Iron-Single Sites Confined by Graphene Lattice for Ammonia Synthesis under Mild Conditions

Development of ammonia synthesis catalysts under mild conditions is highly desirable for carbon neutrality. Herein, we report that an iron-single-site catalyst confined within the graphene lattice derived from ball milling of iron phthalocyanine with graphene and promoted by alkali metal enables ammonia synthesis below 200 C-degrees and atmospheric pressure. It exhibits an activity of 1.1 mu mol(NH3) gc (-1) h(-1) at 150( degrees)C and 10.3 mu mol(NH3) gc( -1) h(-1) at 190( degrees)C, whereas no activity is detected over a physical mixture of iron phthalocyanine and graphene. X-ray absorption fine structure analysis and( 15)N(2)-H-2 periodic switching experiments revealed that iron-single sites are stabilized as FeN4 sites within the graphene lattice and evolve to FeN3 as the active sites during the reaction. N-2 transformation to NH3 follows an associative mechanism, evidenced by the presence of N2H2+ and N2H3 (+) detected by TOF-MS. These findings could guide further development of more active catalysts for mild ammonia synthesis.

Automated summary

In this study, a new method for ammonia synthesis under mild conditions was reported, which involves the confinement of iron single-site catalyst within the graphene lattice and promotion by alkali metal. The catalyst exhibited high activity and the evolution of iron sites during the reaction was characterized. These findings provide important guidance for the development of more active catalysts for ammonia synthesis.