Isotopic transient-kinetic analysis of ammonia synthesis catalysis

The steady state isotropic transient kinetic technique was employed to analyze the catalytic synthesis of ammonia over Fe and Ru based catalysts. Inferences were made regarding the reaction mechanism, the surface coverage in reactive intermediates and the degree of kinetic heterogeneity of the working catalyst surfaces. The roles of Cl and O as catalyst poisons and the nature of alkali promotion were also investigated. The kinetic analyses were performed in a gradientless microreactor of novel design. The commercial triply-promoted Fe catalyst (Haldor Topsoe KMIR) was kinetically non-uniform with respect to the synthesis reaction at 673 K and higher temperatures, total pressure of 204 or 513 kPa, and for H{dollar}sb2{dollar}/N{dollar}sb2{dollar} ratio of 3. N{dollar}sb2{dollar} dissociation was found to be a kinetically significant step and appears to be hydrogen assisted. The surface was covered by no more than one distinguishable pool of reactive intermediates in series. Oxygen selectively poisoned the most highly active sites on KMIR, reduced the degree of the kinetic heterogeneity and attenuated the intrinsic activity of the remaining sites of the surface. The O-poisoning of the reaction over K/Ru/SiO{dollar}sb2{dollar} was more indiscriminate than was the case with KMIR--showing no effect on the intrinsic activity of the remaining sites and no effect on the degree of kinetic heterogeneity. Ru/SiO{dollar}sb2{dollar} sustained a relatively low N-intermediates coverage during ammonia synthesis. Alkali-promotion of Ru led to a substantial increase in the inventory in reactive N-intermediates. Evidence was found for the accentuation of kinetic heterogeneity due to the interaction of K with the ad-layer--K/Ru/SiO{dollar}sb2{dollar} showed a bimodal active-site distribution spectrum as opposed to the unimodal distribution observed over Ru/SiO{dollar}sb2{dollar}. When the Ru/SiO{dollar}sb2{dollar} was doped with chlorine, the result was a kinetically more uniform surface, a diminution of intrinsic activity, and a lowered N-coverage. Cl-poisoning of the Ru-catalyzed reaction appeared to proceed through a site-blocking/electronic modification mechanism.