Experimental and theoretical investigations on the anti-perovskite nitrides Co3CuN, Ni3CuN and Co3MoN for ammonia synthesis

Abstract

The ammonia synthesis activities of the anti-perovskite nitrides Co₃CuN and Ni₃CuN have been compared to investigate the possible metal composition–activity relationship. Post-reaction elemental analysis showed that the activity for both nitrides was due to loss of lattice nitrogen rather than a catalytic process. Co₃CuN was observed to convert a higher percentage of lattice nitrogen to ammonia than Ni₃CuN and was active at a lower temperature. The loss of lattice nitrogen was revealed to be topotactic and Co₃Cu and Ni₃Cu were formed during the reaction. Therefore, the anti-perovskite nitrides may be of interest as reagents for the formation of ammonia through chemical looping. The regeneration of the nitrides was achieved by ammonolysis of the corresponding metal alloys. However, regeneration using N₂ was shown to be challenging. In order to understand the difference in reactivity between the two nitrides, DFT techniques were applied to investigate the thermodynamics of the processes involved in the evolution of lattice nitrogen to the gas phase via conversion to N₂ or NH₃, revealing key differences in the energetics of bulk conversion of the anti-perovskite to the alloy phase, and in loss of surface N from the stable low-index N-terminated (111) and (100) facets. Computational modelling of the density of states (DOS) at the Fermi level was performed. It was shown that the Ni and Co d states contributed to the density of states and that the Cu d states only contributed to the DOS for Co₃CuN. The anti-perovskite Co₃MoN has been investigated as comparisons with Co₃Mo₃N may give an insight into the role structure type plays in the ammonia synthesis activity. The XRD pattern and elemental analysis for the synthesised material revealed that an amorphous phase was present that contained nitrogen. In contrast to Co₃CuN and Ni₃CuN, the material was shown to have steady state activity at 400 °C with a rate of 92 ± 15 μmol h⁻¹ g⁻¹. Therefore, it appears that metal composition has an influence on the stability and activity of the anti-perovskite nitrides.