(327f) CO2 Reduction on Ligand-Protected Au Nanoclusters

Authors: 
Mpourmpakis, G., University of Pittsburgh
Austin, N., University of Pittsburgh
Although Au nanoparticles exhibit size-dependent catalytic activity, their size is usually not well defined, limiting the precise correlation of their electronic and structural properties with the observed catalysis. Atomically precise Au nanoclusters, stabilized by organic ligands, exhibit well-defined structure (size/shape), but the accessibility of reactants to the metal is limited by the presence of ligands. Experimental work has shown that these ligand-protected nanoclusters are active for CO2 electroreduction, albeit, the active catalytic centers are elusive. In this work, we apply Density Functional Theory calculations to assess the CO2 reduction to CO and the competing H2 evolution reaction on thiolate protected Au nanoclusters. Our results demonstrated that the fully protected nanoclusters may not be the active catalysts for CO2 reduction. When we accounted for the removal of a ligand from the nanocluster 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 for electrocatalysis. These sites were found to significantly stabilize the COOH intermediate, resulting to dramatically facilitating CO2 reduction. Detailed charge analysis on the nanoclusters revealed that negative charge stabilized on the nanocatalysts plays an important role in the COOH intermediate stabilization. The generated sites for CO2 reduction are also active for H2 evolution. This work highlights the importance of both the overall charge state and generation of catalytically active surface sites on ligand-protected nanoclusters, while elucidating the CO2 electroreduction mechanisms. Our work rationalizes a series of experimental observations.