Computational screening of antiperovskite nitride materials for nitrogen chemical looping Michael Higham 1,2 , Angela Daisley 3 , Justin S. J. Hargreaves 3 , C. Richard A. Catlow 1,2 1 University College London, UK, 2 Research Complex at Harwell, UK, 3 University of Glasgow, UK 3 . Chemical looping, whereby nitride materials are decomposed to yield ammonia and their corresponding alloys under hydrogenating conditions, and are subsequently regenerated via nitridation of the alloys, are a potentially feasible alternative to catalytic processes for ammonia synthesis, avoiding the challenges faced by traditional ammonia synthesis catalysts that require extreme pressures and temperatures. Ammonia (NH 3 ) synthesis is an essential yet energy-demanding industrial process developing new approaches for NH 3 synthesis that provide high NH 3 yields under milder conditions 1,2 . Hence, there is interest in Metal nitride systems have been investigated for nitrogen looping activity 4–7 , with antiperovskite metal nitrides (AB 3 N) emerging as promising candidates. Clearly, the vast structural phase space spanned by metal nitride materials necessitates a robust theoretical framework to guide computational screening of new materials and subsequently future experimental efforts. Building on the approach developed by Michalsky et al. 4 , we apply plane-wave density functional theory (DFT) methods to screen antiperovskite metal nitrides for nitrogen looping. Searches were performed to obtain structures for antiperovskite nitrides (AB 3 N) listed in the ICSD, along with alloys of corresponding stoichiometry (AB 3 ), since efficient nitrogen looping necessarily requires high bulk lattice N activity. Calculations were performed to obtain the reaction energy for complete reduction of the antiperovskite to the corresponding alloy. yielding both NH 3 under hydrogenating conditions, and N 2 : ∆ E NH3 = E AB3 + E NH3(g) - E AB3N - 1.5 E H2(g) (1) ∆ E N2 = E AB3 + 0.5 E N2(g) - E AB3N (2) By taking the average of (1)and (2), we propose a convenient means of quantifying theoretical nitrogen looping activity; it can be seen that pairs of nitrides and alloys for which the average is closest to zero represent systems that most closely satisfy the condition outlined by Michalsky et al., namely that both Eq.(1) and the reverse of Eq.(2) are exergonic. As a result of the screening process, potentially promising systems were identified, including PdFe 3 N. It is intended that the present work will inform future experimental studies, and provide the groundwork for further theoretical investigations that will explore the impact of finite temperatures and pressures for the most promising systems identified. References 1. Ammonia: zero-carbon fertiliser, fuel and energy store, https://royalsociety.org/-/media/policy/projects/green-ammonia/green- ammonia-policy-briefing.pdf, (accessed 28 September 2022). 2. P. H. Pfromm, J. Renew. Sustain. Energy , 2017, 9 , 034702. 3. J. Humphreys, R. Lan and S. Tao, Adv. Energy Sustain. Res. , 2021, 2 , 2000043. 4. R. Michalsky, A. M. Avram, B. A. Peterson, P. H. Pfromm and A. A. Peterson, Chem. Sci. , 2015, 6 , 3965–3974. 5. R. Michalsky, P. H. Pfromm and A. Steinfeld, Interface Focus , 2015, 5 , 20140084. 6. Y. Goto, A. Daisley and J. S. J. Hargreaves, Catal. Today , 2020, 0–5. 7. A. Daisley, M. Higham, C. R. A. Catlow and J. S. J. Hargreaves, Faraday Discuss. , 2022, 34 , 240–241.
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