Adsorption-Photocatalytic Activities And Mechanism Of TiO₂/Bentonite On Degradation Of Gaseous Toluene

The concentration levels, health risk and purification technologies of indoor VOCs were introduced. Relevant contents of photodegradation were focused, including the principle of photocatalysis, methods of nanometer TiO₂ preparation, influencing factors of the VOCs photodegradation by TiO₂, problems for TiO₂ application and adsorption-photocatalytic degradation of liquid organic compounds. The trends of research on indoor VOCs purification were proposed.Bentonite was used as an adsorbent in this work. Composite and mixed-type catalysts with TiO₂ and bentonite were prepared. The technics of colloidal method for TiO₂ pillared bentonite preparation and their corresponding impacts on the Photocatalytic degradation of toluene were studied. The effects of humidity on the photoactivities of the catalysts with different specific surface areas were focused. Moreover, a mixture of organobentonite and photocatalyst was applied as the catalyst and the activity as well as degradation mechanism were researched. Intermediate products of toluene degradation by catalysts were elementarily investigated. The main conclusions of this dissertation are:(1) In the process of TiO₂ pillared bentonite preparation using sol method, the volume ratio of titanate butyl to alcohol and the interlayer cation had great effects on the catalyst activity. As for the acid sol method, it was cubersome but needful to strictly contol the pH value of the sol. Therefore, the sol method was comparatively simple and the activites of prepared catalysts were not reduced.(2) Titania pillared bentonite prepared by sol method was applied as the catalyst. The relationships between the effect of humidity on the photoactivity and catalyst structure or catalyst support or pollutant character were discussed. In the range of 25%～75% for the relative humidity, competitive adsorption between toluene and water molecules occurred in the gas phase, thus water vapor had inhibiting effect on the catalytic activities. The inhibiting effect of water vapor on the activity was weaker for the catalyst with larger specific surface area. It can be assumed that the adsorption capability of the catalyst increased with the specific surface area, thus the apparent adsorption constant of water was reduced and the resistance to the effect of humidity was stronger. Furthermore, different degradation activity was achieved applying another substrate. It is possible that the competition for adsorption on the catalyst with molecules was stronger for acetone than for toluene. Thus, adsorption affinity on the catalyst was better for toluene and acetone Photocatalytic degradation was more affected by the humidity than toluene.(3) A mixture of cationic surfactant (CPC) modified organobentonite and photocatalyst was applied as the catalyst under high experimental humidity. At the beginning of reaction, the degradation rate mainly depended on the amount of nanometer TiO₂ (P25), and the promotion effect of organobentonite could be neglected, which may attribute to the much toluene adsorbed by catalysts. The adsorption capacity influenced the toluene degradation with the increasing reaction time. Specific surface areas of catalysts were low. Partition, rather than surface adsorpion, is the mechanism for toluene adsorbed into the CPC modified bentonite. Although the organobentonite was able to enrich the pollutant and enhance the photoactivity, it could protect P25 from absorbing light, resulting in negative effect. The experimental data indicated that the promotion effect of organobentonite on Photocatalytic degradation of toluene was strongest when the mass ratio of organobentonite to P25 was 5. Active carbon (AC) was used to adsorb the organic matter of the effluent gas, then CS₂ was applied as the solvent for ultrasound extraction of AC. The extract was analysed by GC-MS, and benzaldehyde was detected as the intermediate product of toluene photodegradation.