(201a) Unveiling Electrochemical Reaction Mechanisms Under Realistic Conditions: An Ab Initio Molecular Dynamics Study of Ammonia Electro-Oxidation on Pt(100) Surface

Electrochemistry plays a crucial role in diverse applications, including energy storage, catalysis, and sensing devices. Utilizing theoretical simulations, such as density functional theory (DFT) and ab initio molecular dynamics (AIMD), has become increasingly popular for elucidating reaction mechanisms, probing electrode-electrolyte interfaces, and guiding electrode catalyst design. These computational methods offer cost-effective and predictive capabilities, complementing time- and resource-intensive experimental approaches. However, the accuracy of computational modeling heavily relies on the precise identification of reaction conditions and appropriate calculation settings.

In the field of electrocatalysis, a significant challenge lies in developing accurate and robust simulation methods to capture the energy changes under solvation and applied potential conditions. Another crucial issue is that the reaction mechanism and active species may change when varying the external potential, limiting the development of computational tools for electrocatalysis.

Conventional computational models have several drawbacks, such as a lack of or inaccurate description of solvent effects, calculations at 0K that cannot represent dynamic structural fluctuations, inaccurate calculation of free energy changes, and a lack of or inaccurate description of constant potential conditions. To address these limitations, we employed an integrated AIMD approach combined with advanced sampling techniques (e.g., the slow-growth method), incorporating explicit solvent models and constant potential correction methods.

By utilizing these integrated strategies, we studied the mechanisms and free energy changes involved in the ammonia electro-oxidation reaction (AOR) on the Pt(100) surface under different applied potential conditions. This reaction has attracted considerable attention in recent decades, but the AOR mechanism on the electrode surface remains ambiguous, and the identification of reactive OH species during dehydrogenation reactions is under debate. Our results revealed that the dehydrogenation assisted by adsorbed OH is almost insensitive to applied potentials, while the dehydrogenation by OH in bulk water is potential-dependent, with the reaction barrier increasing as the potential is lowered. These findings indicate that the adsorbed OH is the reactive species during NH₃ dehydrogenation under reaction conditions rather than OH⁻ in bulk water. Additionally, the main pathway for N₂ formation during AOR is initiated by the dehydrogenation of NH₃ to NH₂, followed by NH₂ + NH₂ coupling to form N₂H₄, which concords with the existence of N₂H₄ observed in AOR experiments. Subsequently, N₂H₄ undergoes stepwise dehydrogenation to form N₂. This integrated approach successfully revealed the reaction pathways and mechanisms for AOR, providing theoretical insights into mechanisms that have been debated for over half a century.

References

1. K. Yang, J. Liu, B. Yang, Electrocatalytic oxidation of ammonia on Pt: Mechanistic insights into the formation of N₂ in alkaline media. Journal of Catalysis 2022, 405, 626-633, doi.org/10.1016/j.jcat.2021.10.029
2. K. Yang, J. Liu, B. Yang, Mechanism and Active Species in NH₃ Dehydrogenation under an Electrochemical Environment: An Ab Initio Molecular Dynamics Study. ACS Catalysis 2021, 11 (7), 4310-4318, doi.org/10.1021/acscatal.0c05247