Ammonia synthesis over ruthenium supported on solid bases

In this work, ruthenium clusters supported on several solid bases (zeolite X, MCM-41, and magnesia, promoted with alkali and alkaline earth cations) were prepared and used to catalyze ammonia synthesis. On zeolite X, ruthenium exists as small particles, 10-13 A in diameter, which is the size of the zeolite supercage. Particles of a similar size can be synthesized on MgO. A Ru/CsX catalyst that is more active than Ru/KX can be made through careful synthesis to maximize the amount of cesium incorporated. Turnover frequencies over Ru/KX increased with metal particle size, confirming the structure-sensitive nature of ammonia synthesis over ruthenium.; The kinetics of ammonia synthesis were considerably different over ruthenium than what has been reported for industrial iron catalysts. The reaction order in H{dollar}sb2{dollar} is positive over Fe, but was negative over Ru in this study. This change in reaction orders is most likely caused by a much greater coverage of the metal surface by dissociated hydrogen on ruthenium.; Alkaline-earth metals were found to be more effective promoters than alkali metals for ruthenium on zeolite X and MCM-41 supports. For example, at 623 K, turnover frequencies were three times larger over Ru/BaX than over Ru/CsX. Since alkaline-earth metals have larger electronegativities, this promotional effect is unexpected. Calculations of the support electronegativity from the elemental compositions confirmed that BaX was more electronegative than either CsX or KX and that BaMCM-41 was more electronegative than CsMCM-41. Thus, the support electronegativity does not serve as the main determinant of the effectiveness of a promoter. In addition, IR spectroscopy of adsorbed NO indicated that Ru/CsX and Ru/BaX had similar propensities for electron donation to adsorbates. These data undermine the theory that promoters for ammonia synthesis function by donating electrons to Ru which in turn donates electrons to N{dollar}sb2{dollar} antibonding orbitals, thereby enhancing N{dollar}sb2{dollar} dissociation. A possible explanation for the promotional effect of alkaline-earth metals was revealed by IR spectroscopy of adsorbed ammonia. Bronsted acid sites were formed on zeolites with no ruthenium and on Ru/CsX, but not on Ru/BaX. Some researchers believe that potassium functions as a promoter for industrial iron catalysts by neutralizing acidic sites introduced by the incorporation of alumina, and a similar mechanism may account for promotion of zeolites by alkaline-earth metals.