Abstract
Several different zeolite-supported Ru catalysts for ammonia synthesis were characterized by X-ray and infrared absorption spectroscopy. The samples were composed of 1−2 wt % Ru supported on either KX or BaX zeolites. In one case, barium was added to Ru/BaX in excess of ion-exchange capacity. For comparison, cesium-promoted Ru/MgO was also examined. The turnover frequency for ammonia synthesis at 20.7 atm over zeolite-supported Ru increased by over an order of magnitude after adding Ba to the catalysts and approached that of the highly active Cs-promoted MgO catalyst. Analysis of Ru K edge EXAFS recorded at low temperature in H2 indicated that the zeolite-supported Ru clusters were about 1 nm in diameter, regardless of barium loading. The average Ru−Ru coordination number was 4.8 at an interatomic distance of 2.61 A. Adding barium to Ru/BaX beyond ion-exchange capacity caused new features to appear in the Ru EXAFS that were attributed to the interaction of oxygen atoms with Ru clusters at a distance of 1....
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