Low-Temperature Ammonia Synthesis with an In Situ Adsorber under Regenerative Reaction Cycles Surpassing Thermodynamic Equilibrium

Abstract

Catalytic NH₃ synthesis is a well-studied reaction, but its use in renewable energy storage is difficult due to the need for small-scale production, requiring greatly reduced operating temperatures and pressures. NH₃ inhibition on supported Ru catalysts becomes more prevalent at low temperatures, decreasing the reaction rates. In addition, promoter species are prone to oxidation at lower temperatures, further depressing the reaction rate. In situ NH₃ removal techniques have the potential to enhance NH₃ synthesis under milder conditions to combat both NH₃ inhibition and thermodynamic limitations, while the regeneration of the adsorber can potentially reactivate promoter species. The deactivation event of 5 wt % Ru/CeO₂ (3.9 nm average Ru particle size) was first explored in detail, and it was found that slight oxidation of Ce³⁺ promoter species is the major cause of deactivation at lower temperatures, which is easily restored by high-temperature H₂ treatment. Ru/CeO₂ was then mixed with zeolite 4A, a substance showing favorable NH₃ capacity under mild reaction conditions. In situ adsorption of NH₃ significantly increased the reaction rate of Ru/CeO₂ at 200 °C with 5 kPa H₂ and 75 kPa N₂, where the reaction rate increased from 128 to 565 μmol g^–1 h^–1 even at low H₂ conversions of 0.25% (average NH₃ yield of 0.01%). The temperature swings that were utilized to measure NH₃ uptake on zeolite 4A were also found to provide a reactivation event for Ru/CeO₂. In situ NH₃ removal went beyond equilibrium limitations, achieving H₂ conversions up to 98%. This study sheds light on the kinetics of the use of in situ NH₃ removal techniques and provides insight into future designs utilizing similar techniques.