB=0.044min-1) and TC (kMB=0.052min-1), the TSC photocatalyst with a Si/Co molar ratio of28, calcined at500℃,(kMB=0-175min-1) exhibited the optimal photocatalytic performance. The photodegradation mechanisms were investigated using isopropanol as an·OH radical scavenger and potassium iodide as a valance-band hole scavenger. The photodecomposition reaction of cationic dyes MB, crystal violet (CV), rhodamine B (RhB)) was governed by·OH radicals, whereas holes played a major role in the overall photodegradation process of anionic dyes (MO, orange Ⅱ (AO7) and congo red (CR)). Based on the successful synthesis of TSC, PTSC spheres were prepared through the photochemical reduction process. The photocatalytic activity of PTSC was evaluated by photocatalytic decomposition of the six above-mentioned dyes under visible-light irradiation. FL was used to capture·OH radicals in reaction. The results showed a positive correlation between·OH radical contents and photocatalytic reaction rate. The·OH radicals in PTSC, HPT, TSC and HT reaction systems decreased in sequence, and so were the reaction kinetic constants (kPTSC=0.019min-1>kHPT=0.013min-1> kTSC=0.0007min-1> kHT==0.0004min-1). The recyclable PTSC photocatalyst has the optimal photoactivity under visible light among these four mentioned photocatalysts in this thesis.In this thesis, we successfully synthesized the magnetically recyclable TiO2photocatalyst PTSC which can effectively decompose dyes in water under visible light irradiation. The research results can provide a theoretical basis and a technical reference for preparation of hollow TiO2photocatalysts as well as structure optimization. |
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