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 C18, monosubstituted C17Si, and nine C16Si2 isomers (C16Si2(0)-C16Si2(8)) identifies C16Si2(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 N2 adsorption and effective activation via Si → N2 back-donation. The activated C16Si2-N2 complex undergoes stepwise hydrogenation via C16Si2-NH2 and C16Si2-(NH2)2 intermediates, ultimately yielding two NH3 molecules through an overall energetically favorable pathway. Importantly, the resulting C16Si2-H2 adduct re-enters the catalytic cycle by reacting with the remaining C16Si2-N2, producing C16Si2-(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.