Enhancement of the catalytic activity of lithium amide towards ammonia decomposition by addition of transition metals
Caitlin Brooker-Davis a , Makepeace JW a , Wood TJ b a School of Chemistry, University of Birmingham, UK b ISIS Pulsed Neutron and Muon Facility, UK
Hydrogen production from ammonia is a catalytic process critical in enabling the use of ammonia as a zero carbon energy store in many of its applications. However, catalyst development has faced issues including high operating temperatures, chemical durability, and economic feasibility, meaning no commercial options currently exist. Lithium-nitrogen-hydrogen (Li-N-H) materials are an emerging family of catalysts with ammonia conversions at moderate temperatures superior to state-of-art ruthenium systems, and capability for small-scale power and transportation applications. In contrast with traditional heterogeneous systems, these materials function via the bulk formation of a solid solution between lithium amide (LiNH 2 ) and lithium imide (Li 2 NH) 1 . The interplay between surface and bulk, for example the ability of the system to transport reactive species to and from the surface, is thought to be integral to the high activity of these materials but has proven difficult to study experimentally. The activity of the system may be further enhanced through the addition of transition metals (TM), however it remains unclear whether these function as co-catalysts 2 , promoters 3 , or as a second active site 4 . Overall, gaining mechanistic insight into these systems is essential for their future design and is therefore of key interest. Ammonia decomposition experiments were carried out to uncover the existence and nature of activity
enhancement in LiNH 2 -TM composite systems. Mass spectrometry and flow measurements were used to quantify changes in the composition and activity of the catalysts with temperature, as according to the reactions: (1+ x )LiNH 2 →Li 1+ x NH 2- x + x NH 3 NH 3 ⇌ 1/2N 2 + 3/2H 2
Results confirmed that a compositional transition from molten LiNH 2 to the imide-dominant solid solution (and corresponding significant increase in x , Fig. 1. b)) was correlated with an increased catalytic activity (Fig. 1. a)). Some transition metals were shown to facilitate a lower formation temperature for this solid solution phase and thus enhanced the catalytic activity, thereby acting as promoters to the Li-N-H system. Synchrotron scanning transmission X-ray microscopy (STXM) experiments revealed the evolution of core-shell structures in variable temperature post-catalytic materials. These results are characteristic of the compositional changes which occur throughout the catalytic interaction with ammonia. This presentation will draw attention to recent work examining the mechanism of Li-N-H-based ammonia cracking catalysts and explore routes to improving their durability and performance. References
1. J. W. Makepeace, T. J. Wood, H. M. A. Hunter, M. O. Jones and W. I. F. David, Chem. Sci. , 2015, 6 , 3805–3815. 2. J. Guo, P. Wang, G. Wu, A. Wu, D. Hu, Z. Xiong, J. Wang, P. Yu, F. Chang, Z. Chen and P. Chen, Angew. Chemie Int. Ed. , 2015, 54 , 2950–2954. 3. J. W. Makepeace, T. J. Wood, P. L. Marks, R. I. Smith, C. A. Murray and W. I. F. David, Phys. Chem. Chem. Phys. , 2018, 20 , 22689–22697. 4. K. Ogasawara, T. Nakao, K. Kishida, T. N. Ye, Y. Lu, H. Abe, Y. Niwa, M. Sasase, M. Kitano and H. Hosono, ACS Catal. , 2021, 11 , 11005–11015.
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