Catalyst alloying enables control over the electrochemical hydrogen transfer route.

An important yet often overlooked catalyst design consideration for electrochemical hydrogenation reactions is the hydrogen transfer route. This can occur either through the Eley-Rideal (ER) mechanism involving proton-coupled electron transfer from solvent water or the Langmuir-Hinshelwood (LH) mechanism using surface-adsorbed *H. Although this can significantly influence catalyst performance, there is a lack of clear guiding principles on how to control the relative contribution of each mechanism. Here we show how catalyst alloying can shift the balance between hydrogenation via the LH vs. ER mechanism. Specifically, we demonstrate this with a model system involving acetylene reduction to ethylene on a series of CuAu alloys of varying composition. Using H2O-D2O mixtures as the electrolyte, we elucidate how varying the Au content can steer hydrogenation towards the LH mechanism. This consumes *H, which lowers hydrogen evolution activity and boosts ethylene selectivity. Using this strategy, the optimal CuAu catalyst achieved an ethylene Faradaic efficiency of 90.90% at 400 mA cm-2 with a production rate of 22.61 mmol h-1 mgcat-1. Our findings illustrate how the hydrogenation pathway can be rationally tuned to control catalytic outcomes.