Importance of Product Readsorption during Isotopic Transient Analysis of Ammonia Synthesis on Ba-Promoted Ru/BaX Catalyst

Steady-state isotopic transient kinetic analysis was used to evaluate the intrinsic turnover frequency (TOFintr) and the fractional surface coverage of nitrogen-containing species (θNHx) on a Ba-promoted Ru/BaX zeolite (∼2 wt% Ru) catalyst during ammonia synthesis at 3 atm, 673 K, and a stoichiometric ratio of N2 and H2. The artifacts due to inter- and intraparticle readsorption of product ammonia on the kinetic parameters were removed by cofeeding ammonia to the system and varying the total flow rate. The values of TOFintr and θNHx determined from the intrinsic lifetime of surface intermediates at zero ammonia pressure were 0.067 s−1 and 0.064, respectively. The zeolite-supported catalyst was compared to poorly active Ru/SiO2 and highly active Cs–Ru/MgO catalysts. Although the TOFintr spanned two orders of magnitude, the value of θNHx was fairly independent of support and/or promoter.