Resolving the Size-Dependent Transition between CO2 Reduction Reaction and H2 Evolution Reaction Selectivity in Sub‑5 nm Silver Nanoparticle Electrocatalysts

We combined ultrahigh vacuum surface science techniques, electrochemical measurements, density functional theory, and microkinetic modeling to finely resolve the size-dependent transition between the CO2 reduction reaction (CO2RR) and H2 evolution reaction (HER) selectivity for Ag nanoparticle electrocatalysts in the sub-5 nm range. We experimentally measured activity and selectivity trends with sub-nm size resolution using a series of Ag nanoparticles with average diameters between 2 and 6 nm. CO2RR activity increased with particle sizes between 2 nm and ∼4 nm, and 3.7 ± 0.7 nm-diameter particles demonstrated the highest combination of CO2RR activity and selectivity. Computational modeling of 1–10 nm Ag particles predicted a nearly identical size-dependent trend with maximum CO2RR activity predicted for 3.7 nm-diameter particles. Smaller diameter particles were predicted to favor HER due to a high population of Ag edge sites. CO2RR activity was predicted to increase for larger diameter particles as the population of Ag(100) surface sites grew, but a growing population of electrochemically inaccessible, interior atoms eventually decreased catalyst utilization for particle diameters above ∼4 nm. Our results resolve the CO2RR behavior of Ag in the critical sub-5 nm range, establish an effective minimum size limit for selective and active Ag catalysts, and provide insights to help guide future catalyst development efforts.