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

ConspectusAmmonia (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 (eNOxRR), 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, eNOxRR 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-eNOxRR 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.