Electron-rich dianion vacancies boost diazenide intermediates for efficient chemical looping ammonia synthesis.

Abstract

Activating the nitrogen molecule under mild conditions remains a great challenge in ammonia synthesis. Electron-rich active sites are highly sought after to lower the activation barrier and enable efficient ammonia formation. In this work, we engineer electron-rich dianion vacancies using rare-earth carbide (LaC₂) as a platform, where N₂ undergoes horizontal adsorption and lattice embedding at the C₂ dianion vacancy sites. In situ experimental characterization, combined with computational calculations, reveals that the adsorbed N₂ molecules accept electrons from the C₂ dianion vacancies, forming highly reactive (N = N)^δ- intermediates. These species are crucial for promoting ammonia synthesis via an associative pathway. Utilizing the chemical looping ammonia synthesis process, LaC₂ emerges as an optimal candidate for N₂ activation and the Ni/LaC₂ catalyst demonstrates exceptional performance, achieving 6.9 mmol·g⁻¹·h⁻¹ at 400 °C and 0.1 MPa, exceeding the activity of previously reported metal catalysts for chemical looping ammonia synthesis process and even surpasses the performance of the state-of-the-art 3d transition metal catalyst for catalytic ammonia synthesis under identical conditions. These findings provide new insights into electron-rich, defect-mediated activation processes and offer a promising strategy for the design of efficient and stable catalysts for ammonia synthesis under mild conditions.