Infrared studies of NO adsorption and co-adsorption of NO and O 2 onto cerium-exchanged mordenite (CeNaMOR)

Abstract

Infrared (IR) studies on NO adsorption and co-adsorption of NO and O 2 onto cerium- and lanthanum-exchanged mordenite (CeNaMOR and LaNaMOR, respectively) were performed in order to elucidate the role of redox properties of cerium ( Ce III Ce IV ) in the oxidation of NO to NO 2, an important preliminary step in the NO reduction catalysis. NO adsorption onto both CeNaMOR and LaNaMOR leads to the formation of N 2O (2247 cm −1), NO + (nitrosonium ion; 2162 cm −1) and NO x − species ( nitrito or/and nitrato, 1300–1500 cm −1). These are thought to arise from disproportionation of NO towards N 2O and N 2O 3 and the subsequent ionization of N 2O 3 towards NO + and NO 2 −. This scheme is supported by the transient observation of molecular N 2O 3. The co-adsorption of NO and O 2 onto CeNaMOR and LaNaMOR resulted in the enhanced formation of NO + and NO 3 − (1515, 1488–1497 and 1333 cm −1), which is accounted for by the formation of NO 2 and its subsequent ionization via N 2O 4 towards NO + and NO 3 −. Combining IR and NO temperature-programmed desorption (TPD) data, it is proposed that the formation of NO + is associated with zeolite acid sites, and that NO 3 − ( nitrato) species are coordinated to lanthanide cations. Furthermore, the NO + and NO x − species were found to desorb more easily from CeNaMOR than from LaNaMOR. The redox properties of cerium ( Ce III Ce IV ) may contribute to the easier desorption of these oxidized NO species.