Structure Characterization Of Mesoporous Titania Wiskers And Studies On The Mechanism Of Low-temperature Synthesis

The photocatalytic technique of nano-semiconductor represented by TiO₂ 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 activities, 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. Potassium titanate wiskers had been obtained when TiO₂·nH₂O was used as the substitute for the crystallized anatase in our previous researches, where the generation temperature is much lower than the counterpart of the crystallized anatase. On the other hand, the mesoporous titania wiskers with high specific surface areas had been gained from potassium titanates by the means of ion-exchange reaction, which solve specific problems in the industrial application and overcome the shortcoming of hard-to-reclaim characterized by nano-photocatalysts due to the micron-scale morphology. Meanwhile, the photocatalytic reactivity is equal to P-25, a famous commercial nano-photocatalyst produced by Degussa, Germany. However, the solid-state synthesis mechanism of potassium titanates at the low-temperature and the reason leading to the high specific surface areas and photocatalytic activity of mesoporous titania wiskers are not clear. In this thesis, the solid-state synthesis mechanism of potassium titanates starting from TiO₂·nH₂O at the low temperature has been investigated on the basis of the previous studies, which can not only confirm the indispensable influence of the incipient mixing status on the solid-state reaction, but also provide the explanation for the higher activity of TiO₂·nH₂O from the microstructure analysis and present a novel route to the research of low-temperature solid synthesis. Meanwhile, the relationship between pore structure and calcined temperatures on the final product-mesoporous titania wiskers has been studied via measuring the pore structural data, the indispensable property parameters, which can not only expain the reason for the high photocatalytic activity, but also make the foundation for the selection of the catalytic system in the future and researches on the relationship between structure and property. The main contents and results of this thesis are as follows: Characterization and analyses have been carried on TiO2·nH2O samples calcined at different temperatures by the means of XRD, TEM and BET. And the results show that the crystallite sizes and crystallinity degrees increase with the raise of temperatures and the partial agglomeration takes place above 600 ℃. The mixing mode of K2CO3 with anatase and TiO2·nH2O has been studied to discuss the effect of mixing status on the reaction by the means of thermoanalytical methods, XRD and TEM characterization. XRD results reveal a rather preferable inter-contact in the TiO2·nH2O-K2CO3 system, in comparison with the anatase-K2CO3 system, explaining the lower generation temperature of potassium titanates. TEM characterization shows that anatase crystallites in the TiO2·nH2O-K2CO3 system tend to disperse in a favourable way and the crystallite sizes are conspicuously smaller, in comparison with TiO2·nH2O, indicating that the potassic salts could enter into the framework of TiO2·nH2O to form the nano-mixing phase. As a result, the further crystallite growth is restricted and the reaction temperature decreases. Furthermore, the thermogravimetric analyses show that the dehydration assigned to the deprivation of mobile water for both TiO2·nH2O and the TiO2·nH2O-K2CO3 system discloses almost the same kinetic process, while the dehydroxylation of TiO2·nH2O-K2CO3 demonstrates the much lower pre-exponential factor in comparison with TiO2·nH2O despite the approximate activation energy and the consistent mechanism. Apparently, the intervention of potassic salts has a tendency to qualify the dehydroxylation process rather than the dehydration in the mixture.The results demonstrate the preferable nano-scale mixing status resulting from the reaction of K2CO3 with hydroxyl in the framework of TiO2·nH2O. The important role of the mixing mode on the solid-state reaction during thesynthesis of potassium titanate wiskers has been confirmed from the results above. The preferable nano-mixng phase in the TiO2·nH2O-K2CO3 system resulting from the combination of K2CO3 with hydroxyl in the framework during the process of mixing can not only slow the dehydroxylation and restrict the crystallite growth, but also decrease he generation temperature of potassium titanate wiskers, which is beneficial to the low-cost and high-quality preparation of titanates. On the other hand, the relationship among crystallite sizes, specific surface areas and pore distribution of mesoporous TiO2 wiskers obtained by different temperatures has been studied by the means of XRD, TEM and N2 adsorption–desorption, which serve as the foundation for the quantitative study on the nano-scale materials. With the application of the revised cylinder skeleton model presented by G W Sherer, where oxides are regarded as materials with cubic framework, the structure of mesoporous titania has been analyzed by the means of quantitative calculation based on several parameters, such as specific surface areas, pore diameters as well as density, etc. And the quantitative model on the crystallite sizes, specific surface areas and pore volume has been founded to interpret the evolution of specific surface areas as well as predict the data about the pore structure. It can be concluded that the high specific areas of mesoporous titania is the synergism effect of nanocrystallines and real pores, not merely the effect of nanocrystallines representd by the nanocatalyst, P-25. Meanwhile, the pore structure data increase in the nanometer scale and the specific surface areas decrease resulting from the increase of the framework size and the smoothness of the surfacial structure during the calcination. The conclusions above can not only explain the high specific surface areas and photocatalytic reactivity, but also serve as the theoretical foundation for the further work.