The effects of aluminum oxide in restructuring iron single crystal surfaces for ammonia synthesis

Abstract

The effect of aluminum oxide and potassium on the ammonia synthesis over the (111), (100), and (110) faces of iron has been investigated. A restructuring of the Fe(110) and Fe(100) surfaces, induced by the presence of aluminum oxide and 20 Torr of water vapor, takes place making the restructured surfaces almost as active as the clean Fe(111) plane in the ammonia synthesis reaction (20 atm reactant pressure of hydrogen and nitrogen). The high activity of the restructured surfaces is maintained for over 4 hr of ammonia synthesis. Without the presence of aluminum oxide, treatment of the Fe(110) and Fe(100) surfaces with 20 Torr of water vapor again produces restructured surfaces which are almost as active as the Fe(111) plane for a short period. However, in this case deactivation of the restructured surfaces into the respective clean, unrestructured surfaces occurs within 1 hr of ammonia synthesis. Restructuring of the Fe(111) with 20 Torr of water vapor produces only a slight decrease in ammonia synthesis activity. The enhancement in rate of the restructured Fe(110) and Fe(100) surfaces, with or without aluminum oxide, might be explained by the formation of active surface orientations for ammonia synthesis (i.e., Fe(111) and Fe(211)), which contain C 7 sites (iron atoms with seven nearest neighbors), during the water vapor treatments. These restructured surfaces are only stable in the ammonia synthesis conditions when aluminum oxide is present. Potassium adsorbed alone or with coadsorbed aluminum oxide exhibits no promotional effects under the water vapor pretreatment conditions used in this study.