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

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 H-2 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-degrees with 5 kPa H-2 and 75 kPa N-2, where the reaction rate increased from 128 to 565 mu mol g-1 h-1 even at low H-2 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 H-2 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.