Catalytic Ammonia Synthesis Over Metal‐Free Silicon Disubstituted Cyclo[18]carbon: A Density Functional Theory Study

In this work, we investigated silicon disubstituted cyclo[18]carbon as a metal‐free catalyst for nitrogen activation and ammonia synthesis using density functional theory at the ωB97XD/def2‐TZVPP level. A systematic comparison of the structural features of pristine C ₁₈ , monosubstituted C ₁₇ Si, and nine C ₁₆ Si ₂ isomers (C ₁₆ Si ₂ (0)–C ₁₆ Si ₂ (8)) identifies C ₁₆ Si ₂ (8) as the most stable configuration, attributed to the maximum Si–Si separation and minimal distortion of the conjugated carbon framework. Electronic structure analysis indicates that dual Si substitution creates active sites that are highly electron‐deficient and a more symmetrically distributed reactive environment, thereby promoting strong N ₂ adsorption and effective activation via Si → N ₂ back‐donation. The activated C ₁₆ Si ₂ –N ₂ complex undergoes stepwise hydrogenation via C ₁₆ Si ₂ –NH ₂ and C ₁₆ Si ₂ –(NH ₂ ) ₂ intermediates, ultimately yielding two NH ₃ molecules through an overall energetically favorable pathway. Importantly, the resulting C ₁₆ Si ₂ –H ₂ adduct re‐enters the catalytic cycle by reacting with the remaining C ₁₆ Si ₂ –N ₂ , producing C ₁₆ Si ₂ –(NH) ₂ and restoring the catalytic pathway, thereby establishing a self‐sustaining mechanism. Overall, our findings show that multisite silicon engineering of cyclo[18]carbon frameworks is an effective strategy for developing efficient, metal‐free catalysts for ammonia synthesis under mild conditions.