(506c) Plasma-Enhanced Catalytic Ammonia Synthesis As a Technology for Decentralized Power-to-Ammonia Applications

Ammonia may also be one of the energy carriers of the future, functioning as a hydrogen carrier [1–7]. Ammonia can be produced from surplus electricity from intermittent renewables, thereby facilitating energy storage over seasons [5]. In decentralized systems, this requires on-demand technologies rather than high temperature, high pressure technologies such as the Haber-Bosch process. While the H₂ production and N₂ production can be scaled-down easily, the ammonia synthesis loop cannot be scaled-down easily to the sub-MW scale due to the severe operating conditions (400-500°C and 100-300 bar). Plasma-catalysis is a potential on-demand technology [8], which may replace the Haber-Bosch process for decentralized applications.

Although some progress has been made regarding plasma-catalytic ammonia synthesis, the plasma-catalyst interactions are not well understood. It is difficult to decouple effects of plasma on the catalyst, and the effects of the catalyst on plasma for ammonia synthesis. However, as recently discussed by Whitehead [9], plasma-catalysis is best regarded as conventional catalysis perturbed by the presence of a discharge.

The status quo of plasma-catalytic ammonia synthesis will be discussed. After the recent cutting-edge article of Mehta et al. [10], plasma-catalytic ammonia synthesis can be understood better. According to the postulate of Mehta et al. [10], the dissociation of N₂ can be enhanced through plasma-induced vibrational excitation of N₂, while the subsequent hydrogenation steps on the catalyst surface and ammonia desorption are not influenced by the plasma. Recently, we have provided additional experimental evidence to substantiate the claim of Mehta et al.

The highest energy efficiencies reported so far for plasma-catalysis (20-35 g kWh^-1 [11, 12]) are two orders of magnitude lower than those for the large-scale conventional Haber-Bosch process (about 500-2000 g kWh^-1 [13]). However, upon scale-down, the Haber-Bosch process becomes less efficient [14], and energy efficiencies drop to 100-200 g kWh^-1 [13]. Thus, some energy efficiency improvements are required for plasma-catalysis (a factor ~2-10) for small-scale, on-demand applications. A major challenge is maintaining the energy efficiency of at least 100-200 g kWh^-1 at high ammonia yields (>1 mol.%), such that ammonia can be separated by absorbents or adsorbents.

Optimization of plasma-catalysis can typically be divided into plasma optimization and catalyst optimization. While much focus has been on the plasma optimization in recent years, catalyst optimization has been given less attention. In the presentation, we show how the catalyst formulation (active metal, support, promoters) may be optimized in order to overcome this energy efficiency gap. While the plasma-catalysis community has frequently stressed how dissimilar the catalysts for thermal catalysis and plasma-catalysis are, experimental results and DFT calculations show there are similar activity trends for various catalyst formulation.

The outlook provides an overview of how plasma-catalytic ammonia synthesis can be integrated into a process design for power-to-ammonia applications on a decentralized scale (<<1 MW). This connects the work we presented on power-to-ammonia-to-power the previous year (1-10 MW systems).

References

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