Mechanism of ammonia synthesis on Fe₃Mo₃N

Abstract

Ammonia (NH3) synthesis is an essential yet energy-demanding industrial process. Hence, there is a need to develop NH3 synthesis catalysts that are highly active under milder conditions. Metal nitrides are...

A Comparison of the Reactivity of the Lattice Nitrogen in Tungsten Substituted Co3Mo3N and Ni2Mo3N

“…The associative Mars‐van Krevelen mechanism for ammonia synthesis relies on the presence and regeneration of surface lattice N vacancies to facilitate N 2 activation, and its subsequent hydrogenation. Indeed, previous computational studies [10,16] determined surface N vacancy formation on Co 3 Mo 3 N and Fe 3 Mo 3 N surfaces to be endothermic (both with respect to evolution of N 2 , and NH 3 under hydrogenating conditions) at 0 K and under vacuum, but well within the bound of possibility under typical reaction conditions at finite temperatures and pressures. Hence, in the present work, it is of interest to determine the surface N vacancy formation energy for W‐doped Co 3 Mo 3 N in order to compare with the previous results for the undoped Co 3 Mo 3 N system to assess the impact of the presence of W, and for both Ni 2 Mo 3 N and W‐doped Ni 2 Mo 3 N to compare more generally with the previously studied ternary metal nitride systems.…”

“…It is interesting, and perhaps surprising, to note that analogous behavior is not observed in the related isostructural Fe 3 Mo 3 N phase where, rather than leading to the lower nitrogen content phase, reduction at increasing temperature ultimately leads to structural decomposition [15] . Recent DFT modelling indicates the nitrogen formation energy to be more endothermic in the case of Fe 3 Mo 3 N than Co 3 Mo 3 N such that vacancy formation may be limited to the surface in the former case [16] . In the case of Ni 2 Mo 3 N, which possesses the filled β‐Mn structure, no bulk reduction occurs upon treatment with hydrogen and, again, surface nitrogen vacancies have been proposed to be of possible importance [17] .…”

“…[21][22][23] During geometry optimization, all atomic positions were allowed to relax. In line with the previous computational studies for Co 3 Mo 3 N and Fe 3 Mo 3 N, [10,16] a Γ-centered (4×4×1) Monkhorst-Pack k-point sampling scheme was used for W-doped Co 3 Mo 3 N, Ni 2 Mo 3 N, and W-doped Ni 2 Mo 3 N, given the relatively similar dimensions of the 2x2 thin film surface supercells. [25] Inner electrons were replaced by projector-augmented waves (PAW), [26,27] and the valence states were expanded in plane-waves with a cutoff energy of 650 eV.…”

“…Previous computational studies [9,10,16] have demonstrated the viability of the associative Mars-van Krevelen mechanism, which relies on the presence of surface N vacancies to serve as active sites for molecular N 2 adsorption. Hence, in the present work, we build on the models utilized in previous studies to explore the impact of W doping on the formation of surface N vacancies, and on N 2 adsorption and activation.…”

“…[15] Recent DFT modelling indicates the nitrogen formation energy to be more endothermic in the case of Fe 3 Mo 3 N than Co 3 Mo 3 N such that vacancy formation may be limited to the surface in the former case. [16] In the case of Ni 2 Mo 3 N, which possesses the filled β-Mn structure, no bulk reduction occurs upon treatment with hydrogen and, again, surface nitrogen vacancies have been proposed to be of possible importance. [17] Indeed, this material was observed to be particularly stable, with its phase being retained at temperatures up to 900 °C.…”