The Studies Of Gas Phase Photocatalytic Degradation Of Pollutants On TiO₂-SiO₂ Compound Semiconductor Photocatalyst

In 1972, Fujishima and Honda discovered the photocatalytic splitting of water on TiO2 electrodes. This event marked the beginning of a new era in heterogeneous photocatalysis. Since then, research efforts in understanding the fundamental processes and in enhancing the photocatalytic efficiency of TiO2 have come from extensive research performed by chemists and physicists. In recent years, applications to environmental cleanup have been one of the most active areas in heterogeneous photocatalysis. This is inspired by the potential application of semiconductor metal oxides and sulfides photocatalysts for the otal destruction of organic compounds in polluted air and wastewaters. TiO2, ZnO, Fe2O3, WO3, CdS, etc, are semiconductors, i.e. they have a moderate energy band-gap (1-3 eV) between their valence and conduction bands. Under illumination by photons of greater than band-gap energies, the valence band electrons can be excited to the conduction band, creating highly reactive electron-hole pairs. After migration to the solid surface, these may undergo electron-transfer processes with adsorbates of suitable redox potentials. Studies involving gas-solid heterogeneous photocatalysis are relatively few in number compared with the substantial literature on photocatalytic water treatment, but are now of growing interest because of the potential application to contaminant control in contained air atmospheres as found in aircraft and spacecraft, office buildings and factories. At moderate conditions (room temperature, one atmosphere pressure and with molecular oxygen as the only oxidant), the above mentioned semiconductors have proved to be effective photocatalysts for the thermodynamically favored transformations of many organics to CO2 and H2O. In this paper, firstly we selected different synthesis conditions (synthesis route, different alcohol, aging time, calcination temperature and Si concentration) to prepare TiO2-SiO2 mixed oxides. The different of the photocatalytic activity performance exhibited by the samples prepared via two different synthesis routes illustrates that the photocatalytic activity of mixed oxides is closely related with its degree of homogeneity. For heptane, the catalyst prepared by using ethanol as the solvent shows higher photocatalytic ability than those prepared by using methanol, i-propanol and n-butanol. In addition, longer aging time is beneficial for obtaining higher photocatalytic activity. In the case of heptane and SO2, the catalyst researches the highest photocatalytic activity when about 9.1 mol% SiO2 is added. The properties of the samples mentioned above are characterized by means of XRD, BET, FT-IR, SPS, XPS and UV-Vis. It was found that the addition of silica can effectively inhibit the growth of crystalline size and the phase transformation from anatase to rutile, moreover, promote the generation the Br?nsted acid.The comparison between the photocatalytic activity of heptane and toluene was made in the chapter three of this paper. It was found that the catalyst surface was kept clean after the photoreaction in that heptane can be rapidly and completely photodegraded into CO2. However, toluene can not be rapidly and completely photodegraded into CO2 and produce less-active intermediates adsorbed on the catalyst surface to block the active sites. Therefore, we suggest that for heptane larger surface area is beneficial for higher photocatalytic activity. On the contrary, smaller surface area may be good for photodegradation of toluene.