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

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journal contribution
posted on 27.12.2019, 20:44 authored by Jiankang Zhao, Chaonan Cui, Hua Wang, Jinyu Han, Xinli Zhu, Qingfeng Ge
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 toward dissociative N2 adsorption while reduces the activation barriers for NHx 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.