Facile preparation of alkali metal-doped Ru/MgO catalysts for ammonia decomposition via one-pot dry ball milling: structural and electronic promoting effect of K, Rb, and Cs

Ammonia decomposition is a promising method for on-site hydrogen generation for fuel cells. Ruthenium is the most active catalyst for this process, and modulation of its geometric and electronic properties is essential for achieving high catalytic performance. This study developed a facile approach for synthesizing alkali metal-doped Ru/MgO catalysts using a one-pot dry ball milling method with acetates as precursors. Characterization results indicate that K, Rb, and Cs act as effective structural and electronic promoters. Notably, the presence of alkali metals significantly inhibits the aggregation of bulk-like RuO₂, leading to the formation of well-dispersed RuO_x, which, upon reduction, produces small Ru nanoparticles. The average Ru particle size is about 2–3 nm, aligning with the theoretical optimal size for maximizing B₅ sites. XAS and DRIFTS analyses confirm that Ru species are stabilized due to the strong metal-promoter interaction between ruthenium and alkali metals via oxygen bonding. The resulting alkali metal-promoted Ru nanoparticles exhibit superior low-temperature activity for ammonia decomposition. The 2 K-3 %Ru/MgO catalyst achieves a high reaction rate of 11.5 mmol-NH₃ g_cat⁻¹ min⁻¹ at 400 °C under 30,000 mL g_cat⁻¹ h⁻¹, demonstrating one of the highest performances among Ru catalysts. Furthermore, this catalyst maintains high activity at 475 °C for 50 h without significant deactivation. The enhanced catalytic performance is attributed to the structural and electronic promotion effects of alkali metals, which facilitate the formation of B₅ site-rich and electron-rich Ru nanoparticles. These findings offer valuable insights into the role of alkali metals and provide helpful guidance for designing efficient alkali metal-promoted ruthenium catalysts for practical applications.