X-ray and IR Spectroscopy of Barium-Promoted, Zeolite-Supported Ruthenium Catalysts for Ammonia Synthesis

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 Å. 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 1.91 Å. The combination of infrared and Ba LIII near edge spectroscopy revealed that the occluded barium was predominantly in a carbonate form prior to treatment in dihydrogen. Most of this carbonate decomposed after heating to 773 K in H2. Since incorporation of Ba into the samples did not alter the structure or chemical state of the Ru clusters, the promotional effect of Ba on ammonia synthesis rate may be due to the creation of highly active sites at the promoter−metal interface.