Correlation between the structural features and intrinsic activity trend of Fe surfaces for ammonia synthesis

Abstract

The Haber–Bosch process, which was developed more than a century ago, remains the primary method for nitrogen fixation on a large scale and Fe is typically the main catalyst used in the process. Despite having been extensively studied, some anomalies regarding the activity trend across various Fe surfaces still exist. To understand the intrinsic activity trend of Fe catalysts, we utilize density functional theory (DFT) to calculate the reaction energetics on various Fe surfaces in conjunction with microkinetic analyses to examine the activity of the Fe surfaces. The catalytic activity order obtained is Fe(111) > Fe(211) > Fe(210) > Fe(100) > Fe(110). We find that the activity trend is correlated to the barrier of the rate-determining step, N₂ dissociative adsorption barrier, and perhaps more importantly to the surface energies. It is also noted that the association barriers of flat surfaces are generally larger than those of stepped surfaces, for which a clear explanation is provided.