The pseudo-catalytic properties of lithium (Li)-based alloys with group 14 elements were investigated for ammonia (NH3) synthesis under ambient conditions. The reaction between the Li alloys and nitrogen (N2) proceeds below 500 °C to form lithium nitride (Li3N). The peak temperature of nitridation was lower in the order of Li4.4Si < Li4.4Ge ≈ Li4.4Pb < Li4.4Sn. The N2 dissociation activity is related to the value of Knight shift observed in the 7Li solid-state magic angle spinning nuclear magnetic resonance (7Li MAS NMR) spectra, indicating that the metallic feature of Li species is an important factor for low-temperature N2 dissociation. Although the reaction yields for NH3 synthesis were depending on the alloys, NH3 was generated at the same temperature around 240 °C because the NH3 formation proceeded by the same reaction between Li3N and hydrogen (H2), generating lithium hydride (LiH) as a by-product. For all the alloys, LiH desorbs H2 to form Li alloys with higher Li composition at lower temperature than that of thermal decomposition of pure LiH. In addition, the conventional catalytic process was also investigated under a mixed gas of H2 and N2. NH3 was synthesized at 150 and 200 °C by using Li4.4Si and Li4.4Ge, respectively.

In this work, the reaction properties of ammonia (NH3) synthesis via the chemical looping process of lithium hydride (LiH) are investigated, and kinetic improvement is carried out. During the heating process up to 500 °C under 0.1 MPa nitrogen flow conditions, LiH reacts with N2 and changes to lithium imide (Li2NH) with hydrogen desorption. However, the kinetics of the reaction between LiH and N2 is slow due to agglomeration of the products. Lithium oxide (Li2O) as a scaffold is effective to drastically improve the reaction kinetics because Li2O suppresses the agglomeration. In this case, the reaction of LiH and N2 is completed within 20 min, which is drastically short compared with that of LiH (more than 1000 min). NH3 can be generated by reaction between Li2NH as the product and 0.1 MPa H2 from about 350 °C. Crushing the agglomerated particles and addition of Li2O can improve the reaction kinetics of NH3 synthesis, and then, the reaction completely proceeds at a lower temperature and shorter time. It is expected from the experimentally obtained reaction products and thermodynamic database that the N2 dissociation and NH3 generation are exothermic reactions. From the abovementioned results, it is concluded that NH3 can be produced at ambient pressure via successive reactions of LiH with N2 and H2 by exothermic processes, and the kinetics can be controlled using scaffolds.

In this work, catalysis for ammonia production of sodium alloys is systematically investigated. Sodium alloys are synthesized from sodium and group 14 elements and characterized by X-ray diffraction, thermal desorption mass spectroscopy, and nuclear magnetic resonance experiments. The catalytic properties of Na alloys are evaluated by heating alloys under flow conditions of mixed gases of hydrogen and nitrogen. As a result, ammonia can be synthesized on sodium alloys below 400 °C under atmospheric pressure, and catalysis is preserved for more than 100 h. From the above experimental results, it is clarified that the ammonia synthesis temperature and catalysis are related to the thermal stability and metallic feature of sodium atoms in the alloys, respectively. Although the sodium alloys are partially decomposed during the ammonia synthesis process, the higher partial pressure of hydrogen would suppress the decomposition. The obtained results indicate that Na alloys are potential ammonia synthesis catalysts.