Detailed Characterization of a Low-Temperature Plasma-Driven Ammonia Synthesis Process

Non-thermal plasma catalysis for ammonia synthesis has recently attracted increasing attention as a promising alternative to the standard ammonia production method, Haber-Bosch process (HB process). Unlike the HB process, which emits over 300 million metric tons of CO₂ annually, non-thermal plasmas could dramatically reduce emissions as plasma-based reactors can be operated entirely by renewable electricity. In addition, the centralized production of ammonia by the massive HB plants could be replaced with a distributed network of small-scale plasma-based plants, enabling the on-site production of feedstock ammonia and synthetic fertilizer. Plasma-based processes are expected to play a crucial role in the transition from a fossil fuel-based to an electricity-based energy and chemical infrastructure [1]. One of the challenges in this effort is the inherent complexity of the reactive environment formed at the interface between the low-temperature plasma and surface of the metal catalyst. In this work, we characterized the yield and energy cost of ammonia synthesis for a low-pressure RF-driven plasma system. The reactor consists of a quartz cylindrical plasma reactor with 1” diameter, a matching network, and a RF power supply. The reaction products were sampled downstream of the plasma reactor through a 50 μm orifice into a Residual Gas Analyzer (RGA). Process parameters such as nitrogen-to-hydrogen ratio and pulse duration have been carefully varied and correlated with ammonia yield and energy cost. Optical Emission Spectrometer (OES) measurements were also performed to estimate atomic nitrogen and hydrogen density with BOLSIG+, a commonly utilized freeware software for the solution of the Boltzmann transport equation, and to determine nitrogen vibrational temperature. We find that nitrogen dissociation in the plasma is an important activation pathway. This is consistent with the results from Born-Oppenheimer Molecular Dynamics (BOMD) simulation, which suggest that the direct surface abstraction pathway is fast and proceeds with near 100% probability [2]. We also find that vibrational activation of the nitrogen plays an equally important role. Interestingly, this pathway is active even when using catalysts such as copper, i.e. a metal that is not active for the case of thermally-driven ammonia production. BOMD simulations show that the hydrogen flux to the metal surface is crucial at stabilizing nitrogen once it is dissociatively adsorbed onto the copper surface. Finally, pulsing of the plasma is a promising approach to increase the energy cost of the process. This is due to the fact that in continuous operation, a significant fraction of the energy input is used to dissociate the ammonia produced at the catalyst surface, lowering the reactor yield and increasing the energy cost.<br/>Reference:<br/>[1] A. Bogaerts, et al., Plasma Technology: An Emerging Technology for Energy Storage, ACS Energy Letters, 3, 1013−1027 (2018).<br/>[2] S. S. R. K. C. Yamijala et al., Harnessing Plasma Environments for Ammonia Catalysis: Mechanistic Insights from Experiments and Large-Scale Ab Initio Molecular Dynamics, The Journal of Physical Chemistry Letters, 11, 10469-10475 (2020).