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 NH_x (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 NH_x 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 toward dissociative N₂ adsorption while reduces the activation barriers for NH_x hydrogenation. Bonding with the N adatoms distorts the Mo₄ ensembles geometrically and modifies the active sites electronically. Microkinetic analysis based on the energetic results indicates that the Mo₄ ensembles modified with N adatoms at the corner site formed in intermediate N coverages are highly active toward ammonia formation. The present study demonstrates the importance of the local structure of the active site for catalytic ammonia synthesis and is helpful to the design of novel active ammonia synthesis catalysts and the selection of optimal operating conditions.