A revised photocatalytic transformation mechanism for chlorinated VOCs: experimental evidence from C2Cl4 in the gas phase

The photodegradation of gaseous perchloroethylene (PCE) was investigated on titanium dioxide under UV light at 365 nm at the gas/solid interface in a CSTR photo-reactor (Continuous Stirred Tank Reactor). The rate and products are strongly affected by oxygen presence. Gaseous products of PCE degradation agree with literature. The production of active chlorine (sum of Cl2, HClO, ClO and ClO2) was investigated both in the presence of oxygen and in anoxic conditions. At low O2 concentration no gaseous active chlorine was determined, while a significant amount was measured in the presence of oxygen. By considering that in the absence of O2 the only possible form of active chlorine is Cl2, this highlights that Cl• is not produced, and that chain reactions promoted by the chlorine radical do not occur on the TiO2 surface. The photocatalytic transformation of C2Cl4 was investigated at different concentrations. The rate follows a first order kinetic that is rationalized with a photocatalytic kinetic model in which the substrate is able to react simultaneously with both photogenerated holes and electrons. In anoxic conditions adsorbed halogenated organic compounds with molecular weights higher than that of PCE were produced and chloride ions accumulated at the surface. Reductive pathways have a key role in PCE degradation. The water vapor has a detrimental role on the PCE transformation rate due to the competition with PCE adsorption on reactive sites with 2:1 stoichiometry. The addition of chloride ions on TiO2 surface slows down the PCE degradation rate and the production of gaseous CCl4 but increments that of C2Cl6 in anoxic conditions. This is rationalized by a mechanism in which direct hole transfer to substrate occurs followed by chloride anion addition to the carbocation.