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, c haracterized, and tested as catalysts for ammonia synthesis at 20 bar. The effects of both cation type (K, Cs, Mg, Ba) and loading (exchange d or occluded) on the kinetics of ammonia synthesis were evaluated. Re sidual protons in the ion exchanged catalysts were detrimental to both zeolite stability and measured turnover frequency. In addition, alkal ine-earth-exchanged materials were more active than alkali-exchanged m aterials, contrary to the ranking of zeolite basicity. Successive impr egnations of Ba cations beyond ion exchange capacity of Ru/BaX further promoted the ammonia synthesis reaction by almost an order of magnitu de to levels near that of Cs-promoted Ru/MgO, a highly active, nonzeol itic catalyst. Impregnation of occluded K cations onto Ru/KX did not i mprove catalyst activity, presumably due to the severe loss in micropo re 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 consisten t with N-2 adsorption being rate-determining and hydrogen atoms coveri ng a substantial fraction of the metal surface. (C) 1998 Academic Pres s