Lithium strontium nitride-hydride as an ammonia synthesis catalyst Ezri McGarrigle, Manoj Raviand, Joshua Makepeace University of Birmingham, Edgbaston, Birmingham, UK, B15 2TT Around 176 tonn s of ammonia is produced each year with uses varying from fertilizers to acting as a hydrogen storage molecule. 1 The main method of forming ammonia uses the Haber Bosch process, with reaction conditions of 350-500°C and 150-300 bar which contributes to 1.8% of the world’s carbon dioxide emissions. 1 With the aim of working towards green ammonia production, mechanisms with milder reaction conditions need to be discovered, especially if the promise of net-zero CO 2 emissions by 2050 is to be met. 2 One limitation to reducing reaction conditions is the scaling relation, which shows that as nitrogen activation energy decreases, the binding strength of resultant N-H species to the catalyst increases. 3 To have a catalyst with high efficiency, this scaling relation must be broken. One such catalyst is a transition metal-lithium hydride composite which breaks the scaling relation by providing separate sites for nitrogen activation and hydrogenation. 3 This has drawn great interest to lithium-nitrogen-hydrogen materials and their ability to catalyse the synthesis of ammonia. Lithium nitride-hydride (Li 4 NH) has great promise as a catalyst with high ammonia synthesis rates at mild reaction conditions. 4 It does this by utilising a looping process where first, under nitrogen, Li 4 NH is partially converted to lithium imide (Li 2 NH) and lithium nitride after which, the product mixture is reacted under H 2 to reform Li 4 NH and produce ammonia. 4 One downfall is that if the hydrogenation step is carried out to completion, particularly at lower temperatures, lower-activity lithium hydride is produced. 4 This process isirreversible and hence the efficiency of the catalyst decreases. This side reaction limits the use of Li 4 NH as an ammonia synthesis catalyst for long periods of time without limiting the hydrogenation period and keeping the temperature high to prevent decomposition of the catalyst. 4 Asimilar nitride-hydride compound, trontium lithium nitride-hydride (Sr 2 LiNH 2 ), has been found to produce ammonia at comparable rates to Li 4 NH at similar reaction conditions using the same looping mechanism. However, no catalyst decomposition to form hydrides has been observed. This would mean that several cycles of ammonia synthesis could be carried out with a maintained catalyst efficiency. References 1. The Royal Society, Ammonia : zero-carbon fertiliser, fuel and energy store , London, 2020. 2. Department for Business, UK enshrines new target in law to slash emissions by 78% by 2035, https://www.gov.uk/ government/news/uk-enshrines-new- target-in-law-to-slash-emissions-by-78-by-2035, (accessed 16 January 2022). 3. P. Wang, F. Chang, W. Gao, J. Guo, G. Wu, T. He and P. Chen, Nat Chem , 2017, 9 , 64–70. 4. M. Ravi and J. W. Makepeace, Chemical Communications , 2022, 58 , 6076–6079.
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