On the rate enhancement of ammonia synthesis over iron single crystals by coadsorption of aluminum oxide with potassium

Abstract

The behavior of doubly promoted iron catalysts utilized for ammonia synthesis is modelled by the coadsorption of aluminum oxide and potassium on iron single crystal surfaces that were employed in high pressure reaction rate studies. The promoter effect of aluminum oxide is due to its interaction with iron oxide during the preparation stage of the industrial catalyst. After reduction, aluminum oxide stabilizes the most active Fe(111) and Fe(211) crystal surfaces. Potassium does not appear to be involved in the structural promotion but its presence on the active iron surfaces increases the rate of dinitrogen dissociation mostly by lowering the concentration of adsorbed ammonia thus, making more catalytic sites available for dinitrogen dissociation. Co-adsorbed potassium and alumina form a potassium aluminate compound that a) inhibits the aluminum oxide induced restructuring of iron and b) covers up the active iron sites for ammonia synthesis.