Systematic Study on Nitrogen Dissociation and Ammonia Synthesis by Lithium and Group 14 Element Alloys

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.