Identification of Active Hydrogen Species on Palladium Nanoparticles for an Enhanced Electrocatalytic Hydrodechlorination of 2,4-Dichlorophenol in Water.

Clarifying hydrogen evolution and identifying the active hydrogen species are crucial to the understanding of the electrocatalytic hydrodechlorination (EHDC) mechanism. Here, monodisperse palladium nanoparticles (Pd NPs) are used as a model catalyst to demonstrate the potential-dependent evolutions of three hydrogen species, including adsorbed atomic hydrogen (H* _ads ), absorbed atomic hydrogen (H* _abs ), and molecular hydrogen (H ₂ ) on Pd NPs, and then their effect on EHDC of 2,4-dichlorophenol (2,4-DCP). Our results show that H* _ads , H* _abs , and H ₂ all emerge at -0.65 V (vs Ag/AgCl) and have increased amounts at more negative potentials, except for H* _ads that exhibits a reversed trend with the potential varying from -0.85 to -0.95 V. Overall, the concentrations of these three species evolve in an order of H* _abs < H* _ads < H ₂ in the potential range of -0.65 to -0.85 V, H* _ads < H* _abs < H ₂ in -0.85 to -1.00 V, and H* _ads < H ₂ < H* _abs in -1.00 to -1.10 V. By correlating the evolution of each hydrogen species with 2,4-DCP EHDC kinetics and efficiency, we find that H* _ads is the active species, H* _abs is inert, while H ₂ bubbles are detrimental to the EHDC reaction. Accordingly, for an efficient EHDC reaction, a moderate potential is desired to yield sufficient H* _ads and limit H ₂ negative effect. Our work presents a systematic investigation on the reaction mechanism of EHDC on Pd catalysts, which should advance the application of EHDC technology in practical environmental remediation.