(685d) Engineered Bifunctional Alloys for Chemical Looping Ammonia Synthesis

Chemical looping ammonia synthesis (CLAS) is an emerging production route with considerable potential for distributed and/or renewable ammonia. CLAS systems can synthesize ammonia at atmospheric pressure from a versatile number of feedstocks and energy sources including the simplest combinations of air, water, and sunlight. CLAS systems use a solid carrier to transfer hydrogen and nitrogen between the reaction steps, mediating ammonia synthesis in the process. However, there is a tradeoff between solid carriers that are thermodynamically favored to fix hydrogen and those that are favored to fix nitrogen, resulting in a limited number of carriers that perform both. Therefore, to expand the solution domain of solid carriers for CLAS and accelerate their development, we propose the use of engineered bifunctional alloys.

To evaluate this hypothesis, we used a theoretical framework to identify bifunctional alloys of hydrogen and nitrogen fixers, and an experimental framework to validate its performance in the following chemical loop:

Mx + H2 = MxH2,	(R1)
3My + N2 = M3yN2,	(R2)
3MxH2 + M3yN2 = 3Mx + 3My + 2NH3,	(R3)

where M^x and M^y represent a nitrogen and hydrogen fixing carrier, respectively. Bifunctional alloys of M^x and M^y were identified in the following series of steps diagramed in Figure 1. The proposed framework identified over 900 thermodynamically feasible bifunctional materials to synthesize ammonia in the chemical loop of R1-R3, of which less than 300 would make a stable alloy. From an extensive literature survey, many of the materials identified are supported in the literature as either being a catalyst, promoter, or support for ammonia synthesis. The theoretical framework developed in this work expanded the solution domain of potential solid carriers for CLAS over 150-fold, which can be used to accelerate development of a practical solution.