Prepared Based On The Supercritical Drying Method Doped Tio <sub> 2 </ Sub> And Its Photocatalytic Properties

Much attention has been paid for heterogeneous photocatalysis owing to its potential application in many areas, especially the treatment of environmental pollutants. Among semiconductor photocatalysts, TiO₂ is most frequently employed because of non-toxic, low cost, high activity and strong stability in aqueous media. Usually, TiO₂ particles are prepared in liquid phase followed by a directly dry process. However, the thermal drying of the TiO₂ gel can cause particles agglomeration and collapse of the pore structure resulting in the poor photocatalytic activity. The modification of TiO₂ with ions to improve the photocatalytic activity is widely used. If the combination between the ions and TiO₂ is not strong enough, the ions may break off from the catalyst during the photocatalytic reactions. In the present paper, TiO₂ photocatalysts was prepared by metal ion doped druing the sol-gel process and non-metal ion doped under supercritical condition.The photocatalytic degradation of phenol and methylene blue (MB) in aqueous solution is used as a probe to evaluate their photocatalytic activities. XRD, XPS, TEM, SEM, BET, PLS, Raman, and FTIR, were used to characterize these photocatalysts. The structural characteristics and surface electronic states of photocatalysts and thire photocatalytic performances were discussed briefly as following:1. The undoped and the Ce4+-doped TiO₂ catalysts obtained from the sol-gel process with supercritical drying have well crystallized anatase phase, higher surface area, larger pore size and pore volume as well as more oxygen vacancies and/or defects, which were favorable for the photocatalytic degradation of phenol. The deposit of Ce4+-dopant can enhance the activity of TiO₂ owing to the increase of the crystallization degree of anatase, the surface area, the pore size, the pore volume, and especially, the thermal stability of anatase phase. The particle diameter, per-surface area of the prepared catalyst particles will enhance photoactivity in the degradation of phenol.2. N-doped TiO₂ nanoparticles with higher N-content up to N/Ti = 4.12% were prepared by supercritical treatment of the TiO₂ precursor in Et3N/EtOH fluid. During liquid phase phenol degradation under 365 nm light irradiation, the as-prepared TiO₂N(SC) exhibited much higher activity than the undoped TiO₂ and even the N-doped TiO₂ obtained under supercritical conditions with NH3/EtOH fluid. The optimum activity was obtained at the N/Ti molar ratio of 3.37%, which was nearly 4 times higher than the commercially available P 25 TiO₂. According to various characterizations, the promoting effects of both the supercritical treatment and the N-modification on the activity were discussed based on the considerations of (1) the higher surface area (SBET) and pore volume (VP) which facilitated the adsorption and diffusion of reactant molecules, (2) the higher crystallization degree of the anatase, the increased number of surface defects and oxygen vacancies as well as electron-deficient N atoms which might inhibit the electron-hole recombination, (3) the increase of the light absorbance, especially at longer wavelength, which could generate more photo-induced holes for phenol oxidative degradation.3. Highly active sulfur-promoted anatase TiO₂ (TiO₂S(SC)) was prepared by pretreatment the TiO₂ precursor under CS2/Ethanol supercritical conditions. The highly photocatalytic activity was evaluated by the liquid-phase oxidative degradation of phenol under irradiation with UV light characteristic of 365 nm. The results showed that the crystallinity of anatase was improved by S-doping. Moreover, sulfur not only enhance the crystallized of anatase but also prevented phase transition of anatase to rutile. The highly photocatalytic activity of S-doped TiO₂ powders could be attributed to its higher surface area, larger pore volume, well-crystallized anatase, for the more, the S-doped TiO₂ samples exhibited stronger absorption in the UV-visible range with a red shift in the band gap transition. It could be due to the formation of S-Ti-O bonds. The electron-deficient sulfur atoms in S-Ti-O sulfurization could account for the higher activity since the S-doped samples could exhibit the recombination between the photoinduced electrons and holes by capturing the photoinduced electrons. The S-doped TiO₂ samples with S/Ti molar of 1.83 % showed maximum activity, which was even much higher than either P25 TiO₂or obtained via direct calcinations.4. In our study, nitrogen and fluorine atoms were simultaneously codoped into the TiO₂ crystal lattice under supercritical conditions with the aim of introducing new active sites to improve the visible-light absorption. As a result, a highly reactive, visible-light-driven photocatalysis would be achieved.