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 PN2/PH2 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 through reduction revealed a shift of the Ru 3d5/2 peak to lower binding energy by about 1 e V in the presence of Cs suggesting an electronic promoter effect