Ruthenium as catalyst for ammonia synthesis

Five Ru-based catalysts were prepared to study the effect of the support and the role of the alkali promoter in NH₃ synthesis: Ru/Al₂O₃, Ru/MgO, Cs-Ru/Al₂O₃ and Cs(K)-Ru/MgO. The catalysts were characterized by N₂ physisorption, H₂ chemisorption and XPS. The absence of chlorine- and sulphur containing compounds turned out to be important for the preparation of highly active catalysts. Power law expressions were derived from conversion measurements at atmospheric pressure and at 20 bar. For all catalysts, the reaction order for H₂ was found to be negative suggesting that a P_N2 / P_H2 ratio in the feed gas higher than 1 / 3 would be favourable for industrial NH₃ synthesis at high pressure. The microkinetic analysis of the temperature-programmed desorption and adsorption of N₂ and of the kinetics of isotopic exchange demonstrated the enhancing influence of the Cs promoter on the rate of N₂ dissociation and recombination. XPS measurements after thorough reduction revealed a shift of the Ru 3d_5/2 peak to lower binding energy by about 1 eV in the presence of Cs suggesting an electronic promoter effect.