1985 Volume 20 Issue 2 Pages 82-88
Photocatalytic effects of TiO2, one of metal elements in atmospheric particles, on trans-2-C4H8-02 and trans-2-C4H8-N02-air photochemical reactions have been investigated using a flow-type reactor.
Trans-2-C4H8 was rapidly oxidized to CO2 by O2 in the presence of TiO2 only under photoirradiation. From the O2 partial pressure dependence of CO2 formation rate, O2-species adsorbed on TiO2 surface generated under photoirradiation may contribute to the complete oxidation of trans-2-C4H8. Addition of NO2 to the reaction system remarkably reduced the CO2 yield, indicating that NO2 depressed the photocatalytic oxidation activity of TiO2. On the other hand, from the comparison of the results with those obtained for a gas-phase homogeneous photochemical reaction (trans-2-C4H8-NO2-air) (the order in the yield of the products: CH3CHO>MEK EPB>CH3ONO2-CO>CO2), due to the presence of Ti02, MEK+EPB yield remarkably decreased but CH3ONO2 formation was enhanced and HCN appeared. It is inferred that active species arising from NO2 (or NO) adsorbed on TiO2 may have lower activity in complete oxidation than O2-, but may retain some activity in further oxidation of the partially oxidized compounds and in formation of CH3ONO2 and HCN. This reasoning was not in conflict with the experimental results obtained for a heterogeneous photochemical reaction between TiO2 and a reactant which has passed through a gas-phase photochemical reaction beforehand.
The results presented here may suggest the importance of the heterogeneous chemical reactions in the atmosphere to understanding the transformation of hydrocarbons (especially, decomposition rate of hydrocarbons to CO2) and the cycle of nitrogen on a global scale.
