Unraveling the active sites of Cs-promoted Ru/γ-Al2O3 catalysts for ammonia synthesis

The development of efficient and stable Ru catalysts is crucial for synthesis of decarbonized NH3. Herein, a series of γ-Al2O3-supported Cs-promoted Ru catalysts were prepared, among which a 1.5Cs-Ru/γ-Al2O3 catalyst with a Cs/Ru molar ratio of 1.5 and a Ru size of ~2 nm exhibited high NH3 synthesis rates (6.9–30 mmol gcat–1 h–1) at ~400 °C and 1 MPa. We unraveled that the surface acidity/basicity could be changed, and new active sites could be generated at the interfaces between the Ru particles and the CsOH-Cs0 species by tuning the Cs/Ru molar ratio. Among the active sites, Cs0 (minor), which transferred electrons to Ru, was present at the boundaries of the Ru particles and CsOH (major), which attracted the dissociative hydrogen atoms from the metallic Ru surfaces via a spillover effect and changed the surface acidity/basicity. This facilitated the adsorption/desorption of reactant species, thus promoting NH3 synthesis.