(188b) Effect of Potential and Explicit Aqueous Media on Ammonia Oxidation on Pt(111) Using Computational Multi-Scale Modeling.

Fossil fuels as a conventional source of energy have caused many environmental problems. Hydrogen is a clean and renewable source of energy; however, challenges regarding storage and transportation make it infeasible. Alternatively, water and ammonia contain significant amounts of hydrogen; further, they bypass challenges in transportation and storage since they can easily be converted to liquids. Ammonia oxidation requires only about 5% of the cell voltage (0.06 V) of water oxidation to produce hydrogen, which makes it more viable. It is also a promising way to denitrify wastewater. Additionally, ammonia can be used in a fuel cell to produce energy directly. However, catalyst costs and poisoning impede the broader adoption of NH₃ as a fuel. In electrochemical systems, there is another challenge; there is no electroneutrality due to presence of bias in the system. Solving these problems requires optimizing electrodes, electrolytes, and operating conditions. Molecular simulations play an integral role in this regard by providing molecular-level understanding of the electrocatalytic thermodynamics and kinetics. In this work, we use multiscale modeling to interrogate how system bias influences the free energies of electrocatalytic species. Specifically, we use a combination of density functional theory (DFT) and classical molecular dynamics (MD) to calculate the enthalpies, entropies, and free energies of species on a platinum electrode surface in the presence of both solvent and potential. The potential in the system is simulated using an explicit method capable of exchange of ions and electrons. We demonstrate the influence that electrode potential and pH have on the free energy of species in the NH₃ dehydrogenation pathway and compare these results with the analogous systems under thermal catalytic conditions. Through this exercise, we demonstrate creation of electrical double layer and the intense electric field in the vicinity of the surface and their influence on the free energies of catalytic species.