Understanding Trichloroethylene Chemisorption to Iron Surfaces Using Density Functional Theory

摘要

This research investigated the thermodynamic favorability and resulting structures for chemical adsorption of trichloroethylene (TCE) to metallic iron using periodic density functional theory (DFT). Three initial TCE positions having the plane defined by HCC atoms parallel to the iron surface resulted in formation of three different chemisorption complexes between carbon atoms in TCE and the iron surface. The Cl-bridge initial configuration with the HCC plane of the TCE molecule perpendicular to the iron surface did not result in C–Fe bond formation. The most energetically favorable complex formed at the C-bridge site where the initial configuration had the C=C bond in TCE at a bridge site between adjacent iron atoms. In the C-bridge complex one C atom formed two sigma bonds to different Fe atoms while the second C atom formed a sigma bond with a second Fe atom. Surface complexation at the C-bridge site resulted in scission of all three C–Cl bonds, and also resulted in a shortening of the C=C bond to a distance intermediate between a double and a triple bond. Initial configurations with the C=C bond adsorbed at top or hollow sites on the iron surface resulted in formation of C–Fe sigma bonds between a single C and two adjacent Fe atoms, and the scission of only two C–Cl bonds. Bond angles and bond lengths indicated that there were no changes in bond order of the C=C bond for top and hollow adsorption. Chemisorption at the C-bridge site had an early transition state in which all three C–Cl bonds were activated from ~1.7 to ~2.2 Å, with an activation energy of 49 kJ/mol. The early transition state and the loss of all three Cl atoms upon chemisorption are consistent with most experimental observations that TCE undergoes complete dechlorination in one interaction with the iron surface. The absence of chemisorption and scission of only two C–Cl bonds at the Cl-bridge site is consistent with experimental observations that trace amounts of chloroacetylene may also be produced from reactions of TCE with iron.