Graphitic carbon nitrides were employed to prepare the Ruthenium-based ammonia synthesis catalysts by thermal decomposition of Ru₃(CO)₁₂ under N₂ or H₂ atmosphere. Different pretreatment atmospheres significantly affected the interaction between Ru nanoparticles and the graphitic carbon nitride. The strong metal support interaction enhanced the electronic density transfer from rich-electron π-plane of the support to the Ru nanoparticles. And it further caused the upshift of d-band center of Ru nanoparticles. The upshift of d-band center improved the ability of surface to bond to N₂, and consequently the ammonia synthesis activity was enhanced.