Selective Catalytic Oxidation (SCO) of Ammonia to Nitrogen over Fe-Exchanged Zeolites

Abstract

Fe-exchanged zeolites were prepared and studied as catalysts for selective catalytic oxidation (SCO) of ammonia to nitrogen in the presence of excess oxygen. Fe–ZSM-5, Fe–mordenite (MOR), and Fe–clinoptilolite (HEU) showed excellent SCO performance at a very high space velocity (GHSV=2.3×105 h−1). Greater than 97% NH3 conversion and nearly 100% N2 selectivity were obtained. Among different zeolites, the catalytic performance decreased in the sequence of Fe–ZSM-5, Fe–MOR, Fe–HEU > Fe–Y, Fe–Beta > Fe–ferrierite, Fe–chabazite. High iron content, low Si/Al ratio, and low ammonia concentration were favorable for high activities for NH3 oxidation to N2. For the Fe-exchanged zeolites, there existed a good correlation between the N2 selectivity for the SCO reaction and the activity for the SCR (selective catalytic reduction) of NO with ammonia; i.e., the higher the SCR activity, the higher the N2 selectivity in SCO. The variable valence of iron cations might be beneficial to oxygen activation and thus result in the high SCO performance for the Fe-exchanged zeolites. A kinetic study indicated that the reaction was first order with respect to NH3 and the apparent reaction activation energy (Ea) was 88 kJ/mol.