Dissertation > Excellent graduate degree dissertation topics show

Nanocrystalline titanium dioxide phase , texture and photocatalytic properties

Author: ZhuJian
Tutor: FanKangNian
School: Fudan University
Course: Physical and chemical
Keywords: Photocatalytic Activity Calcination temperature Photocatalytic reaction Rutile Specific surface area Crystalline structure Catalyst structure Degree of crystallinity The structure of the mesoporous Photocatalyst
CLC: O644.1
Type: PhD thesis
Year: 2006
Downloads: 1105
Quote: 5
Read: Download Dissertation

Abstract

Semiconductor photocatalysis is a comprehensive discipline which combines optics, electrochemistry, material science, surface chemistry and catalysis chemistry. The semiconductor materials can absorb the optical energy and produce the photo-induced holes and electrons which have strong oxidizability and reducing power as well, thus the direct and indirect oxidation and reducing reaction can be occurred at the surface of the catalyst. What causes the great interest of the semiconductor photocatalysis in the scientific research area is the potential value to the problem of the energy utility and environmental purification.The semiconductor photocatalysis research has been developed for 30 years and a lot of achievements have been made but there remain many key problems waited to be solved. Efficient semiconductor photocatalyst need to be improved on the basis of fundamental research. As far as the general TiO₂ photocatalyst, the broad band gap and low quantum efficiency confine its development. There isn’t the unitized cognition to the real reaction mechanism of photocatalysis. With the development of the modern physical technique, the in-situ technique should be developed to reveal the nature of photocatalysis. In practice, the separate process of ultra fine or nano-semiconductor photocatalyst is quite difficult, the activity decrease after being supported and the leaking problem of effective component are waited to be settled up. Furthermore, the device of the reactor can not be ignored because it involves both the heat transfer and mass transfer problem. How to take advantage of the incident light efficiently should be carefully considered. As to the field generated （electric field, micro-waved and magnetic field） assisted photocatalysis reactor, the device is even complex and needs comprehensive consideration.Our work begins from the structure and morphology research of the catalyst to optimizing the preparation methods and conditions to prepare a series of photocatalyst. The effect of crystal phase, texture and surface property to activity was explored.Thus provides a lot of experience to further develop practical photocatalysis technique. The detailed works are as follows:1. Preparation and photocatalytic activity of hierarchically mesostructured grape-like titania materials at low temperature.We use P₁₂₃ as template and present a novel bubbling-mediated hydrolysis approach. We synthesis the hierarchically mesostructured grape-like titania materials composed of microphere aggregates by controlling the bubbling time, bubbling temperature and bubbling rate. The hierarchical grape-like morphology was preserved after cycles of recovery from the aqueous reaction system, and the recycled materials exhibited almost fully retained photocatalytic activity.The amorphous titania transformed to anatase titania when temperature rises. When the sample calcined at 350 °C, it showed good crystalline of anatase. Rutile formed at 500 °C during the calcination while at 700 °C all the sample is rutile. The surface area and pore size decreased with the temperature rising. The template determines the crystalline of titania, and it is crucial to the destruction of mesoporous titania. The surface area of Ti-8-400 without template is only 27 m²·g^-1.When the molar ratio of P123 and tetrabutyl titanate is 1.5×10^-3, the surface area of TiC-8-400 is 85 m²·g^-1.But high concentration is not favorable to the stability of the pore structure of mesoporous titania. When the bubbling time is 3 h, the sphere is highly dispersly. When the bubbling time is 8 h, the mesostructured titania is grape-like and its average size is 25-30 um. This grape-like titania materials is composed of microphere aggregates which average size is 2-3um. Such kind of size ensures the catalyst recycled easily in the aqueous solution.We use degradation of phenol as our probe experiment. TiC-3-400 shows the best photocatlytic activity, but because of its small size and its little precursor, it cannot be easily recycled and put into practice application. The average size of sample TiC-8-400 is 25 um. Its anatase crystalline and worm-like mesoporous structured make it a good photocatalytic activity and recycle. TiC-8-400 indicates a better prospect of application.2. Preparation and photocatalytic activity of well-ordered mesoporous titania photocatalyst.According to the reported EISA method, we successful prepared a series of well-ordered mesoporous titania photocatalyst by adjusting the ratio between TiCl₄ and Ti（OC₄H₉）₄. We found the change of the ratio between TiCl₄ and Ti（OC₄H₉）₄ would result in the different structure of titania including both crystal and pore parameters. When the precursors were composed of just TiCl₄ or Ti（OC₄H₉）₄ the obtained sample is poor-ordered. The well-ordered sample can be prepared while the proper ratio is reached. Besides the pore structure, the crystal structure has also been influenced by the "in-situ" acid. In the sample prepared by only TiCl₄, there is lots of rutile. With the decrease of TiCl₄, the content of rutile is reduced. And when the ratio is 0.3 the obtained sample is composed only by anatase. The phase transform from anatase to rutile will make the collapse of pore wall and result in loss of well-ordered structure.According to the result obtained from photocatalytic reaction, the sample TiM-1.8-350 has the best photocatalytic activity, which is decided by both crystal and pore structure. The crystalline of anatase is superior to order structure in influencing the finally reaction activity. The crystalline of anatase decide transfer speed of the photo-generated carrier, which is limited step of photocatalytic reaction. On the other hand, the order structure just influences the absorption ability. However, the order structure also has some positive influence on the activity which is approved by our experiment.2. The application of nonhydrolytic sol-gel method in the preparation of titaniaphotocatalyst.（1） Low temperature alcoholyisis of TiCl₄ in benzyl alcohol.Nanocrystalline anatase TiO₂ photocatalysts prepared by a facile nonhydrolytic sol-gel （NSG） reaction of TiCl₄ and benzyl alcohol at low temperature followed by subsequent calcination at elevated temperatures were investigated in relation to their performance in the photocatalytic degradation of phenol. A variety of techniquesincluding N₂ adsorption, XRD, DRIFTS, TG/DTA, TEM, DR UV-Vis, Raman and XPS were employed to characterize the resulting materials. It is shown that the fresh nanocrystalline TiO₂ sample obtained by the low temperature NSG process exhibits considerable activity comparable to that of commercial photocatalyst Degussa P-25, although which is evidenced to be surface-capped with appreciable amount of organic moieties. Moreover, it is demonstrated that the catalytic efficiency of the as-prepared nanocrystalline TiO₂ sample can be further markedly enhanced by subsequent thermal treatment at elevated temperatures ranging from 300 to 600℃. Both the calcination temperature and calcination time appear to be crucial factors in influencing a number of critical properties of the calcined TiO₂ samples such as the surface area, particle size, crystallinity, amount of surface hydroxyl groups as well as carbonaceous residues. The TiO₂ photocatalyst obtained by calcination at 400℃ for 3 h exhibits the highest activity toward photocatalytic degradation of phenol.（2） Low temperature alcoholyisis of TiCl₄ in tert-butyl alcohol.A new type of nanocrystalline mesostructured TiO₂ （NMT） predominantly in the anatase phase with a high specific surface area up to 269 m²·g^-1 was prepared by a novel integrated nonhydrolytic sol-gel/UV-illumination technique at low temperature. During the gaseous phase photocatalytic degradation of acetone, the material exhibited excellent photoactivity, much higher than commercial Degussa P-25, which closely related to the markedly enhanced crystallization degree and the beneficial formation of surface Ti-OH groups on the NMT sample during the post UV-illumination process. The significance of the present method for preparation of nanocrystalline mesoporous TiO₂ with favorable structural and surface properties lies mainly in its simplicity, feasibility, and the control of several key factors that determine the photoactivity of TiO₂ materials.（3） Solvothermal process of TiCl₄ in tert-butyl alcohol.we successful synthesize bi/triphasic TiO₂ nanoparticles via a nonhydrolytic solvothermal approach using TiCl₄ and tert-butyl alcohol （TBA） as precursors. The results show the phase content （anatase, brookite and rutile） of TiO₂ can be will well controlled by varying the mol ratio between TiCl₄ and TBA. And the highestphotocatalytic performance of the biphasic TiO₂ sample composed of brookite and rutile has been firstly exhibits, than both commercial P-25 TiO₂ and the mixture of anatase and brookite, in the following photocatalytic oxidation of phenol. The phase content as well as the synergetic effect between different phases have been regarded as the most important factors than surface area in determining the final photoactivity.According to the obtained photocatalytic activity, the synergetic effect between the different phases must be taken into account to get a reasonable explain for the interesting change of activity. The sample NSTi-1 is mainly composed of anatase and brookite, which has a lower activity. When a large scale of core-shell structure composed of anatase and rutile forms the activity of sample NSTi-2 increases quickly. Despite the similar content of brookite found in these two samples, it is proved the core-shell structure composed of anatase and rutile improved the activity more than anatase phase. In another word the synergetic effect between brookite and rutile willl gradually take place the effect between anatase and rutile. Therefore, the observed increase of activity from sample NSTi-2 to NSTi-3 may be attributing to the synergetic effect between brookite and rutile. And The decrease of activity from sample NSTi-4 to NSTi-5 is related to the decrease of the amount of brookite showing the proper ration of brookite and rutile is required to achieve a superior photocatalytiv activity.

Related Dissertations

CLC: > Mathematical sciences and chemical > Chemistry > Physical Chemistry ( theoretical chemistry ),chemical physics > Photochemical,radiation chemical,ultrasonic,the role of chemical processes > Photochemical
© 2012 www.DissertationTopic.Net Mobile