Molten multi-phase catalytic system comprising Li–Zn alloy and LiCl–KCl salt for nitrogen fixation and ammonia synthesis at ambient pressure†

Abstract

Ammonia (NH₃) is one of the most important synthetic inorganic commodities. The current industrial NH₃ production is dominated by the Haber–Bosch process with high energy cost and CO₂ emission as well as the need for large-scale centralized operation. Liquid metals and molten salts have recently emerged as promising catalytic materials for NH₃ synthesis. Herein, we present a molten system comprising Li–Zn alloy and eutectic LiCl–KCl salt for effective NH₃ synthesis at 400 °C and 1 bar. The 70 mol% Li–Zn liquid alloy activates N₂ dissociation more easily than the pure liquid Zn and the 60 mol% Li–Sn liquid alloy. Effective N₂ fixation by the liquid Li–Zn alloy is followed by the hydrogenation of Li₃N dissolved in the molten salt above. For the first time, this work reports a volcano-type relationship between the Li₃N concentration in the molten salt and the NH₃ synthesis rate when feeding H₂ to the molten salt. Ab initio molecular dynamics simulations suggest that, within this system, both N₂ cleavage and Li₃N hydrogenation are quite reactive. Through combined experiments and simulations, this work unravels the molecular mechanisms of nitrogen fixation and ammonia synthesis in the liquid alloy–salt catalytic system, and also demonstrates effective strategies for improving the ammonia synthesis rate. Such a hybrid molten catalytic system offers a promising solution for distributed NH₃ production with low energy cost and CO₂ emission.