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

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. Activating dinitrogen under mild conditions remains a central challenge in ammonia synthesis, driving the search for electron-rich active sites that can facilitate N₂ activation and lower reaction barriers. Here, the authors create electron-rich dianion vacancies in a rare-earth carbide catalyst, providing a promising route to efficient and stable ammonia synthesis under mild conditions.