Catalytic studies of imide–transition-metal composites for ammonia decomposition
Caitlin Brooker-Davis 1, T. J. Wood 2 , J. W. Makepeace 1 , W. I. F. David 2,3 1 University of Birmingham, UK, 2 ISIS Pulsed Neutron and Muon Facility, UK 3 University of Oxford, UK
The development of high-performing ammonia decomposition catalysts is crucial towards utilizing ammonia as a zero-carbon chemical energy store, since its full or partial decomposition to hydrogen is required to enable many of its energy applications 1 . Recently, the Li-N-H system has been identified as possessing superior ammonia conversion to state-of-the-art Ru at moderate temperatures, and the formation of a solid solution between lithium amide (LiNH 2 ) and lithium imide (Li 2 NH) has been identified as key to this activity 2 . There have been conflicting accounts on the activity enhancement of Li 2 NH through the addition of transition metals (TM), and the mechanism of this continues to be debated 3,4 . This study aimed to characterize the mechanism of activity improvement in LiNH 2 -TM composites, critical towards the future design of these catalysts.
LiNH 2 -TM composite materials were placed under ammonia flow and temperature raised incrementally through 300–540°C, with mass spectrometry and flow measurements carried out on the post-sample gas stream. Since sample composition and catalyticactivitychange as temperature is increased,component flows vary according to reactions 1 and 2: 2 LiNH 2 → Li 2 NH + NH 3 2 NH 3 → N 2 + 3 H 2 Characteristic gas releases were used to quantify the temperature of compositional change from molten LiNH 2 to an Fm3̅m solid solution phase, Li 1+ x NH 2- x (0 ≤ x ≤ 1, reaction 1) 2 , and the corresponding significant rise in x (i.e. transition from lithium amide to an imide-dominant solid solution). This compositional transition occurred at 440°C for LiNH 2 -Cr and LiNH 2 -Mn systems compared with 460°C for LiNH 2 and LiNH 2 -Fe (Fig. 1.b). All systems demonstrated a steep rise in ammonia conversion upon formation of the Fm3̅m phase, however conversion was further elevated for LiNH 2 -Cr and LiNH 2 -Mn (Fig. 1.a). It is concluded that these systems function as promoted Li 2 NH catalysts, with Cr and Mn ameliorating the system’s activity by stabilising Li 2 NHrelative to LiNH 2 and thusreducing the onset temperature for solid solution formation. References 1. J. W. Makepeace, T. He, C. Weidenthaler, T. R. Jensen, F. Chang, T. Vegge, P. Ngene, Y. Kojima, P. E. de Jongh, P. Chen and W. I. F. David, Int. J. Hydrogen Energy , 2019, 44 , 7746–7767. 2. J. W. Makepeace, T. J. Wood, H. M. A. Hunter, M. O. Jones and W. I. F. David, Chem. Sci. , 2015, 6 , 3805–3815. 3. 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. 4. 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.
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