Study On The Preparation And Performance Of TiO₂/ZSM-5 Composite Photocatalyst

semiconductor as a typical photocatalyst, which has been extensively concerned due to its many advantages such as high photosensitivity and chemical stability, has become a most widely-used photocatalytic material. Fine TiO2 particle shows higher photocatalytic activity than bulk TiO2 particle for it's easy to increase photon yield. However, the fine TiO2 particles have the tendency of aggregation. In addition, the bare TiO2 generally has very small adsorption capacity toward organic compounds. These inherent disadvantages make it less efficient in photocatalysis. The load of TiO2 on porous solid supports with large surface area carries out the combination of dispersion, adsorption and catalysis. It is instructively significant for the development of photocatalyst and the modulation of reaction kinetics to systematically investigate the relationship of support's properties with the characters of TiO2 active constituent and substance molecule. This paper concerns the effect of ZSM-5 zeolitic support's properties (ie. crystal size and acidity) on the preparation and performance of TiO2/ZSM-5 composite photocatalyst. Various characterization techniques were employed to investigate the physicochemical and photoresponse properties of the samples. The adsorption capacity and photodegradation activity of the samples are measured using methyl orange (MO) aqueous solution. Focuses are given to the nature of zeolite supports'adsorption toward MO, and the effect of the strength of adsorption bond on TiO2 photocatalytic activity. The main results obtained in this dissertation are as follows:1. ZSM-5 zeolite was found to be the optimal support in comparison to SiO2, Al2O3,5A and Y zeolite.2. TiO2 loading and dispersion can be increased by the capillarity of zeolitic micropore structure which can adsorb and gentlely release water to control TiO2 sol-gel process. As-prepared TiO2/ZSM-5 composite phototcatalyst has much higher MO photodegradation activity.3. MO adsorption mainly takes place on the relatively strong acid sites of the ZSM-5 external surface. The acid sites of ZSM-5 grow more with SiO2/Al2O3 decreasing, and the adsorption capacity and TiO2 specific photoactivity of TiO2/ZSM-5 are enhanced. The decrease in zeolite crystal size can increase the external surface of zeolite. The high adsorption capacity of nanosized zeolitic support can be attributed to the large amount of acid sites on the large external surface of nanosized zeolite, which can promote photodegradation in dilute substance solution. In addition, large external surface is helpful to increase TiO2 loading and dispersion. The highest TiO2 specific activity of TiO2/ZSM-5 nano-composite depends on the proper TiO2 loading, and the optimal value is 6.5 wt% when TiO2 is supported on ZSM-5 with SiO2/Al3O2 molar ratio of 68 and crystal size of less than 100 nm.4. Results of the adjustment of ZSM-5 external suface acidity by Na+modification indicate: MO molecules adsorbed by strong acid sites are difficult to desorb, which is not helpful to enhance the photoactivity of supported TiO2. Whereas the adsorption of MO by medium strong acid sites (the range of 350-400℃in NH3-TPD profile) can desorb at room temperature, belonging to reversible adsorption. The reversible adsorption of the substrate is significant in promoting the photocatalytic activity of supported TiO2. The MO adsorption by the support is mainly reversible adsoption at Na+content of 1.1 wt%, so TiO2/1.1%Na-HZSM-5 exhibits the highest photodegradation activity among prepared nano-composite photocatalysts.