Daisley, Angela; Higham, Michael; Catlow, C Richard A; Hargreaves, Justin SJ; (2022) Experimental and theoretical investigations on the anti-perovskite nitrides Co3CuN, Ni3CuN and Co3MoN for ammonia synthesis. Faraday Discussions 10.1039/d2fd00151a. (In press).

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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.

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