In-situ characterization of Ni-BaH2 catalyst for low-temperature ammonia production through chemical looping

Ammonia criticalis critical in the global food security as the key component for fertilizers and as a potential energy carrier 1,2. By 2019, its production accounted for 1.2% of global CO2 emissions, almost a third of the chemical sector’s footprint 3,4. Amid the climate crisis, the need for low-carbon ammonia production alternatives appears unavoidable. As part of the solutions proposed over the years, the transition of the hydrogen production unit from steam methane reforming to water electrolysis is at the center of research 1. However, such a change will inevitably raise process challenges, such as the heat management at the junction between a low-temperature and pressure alkaline electrolyzer and the conventional Haber-Bosch unit producing ammonia at harsh conditions 5. In that context, the current work further reassesses NH3 production processes by investigating a novel technique, replacing the Haber-Bosch unit with a two-step low-temperature and pressure chemical looping process. Chemical Looping relies on splitting a conventional catalytic reaction in sub-reactions through an intermediate reacting and regenerating in cycles. It has shown promising applications in recent years 6, including the synthesis of ammonia at mild conditions. Among recent studies, Gao et al. demonstrated unprecedented production rates using Ni-BaH2/BaNH as a catalyst, as shown in reactions (1) and (2) 7. 2BaH2(s) +N2(g) = 2BaNH(s) + H2(g) (1) 2BaNH(s) +4H2(g) = 2BaH2(s) + 2NH3(g) (2) Operating at ambient pressure, ammonia synthesis started at 100°C. Such mild conditions would align well with alkaline water electrolysis, a promising aspect for the electrification and decarbonization of NH3 synthesis. Despite encouraging outcomes, and research on barium hydride for ammonia production remains particularly scarce. This study aims to expand current understanding through an in-depth in situ characterization of Ni-BaH2. Experiments were conducted using thermogravimetric-analysis (TGA) and in-situ transmission electron microscopy (TEM) to study the catalyst behavior during the reaction process at isothermal conditions, with a focus on nitridation (reaction (1)).