Construction of Reaction System and Regulation of Catalyst Active Sites for Sustainable Ammonia Production

Abstract

Ammonia (NH₃) is widely used for human life and considered a green energy carrier without CO₂ emissions; thus, green and sustainable NH₃ synthesis is of great importance. The traditional Haber-Bosch process requires harsh conditions with serious environmental implications. Therefore, numerous research is focused on the efficient synthesis of NH₃ from abundant N₂/air and water under ambient conditions, utilizing renewable energy sources. Despite the fact that the electrocatalytic N₂ reduction reaction (eNRR) is an ideal method for NH₃ synthesis, the NH₃ yield and Faradaic efficiency (FE) are severally hampered by the inertness of N₂, impeding its industrial application. Various strategies have been proposed to synthesize highly efficient heterogeneous catalysts for N₂ adsorption and dissociation to improve NH₃ yield and FE. Besides, benefiting from the nonthermal plasma N₂ oxidation reaction (pNOR) and electrocatalytic nitrate/nitrite reduction reaction (eNO_xRR), the two-step approach overcomes the limitations of eNRR, attracting significant interest. This strategy facilitates N₂ splitting, which is a crucial step in the synthesis of NH₃. Additionally, eNO_xRR involves complex intermediates, making it essential to investigate catalysts with high selectivity of NH₃. Overall, through the optimization of catalysts and reaction systems, NH₃ can be synthesized with high efficiency. The two-step strategy is the most realistic process for mass NH₃ production, but several challenges still need to be addressed, including improving the overall energy efficiency and scaling up the technology for industrial applications.

In this Account, we present an overview of our recent efforts in the construction of the reaction system and regulation of catalyst active sites for sustainable and efficient NH₃ synthesis. First, we introduce the design principles of the catalysts, which should possess abundant stable active sites and moderate adsorption strength. Subsequently, a range of strategies is proposed to enhance the NH₃ synthesis performance of Au, Bi, Co, Cu, and other catalysts, including coordination tuning, defect construction, elemental regulation, and structural design for direct eNRR and the two-step method of pNOR-eNO_xRR at ambient conditions. Additionally, we explore the NH₃ synthesis process at a large scale by scaling up the electrode and reactor. Furthermore, the separation and collection routes of NH₃ from electrolytes are also investigated to meet the requirements of various applications. Finally, a brief outlook is provided to discuss the catalyst optimization method, remaining challenges, and future perspectives of expanding production. This Account will offer valuable insight into the in-depth study of catalytic electrode preparation and system optimization, widening the way for efficient and sustainable NH₃ synthesis.