On the mechanism of electrochemical ammonia synthesis on the Ru catalyst

We theoretically investigate the electrochemical N-2 reduction reaction (NRR) mechanism to produce NH3 on the Ru catalyst. All possible N-N dissociation steps during the reduction processes were evaluated along with the conventional associative and dissociative pathways. Based on the calculated free energy diagrams, it is revealed that the kinetically facile intermediate dissociative pathways during the NRR require a thermodynamic limiting potential (-0.71 V) similar to the associative pathway (-0.68 V), although the initial dissociative pathway as in the Haber-Bosch process has a substantial kinetic barrier for the N-N bond dissociation. The competitive hydrogen evolution is found to be a major hurdle for achieving a high efficiency for the electrochemical nitrogen reduction. In the low overpotential region, the hydrogen adsorption is thermodynamically more favorable than the protonation of N-2, thereby reducing the number of active sites for the N-2 activation. A comparison of free energies in the presence of different H-coverages on the Ru further demonstrates that the H-coverage can significantly increase the energy barrier for the first protonation of N-2, resulting in a change of the potential determining step and an increase in the overpotentials.