|1) Volatile Organic Compounds and Hazardous Air Pollutants|
As can be surmised from Table 1, olefins decompose more rapidly on TiO2 than saturated hydrocarbons. They form the corresponding aldehydes and then are completely oxidized into CO2. Much research is being done on the decomposition of aldehydes from the perspective of offensive odors and indoor pollution.
Research is also heavy into the decomposition of the volatile organic compounds TCE, PCE and 1,1,1- trichloroethane. TCE rapidly oxidizes into CO2 after coming in contact with a TiO2 photocatalyst, while forming small quantities of products such as CO, COCl2 and CHCl3. Reactions under dry conditions progress very rapidly, suggesting that the chlorine atoms formed during the decomposition process have something to do with it. The reaction speed slows as humidity increases, but reactions can be supported by increasing the intensity of radiated light.
Ground water pollution and soil contamination by TCE and PCE are a problem. Studies are looking into the possibilities of contacting polluted water with a photocatalyst, but attention is directed at aerating the water and treating it in the gaseous phase. An MC forms 1,1- dichloroethylene by a thermal dehydrochlorination reaction that does not require light, which is then rapidly decomposed by photocatalytic reaction. With this kind of compound, a photocatalytic decomposition system that can set reaction temperature to some degree is necessary. In fact, investigations are increasing into compounds of slow reaction speed in reactions with OH, such as alcohols and saturated hydrocarbons.
|Fig. 3 : Change in benzene over time by photocatalytic decomposition reaction |
Benzene: 80 ppm. Air containing 2.2% H2O continually supplied at 0.1 L/min.
Airborne benzene (80 ppm) can be broken down almost entirely (Fig. 3) because of water vapor in the air. In examinations of the carbon mass-balance, it was confirmed that 90% or more was converted into CO2 and about 7% into CO. In experiments using a flow- through reactor, it was learned that some time was needed for the decomposition reaction and that small amounts of carbon matter remained behind on the TiO2 surface, because products gradually increased during the initial stages of reaction and because small quantities of CO2 and CO formed after benzene flow was stopped. Gaseous products other than CO2 and CO were not detected, but minute traces of phenol were found after the experiment when the TiO2 was washed with organic solvent and analyzed by GC- MS. It was also made clear that water vapor plays a big role in the decomposition of benzene by oxidization, since the TiO2 surface gradually colored and decomposition rate seriously declined when the reaction was continued in dry air. Recently, it was confirmed that, when TiO2 that supports platinum was applied to the reaction, activity that rapidly oxidized the CO byproduct into CO2 was seen. Investigations by ESR and other means suggested a mechanism whereby the platinum on the TiO2 stabilized O- and CO was oxidized by O3-.