The capability of low temperature plasmas to produce ammonia from a nitrogen-hydrogen mixture is relevant for the realization of distributed, small-scale systems for ammonia synthesis. Still, the complex chemistry associated with these systems has hampered the development of plasma-driven processes, and clear guidelines on the optimization of catalysts for application in a plasma environment are still missing. In this talk, our most recent results on the synthesis of ammonia in a low-temperature, radio-frequency (RF) sustained plasma will be presented. The study is performed in the mid-pressure regime, in a flow-through reactor whose inner surface is lined by an aluminum foil. Several materials (Pt, Cu, Co, Pd, Ag, Ti, Mo, Al2O3, Fe, Ni) are sputtered onto the Al substrate to provide a direct comparison of their activity while in contact with the plasma. We observe a clear increase in the degree of nitrogen fixation when the metal foil is present, but we also find no statistically meaningful difference between the tested metals. In addition, sequential exposures of the surface to hydrogen-only and nitrogen-only plasmas indicate that ammonia is produced when first treating the surface with a hydrogen discharge, followed by exposure to a nitrogen one. To get a detailed understanding of our experimental findings, we performed ab-initio Born-Oppenheimer molecular dynamics (BOMD) simulations. BOMD simulations naturally account for both the dynamic and steric aspects of a collision reaction that cannot be captured by the static electronic structure (DFT) calculations. We considered both copper and iron surfaces preadsorbed with a monolayer of hydrogen and impinged them with either a nitrogen atom or a nitrogen molecule at various angles to the surface normal. For the case of a nitrogen atom collision, we observe the formation of either NH3" or "NH2 and H2" through both Eley-Rideal and hot-atom mechanisms. On the contrary, our collision simulations with a nitrogen molecule always lead to its reflection from the surface (i.e., no reaction with the surface). We verified that our findings are robust against the kinetic energies of the incident atom/molecule in the range of 0.03 eV - 3 eV. Collectively, we find that the plasma effectively hydrogen-terminates the surface of the catalyst, increases the probability of the subsequent hydrogenation of the impinging nitrogen radicals, and makes the overall process nearly independent of the catalyst material choice.

Plasma-surface interaction: Case of ammonia production

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.