Effect of cationic promoters on the kinetics of ammonia synthesis catalyzed by ruthenium supported on zeolite X

Zeolite-X-supported ruthenium materials (2 wt% Ru) were synthesized, characterized, and tested as catalysts for ammonia synthesis at 20 bax. The effects of both cation type (K, Cs, Mg, Ba) and loading (exchanged or occluded) on the kinetics of ammonia synthesis were evaluated. Residual protons in the ion exchanged catalysts were detrimental to both zeolite stability and measured turnover frequency. In addition, alkaline-earth-exchanged materials were more active than alkali-exchanged materials, contrary to the ranking of zeolite basicity. Successive impregnations of Ba cations beyond ion exchange capacity of Ru/BaX further promoted the ammonia synthesis reaction by almost an order of magnitude to levels near that of Cs-promoted Ru/MgO, a highly active, nonzeolitic catalyst. Impregnation of occluded K cations onto Ru/KX did not improve catalyst activity, presumably due to the severe loss in micropore volume during reaction. Orders of reaction for the Ru/BaX catalysts were about 1 and -1 for N-2 and H-2, respectively, which is consistent with N-2 adsorption being rate-determining and hydrogen atoms covering a substantial fraction of the metal surface. (C) 1998 Academic Press