Synthesis of nitrogen-rich binary nitrides as new catalysts for ammonia synthesis Marianna Casavola 1 , Min Zhang 1 , Angela Daisley 2 , Justin Hargreaves 2 and Andrew L. Hector 1 1 University of Southampton, UK, 2 University of Glasgow, UK Ammonia plays a key role in the transition to a carbon neutral, renewable energy production and significant efforts in the scientific community are devoted to develop new catalysts to synthesise ammonia in a cost-effective, scalable and safe way. 1 Recent studies have suggested that ammonia synthesis by nitride catalysts may occur via the Mars van Krevelen mechanism, in which intrinsic lattice nitrogen is hydrogenated, generating transient vacancies which could be replenished from the N 2 feed. The occurrence of associative reaction routes would allow to perform catalytic reactions with considerably lower energy expense compared to the most common dissociative pathways and could pave the way to a sustainable production of ammonia. 2-3 Metal nitrides with high nitrogen content, i.e. N:M ratios >1, are good candidates as catalysts for ammonia synthesis with possible looping effects. Nevertheless, transition metals usually form nitrides with lower nitrogen content, while the synthesis of higher nitrides requires more synthetic ingenuity. 4 In our group we developed wet-chemical processes at low pressure and moderate temperature to synthesise different N-rich nitrides such as Sn 3 N 4 , 5-6 Hf 3 N 4 , 7 and Zr 3 N 4 . The methods, based on a solvothermal and a combined ammonolysis/pyrolysis process, respectively, allowed for the production nitrogen-rich nitride nanopowders, which were characterised by powder XRD, SEM/EDS, TGA and ammonia synthesis catalytic tests to directly correlate structure and catalytic performance. In order to improve the catalytic properties of the nitrides, we developed a method to incorporate transition metals such as Fe in the nitrides, which could promote the reactivity of feed hydrogen and improve catalyst stability. 8-9 References 1. M. El-Shafie, S. Kambara, Recent advances in ammonia synthesis technologies: Toward future zero carbon emissions , Int. J. Hydrogen Energy, ttps://doi.org/10.1016/j.ijhydene.2022.09.061. 2. C. D. Zeinalipour-Yazdi, J. S. J. Hargreaves, and C. R. A. Catlow, J Phys Chem C, 2018, 122, 6078. 3. A. Daisley, J.S.J. Hargreaves, Metal nitrides, the Mars-van Krevelen Mechanism and Heterogeneously Catalysed Ammonia
Synthesis , Catalysis Today, 2022, doi: https://doi.org/10.1016/j.cattod.2022.08.016. 4. A. Salamat, A. L. Hector, P. Kroll, P. F. McMillan, Coord. Chem. Rev., 2013, 257, 2063. 5. X. Li, A. L. Hector, J. R. Owen and S. I. U. Shah, J. Mater. Chem. A, 2016, 4, 5081.
6. S. D. S. Fitch, G. Cibin, S. P. Hepplestone, N. Garcia-Araez and A. L. Hector, Dalton Trans., 2019, 48, 16786. 7. A. Salamat, A. L. Hector, B. M. Gray, S. A. J. Kimber, P. Bouvier, and P. F. McMillan, J. Am. Chem. Soc. 2013, 135, 9503. 8. T.-N. Ye, S.-W. Park, Y. Lu, J. Li, M. Sasase, M. Kitano, and H. Hosono, J. Am. Chem. Soc. 2020, 142, 14374. 9. T.-N. Ye, .S-W. Park, Y. Lu, J. Li, J. Wu, M. Sasase, M.i Kitano, and H. Hosono, J. Am. Chem. Soc. 2021, 143, 12857.
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