Elucidating the stability of ligand-protected Au nanoclusters under electrochemical reduction of CO2

Ligand-protected gold nanoclusters are a novel class of particles that have attracted great interest in the field of catalysis due to their atomically-precise structure, high surface-to-volume ratio, and unique electronic properties. In particular, the anionic thiolate-protected Au-25 nanocluster (NC), [Au-25(SR)(18)](1-), with partially lost ligands, has been demonstrated to act as an active catalyst for the electrochemical reduction of CO2. However, the stability of this and other thiolate-protected NCs after partial ligand removal remains elusive. Using density functional theory (DFT) calculations and the recently developed thermodynamic stability model, we investigate the stability of [Au-25(SR)(18)](1-), [Au-18(SR)(14)](0), [Au-23(SR)(16)](1-), and [Au-28(SR)(20)](0) with ligand loss. Additionally, we examine the stability of the partially protected NCs upon adsorption of CO2 reduction reaction intermediates (i.e. H, CO, and COOH) on the different active sites generated after ligand removal. Our results reveal that the partially protected Au-25 NC shows the highest stability compared to the other partially protected NCs in the presence of electrochemical reduction intermediates. We find that the presence of the COOH intermediate on the generated active sites stabilizes the Au-25 NC almost as well as the removed ligands. Moreover, time-dependent DFT calculations and UV-Vis/Raman experiments suggest that the most probable ligand removal mode under electrochemical conditions is the one that generates S active sites, in agreement with the DFT ligand removal thermodynamic analysis. Importantly, this study demonstrates the robustness of the Au-25 NC and offers a novel way to address stability of ligand protected NCs during electrocatalytic reaction conditions.