Kinetic simulation of ammonia synthesis catalysis

Abstract

Kinetic simulation studies were carried out and showed that kinetic and spectroscopic data collected on iron single crystals under ultrahigh vacuum conditions can be used to predict the rate of ammonia synthesis over a commercial iron catalyst in a plug-flow reactor operating under industrial reaction conditions (i.e., 720 K, 107 atm). The results agree with earlier calculations by P. Stoltze and J. K. Norskov [Surf. Sci. Lett. 197, L230 (1988); Phys. Rev. Lett. 55, 2502 (1985); Phys. Scr. 36, 824 (1987); J. Catal. 110, 1 (1988)]. Furthermore, the kinetic simulations confirm the fact that the dissociation of molecular dinitrogen precursor species is a slow step in the mechanism; however, other steps may also become slow depending on reaction conditions. Atomic nitrogen appears to be the most abundant reactive intermediate on the iron surface, except at the reactor inlet, where a variety of adsorbed species may compete for surface sites. Finally, the kinetic simulation results are also shown to describe the performance of iron catalysts in laboratory reactors operating at pressures from 1 to 20 atm.