Mechanistic insight into the electrocatalytic performance of reduced graphene oxide supported palladium, silver and palladium–silver nanodeposits toward electro-dehalogenation of halocarbons in room temperature ionic liquids

Abstract

Herein, we report the results from our extensive voltammetric investigations designed to explore, assess and explain the electrocatalytic performance of reduced graphene oxide supported metal nano-deposits toward the electro-dehalogenation of halocarbons in room temperature ionic liquids (RTILs). Specifically, we investigated the electro-reductive dechlorination of the model halocarbon, carbon tetrachloride over glassy carbon electrode (GCE) and palladium–graphene (Pd–Gr), silver–graphene (Ag–Gr) and palladium–silver–graphene (PdAg–Gr) nanocomposites in 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([BMIM][NTf₂]). Analysis of the voltammetric data in light of Marcus–Hush formulation reveals that the electro-reductive cleavage of the C–Cl bond of CCl₄ over GCE in [BMIM][NTf₂] follows a sticky dissociative electron transfer (SDET) pathway. The significantly stronger interaction energy between electrogenerated Cl⁻ and CCl₃˙ (radical) fragments in RTILs makes electroreduction of CCl₄ in [BMIM][NTf₂] much easier than in organic solvents. The activation-driving force relationship for electro-catalytic dechlorination of CCl₄ over Pd–Gr was observed to follow a modified sticky dissociative electron transfer model wherein apart from the ion–radical interaction, the adsorptive interaction of chlorinated species with the electrocatalytic surface needs to be taken into consideration to account for the apparent activation energy-driving force dependence. Interestingly the activation energy-driving force relationships for the electroreduction of CCl₄ over Ag–Gr and PdAg–Gr were observed to fit a modified stepwise ET (MSET) pathway. In the MSET pathway, the adsorption and the implied free energy change of the electroreducible halocarbon significantly alter the solvent re-organization energy and the inherent barrier for the heterogeneous ET process. The adsorptive interaction and hence the electrocatalytic performance of PdAg–Gr were observed to be more than that observed for Ag–Gr. This is attributed to the Ag to Pd charge transfer in the PdAg–Gr nanodeposits. Our results besides underlining the positive influence of RTILs in facilitating the electroreductive detoxification of halocarbons, very well establish the mechanistic basis for the electrocatalytic performance of graphene based nanodeposits toward electrodehalogenation of halocarbons.