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

Abstract

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 and atmospheric pressure. It exhibits an activity of 1.1 μmol NH3 g c –1 h –1 at 150 °C and 10.3 μmol NH3 g c –1 h –1 at 190 °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 FeN 4 sites within the graphene lattice and evolve to FeN 3 as the active sites during the reaction. N 2 transformation to NH 3 follows an associative mechanism, evidenced by the presence of N 2 H 2 + and N 2 H 3 + detected by TOF-MS. These findings could guide further development of more active catalysts for mild ammonia synthesis.