Density functional theory studies on the relative reactivity of chloroethenes on zerovalent iron

The gas-phase dissociation of perchloroethene (PCE), trichloroethene (TCE), and cis-dichloroethene (cis-DCE) on zerovalent iron Fe(110) was investigated using periodic density functional theory (DFT) with the generalized gradient approximation (GGA) and climbing image nudged elastic band method (CI-NEB). Activation energies and dechlorination rate constants for reductive beta-elimination of the chloroethene compounds were calculated using an Arrhenius equation with theoretically calculated vibrational frequencies for the compounds. Activation energies were found to decrease as the chlorination number increases. The reaction rate-limiting step for PCE dissociation occurs at the second chlorine cleavage, while the rate-limiting steps for TCE and cis-DCE occur at the first chlorine cleavage. The activation energies of PCE, TCE, and cis-DCE at their rate-limiting steps are 9.9, 16.6, and 23.8 kJ/mol, respectively. Energy profiles along the reaction coordinate for the dechlorination paths are presented. The relative gas-phase reactivity order among chlorothenes on Fe(110) was found to be PCE > TCE > cis-DCE. At room temperature (300 K), the PCE dechlorination rate is 14 and 338 times faster, respectively, than that of TCE and cis-DCE. Details regarding the electronic properties of the transition states of the dechlorinated compounds are reported.