Pd-catalyzed hydrodehalogenation of chlorinated olefins: Theoretical insights to the reaction mechanism

Density functional theory calculations are applied to study energetics of trichloroethene (TCE) hydrodechlorination over pure Pd(1 1 1), chlorine-covered Pd(1 1 1), and Pd island supported by Au(1 1 1). Our results show that in all cases Csingle bondCl bond breakings take place more readily than Csingle bondH bond formations and that TCE dechlorinates fully producing single bondCCH precursors for the hydrogenations. The reaction pathway through radical-like species provides a possible explanation to the experimental product distributions that show a nominal amount of lesser chlorinated species in the presence of excess hydrogen. The surface chlorine resulted from the TCE decomposition weakens the binding of the adsorbates and reduces the gap between the dechlorination and hydrogenation barriers but is not expected to affect the overall reaction route. The underlying gold induces stronger adsorption and decreases the activation barriers considerably compared to the pure Pd(1 1 1). The change in Gibbs free energy between the gas-phase molecule and the activated surface species explains the experimentally observed reactivity ordering among chlorinated ethenes. Although vinyl chlorine has a stronger Csingle bondCl bond than TCE, it dechlorinates faster when introduced from the gas phase to the catalyst surface.