Y.J. Mergler

Infrared studies of NO adsorption and co-adsorption of NO and O2 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 2 O (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 2 O and N 2 O 3 and the subsequent ionization of N 2 O 3 towards NO + and NO 2 − . This scheme is supported by the transient observation of molecular N 2 O 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 2 O 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.

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

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.

Comparison of Pt/MnOx/SiO2 and Pt/CoOx/SiO2 catalysts for the CO oxidation with O2 and the no reduction with CO

Abstract Pt/MnO x /SiO 2 and Pt/CoO x /SiO 2 catalysts are efficient catalysts for the reduction of NO with CO and the oxidation of CO with O 2 at low temperature, with a much lower on-set temperature for the reactions compared with Pt/SiO 2 . An oxidative pretreatment lead to a shift to higher light-off temperatures for O 2 and NO conversion. For a pure CoO x /SiO 2 catalyst primarily N 2 O was formed following an oxidation step. Over Pt/CoO x /SiO 2 CO is already oxidised at room temperature. Three possible models to account for the high activity of Pt/CoOP x /SiO 2 in the CO/O 2 reaction are presented and discussed. The highest NO conversion was reached by using the Pt/MnO x /SiO 2 catalyst. It was found that N 2 O is an important intermediate compound in CO/NO reactions over the Pt/MnO x /SiO 2 catalyst. The performance of Pt/CoO x /SiO 2 in CO/NO reactions is only slightly better than Pt/SiO 2 .

Reduction of NO by H2 and CO on Pt/TiO2/SiO2

Different catalysts based on platinum and a silica, titania or mixed titania/silica support were studied in NO reduction reactions by CO and H2 in the temperature range of 25–400‡C. The mixed oxide catalysts showed considerably lower onset temperatures in NO/CO reactions but this coincided with a maximum in N2O formation. In NO/H2 reactions all titania containing catalysts produced more N2O than silica supported platinum at low temperatures but were more selective to N2 at high temperatures.

NOx reduction with ammonia over cerium exchanged mordenite in the presence of oxygen. An ir mechanistic study

Summary An IR mechanistic study of NO reduction with ammonia in the presence of oxygen was performed over cerium-exchanged mordenite (CeNa-MOR) and compared with lanthanum-exchanged mordenite (LaNa-MOR), which does not possess redox properties. The formation of NO x − (x = 2 or 3) species was observed upon NO adsorption, which was obviously enhanced in the presence of oxygen over both mordenites. These NO x − species were found to be reactive towards ammonia over cerium already at 300°C, while the same species remained unreacted over lanthanum. The importance of redox properties in this reaction is indicated.