Ammonia-pretreatment-induced restructuring of iron single-crystal surfaces: Its effects on ammonia synthesis and on coadsorbed aluminum oxide and potassium

Abstract

The effects of ammonia pretreatment on the structure and activity of iron single-crystal surfaces for ammonia synthesis are examined. Treatment of the (110), (100), and (111) planes of iron with 5 or 50 Torr of ammonia (723 K, 30 min) causes surface restructuring as evidenced by changes in ammonia synthesis activity (20 atm total pressure), and in temperature-programmed desorption, and by the appearance of new surfaces as seen by scanning electron microscopy. The (110) plane of iron is inactive for ammonia synthesis under the conditions used in this study, but the ammonia-pretreatment-restructured Fe(110) surface becomes about twice as active as the Fe(100) face. The restructured Fe(100) surface becomes about four times more active than the clean, unrestructured (100) surface. Restructuring the active Fe(111) face in ammonia results in a slight decrease in ammonia synthesis activity. The kinetic data, temperature-programmed desorption, and scanning electron microscopy suggest that Fe(110) and Fe(100) are restructured to surfaces containing C 7 sites, such as Fe(111) and Fe(211). A stereographic analysis of the restructured Fe(111) surface, using scanning electron microscopy, taken together with the reaction rate measurements suggests that {211} surface planes are formed upon ammonia treatment. The high concentrations of near-surface nitrogen that is deposited by the ammonia pretreatment does not block catalytic sites for the synthesis of ammonia. The presence of aluminum oxide on any of the iron single-crystal surfaces inhibits the ammonia-induced restructuring process, while the presence of potassium has no observable effect on the process. These results suggest a method of activating iron with both ammonia and water vapor restructuring in the presence of aluminum oxide and potassium to achieve a catalyst with optimal activity for ammonia synthesis.