Effect of H/N ratio control in a multibed ammonia synthesis system with Ru-based catalysts

Abstract

Ammonia has recently attracted attention as a hydrogen carrier and fuel, based on the power-to-fuel concept. This concept can be realized using Ru-supported rare-earth oxides for the synthesis of ammonia from hydrogen and nitrogen (3H₂ + N₂ → 2NH₃) under mild conditions. However, at a high H/N ratio, Ru catalysts exhibit hydrogen poisoning, which reduces their activity for ammonia synthesis. This study investigates the effect of the H/N ratio on the ammonia synthesis activity of the developed Ru catalyst Ru(5 wt%)/Ce_0.5La_0.4Si_0.1O_1.8 under isothermal conditions (350−500 °C). The optimal H/N ratio for achieving the highest catalytic activity decreases as the temperature is lowered (H/N = 0.5 at 350 °C; H/N = 2.0−2.5 at 450 °C). In a multibed reactor, adjusting the H/N ratio to a lower value in the downstream catalyst beds—where the temperature decreases along the gas flow path—can enhance the overall rate of ammonia production by optimizing the reaction conditions in these cooler stages. We propose a system to control the H/N ratio for each catalyst bed in a multibed reactor and demonstrate an increase in the rate of ammonia production when using a double-bed reactor containing the Ru/Ce_0.5La_0.4Si_0.1O_1.8 catalyst. The proposed system offers various opportunities to accelerate the use of ammonia as a hydrogen carrier and fuel.