Active sites of ligand-protected Au₂₅ nanoparticle catalysts for CO₂ electroreduction to CO

Recent experimental studies have reported the electrochemical reduction of carbon dioxide (CO₂) into CO at atomically precise negatively charged Au₂₅⁻ nanoclusters. The studies showed CO₂ conversion at remarkably low overpotentials, but the exact mechanisms and nature of the active sites remain unclear. We used first-principles density functional theory and continuum solvation models to examine the role of the cluster during electrochemical CO₂ reduction and analyze the free energies of proposed intermediate species. Contrary to previous assumptions, our results show that the fully ligand protected cluster is not an active CO₂ reduction catalyst because formation of the crucial carboxyl intermediate required very high electrochemical potentials. Instead, our calculations suggest that the reduction process likely occurs on a dethiolated gold site, and adsorbed carboxyl intermediate formation was significantly stabilized at dethiolated gold sites. These findings point to the crucial role of exposed metal sites during electrochemical CO₂ reduction at gold nanocluster catalysts.