Elucidating the Electrochemical Hydrogenation Route by Spatially Isolating Competing Pathways

Electrochemical hydrogenation can, in principle, occur through two different possible pathways. One is through the Eley-Rideal mechanism, which involves proton-coupled electron transfer directly from solvent water. Alternatively, hydrogenation can also occur through the Langmuir-Hinshelwood mechanism using surface adsorbed *H. Presumably, the competition between these pathways could exert a considerable influence on product selectivity and reaction rates, however much remains unknown regarding the nature of these processes. Here by employing a Pd membrane reactor to spatially isolate the Langmuir-Hinshelwood pathway, we demonstrate that this has a larger kinetic isotope effect (KIE) as compared to the Eley-Rideal pathway. Hence, we find that hydrogenation through the Eley-Rideal pathway results in a relatively higher incorporation of D when a H₂O/D₂O mixture is used as the electrolyte. Finally, we show that increased steric hindrance in the reactant molecule favors the Langmuir-Hinshelwood pathway, which was supported by our theoretical simulations. Our results have important implications on computational modeling of mechanistic pathways and catalyst design for electrochemical hydrogenation reactions.