(463c) Tuning the Structure and Electronic Properties of Manganese Nitrides for Ammonia Production from First-Principles Modeling

The conventional Haber-Bosch ammonia synthesis process plays a significant role in the global food chain by feeding approximately 40% of world’s population, but is also extremely energy intensive, consuming 1-2% of the world’s energy supply. Ideally, simple technology enabling intermittent operation using renewable energy will be made available through step catalysis by exploiting manganese (Mn) based alloy materials to facilitate both nitrogen activation and ammonia production.

In this talk, a first-principles investigation based on periodic density functional theory (DFT) calculations has been carried out by focusing on the elucidation of molecular-level mechanisms for ammonia formation using a model based on the Mn₄N nitride phase, which can be readily obtained through the nitridation of metallic manganese. It is hypothesized that the reduction will follow a Mars-van Krevelan-type mechanism. Under a reducing environment at 1 atmospheric H₂ pressure, this computational study has shown that it is thermodynamically favorable to form NH utilizing the surface N species. Afterwards, the reaction becomes endothermic for ammonia evolution from NH. The diffusion of N in the nitride crystal lattice can influence the overall kinetics, and has been considered in this study as well. It turns out that both the overall thermodynamics and the N diffusion kinetics are the key for ammonia production and should be considered as the parameters to tune the materials performance. Also in this talk, the simple strategy to dope and alloy manganese with other transition metals to validate the research hypothesis has been explored computationally, and will be presented and discussed.