Elucidating the active sites for CO2 electroreduction on ligand-protected Au25 nanoclusters

Abstract

Using density functional theory (DFT) calculations, we investigated the electrochemical reduction of CO₂ and the competing H₂ evolution reaction on ligand-protected Au₂₅ nanoclusters (NCs) of different charge states, Au₂₅(SR)₁₈^q (q = −1, 0, +1). Our results showed that regardless of charge state, CO₂ electroreduction over Au₂₅(SR)₁₈^q NCs was not feasible because of the extreme endothermicity to stabilize the carboxyl (COOH) intermediate. When we accounted for the removal of a ligand (both –SR and –R) from Au₂₅(SR)₁₈^q under electrochemical conditions, surprisingly we found that this is a thermodynamically feasible process at the experimentally applied potentials with the generated surface sites becoming active centers for electrocatalysis. In every case, the negatively charged NCs, losing a ligand from their surface during electrochemical conditions, were found to significantly stabilize the COOH intermediate, resulting in dramatically enhanced CO₂ reduction. The generated sites for CO₂ reduction were also found to be active for H₂ evolution, which agrees with experimental observations that these two processes compete. Interestingly, we found that the removal of an –R ligand from the negatively charged NC, resulted in a catalyst that was both active and selective for CO₂ reduction. This work highlights the importance of both the overall charge state and generation of catalytically active surface sites on ligand-protected NCs, while elucidating the CO₂ electroreduction mechanisms. Overall, our work rationalizes a series of experimental observations and demonstrates pathways to convert a very stable and catalytically inactive NC to an active electrocatalyst.