Isotopic Transient Kinetic Analysis of Cs-Promoted Ru/MgO during Ammonia Synthesis

Steady-state isotopic transient kinetic analysis was used to examine a cesium-promoted Ru/MgO catalyst (1.75 wt% Ru) for ammonia synthesis. For comparison, an unpromoted Ru/SiO2 (22 wt% Ru) catalyst was also studied. Steady-state and isotopic transient measurements were conducted at temperatures between 603 and 673 K with a stoichiometric ratio of N2 and H2 at a total pressure of 3 atm. The fractional surface coverage of nitrogen-containing intermediates (θNHx) based on total H2 chemisorption was found to range from 0.02 to 0.05 on both catalysts under the conditions used in the current study. However, the intrinsic turnover frequency of ammonia synthesis over Cs–Ru/MgO was two orders of magnitude greater than that of the unpromoted Ru/SiO2. The combination of the Cs promoter and MgO support enhanced the intrinsic activity of Ru, presumably by lowering the barrier for dinitrogen dissociation.