Supercritical Drying Preparation Of New Type Of Tio <sub> 2 </ Sub> Photocatalytic Materials And Sewage Governance

Much attention has been paid for heterogeneous photocatalysis owing to itspotential application in many areas, especially the treatment of environmentalpollutants. Among semiconductor photocatalysts, TiO₂ is most frequently employedbecause of its non-toxity, low cost, high activity and strong stability in aqueous mediaUsually, TiO₂ particles are prepared in liquid phase followed by a dry process.However, the thermal drying of the TiO₂ gel can cause the gathering of the smallparticles and more severely, the collapse of the pore structure resulting in the poorphotocatalytic activity. The modification of TiO₂ with ions to improve thephotocatalytic activity is widely used. If the combination between the ions and TiO₂ isnot strong enough, the ions may break off from the catalyst during the photocatalyticreactions. To improve photocatalytic performance of TiO₂, it was prepared in thepresent paper that metal/nonmetal ion doped TiO₂ photocatalysts with the sol-gelmethod and TiO₂ with large surface area combined with surfactant self-assemblymethod both followed by supercritical drying. Besides, other methods such as thebubbling-mediated hydrolysis, solvent evaporation and alcohol hydrolysis withsurfactant free self-assembly methods are also used to prepared TiO₂ photocatalystswhich can be of novel morphology and high photoactivity. The photocatalyticdegradation of phenol in aqueous solutios and acetone in gas is used as a probe toevaluate their photocatalytic activities. XRD, XPS, TEM, SEM, BET, FLS, RAMAN,IR, DRS of pyrodine adsorption and nitrogen-adsorption were used to characterizethese photocatalysts. Their photocatalytic performances were discussed briefly fromeither their structural characteristics or surface electronic states.1. The undoped and the SO^2-₄ -doped TiO₂ catalysts obtained from the sol-gelfollowed by supercritical drying are of well crystallized anatase phase, higher surfacearea, larger pore size and pore volume as well as more oxygen vacancies and/ordefects, which were favorable for the photocatalytic degradation of phenol. Thedeposit of SO4 -dopant can enhance the activity of TiO2 owing to the increase of the 2-crystallization degree of anatase, the surface area, the pore size, the pore volume, andespecially, the surface acidity. Meanwhile,modification of SO4 is more powerful for 2-supercritical drying than thermal drying, because nearly all SO4 species could 2-coordinate with TiO2, which could generate more Lewis acidic sites for capturingphoto-induced electrons and thus, enhanced quantum yield as well as inhibited therecombination between the photo-induced electrons and holes. This could alsoaccount for the longer lifetime of the SO4 /TiO2(SC) than the SO4 /TiO2(DC) 2-2-owing to their stronger coordination bond. 2. The La2O3/TiO2 prepared by using a combined method of sol-gel andsupercritical drying shows high photocatalytic activity because of much larger surfacearea. Supercritical drying process can effectively avoid the collapse of the gelstructure and improve the crystallinity and the thermal stability of the catalyst. 3. The TiO2 nano-particles prepared by another combined method of self-assemblyusing Triton X-100 as template and supercritical extraction is of slightly higherphotocatalytic activity than that of P25 also because of much larger surface area.Beside supercritical extraction process can avoid the collapse of the TiO2 gel structure,it could effectively remove the surfactant from gel of TiO2. Furthermore, as thetemplate, the Triton X-100 surfactant could enhance the crystallization degree ofanatase phase, surface area and pore volume, which may be responsible for their highphotoactivity. 4.(1)Anovel bubbling hydrolysis method was developed to synthesize highlycrystalline anatase titania microspheres with well-defined worm-like mesoporousstructure. The optimatic values of bubbling time and calcination temperature werealso determined. These mesoporous titania catalysts exhibit a high activity forphoto-degradation of phenol, which could be contributed to their well-definedmesoporous structure, large surface area and nanometer scale sub-structure.