(149b) Density Functional Theory Studies On the Relative Reactivity of Chloroethenes On Fe(110)

The adsorption and dissociation of perchloroethene (PCE), trichloroethene (TCE), and cis-dichloroethene (cis-DCE) on Fe(110) were investigated using density functional theory (DFT) and the generalized gradient approximation (GGA) to evaluate hypotheses concerning the relative reactivity of these compounds on zerovalent iron. Of adsorption sites investigated, an atop site, where the chloroethene C=C bond straddles a surface iron atom, was the most energetically favorable site for the adsorption of all three chloroethenes. Electronic structure and property analyses provide an indication of the extent of sp2-sp3 hybridization. The strong hybridization of the π-bonding orbital between the chloroethene C=C bond and the iron surface suggests that adsorbed chloroethenes are strongly activated on Fe(110) and are likely precursors for subsequent chloroethene dissociation on the Fe surface. The climbing image nudged elastic band (CI-NEB) method was employed to calculate activation energies of the chloroethene compounds according to the principal dechlorination mechanism of β-elimination. Chloroethenes with a higher number of chlorine atoms were found to have lower activation energies than those with fewer chlorine atoms. The activation energies of PCE, TCE, and cis-DCE at their rate-limiting steps in the gas phase 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 reactivity order among the chlorothenes was found to be PCE > TCE > cis-DCE. Furthermore, defects of the iron surface will be employed in the same simulation study to better understand how the degree of chlorination affects the reactivity of chloroethenes on iron in the remediation environment.