Chloroform Hydrodechlorination on Palladium Surfaces: A Comparative DFT Study on Pd(111), Pd(100), and Pd(211)

Abstract

Palladium has been shown to be an effective catalyst for chloroform hydrodechlorination, which serves as a promising treatment method for industrial chloroform waste. To investigate the structure sensitivity of this chemistry on Pd surfaces, we performed a density functional theory (DFT, GGA-PW91) study of the chloroform hydrodechlorination reaction on three distinct facets: Pd(111), Pd(100), and Pd(211). Based on the DFT results, the binding strengths of most surface intermediates generally increase in the following order: Pd(111) < Pd(100) < Pd(211). On all three Pd facets, methane is formed as the preferred reaction product through a pathway in which CHCl3* is fully dechlorinated to CH* first, and then hydrogenated to CH4. We constructed potential energy diagrams (PED) and compared the reaction energetics for chloroform hydrodechlorination on the three Pd facets. We propose that the competition between the desorption of chloroform and its initial dechlorination to form CHCl2* likely determines the hydrodechlorination activity of the catalyst. On Pd(111), the desorption of chloroform is energetically favored over its dechlorination while the dechlorination barriers are lower than the desorption barriers on Pd(100) and Pd(211). On the other hand, Pd(100) and Pd(211) bind atomic chlorine stronger and also catalyze the formation of atomic carbon effectively; both are potential site-blocking species. Our results suggest that the more open facets and step edge sites of a Pd nanoparticle may carry higher intrinsic activity towards chloroform hydrodechlorination than the close-packed facets, yet these under-coordinated sites are more prone to catalyst poisoning.