Adsorption and decomposition of ammonia on Fe surfaces: a molecular dynamics study

Ammonia is considered a potential hydrogen vector due to its high hydrogen content and ease of transportation and storage. However, on-site carbon-free hydrogen production requires the development of efficient methods of hydrogen extraction from ammonia. To this end, iron-based catalysts are widely employed in ammonia cracking. Despite more than one century of studies, a microscopic characterization of the reaction is still lacking, hindered by extreme reaction conditions (700-800 K).[1] So far, studies based on density functional theory (DFT) calculations often neglect dynamic behaviours, which play a critical role in heterogenous catalysis, explaining the activity and long-term stability.[2-4] Leveraging state-of-the-art molecular dynamics, machine learning potentials, and enhanced sampling methods, we offer an atomistic view of the adsorption, diffusion, and dehydrogenation processes of NHx (x=1,3) on two representative surfaces at operando temperature of 700 K. Dynamics pervasively affects all steps of decomposition, including on the stable (110) surface where the high mobility of reaction intermediates influences the reactivity. The role is even more dramatic on the (111) surface, where the mobility of Fe surface atoms introduces new adsorption sites and alters the dehydrogenation barriers. Our study establishes a correlation between diffusion and dehydrogenation free energy barriers, highlighting the intricate interplay between surface dynamics and catalytic activity. A detailed analysis of reactive events shows that there is never a single transition state, but it is always an ensemble composed of at least two pathways. Notwithstanding, a unified mechanism can be identified by following the charge transfer along the different reaction pathways.[5] The framework used is general and can be employed to study other heterogeneous catalysis processes with quantum mechanical-like accuracy. References 1. I. Lucentini, et al., Ind. Eng. Chem. Res., 2021, 60(51), 18560-18611. 2. M. S. Spencer, Nature 1986, 323, 685-687 3. L. Bonati, et al., Proceedings of the National Academy of Sciences 2023, 120, e2313023120. 4. S. Tripathi, L. Bonati, S. Perego, M. Parrinello, ACS Catal. 2024, 14, XXX, 4944–4950 5. S. Perego, L. Bonati, S. Tripathi, M. Parrinello, ChemRxiv., 2024