Identification of Active Sites for Ammonia Electrosynthesis on Ruthenium

Electrochemical N2 reduction reaction (NRR) provides an attractive approach toward sustainable NH3 production, while the design of electrocatalysts for NRR is hindered by the lack of knowledge of the structure–activity relationships and active sites. Here we report a prominent size-dependent activity for the NRR on Ru nanoparticles prepared by atomic layer deposition. As the Ru particle size increased from 2.1 to 8.4 nm, the mass activity and Faradaic efficiency for NH3 production both decreased monotonically, while the specific (Ru-surface-area-normalized) activity reached the highest value on 3.8 nm Ru nanoparticles but declined by 5-fold on 8.4 nm Ru nanoparticles. Density functional theory (DFT) calculations and free energy analysis of elementary steps revealed the Ru D5 step site, with its maximal population at ∼4 nm particles, as the active site for the NRR on Ru, because it favors the adsorption of the *N2H intermediate compared to other surface sites while not getting poisoned by the *NH2 intermediate.