Characterization and Ammonia Synthesis Activity of Ruthenium Zeolite Catalysts

Ruthenium metal in a series of alkali-metal-exchanged faujasite-type zeolites was found to be active for the catalytic synthesis of ammonia from dihydrogen and dinitrogen at temperatures between 300 and 450°C and at atmospheric pressure. A triply promoted iron catalyst exhibited a lower activity under similar conditions. Following reduction in hydrogen, the ruthenium metal crystallites were located within the pore structure of the zeolite microcrystals. Variation of the exchangeable alkali-metal cations and the silicon-to-aluminum ratio altered the activity of the ruthenium catalysts, with RuKX being the most active material. By applying Sanderson′s electronegativity calculations to the ruthenium zeolites, a relationship was found between the turnover frequency for ammonia synthesis and the effective oxygen charge, which was a function of the cation and the Si/Al ratio. Presumably negative charge is transferred from the oxygen to the metal, thereby enhancing the rate of N 2 dissociation. The metal particle size also influenced the activity; larger particles were more active. A RuHY zeolite was the least active among the catalysts studied.