Studies On The Synthesis And Reaction Of Novel Nano-scale TiO₂ Photocatalysts

Studies on the synthesis and reaction of novel nano-scale TiO₂ photocatalystsAccompany with the development of the globlal industrialization, severer environmental pollution have been occurred. Photocatalytic technology, as a typical representative of green chemistry, attracts increasingly interest. There are many methods for disposing contaminations, among which the multiphase photocatalysis for the degradation of contaminations of nano-scale TiO₂, as a popular inorganic material, has been widely used because of its various merits, such as optical and electronic properties, low cost, high photocatalytic activity, chemical stability and non-toxicity. This method has high activity, no selectivity, complete oxidation, sunlight and oxygen's application in water phase. Proper control the properties of TiO₂, especially morphology and crystallite structure, in the preparation of nano-scale TiO₂, represents some of the key issues in this area. Because the crystalline state and structure of the support strongly affect the catalytic activity and selectivity, the design and selection of novel highly active catalysts places many requirements on the supports used.Up to now, the research level of this technology is limited in the laboratories in China. Two main problems happened to be in the way of practical application of titania photocatalyst. On the one hand, the nanoparticles of titania photocatalyst would be very easy to run out with the pollutant, it is difficult to collect and callback the catalyst after photodegradation; on the other hand, TiO₂ can be activated only under UV light of wavelengths less than 387 nm irradiation due to its large band gap of 3.2 eV. The solar spectrum usually contains about 4% UV light. Therefore, it is important to shift the optical response of TiO₂ into the visible-light region in order to enhance its photocatalytic activity under daylight or solar irradiation.In the present work, in allusion to the existing problems, we try to improve on the fabrication methods and conditions so as to get series of TiO₂ photocatalyst with various structures and morphology. The morphology and microstructure characteristics of TiO₂ photocatalyst were characterized by means of the N₂ adsorption-desorption measurements, thermalgravimetric and differential thermal analysis (TG-DTA), transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), UV-visible diffuse reflectance spectra (DRS) and X-ray powder diffraction (XRD).Main results of the present work were as follows:1. The nano-scale TiO₂ and Nb/TiO₂ catalysts prepared by hydrolysis of titanium tetrachloride and their ocatalytic activityTitanium tetrachloride (TICl₄, analytical reagent grade) was used as titanium precursor. Nano-scale TiO₂ catalysts were fabricated by directly hydrolysis of titanium tetrachloride. Some of the hydrolysis conditions were studied to get the rule of sample performance. Traditionally, heterogenic TiO₂ materials are prepared by using surfactant templating in which supramolecular aggregates are considered to direct inorganic deposition across a range of length scales. The organic templates must then be removed by thermal treatment, resulting in high energy consumption, environmental pollution and most important the agglomeration and collapse of pore structure in many cases. So most reported mesoporous TiO₂ have a low thermal stability. The direct hydrolysis method can avoid these shortcomings. Larger specific surface area, bigger pore volumn and better anatase crystallization means better photocatalytic performance. The photocatalytic activities of the TiO₂ samples were measured by the degradation of phenol in an aqueous solution with concentration at 0.060g L^-1. Flowing air was bubbled into the solution before irradiation for 30 min in order to establish the adsorption equilibrium and throughout the experiment. Four 8 W lamp were used as ultraviolet and visible light source. 0.050 g of photocatalyst was suspended in a 50 mL aqueous solution of organic reactants. The temperature was controlled at 298 K during the overall degradation process. The concentrations of which were measured with a UV-Visible spectrophotometer (Shimadzu UV-2450). Millipore discs were used to separate the catalysts before the analysis. The measurements were repeated for each catalyst and the experimental error was found to be within±3%. The TiO₂ samples was proved to have comparatively high photocatalytic ability, additionally, Nb modified TiO₂ catalysts gives better catalytic activity in both photoreaction and heterogeneous reactions.2. The one-dimensional nano-scale TiO₂ catalysts and their photocatalytic activity Some of the one-dimensional nano-scale TiO₂ catalysts have been prepared by hydrothermal or hydrolysis methods. TiO₂ nanotube is prepared by using the alkali hydrothermal treatment of commercial P25 TiO₂ nanoparticles. Studies on the nanotube formation process indicate that the nanotube is formed during the alkali hydrothermal process, rather than washing process. XRD and BET results indicate that the as-synthesized TiO₂ nanotube shows only anatase phase with 209 m²/g BET surface area and 15.0 nm diameters calculated by using the BJH equation from the desorption branch of the isotherm.Apertured N-doped TiO₂ microtubes and fibrous TiO₂ have been fabricated by simple hydrolysis of titania tetrachloride using ammonia without any external templates. The morphology and microstucture characteristics of apertured N-doped TiO₂ microtubes were characterized by means of the specific surface area, transmission electron microscopy, scanning electron microscopy, X-ray photoelectron spectroscopy, UV-visible diffuse reflectance spectra and X-ray powder diffraction. The unique morphology of microtubes and mesoporous microstructrure was maintained after a heat treatment at 723 K for 3 h, exhibiting significantly thermal stability. The catalysts exhibit high ultraviolet and visible light photocatalytic activity in degrading phenol and methol orange. The facile and template-free method for the fabrications of apertured TiO₂ microtubes were proved to be remarkable thermal stability. The new photocatalysts show high visible light photocatalytic activity on the degradation of phenol and methyl orange owing to their large specific surface area, surface permeability and well anatase crystallization. The doped nitrogen species play a key role in expanding the photoactivity to visible light region and improving the anatase crystallization and thermal stability. Further work for the examination of the formation mechanism of these apertured N-TiO samples is being under way.3.The microsphere nano-scale TiO₂ catalysts and their photocatalytic activityThe mesoporous TiO₂ microspheres was prepared by a hydro-alcohol thermal method with urea in ethanol/water solution in the presence of sodium sulfate. The mesoporous core-shell structured TiO₂ microspheres was prepared by a hydro-alcohol thermal method with urea in ethanol/water solution in the presence of ammonium sulfate. Anatase TiO₂ powders was synthesized by hydrothermal method which has been applied to synthesize nanosized materials already, since products prepared by this method have well crystalline phase, which benefits to thermal stability of the nanosized materials. The samples have been characterized by X-ray diffractometry, specific surface area determinations, transmission electron microscopy and scanning electron microscopy. The photodegradation of phenol was chosen as a probe reaction to measure the photocatalytic activity of different samples. The photocatalytic properties of commercial photocatalyst Degussa P25 was also presented with that of the nanocrystalline TiO₂ samples for the comparison purpose. The TiO₂ microspheres with high specific surface areas exhibit considerable activity in photocatalytic degradation of phenol. Moreover, the photocatalytic activity of TiO₂ microspheres increases with the calcinations temperature at interval of 673 to 823 K due to the increase in the crystallization degree of the anatase phase. The sample calcined at 733 K gives the best photocatalytic activity and demonstrates to be far superior to that of the commercial Degussa P25 counterpart. The TiO₂-MS samples present high photocatalytic activity as well as good recycling ability, and the unique single dispersed spherical sample showed not only regular morphology. In fact, comparably uniform microspheres with diameter of several microns could be easily separated from the reaction solvent and recycled for more than 10 times. During the cycling process, the catalytic activity keeps almost the same. The excellent recycling ability and its easy separation from the reaction mixture all make the core-shell structured TiO₂ microspheres a promising candidate in further industrial application. Furthermore, Fe(â…¢) doped core-shell structured titania microspheres show comparatively high visible-light photoactivity in degrading phenol.In summary, mesoporous TiO₂ photocatalysts with highly thermal stability and high surface area was prepared by novel hydrolysis and hydro-alcohol thermal route. We found that high specific surface area, large pore volume, narrow pore distribution and good crystallization degree of the anatase phase plays key roles in high performance photocatalyst. The catalysts demonstrated to be far superior to the commercial Degussa P25 in degrading organic pollutants. The excellent recycling ability and its easy separation from the reaction mixture all make the novel TiO₂ photocatalysts a promising candidate in further industrial application.