Direct VUV photolysis of chlorinated methanes and their mixtures in a nitrogen stream

The gas-phase decomposition of CCl4, CHCl3 and CH2Cl2 and their binary mixtures was studied in a flow-type reactor in a nitrogen gas stream, using a low-pressure mercury vapour lamp covered with a high-purity silica quartz sleeve. The 184.9 nm vacuum-ultraviolet (VUV) light emitted is able to rupture the C–Cl bond in these target substances. For H-containing compounds, the decomposition takes place not only by direct photolysis, but also by H abstraction by Cl formed during the direct photolysis of the target substances. The relative contributions of direct photolysis and Cl-sensitized reactions to the decomposition were estimated at different initial concentrations. The addition of CCl4 to CHCl3 or CH2Cl2 increased their decomposition rates via increase of the Cl concentration, whereas the addition of CH2Cl2 to CHCl3 decreased its degradation rate, suggesting that CH2Cl2 acts as a Cl radical scavenger. The variation of the product distribution confirms the effect of the composition of the irradiated gas mixtures on the relative contributions of Cl-sensitized reactions and direct photolysis.