Active and stable Fe-based catalyst, mechanism, and key role of alkali promoters in ammonia synthesis

•Cs and K impact drastically the Fe NPs dispersed on N-C support material toward ammonia synthesis.•A core–shell structure of the Fe NPs obtained with Fe(0) located on core and surrounded by Cs or K oxides.•The DFT reveals a geometrical repartition of alkali, leading to a larger number of exposed iron atoms.•A non-dissociative mechanism proposed by DFT, a stepwise addition of H on N leading to NH3.