Unraveling the size-dependent effect of Ru-based catalysts on Ammonia synthesis at mild conditions

Identifying optimal Ru size in NH₃ synthesis can improve reaction activity and maximize the utilization of Ru to reduce catalyst cost. However, previous researches are focused on large Ru particle size (≥2 nm) while that below 2 nm in NH₃ synthesis is unclear. Here we synthesized a series of Rux/BaCeO₃ with different Ru sizes (x = 1.1–3.0 nm) through size-controlled Ru colloid. With the decrease of Ru size, NH₃ synthesis rate over Ru1.1/BaCeO₃ increases to 19.4 mmol g_cat⁻¹h⁻¹ at 400 °C and 1 MPa, which is 5.7 times that of Ru3.0/BaCeO₃ and superior to most of Ru-based catalysts previously reported. It reveals that the reduction of Ru size enhances the generation of Ce³⁺ and oxygen vacancies in BaCeO₃, which can donate electron to Ru centers and promote N₂ dissociation. Moreover, the small Ru size enhances hydrogen spillover from Ru to BaCeO₃ to alleviate hydrogen poisoning, resulting in efficient NH₃ synthesis.