New routes to low temperature ammonia synthesis Selin Ernam , Anastasiia Karabanova, Xiufu Sun, Peter Vang Hendriksen Technical University of Denmark, Denmark
Currently, ammonia is produced by the Haber Bosch process which involves high pressures (100 - 300 bar) and high temperatures (350 - 500 o C). Catalysts used in the Haber Bosch process show adequate activity only at higher temperatures. However, the reaction is exothermic and higher temperatures shift the equilibrium towards the reactants. To counter act this shift, pressure is increased 1 . These conditions as well as production processes for hydrogen an nitrogen, favors production in large scale facilities 2 . In contrast, a catalyst active at lower temperatures would alleviate the need for higher pressures. Milder operating conditions would facilitate small scale production of ammonia, which is highly beneficial when also considering sustainable routes to the hydrogen needed for the process. This will likely be provided via water splitting by electrolysis, which open up for decentralized production units operating on intermittently available electricity. Conventional high temperature catalysts are alkali metal promoted iron or ruthenium-based catalysts. Nitrogen and hydrogen dissociation over the catalysts are primary steps where nitrogen dissociation is the rate determining step and has a high activation barrier. The inherent problem is the scaling relation that comes with this activation barrier and the bond strength of the intermediates to the surface, making low temperature activation of such catalysts challenging. Recent advances for low temperature/pressure catalysts include ternary transition metal hydrides 3 , electrides 4 and nitrides 5 . Nitrogen activation is carried out differently over these catalysts which include associative adsorption of nitrogen followed by hydrogenation, altering the mechanism so that nitrogen dissociation is no longer the rate determining step and the Mars van Krevelen mechanism where lattice nitrogen are used and replenished by the synthesis gas respectively. Different approaches to nitrogen activation and an overview of the current progress will be discussed. Ultimately, decentralized and small-scale ammonia production units working at low temperature is the objective. Ideally, these units would operate on renewable energy. Due to the intermittent nature of renewable electricity, procedures that work well with continuous ammonia production would need modification. One such issue is the collection of produced ammonia. The poster will present our ideas for novel nitride/hydride catalysts, discuss how ammonia absorbing materials such as metal halide salts can benefit ammonia synthesis and how they can be integrated to the process. References 1. V. B. Shur and S. M. Yunusov, Russ. Chem. Bull. , 1998, 47 , 765–776. 2. M. Yoshida, T. Ogawa, Y. Imamura and K. N. Ishihara, Int. J. Hydrogen Energy , 2021, 46 , 28840–28854. 3. Q. Wang, J. Pan, J. Guo, H. A. Hansen, H. Xie, L. Jiang, L. Hua, H. Li, Y. Guan, P. Wang, W. Gao, L. Liu, H. Cao, Z. Xiong, T. Vegge and P. Chen, Nat. Catal. , 2021, 4 , 959–967. 4. M. Hara, M. Kitano and H. Hosono, ACS Catal. , 2017, 7 , 2313–2324. 5. J. S. J. Hargreaves, Appl. Petrochemical Res. , 2014, 4 , 3–10.
P16
© The Author(s), 2023
Made with FlippingBook Learn more on our blog