(645f) Rational Catalyst Design for the Remediation of Chlorinated Compounds from Drinking Water

The presence of chlorinated organic compounds in drinking water, even in trace amounts, is found to severely affect human health and the environment. Remediation of these chlorinated contaminants using palladium catalysts (catalytic treatment) is considered to be an effective and economic method. Herein, we have used periodic density functional theory calculations to gain molecular level insights into the reactivity of a palladium catalyst for hydrodechlorination. In order to accomplish this, the Pd surface is modelled as terrace sites (Pd(111)) and undercoordinated sites (Pd (211), Pd (100) and Pd (110)). Catalytic hydrodechlorination follows dechlorination and hydrogenation steps to yield non-chlorinated compounds as main products. The chlorine released as a result of dechlorination tends to block the active sites thereby poisoning the surface. Chlorine binds to the Pd facets as follows: Pd (110) > Pd (211) > Pd (100) > Pd (111), indicating rapid poisoning of the undercoordinated sites in comparison to the terrace site. The removal of surface chlorine is facilitated by its reaction with surface hydrogen to form hydrogen chloride. The activation barrier for hydrogen chloride formation was calculated to be 90 kJ/mol and 88 kJ/mol on Pd (111) and Pd (100) terrace sites, respectively; while on the stepped Pd (211) facet and corrugated Pd (110) facet, the activation barriers were 109 kJ/mol and 118 kJ/mol, respectively. This suggests the ease of removal of Cl as HCl from the terrace sites relative to the step and corrugated sites. The structure sensitivity could possibly arise due to competitive adsorption of chlorinated compounds and desorption of chlorine on the Pd facets. This mechanistic understanding would provide a rationale for designing suitable catalysts for hydrodechlorination reactions.