AMMONIA-SYNTHESIS CATALYZED BY RUTHENIUM SUPPORTED ON BASIC ZEOLITES

Ammonia synthesis was catalyzed by ruthenium metal clusters, promoted by alkali and alkaline earth elements, supported on zeolite X, magnesi a, and pure silica MCM-41. At atmospheric total pressure and temperatu res ranging from 623 to 723 K, the turnover frequencies of ammonia syn thesis on Ru/KX varied significantly with Ru cluster size, demonstrati ng the known structure sensitivity of the reaction. Therefore, zeolite and magnesia catalysts were prepared with similar Ru cluster sizes, a bout 1 nm in diameter, in order to properly evaluate the effect of pro moters. The same high degree of metal dispersion could not be obtained with Ru/MCM-41 catalysts. The turnover frequency for ammonia synthesi s over Ru/CsX exceeded that over Ru/KX, consistent with the rank of pr omoter basicity. However, alkaline earth metals were more effective pr omoters than alkali metals for Ru supported on both zeolite X and MCM- 41. Since alkaline earth metals are less basic, this promotional effec t was unexpected. In addition, the turnover frequency for ammonia synt hesis on Ru/BaX exceeded that of Ru/MgO, a nonzeolitic material. Pore volumes for Ru/BaX and Ru/KX measured by N-2 adsorption were essential ly identical, suggesting that pore blockage by ions within the zeolite s does not account for the differences in reaction rates. The kinetics of ammonia synthesis over ruthenium differed considerably from what h as been reported for industrial iron catalysts. Most significantly, th e order of reaction in H-2 was negative over Ru but is positive over F e. A likely cause of this change in reaction order is that dissociated hydrogen atoms cover a greater fraction of the Ru clusters compared t o Fe under reaction conditions. (C) 1996 Academic Press, Inc.