Nanoscale Au exhibits high catalytic activity, whereas bulk Au is inert. Although Au nanoparticles display exceptional 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. However, experimental work has shown that these ligand-stabilized nanoclusters are active for CO2 reduction. Thus, in this work, we apply Density Functional Theory calculations to assess CO2 reduction to CO and the competing H2 evolution reaction on Au25(SR)18- and Au25(SeR)18- (R=organic chain) nanoclusters. Analysis of the reaction pathway displayed unfavorable thermodynamics for the formation of the carboxyl intermediates in CO2 reduction. This indicated that the fully protected nanoclusters may not be the active catalysts for CO2 reduction. However, we demonstrate that the free energy for the removal of one ligand from the nanocluster surface competes with carboxyl formation. As a result, upon removal of one ligand the free energies for carboxyl formation and H adsorption (from the H2 evolution reaction) become less endothermic compared to the fully protected nanoclusters. Thus, an active nanocluster for CO2 reduction could have at least one ligand removed and pre-adsorbed hydrogen. Importantly, our revealed CO2 reduction behavior on the ligand-removed nanoclusters correlates with observed experimental results on the Au25 nanoclusters.

Engineering ligand-protected Au nanoclusters for CO2 reduction

254th National Meeting (2017)

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