Effect of basic promoters on the kinetics of ruthenium-based ammonia synthesis catalysts.

摘要

The second-generation ammonia synthesis catalyst is thought to be a ruthenium-based material. Supported ruthenium catalysts for the production of ammonia were commercialized in the 1990's. Harsh industrial operating conditions typically associated with the process utilizing the traditional iron catalyst have been reduced with the incorporation of a ruthenium catalyst, resulting in substantial energy savings.; Basic promoters are known to substantially increase the activity of the supported Ru system. Unfortunately, Ru catalysts are often strongly inhibited by dihydrogen, with the order of reaction sometimes approaching −1, suggesting that the catalysts should be operated at the less thermodynamically favorable non-stoichiometric conditions. A supported Ru catalyst that is less inhibited by H₂ is therefore highly desirable. Lanthanides used both as supports and promoters have been shown to decrease the inhibition by dihydrogen.; The main goal of this work was to determine why basic promoters affect the activity of Ru catalysts and the dependence of the rate on dihydrogen concentration. As of this writing, it is not clear whether or not the role of promoters is electronic or structural in nature.; Magnesium oxide was chosen as the support for a majority of the studies due its stability and lack of microporosity. Steady state isotopic transient analysis and microkinetic modeling revealed that the intrinsically active cesium-promoted Ru/MgO exhibited the lowest activation energy for N₂ dissociation but a rather high enthalpy of H₂ adsorption. Promotion with Ba or La resulted in a lower enthalpy of H₂ adsorption (less inhibition by H₂) and a higher activation barrier for N ₂ dissociation. Therefore, the electronic promotion of the surface that results in more effective dissociation of N₂ also causes a stronger interaction with hydrogen. The activity of base promoted Ru/MgO system involves a competitive adsorption of N₂ and H₂.