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.