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.