Abstract
Atomically precise nanoclusters (NCs) can be designed with high faradaic efficiency for the electrochemical reduction of CO(2) to CO (FE(CO)) and provide useful model systems for studying the metal-catalysed CO(2) reduction reaction (CO(2)RR). 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 [Au(n)Ag(46-n)(C[triple bond, length as m-dash]CR)(24)Cl(4)(PPh(3))(2), Au(24)Ag(20)(C[triple bond, length as m-dash]CR)(24)Cl(2), and Au(43)(C[triple bond, length as m-dash]CR)(20)/Au(42)Ag(1)(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 CO(2)RR activity and selectivity. We develop a simple method to determine the number of CO(2)-accessible sites for a given NC then use this to probe relationships between surface accessibility and CO(2)RR performance for atomically precise NC catalysts. Specifically, the NCs with the highest number of accessible metal sites [Au(43)(C[triple bond, length as m-dash]CR)(20) and Au(42)Ag(1)(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, CO(2)RR 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.