(220e) Chemical Looping Ammonia Synthesis from a Bifunctional Alloy of Lanthanum and Nickel

Industrial ammonia synthesis primarily relies on the Haber-Bosch process, a method developed in the early 20th century that revolutionized the production of ammonia at an industrial scale. This process combines nitrogen from the air with hydrogen, derived from natural gas or other sources, under high pressure (>150 atm) and temperature (>400°C) in the presence of a catalyst to produce ammonia [1]. Despite its widespread adoption, the Haber-Bosch process is a carbon- and energy-intensive process contributing to 2% of global CO₂ emissions and 1.7% of global energy demand [1]. Consequently, there has been increasing research into mild condition ammonia synthesis, aiming to produce ammonia under less extreme temperatures and pressures.

Chemical looping ammonia synthesis (CLAS) is a new mild condition ammonia production method that can produce ammonia at atmospheric pressure (1 atm) and temperatures below 300°C [2]. This method involves a multi-step, pseudo-catalytic process in which nitrogen is first bonded to a carrier material to form a nitrogen-containing compound. This compound is subsequently reduced with hydrogen to generate ammonia [2]. In this work, we demonstrated the feasibility of using a bifunctional alloy of lanthanum and nickel as a carrier material to facilitate CLAS, informed by prior theoretical work [2]. Lanthanum-nickel alloy nanoparticles were produced from ball milling commercially available lanthanum-nickel alloy and used in experiments. The nitrogen fixing properties of the alloy were assessed using Thermogravimetric Analysis under Nitrogen Atmosphere (TGA-N₂). The changes in the alloy’s crystalline structure before and after experimentation was analyzed using X-Ray Diffraction (XRD), X-Ray Photoelectron Spectroscopy (XPS), and Scanning Electron Microscopy (SEM). Chemical looping experiments were performed in a fixed bed reactor, and ammonia quantification was done over several chemical looping cycles. Studies were performed to investigate the effect of system parameters on ammonia production rates examining the effects of cycle duration and nitrogen fixation temperature.

In the TGA-N₂ experiment the alloy exhibited a significant weight increase of 15.6 w.t%. XRD analysis of the nitrified alloy showed the formation of a new crystalline phase, consistent with lanthanum nitride, which was further confirmed with XPS analysis. The nitrified alloy was then reduced with hydrogen and XPS analysis showed the lanthanum nitride peak disappear, suggesting that ammonia production occurs through the formation and reduction of lanthanum nitride. The chemical loop demonstrated a peak ammonia production rate of 720 μmol-NH₃/g.h. at atmospheric pressure using 15-minute cycles of nitrogen and hydrogen. This work demonstrates the novel approach of using bifunctional alloys to enable efficient ammonia production at mild conditions. Future work will focus on optimizing ammonia production from the chemical loop and studying the long-term stability of the alloy after successive cycling.

Acknowledgements

This work was partially sponsored by the Pratt & Whitney Institute for Advanced Systems Engineering of the University of Connecticut. Any opinions expressed herein are those of the authors and do not represent those of the sponsor.

References

[1] L. Wang et al., “Greening Ammonia toward the Solar Ammonia Refinery,” Joule, vol. 2, no. 6, pp. 1055–1074, 2018, doi: 10.1016/j.joule.2018.04.017.

[2] L. Burrows, P.-X. Gao, and G. M. Bollas, “Thermodynamic feasibility analysis of distributed chemical looping ammonia synthesis,” Chemical Engineering Journal, vol. 426, p. 131421, 2021, doi: https://doi.org/10.1016/j.cej.2021.131421.