Advancing ammonia (NH3) production under milder conditions

The Haber-Bosch (HB) process, developed over a century ago for ammonia (NH3) production, has played a pivotal role in modern civilization, particularly through the large-scale production of nitrogen-based fertilizers. This process has been crucial for global food production and agricultural growth. Today, NH3 is used across various industries and is gaining attention as a potential carbon-free fuel, efficient energy carrier, and renewable energy storage medium. However, the HB process faces growing scrutiny due to its environmental and energy related impacts, underscoring the need for technological innovations to decarbonize NH₃ production. In this context, the exploration of small-scale, decentralized NH3 production powered by renewable energy has gained momentum over the past decade with aims to reduce reliance on large, fossil fuel-based centralized plants and promote sustainable, green manufacturing. This research outlined in this dissertation focuses on both process and catalyst development that can realize the path to small-scale NH3 production. Small-scale ammonia synthesis requires operating at lower temperatures and pressures (<400°C and <100 bar) compared to industrial practices. Under these mild conditions, the low partial pressure of ammonia makes conventional separation techniques inefficient due to operational constraints and costs. To address this issue, absorptive-enhanced ammonia production was considered. This process was systematically studied in a bench-scale experimental apparatus, consisting of three main units: synthesis, separation, and recycling compressor (referred to as pump). The goal was to maximize ammonia production rate by optimizing the key operating units. Instant ammonia separation at 25°C increased overall ammonia production by 45 times compared to single-pass production and 15 times, compared to the earlier work that demonstrated the absorptive-enhanced ammonia synthesis. Following an extensive evaluation of process parameters, an optimal set of conditions was identified using a CaCl2-based absorbent, allowing stable operation of the absorption-enhanced HB process. A subsequent investigation focused on optimizing regeneration conditions using pressure swing adsorption (PSA) and a combination of PSA and temperature swing adsorption (PTSA). The effect of sweep gas during regeneration was also evaluated by varying the sweep flow rate. The combination of PSA and TSA yielded higher ammonia production rates across successive cycles. PSA alone at moderate temperatures (e.g., 120°C for CaCl2 absorbent) reduced desorption time and improved ammonia recovery when vacuum was applied. Additionally, 100% pure NH3 was recovered from the column when regenerated at 300°C without sweep gas. To alleviate the reaction temperature, a unique composite catalyst consisting of cobalt (Co) and lithium hydride (LiH) was pursued. Despite being reported to overcome the well-known scaling relations, this study revealed concerns about the stability of the catalyst. It was found that Co-LiH catalyst was fully deactivated within 150 hours. Characterization revealed that particle agglomeration and intermediate reactions that required higher operating temperature for reversible uptake and release of ammonia were the key obstacles that led to deactivation of the catalyst. To mitigate the deactivation challenge, it is suggested that dispersing the active metals on a support material could prevent agglomeration and enhance catalyst stability. Additionally, a novel ruthenium (Ru)-based catalyst supported by laser-induced graphene (LIG) and promoted with barium (Ba) was developed. This is the first reported use of LIG in heterogeneous catalysis as a structural promoter. After optimizing the Ru and Ba loading on LIG, the catalyst loaded with 5 wt.% Ru and electronically promoted with Ba with the ratio of 2.5 (Ba:Ru), referred to as 5 wt.% Ru-2.5 Ba/LIG, demonstrated excellent ammonia synthesis activity under mild conditions (T: 400℃ and P: 10 bar). The developed catalysts displayed excellent resilience toward hydrogen and ammonia poisoning and maintained performance under intermittent conditions, making it a promising candidate for small-scale ammonia production powered by intermittent renewable energy.