Iron single crystals as ammonia synthesis catalysts: Effect of surface structure on catalyst activity

The activity of three iron single-crystal faces with respect to their ability to catalyze ammonia synthesis was investigated. At 798 K and a total pressure of 20 atm of a stoichiometric mixture of hydrogen and nitrogen, the relative rates of ammonia formation were found to be 418:25:1 for Fe(111), Fe(100), and Fe(110) samples, respectively. The activation energy for ammonia synthesis on the Fe(111) surface was determined as 19.4 kcal mole−1. Significant quantities of nitrogen were found to be present in the bulk and on the surface of the iron samples after catalytic runs. This is the first study in which ammonia synthesis activity has been directly correlated with surface crystallographic structure under well-defined, high-pressure conditions.