| The photocatalytic technique of nano-semiconductor represented by titania has demonstrated the appealing practical value, including hydrogen evolution from water, environmental treatment, and sterilization. Although the synthesis of nano-structured titania catalysts with high photoactivities, including nano-powder, film and porous titania, has made rapid progress and applied in the lab to a great extent, many problems still exist when the catalysts are used for the treatment of the hard-to-degradation organic pollutants in large-scale application in view of the factors of separation and costs. The titania whisker obtained from potassium titanate has demonstrated appealing applied prospect due to its micron-scale morphology which makes it easy to separate. However, the lower specific surface area of titania whisker leads to poor photocatalytic efficiency, which restricts its application in the actual system. It is an important tendency of the development of chemical engineering researches that the preparation of the chemical products with beneficial performance should be a requirement-oriented process. In this paper, we have designed and synthesized a novel mesoporous titania whisker with high photocatalytic activity according to the composition-structure-property relationships, which is guided by the ideology of product engineering. Consequently, the synthesis technics has been magnified to the scale of kilogram, and applied to the sewage-treatment photocatalysis equipments with 50 ton/day capacity. The successful application of such theories and technologies indicates the industrialization of advanced technology with autonomous intellectual property rights and demonstrates the characteristics of cross-discipline between material science and chemical engineering. In this thesis, the incipient mixing status during the preparation process of potassium titanate starting from TiO2?nH2O has been firstly investigated, making the foundation for the low temperature solid-state reaction mechanism. Furthermore, various methods to synthesize titania with different types have been explored from ion-exchange, hydrothermal reaction and ambient microstructure transition, respectively. The main contents of this thesis are briefly summarized as follows: The effects of the incipient mixing status of the raw materials on the solid-state reaction are investigated. The thermogravimetric analyses demonstrate that TiO2·nH2O, in comparison with anatase, shows a higher reactivity. The TiO2·nH2O-K2CO3 system shows the preferable nano-mixng status resulting from the reaction of K2CO3 with hydroxyl in the framework of TiO2·nH2O during the process of mixing. Meanwhile, the crystallite growth of titania is prohibited with the increase of temperature. As a result, the generation temperature of potassium titanates decreases which is beneficial to the low-cost and high-quality preparation of titanates. A thermodynamic model of ion-exchange reaction from potassium tetratitanate whisker (K2Ti4O9) is proposed based on the previous study of our lab. From the prediction results of the model, the ion-exchange conditions for the phase-pure anatase titania whisker and other titanate derivatives, K2Ti6O13 and K2Ti8O17 whiskers, are predicted. The temperature for heat treatment of ion-echanged intermediates is determined by thermogravimetry (TG) and differential scanning calorimetry (DSC). These results indicate that the contents of solid product can be achieved only by controlling the pH value and the equilibrium concentration of potassium ion. The hydrothermal synthesis of polymorphic titania crystals from potassium titanates, K2Ti6O13, K2Ti4O9 and K2Ti8O17 whiskers, has been studied systematically. Dispersed, regular twinning rutile TiO2 has been firstly generated by hydrothermally treating K2Ti6O13 whisker in an acid medium. The crystal phase, particle sizes, and morphologies varied with different reaction temperatures and times in diverse mediums have been investigated using XRD, SEM and TEM. Structure-sensitive mechanism is proposed that the crystal structures of starting materials and the solution conditions have profound influences on the nucleation process, the crystal growth, and the morphology. This method may be extended to synthesize other oxide materials with controlled morphology directly from the bulky crystals and provide a model for understanding the formation and growth of natural crystals. A novel mesoporous titania whisker with high surface area is synthesized via controlling microstructure transition process of the sintering product of K2Ti2O5 whisker. With a phase transformation of K2Ti2O5 in a hydrolytic reaction, themesoscopic microphase separation of potassium-rich phase can spontaneous occurs. The resulting mesostructure is constructed by amorphous titania nanoparticles and KOH·nH2O nanophase. The unordered mesopores, which are about 6 -10 nm, are produced by a subsequent dissolving of the resulting nanophase with acid solution. The specific surface area of final mesoporous titania whisker is more than 200 m2/g and the three-dimensional mesoporous structure can be preserved after calcination at high temperature even than 500°C. The structure and relationship between crystallite size and specific surface area of mesoporous titania whisker obtained by different temperatures are studied, which make a foundation for the subsequent study of the relationship between structure and catalytic property. With the application of the cylinder skeleton model, the structure of mesoporous titania whisker is analyzed, and preferable results are gained. It is concluded that the high specific area of mesoporous titania is the synergism effect of nanocrystalline and real pore framework. And the quantitative model on the crystallite size, specific surface area and pore volume is founded to interpret the change of specific surface area as well as make it possible to predict the data about the pore structure. In addition, diffuse reflectance spectra and infrared spectra of mesoporous titania whisker are tested to investigate the surface properties. The photocatalytic activity of titania whisker has been tested via three model systems: the benzene system represented by phenol, the dye system represented by methyl orange, and the industrial pollutant system represented by DMF. And the difference of photocatalytic activity between mesoporous titania whisker and P-25 has been investigated, providing the evidence for the applications of the titania whisker.
|
PDF Full Text Request