Direct VUV photolysis of chlorinated methanes and their mixtures in an oxygen stream using an ozone producing low-pressure mercury vapour lamp Original Research Article

The gas-phase photooxidations of CCl4, CHCl3, CH2Cl2 and their binary mixtures in an O2 stream were studied in a flow reactor under various experimental conditions using a low-pressure mercury lamp as light source covered with a high-purity silica sleeve being used. The 184.9 nm VUV irradiation emitted is responsible for the Cl–C bond rupture in the chlorinated methanes and for the formation of O3 from O2. The rate of degradation of H-containing chlorinated methanes increased sharply on increase of their initial concentrations, most probably of a radical dotCl chain reaction, as indicated by the increase in the molar ratio of the amount of HCl formed to the amount of H-containing target substance decomposed. The experimental results suggested that the further transformations of the radicals and products formed play an important role as radical dotCl sources, causing a considerably higher rate of decomposition of the H-containing target substances. In a humidified O2 stream, the radical dotOH formed opens up another route for oxidation of the target substances. Thus, the rates of degradation of CH2Cl2 and CHCl3 increased on increase of the relative humidity, whereas the water vapour had no effect at all on the decomposition of CCl4. At the same time, competition occurs between radical dotCl or radical dotOH for reactions with the target substance. The photooxidation of binary mixtures was investigated too. The addition of CCl4 or CHCl3 to CH2Cl2 strongly increased its degradation rate. The addition of CH2Cl2 did not have a considerable effect on the rate of degradation of CHCl3.