Step-Edge Functionalization by N-Heterocyclic Carbenes Enhances Catalytic Activity in Electrochemical CO₂ Reduction

Atomic step-edges on metallic surfaces are highly active catalytic sites due to their reduced coordination and modified electronic structure. Yet, approaches to organic ligand functionalization on the single-molecule level have largely targeted flat terrace geometries, whereas site-specific step-edge functionalization remains unaddressed. This study shows that decorating the atomically defined step-edges of Au(788) with N-heterocyclic carbenes (NHCs) enhances their catalytic activity toward ${\mathrm{CO}}_2$ reduction compared to undecorated metallic step-edges. Using high-resolution scanning probe microscopy, an upright-tilted adsorption geometry and a unified binding mode of three different NHCs at step-edges are revealed. The exceptional stability of these well-defined nanostructures allows the use of the single-crystalline samples as working electrodes in electrochemical experiments. Photoelectron spectroscopy and theoretical simulations correlate charge transfer and conformational details with their catalytic performance. By combining macroscopic electrochemical experiments with single-molecule microscopy, this study highlights NHC step-edge functionalization as an effective approach to design highly selective and efficient catalysts.