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 NOx species (NO+, NO3– and NO2–) resulted in a substantial reduction of the NO+ band, while the NOx– 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.