A Kinetic Study of Plasma-Assisted Ammonia Synthesis with Ru/γ-Al2O3

Ammonia production from nitrogen and hydrogen via the Haber-Bosch process uses a thermal catalytic reactor which operates at high temperature (673-773 K) and high pressure (300 bar), consuming almost 2% of the world’s total energy supply [1]. Ammonia synthesis assisted by non-thermal plasmas has gained increasing attention because non-thermal plasmas allow ammonia synthesis to occur at atmospheric pressure and lower temperatures compared to thermal catalytic reactions [1].

Plasma-assisted ammonia synthesis is typically studied in dielectric barrier discharge (DBD) reactors with catalysts packed in the discharge zone [2, 3]. Metal nanoparticles contained in the catalysts can affect the reaction rates by (a) altering the plasma characteristics, and/or (b) providing catalytic sites for the excited species in the plasma to adsorb and react. In efforts to elucidate the mechanism by which the presence of Ru on γ-Al₂O₃ affects the reaction rates, we have performed a series of kinetic experiments at different plasma power levels and flow rates, with γ-Al₂O₃, 5 wt% Cu/γ-Al₂O₃ or 5 wt% Ru/γ-Al₂O₃, and have estimated ammonia decomposition and synthesis rates (the latter defined as the sum of ammonia observed and decomposition rates) from these experiments. Ru/γ-Al₂O₃ gives higher ammonia decomposition and synthesis rates than γ-Al₂O₃ and Cu/γ-Al₂O₃, indicating that Ru has a catalytic effect on both the forward and the reverse reactions. However, when these two catalytic effects are combined, the ammonia observed rate is promoted only by a slight amount in the presence of Ru/γ-Al₂O₃, suggesting that the overall benefit of using Ru/γ-Al₂O₃ over plain γ-Al₂O₃ may not be significant.

References

[1] Bogaerts, A., & Neyts, E. C., ACS Energy Lett., 3(4), 1013-1027 (2018).

[2] Kim, H. H. et al., Plasma Process. Polym., 14(6), 1600157 (2017).

[3] Mehta, P., et al., Nat. Catal., 1(4), 269-275 (2018).