(562c) Catalytic Materials and Process Development for Microwave-Assisted Ammonia Synthesis

Yuxin Wang¹, Xinwei Bai¹, Christina Wildfire², Shekhawat Dushyant², Opeyemi Ogunniyan¹, Chirag Mevawala¹, Debangsu Bhattacharyya¹, Tuhin Khan¹, Jianli Hu¹*

1. Chemical & Biomedical Engineering Department, West Virginia University, Morgantown, WV 26506, USA
2. National Energy Technology Laboratory, Morgantown, WV 26506, USA

Corresponding author*

E-mail: Jianli Hu: john.hu@mail.wvu.edu

Electromagnetic radiation in the form of microwaves has become a focus for chemical synthesis in recent years due to the increased selectivity and decrease in reaction times and temperatures. This paper presents an innovative approach of producing carbon neutral energy-dense liquid ammonia. The approach synergistically integrates microwave reaction chemistry with novel heterogeneous catalysis that decouples N₂ activation from high temperature and high pressure reaction, altering reaction pathways and lowering activation energy. A series of Ru-based catalysts with different supports and promoters have been synthesized, characterized, and evaluated for microwave-assisted ammonia synthesis. As shown in Figure 1, both support and promoter can affect the ammonia yield. All CeO₂ supported catalysts (shown in blue) show higher ammonia yield than the corresponding MgO supported catalysts (shown in red) under the same metal loading and reaction conditions. Ammonia yield over Ru/CeO₂(4%) catalyst reaches to 7.02 ml/h^.g_cat, which is much higher than 3.9 ml/h^.g_cat obtained over Ru/MgO(4%). With the increase in Ru loading, ammonia yield is increased over both MgO and CeO₂ supported catalysts. The presence of K and Cs promoters largely increases ammonia yield on MgO and CeO₂ supported catalysts, in which Cs shows better improvement effect than K. This is largely because Cs has higher electron donating ability to the Ru metal. Compare with MgO support, the Ru/CeO₂ catalyst has much smaller metal particle size, lower reduction temperature, and lower binding energy. The presence of promoters reduce Ru particle size and binding energy. Specifically, Cs shows better promoting effect due to the higher electron donating ability. Density functional theory (DFT) calculating was used in our study to elucidate the interaction between promoters and supports.

On process development side, a high-pressure microwave reactor was designed and tested in this study. High pressure microwave reactor is the first reporting in the field of microwave catalytic ammonia synthesis. Figure 2 shows the effect of reaction pressure on ammonia yield at different H₂/N₂ ratio. Specifically, the highest ammonia concentration is achieved at 1:1 H₂/N₂ ratio. The reaction mechanism under the pressure was explained by DFT modeling work.

Overall, microwave catalytic ammonia synthesis is fundamentally different from commercial Haber-Bosch process, having cost advantages at small scale that is comparable with commercial ammonia process of 10-20 times larger. It can be tolerant to intermittent renewable energy supply, therefore effectively operated at variable rates of production.