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The electrochemical CO reduction reaction (CO RR) is a promising approach for converting fossil fuel emissions into environmentally sustainable chemicals and fuels. The ability to control the surface structure of CO RR nanocatalysts provides an opportunity to tune product selectivity. Bimetallic gold-copper catalysts have been identified as emerging electrocatalyst candidates, but Cu incorporation typically lowers product selectivity compared with pure Au. Here we show sustained CO selectivity and activity up to 49% Cu content in small (<2 >nm), thiol-capped Au/Cu nanoparticles (NPs). Bimetallic NPs containing 49% Cu selectivity converted CO into CO with 100 ± 6% CO Faradaic efficiency and average mass activity of ∼500 mA/mg during a 12 h electrolysis experiment at -0.8 V vs RHE. Au/Cu NPs synthesized without thiol ligands selectively produced H , whereas larger (>10 nm), thermally dethiolated Au/Cu NPs produced a wider product distribution including H , CO, and C H . Density functional theory (DFT) modeling of CO RR and H evolution at realistic, thiol-capped Au/Cu NP structures indicated that copper-thiol surface structures sustained CO selectivity by stabilizing key∗CO intermediates while making∗H binding less favorable. Calculations also predicted that removing a significant fraction of the thiol ligands would increase∗CO binding strength such that desorption of CO product molecules could become the most thermodynamically challenging step. This result, coupled with increased∗H stability on dethiolated nanoclusters, points to decreased CO RR selectivity for small, ligand-free catalysts, which is in line with experimental observations from our group and others. Our results demonstrate that thiol-ligand surface structures can sustain the CO selectivity of bimetallic Au/Cu NPs and reduce precious metal requirements for CO RR. 2 2 2 2 2 2 2 4 2 2 2 2

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Journal of Physical Chemistry C

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