Ruthenium as Catalyst for Ammonia Synthesis

摘要

Five Ru-based catalysts were prepared to study the effect of the support and the role of the alkali promoter in NH_3 synthesis: Ru/Al_2O_3, Ru/MgO, Cs-Ru/Al_2O_3 and Cs(K)-Ru/MgO. The catalysts were characterized by N_2 physisorption, H_2 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 cnversion measurements at atmospheric presusre and at 20 bar. For all catalysts, the reaction order for H_2 was found to be negative suggesting that a P_(N_2) / P_(H_2) ratio in the feed gas higher than 1 / 3 would be favourable for industrial NH_3 synthesis at high pressure. The microkinetic analysis of the temperature-programmed desorption and adsorption of N_2 and of the kinetics of isotopic exchange demonstrated the enhancing influence of the Cs promoter on the rate of N_2 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 suggestign an electronic promoter effect.