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A series of ruthenium catalysts deposited on graphitized carbon and promoted with barium, caesium or both Ba and Cs have been studied in ammonia synthesis. Under experimental conditions (90 bar, H2:N2=3:1, 400 °C, 10% NH3), the reaction rate over the co-promoted catalyst (9.1 wt.% Ru in Ru+C) was found to be higher than those over singly promoted specimens, the overall effect from Ba + Cs in Ba-Cs-Ru/C being almost as high as the sum of individual effects from Ba (Ba-Ru/C) and Cs (Cs-Ru/C), respectively. The co-promoted Ru catalysts, especially that of high ruthenium loading (23.1 wt.% Ru) were shown to be significantly more active in NH3 synthesis than the conventional fused iron catalyst (KMI, H. Topsoe). The oxygen chemisorption studies have shown that the amounts of O2 taken up by the Cs-containing samples (Cs-Ru/C, Cs-Ba-Ru/C) are considerably larger than those for Ru/C and Ba-Ru/C, thus indicating caesium to be in a highly reduced, most likely zero valent form, when operating under ammonia synthesis conditions. It is suggested that barium ((Ba+O)adlayer) is located on the ruthenium surface and it acts as a structural or electronic promoter. Caesium (Cs0) is suggested to be localised on the carbon surface: the Cs promotion occurs at contact points between Ru and the Cs atoms adsorbed on carbon (“hot ring promotion”—electronic). |
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