IR mechanistic studies on NO reduction with NH3 in the presence of oxygen over cerium-exchanged mordenite

Abstract

An in situ IR study on NO reduction with ammonia in the presence of oxygen has been performed with cerium-exchanged mordenite (CeNaMOR) in order to examine the reaction intermediates involved in the reaction. Co-adsorption of NO and O₂ on fresh CeNaMOR resulted in the formation of NO⁺(nitrosonium ion: 2161 cm^–1), NO₂^–(nitrito: 1416 cm^–1) and NO₃^–(nitrato: 1510, 1487 and 1337 cm^–1), while NH₃ adsorption on fresh CeNaMOR led to the formation of NH₄⁺(1470, 1730, 2500–3500 cm^–1) and coordinatively bonded NH₃ species (1603, 2500–3500 cm^–1). Under a flow of the three reactants (NO, NH₃ and O₂) as the case for the selective catalytic reduction (SCR) reaction in practice, the adsorbed ammonia species turned out to be dominantly present on CeNaMOR. The admission of NO and O₂ over NH₃-preadsorbed CeNaMOR at 100 °C led to the preferred disappearance of coordinatively bonded NH₃ species, prior to NH₄⁺, and at the same time, the appearance of the bands due to water and NO₂^– was observed. The admission of NH₃ at 100 °C over CeNaMOR containing preadsorbed NO_x species (NO⁺, NO₃^– and NO₂^–) resulted in a substantial reduction of the NO⁺ band, while the NO_x^– species were found to disappear simultaneously with NH₄⁺ only at and above 300 °C, leaving water as a product. Based on these results, a nitrosation reaction scheme is proposed, where NO⁺ and NH₃ react easily, and the less reactive adsorbed species, NO₂^– and NH₄⁺, react with each other only at a higher temperature. The proposed scheme can agree with the reaction stoichiometry and reaction orders known for NO reduction with NH₃ over CeNaMOR.