N 2 activation in ammonia synthesis and cracking Collin Smith, Laura Torrente-Murciano, Joseph El-Kadi University of Cambridge, UK
Ammonia’s use in long-term renewable energy storage requires the development of both ammonia synthesis and cracking catalysts. In a ‘green ammonia’ energy cycle, excess renewable energy is used to produce hydrogen via water splitting, the hydrogen is then catalytically combined with nitrogen from the air via the Haber-Bosch process to synthesise ammonia. To unlock the stored energy in ammonia, total or partial catalytically cracking is required to produce pure hydrogen (for use in a fuel cell) or a combustible, low NO x generating, hydrogen-ammonia mixture (for use in a turbine). On the synthesis side, redefining the Haber-Bosch process requires considering the limitations of the overall process. In particular, innovative catalyst design should be targeted to align with the optimal operating conditions of the process and the feasible separation of synthesized ammonia at pressures above atmospheric 1 . Under such conditions, the catalyst frequently becomes inhibited by hydrogen and/or ammonia, requiring the development of unique promoters/supports to remove intermediates from the catalyst surface 2 . On the cracking side, coupling ammonia cracking with proton exchange membrane (PEM) fuel cells requires catalysts which unlock hydrogen at temperatures aligned with the PEM fuel cells (60-100 °C) according to the US Department of Energy, making low temperature cracking a key goal 3 . The most active metal for ammonia cracking is ruthenium 4 , an expensive, rare-earth metal, explained by its optimum nitrogen-binding energy; it does not bind to the nitrogen of the ammonia molecule too strongly or too weakly as per the Sabatier principle 5 . However, the cost and scarcity of ruthenium has driven interest into alternative, non-noble metals including cobalt, nickel, or iron [6], which may be more sustainable for large, centralised ammonia cracking. There are opportunities to replace ruthenium with non-noble metals by alloying metals to achieve a synergetic effect [7]. References 1. Smith, C. and L. Torrente-Murciano, Guidance for targeted development of ammonia synthesis catalysts from a holistic process approach. Chem Catalysis, 2021. 1 (6): p. 1163-1172. 2. Smith, C. and L. Torrente-Murciano, Exceeding Single-Pass Equilibrium with Integrated Absorption Separation for Ammonia Synthesis Using Renewable Energy-Redefining the Haber-Bosch Loop. Advanced Energy Materials, 2021. 11 (13): p. 2003845. 3. Rees, N.V. and R.G. Compton, Carbon-free energy: a review of ammonia- and hydrazine-based electrochemical fuel cells. Energy &; Environmental Science, 2011. 4 (4): p. 1255-1260. 4. Hansgen, D.A., D.G. Vlachos, and J.G. Chen, Using first principles to predict bimetallic catalysts for the ammonia decomposition reaction. Nature Chemistry, 2010. 2 (6): p. 484-489. 5. Turek, O.D.H.K.K.K.T., Heterogeneous Catalysis and Solid Catalysts, 2. Development and Types of Solid Catalysts , in Ullmann's Encyclopedia of Industrial Chemistry . 2011. Bell, T.E. and L. Torrente-Murciano, H-2 Production via Ammonia Decomposition Using Non-Noble Metal Catalysts: A Review. Topics in Catalysis, 2016. 59 (15-16): p. 1438-1457. 6. Kirste, K.G., et al., COx-free hydrogen production from ammonia – mimicking the activity of Ru catalysts with unsupported Co-Re alloys. Applied Catalysis B: Environmental, 2021. 280 : p. 119405.
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