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

Abstract

The gas-phase decomposition of CCl 4, CHCl 3 and CH 2Cl 2 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 CCl 4 to CHCl 3 or CH 2Cl 2 increased their decomposition rates via increase of the Cl concentration, whereas the addition of CH 2Cl 2 to CHCl 3 decreased its degradation rate, suggesting that CH 2Cl 2 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.