There is an enormous interest in the commercialization of the electrochemical synthesis of ammonia, which critically depends on the development of new efficient electro-catalysts and suitable electrolytes for the electrolyzers. Metal nitrides and metal carbides have shown a great promise as electro-catalysts for a wide range of reactions; however, little is known about the structure of potential active sites that promote a specific electro-chemical process [1].

We report herein a density functional theory (DFT) study of the electro-reduction of nitrogen to ammonia on different surfaces of molybdenum carbides, Mo₂C and MoC, in order to investigate the viability of using these materials as catalysts for electro-reduction of nitrogen. We first systematically scanned non-equivalent adsorption sites and studied the energetics of adsorption of molecular nitrogen, atomic nitrogen, and ammonia on various crystallographic surfaces of Mo₂C and MoC as a means to identify the most favorable binding sites/surfaces that i) activate nitrogen by binding the nitrogen molecule in side-on position, which activates N-N bond, ii) have high affinity for N-adatoms, which prevents accumulation of H adatoms on the catalytic surface at low cell potentials and iii) do not have strong interaction with ammonia, which promotes the release of the product from the catalytic surface. We further studied the electro-reduction of nitrogen on the most active Mo₂C and MoC surfaces by calculating the free energy diagrams for nitrogen electro-reduction to ammonia via associative and dissociative Heyrovsky mechanism. The free energy diagrams are helpful in determining potential-limiting step and predicting the potentials necessary to drive the reduction of nitrogen towards formation of ammonia. We further studied the interaction of various crystallographic surfaces of Mo₂C and MoC with hydrogen adatoms. Namely, formation of hydrogen gas competes with formation of ammonia and can severely reduce the efficiency of the electrochemical reduction of nitrogen if the catalytic surface is covered with H-adatoms rather than N-adatoms. Kinetic volcano plots for hydrogen evolution on MoC and Mo₂C were used to correlate experimentally measured exchange currents for hydrogen evolution over different metal surfaces with DFT calculated hydrogen adsorption free energies [2,3]. The kinetic volcano plots are then used to identify MoC and Mo₂C surfaces on which hydrogen production is suppressed and that have the highest selectivity for nitrogen electro-reduction.

[1] I. Matanovic, F. H. Garzon, and N. J. Henson, Electro-reduction of Nitrogen on Molybdenum Nitride: Structure, Energetics, and Vibrational Spectra from DFT, Phys. Chem. Chem. Phys. 2014, 16: 3014-3026.

[2] S. Trasatti, Work function, electronegativity, and electrochemical behaviour of metal: III. Electrolytic hydrogen evolution in acid solutions, J. Electroanal. Chem. 1972, 39, 163.

[3] J. K. Nørskov, T. Bligaard, A. Logadottir, J. R. Kitchin, J. G. Chen, S. Pandelov, U. Stimming, Trends in the Exchange Current for Hydrogen Evolution, J. Electrochem. Soc. 2005, 152(3):J23-J26.