Precise Vacancy Fitting of Horizontal Dinitrogen for Ammonia Synthesis

Abstract

Efficient ammonia synthesis under mild conditions remains a significant challenge in modern chemistry. Chemical looping ammonia synthesis (CLAS) presents a sustainable alternative for ammonia production under near-ambient pressure conditions. In this study, we introduce a "horizontal N₂" approach in the CLAS process, where an N₂ molecule is horizontally incorporated into the surface lattice of the catalyst, facilitating effective N₂ activation and subsequent NH₃ production. Utilizing barium carbide (BaC₂) as a model material, we show that the surface dianion vacancy sites of BaC₂ provide optimal spacing for N₂ activation, while the loaded Ni nanoparticles are responsible for H₂ activation, enabling N₂ hydrogenation from both ends to produce NH₃ under mild conditions. Resonant inelastic X-ray scattering (RIXS) analysis combined with computational calculations reveals that the rate-determining step of the CLAS process is the hydrogenation of *HNNH, significantly lowering the activation energy required for N₂ dissociation. The NH₃ production rate for Ni/BaC₂ is one order of magnitude higher than that reported for conventional CLAS catalysts and even surpasses the performance of the state-of-the-art 3d transition metal catalyst for catalytic ammonia synthesis at mild conditions of 100 °C and atmosphere. Unlike traditional CLAS catalysts, which typically suffer from deactivation, the dianion vacancy sites in Ni/BaC₂ demonstrate exceptional catalytic stability, maintaining consistent activity over 20 h without noticeable degradation. These findings highlight the potential role of surface vacancy sites in CLAS reaction cycles and offer a new catalyst design concept for developing earth-abundant catalysts in ammonia synthesis.