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.