Structures and ammonia synthesis activity of hexagonal ruthenium iron nitride phases

A series of ruthenium iron nitride phases with Ru:Fe ratios of ca. 1:3 were synthesized by ammonolysis. When the ammonolysis temperature was above 500°C, the obtained Ru_xFe₃N_y materials had a ε-Fe₃N (P6₃22) structure, while two similar phases were present when the ammonolysis was lower than 500°C. Powder neutron diffraction identified one phase as relating to the ε-Fe₃N structure, while the other had a disordered NiAs-type (P6₃/mmc) structure. These ternary metal nitrides show ammonia synthesis activity at low temperature (200°C-300°C) and ambient pressure, which can be related to the loss of lattice nitrogen. Steady state catalytic performance at 400°C is associated with ruthenium-iron alloy. Additionally, density functional theory calculations were performed using an approximate model for the disordered hexagonal phase, revealing that this phase is more stable than a cubic anti-perovskite phase which has been previously investigated computationally, and corroborating the experimental findings of the present work.