Kinetic analysis of ammonia synthesis catalyzed by barium-promoted ruthenium supported on zeolite X

The kinetics of ammonia synthesis from dinitrogen and dihydrogen were studied at 20.7 atm total pressure over a series of zeolite X supported Ru (similar to2 wt.%) catalysts that were promoted by occluded barium. For comparison, two different Cs-promoted Ru/MgO catalysts were also included. In general, the reaction was first order in dinitrogen, about negative one order in dihydrogen, and weakly inhibited by ammonia. The average turnover frequencies of the Ba-promoted Ru/zeolite catalysts were within a factor of two of the highly active Cs-promoted Ru/MgO catalysts. The effect of Ba loading on an average turnover frequency and overall Ru metal utilization indicated an optimal content of 20 excess Ba cations per unit cell. A uniform surface model, assuming dinitrogen adsorption is the rate-determining step and both nitrogen and hydrogen atoms are present on the surface, reproduced well the measured kinetics over a wide range of temperatures and feed ratios. Results from fitting the kinetic parameters suggest that addition of basic promoters to supported Ru lowers the activation barrier for dinitrogen adsorption. (C) 2000 Elsevier Science B.V. All rights reserved.