Isotopic transient analysis of ammonia synthesis over Ru/MgO catalysts promoted by cesium, barium, or lanthanum

The global and intrinsic kinetics of ammonia synthesis over ruthenium supported on MgO (similar to 2 wt% Ru) and promoted with Cs, Ba, or La were examined at 3 atm and various dihydrogen pressures with steady-state and isotopic transient measurements. Steady-state, global measurements revealed that Cs-Ru/MgO was strongly inhibited by dihydrogen whereas Ba-Ru/MgO and La-Ru/MgO were weakly inhibited by the reactant. However, the isotopic transient measurements showed that the intrinsic turnover frequency was a weak positive function of dihydrogen pressure regardless of the added base. Promotion by these compounds appeared to be electronic in nature because their presence increased the intrinsic turnover frequency of an active site, with Cs being the most effective. However, the coverage of nitrogen-containing intermediates was greater on Ba- and La-promoted catalysts than on Cs-Ru/MgO at 673 K and stoichiometric conditions. Therefore, the global activity of a promoted catalyst is a competition between inhibition by dihydrogen and enhancement of dinitrogen dissociation. Furthermore, promotion with Cs, Ba, and La introduced a new and highly active class of sites to the Ru/MgO system. (C) 2003 Elsevier Inc.