Dissecting the Essential Role of a Molecular Promoter C60 on a Ru Catalyst for Ammonia Synthesis

Abstract

The major bottlenecks for ammonia (NH₃) synthesis under mild conditions are the activation of the extremely inert N≡N bond (941 kJ mol^–1) and/or the desorption of NH₃ from the catalyst surface. Electron donation from the appropriate promoters is essential to enhance N₂ activation over Fe or Ru catalysts. Nevertheless, despite typical element promoters enhancing the N₂ activation efficiency, they induce strong NH₃ binding on the catalyst surface, leading to the need for high temperatures and pressures for the reaction to complete. Herein, we propose the use of a molecular promoter (C₆₀) to tackle the difficulties. The positioning role of C₆₀ at a 1 nm scale on small Ru nanoclusters drives the exposure of more terrace sites (geometric effect) and induces d–π interactions at the Ru–C₆₀ junctions. The latter electronically modifies the Ru sites (electronic effect), thereby synergistically contributing to N₂ and H₂ activation as well as to the release of NH₃ on the Ru sites. The significant electron buffer attribute of C₆₀ and the strong electronic interaction between Ru and C₆₀ facilitate a shift of d-band center toward the Fermi level and a decrease of work function, simultaneously satisfying the electronic requirements for N₂ activation enhancement and NH₃ binding weakening. Consequently, the C₆₀-promoted Ru/LaN catalyst exhibits a high NH₃ synthesis rate. It is envisioned that our findings have significant implications for the rational search of molecular promoters for high-efficiency NH₃ synthesis.