Anisotropic N-Modification of the Mo4 Ensemble for Efficient Ammonia Synthesis on Molybdenum Nitrides

Density functional theory calculations are employed to elucidate the mechanism of ammonia synthesis on the nitridated Mo2N(111) surfaces. An ensemble consisting of four Mo atoms arranged in roughly a rhombic structure (Mo4) was found highly active for N2 adsorption and activation, with an activation barrier of 0.58 eV to break the N-N bond. However, subsequent hydrogenation of the adsorbed NHx (x=0, 1 and 2) species on the Mo4 site becomes rate-limiting as the activation barriers increase up to 1.47 eV. Nitridation in close proximity to the Mo4 site can significantly improve the activity for NHx hydrogenation, and the activity is more sensitive to the location rather than the overall coverage of N. The Mo4 site modified anisotropically by surface N adatoms next to the site maintains its high reactivity towards dissociative N2 adsorption while reduces the activation barriers for NHx hydrogenation. Bonding with the N adatoms distorts geometrically the Mo4 ensembles and modifies electronically the active sit...