Resolving the Size-Dependent Transition between CO2 Reduction Reaction and H-2 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 H-2 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 similar to 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 similar to 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.