Activation of nitrogen by alkali metal promoted transition metal I. Ammonia synthesis over ruthenium promoted by alkali metal

Abstract

The rate of ammonia synthesis over ruthenium was found to be remarkably promoted by addition of alkali metal, particularly when ruthenium is supported by active carbon (AC) or alumina. The synthesis rate over the 5% RuACK catalyst can be raised to about 10 times that of conventional doubly promoted iron (Fe Al 2O 3K 2O) catalyst at 250 °C. RuACCs gives even greater activity than RuACK. The catalytic activity increases with decrease in ionization potential of added alkali (Cs > K > Na) and with increase in the added amount of alkali metal, approaching a plateau value at around 3–4 mg-atom alkali/g-catalyst. Unsupported ruthenium is also remarkably promoted by addition of alkali metal. It is accordingly concluded that the promoter action is provided by a charge transfer from alkali metal to transition metal which brings about a higher electron density in the transition metal, a favorable state for the activation of nitrogen. The specific synthesis rates per surface ruthenium are higher over active carbon and alumina than other supports, suggesting a role of these supports as a medium of electron transfer.