(495c) Microwave-Enhanced Catalytic Ammonia Synthesis over Csru/CeO2 Under Moderate Pressure and Temperature

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. The presence of promoters largely increases ammonia yield on MgO and CeO₂ supported catalysts, in which Cs shows better improvement effect than K. The presence of promoters reduces 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.

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.