Ammonia synthesis kinetics: Surface chemistry, rate expressions, and kinetic analysis

Abstract

The results of surface science studies of nitrogen adsorption and desorption on iron single crystals are summarized with respect to ammonia synthesis reaction kinetics. Analytical rate expressions for ammonia synthesis on high surface iron catalysts at industrial reaction conditions are presented and compared to the behavior of microkinetic models based on the surface science results. Microkinetic models based on uniform surfaces are successful in extrapolating results from surface science studies at low surface coverages to describe the performance of iron catalysts at industrial ammonia synthesis reaction conditions. We suggest that the value of rate constant for recombination of nitrogen atoms on the surface is a weak function of surface coverage. Furthermore, modest variations in the heat of N2 adsorption are sufficient to achieve fractional ammonia orders that are relatively insensitive to the ammonia pressure.