The role of metal accessibility on carbon dioxide electroreduction in atomically precise nanoclusters

Atomically precise nanoclusters (NCs) can be designed with high faradaic efficiency for the electrochemical reduction of CO₂ to CO (FE_CO) and provide useful model systems for studying the metal-catalysed CO₂ reduction reaction (CO₂RR). While size-dependent trends are commonly evoked, the effect of NC size on catalytic activity is often convoluted by other factors such as changes to surface structure, ligand density, and electronic structure, which makes it challenging to establish rigorous structure-property relationships. Herein, we report a detailed investigation of a series of NCs [AuAg_46-(C[triple bond, length as m-dash]CR)₂₄Cl₄(PPh₃)₂, Au₂₄Ag₂₀(C[triple bond, length as m-dash]CR)₂₄Cl₂, and Au₄₃(C[triple bond, length as m-dash]CR)₂₀/Au₄₂Ag₁(C[triple bond, length as m-dash]CR)₂₀] with similar sizes and core structures but different ligand packing densities to investigate how the number of accessible metal sites impacts CO₂RR activity and selectivity. We develop a simple method to determine the number of CO₂-accessible sites for a given NC then use this to probe relationships between surface accessibility and CO₂RR performance for atomically precise NC catalysts. Specifically, the NCs with the highest number of accessible metal sites [Au₄₃(C[triple bond, length as m-dash]CR)₂₀ and Au₄₂Ag₁(C[triple bond, length as m-dash]CR)₂₀] feature a FE_CO of >90% at -0.57 V vs. the reversible hydrogen electrode (RHE), while NCs with lower numbers of accessible metal sites have a reduced FE_CO. In addition, CO₂RR studies performed on other Au-alkynyl NCs that span a wider range of sizes further support the relationship between FE_CO and the number of accessible metal sites, regardless of NC size. This work establishes a generalizable approach to evaluating the potential of atomically precise NCs for electrocatalysis.