Elucidating the Mechanism of Electrochemical N₂ Reduction at the Ru(0001) Electrode

Density functional theory calculations are used to elucidate the mechanism of the nitrogen reduction reaction (NRR) in an electrochemical double layer on the Ru(0001) electrode, where all possible proton–electron transfer steps and N–N scission steps during NRR are considered. The model includes a negatively charged electrode and an explicit solvation bilayer of water including hydronium ion(s). We find that all elementary steps in which a proton is transferred to an adsorbate have small additional barriers on the order of 0.0–0.25 eV at an applied potential of −0.6 V versus the reversible hydrogen electrode. The first proton–electron transfer step where an end-on N2 admolecule is reduced to NNH has a negligible additional barrier, whereas the thermochemical barrier is 0.8 eV at −0.6 V. This elementary reaction step is found to be rate-limiting and potential-limiting for NRR on Ru(0001). We predict that NRR will follow an associative distal pathway where after the third proton–electron transfer, the N–N bond is broken in N–NH3, and the first ammonia molecule is released and a nitrogen atom occupies the surface. The nitrogen atom is reduced further to form the second ammonia molecule. The predicted reaction pathway here on Ru(0001), including all possible electrochemical and nonelectrochemical energy barriers, is the same as predicted from the thermochemistry of adsorbed intermediates. This indicates that the less-expensive thermochemical calculations and the computational hydrogen electrode can be used quite reliably to predict catalytic trends such as reaction pathways and overpotentials of NRR on at least the Ru electrode and presumably also on other types of catalysts.