Gold Nanoparticles Grafted with Molecular N-Heterocyclic Carbenes for CO₂ Electroreduction: Combining Electronic and Hydrophobic Effects

Tuning the surface microenvironment offers a powerful strategy to enhance the electrocatalytic performance of presynthesized metal nanocatalysts. Here, we demonstrate that N-heterocyclic carbenes (NHCs), as a self-assembled monolayer on gold nanoparticles (AuNPs), synergistically promote the electrochemical reduction of CO₂ to CO. This NHC functionalization boosts the partial current density for CO formation by ∼30% while concurrently suppressing competing proton reduction. The change of NHC electronic properties through electron-donating or -withdrawing substituents has a minimal impact on the catalytic activity and selectivity. In situ infrared spectroscopy reveals that NHCs suppress the proton reductions while enhancing the CO₂ selectivity. Furthermore, NHC ligands exhibit high electrochemical stability under a reductive potential. Their surface coverage remains about 70-90% at -1.1 V, in contrast to the near-complete detachment of organic thiol ligands. These findings highlight ligand engineering as a promising approach for optimizing selectivity and efficiency in CO₂ electroreduction.