E. Ito

Selective reduction of NOx with ammonia over cerium-exchanged mordenite

Abstract Cerium-exchanged sodium-type mordenite (CeNa-MOR) was studied in comparison with a non-redox lanthanide mordenite (LaNa-MOR) and an acid-type mordenite (H-MOR) for nitric oxide reduction with ammonia in the presence of excess oxygen. The activity of CeNa-MOR was found to be high over a wide temperature range of 250–560°C. LaNa-MOR showed a similar nitric oxide conversion profile versus temperature as H-MOR, becoming substantially active only above 400°C. Bronsted acid sites are assumed to be responsible for the catalysis by LaNa-MOR, while over CeNa-MOR, two plausible reaction pathways involving the redox couple (Ce III /Ce IV ) are proposed and discussed.

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.

Cerium-exchanged zeolites grown on metal gauze : A new catalyst system applicable for NOx reduction in mobile engine exhaust

Abstract ZSM-5 and mordenite (MOR), directly grown on a stainless-steel metal gauze support and subsequently exchanged with cerium ions, were applied as a NO reduction catalyst. These supported cerium-zeolite catalysts were found to show turnover frequencies comparable to that of an unsupported cerium-exchanged mordenite. Furthermore, the thermostability of these materials was examined by a calcination-activity test cycle and Ce-ZSM-5 was found to be more stable than Ce-MOR. The present approach to grow zeolite directly on a support offers a new possibility to apply zeolite efficiently on a metal monolith applicable for NO x emission control of mobile sources.

Selective reduction of NOx with ammonia over cerium exchanged zeolite catalysts: Towards a solution for an ammonia slip problem

Abstract Cerium-exchanged ZSM-5 and modenite showed a high NO conversion (> 70%) and a high selectivity to N2 (>97%) at 300–600 °C for NO reduction with ammonia in the presence of oxygen. Ammonia was found to be oxidized by oxygen over these cerium zeolite catalysts exclusively towards N2 without production of N2O and NO. NO reduction with up to 30% excess of ammonia exhibited a high NO conversion and complete conversion of NH3 at 300–500 °C at a gas space velocity of 12,000 h−1. This offers a possible solution for the ammonia slip problem in a selective catalytic reduction (SCR) system with NH3. Strong NH3 adsorption up to 600°C and a high amount of adsorbed reactive NO species in associated with the redox property of cerium (CeIII/CeIV) are assumed to be responsible for the high NO reduction activity of cerium exchanged zeolite catalysts.

Selective catalytic reduction of NOx in diesel exhaust gases with NH3 over Ce & Cu mordenite and V2O5/TiO2/WO3 type catalysts: Can Ce solve the NH3 slip problem?

The performance of two zeolite type catalysts on their NOx-SCR activity and their SO2 oxidative activity is compared to a vanadium type catalyst. Home made CeC selectivity decline above 450°C and SO3 formation above 400°C. However, the Ce mordenite catalyst showed high temperature (>400°C) activity without oxidation of SO2 to SO3 and NH3 to NO; a large excess of NH3 resulted only in a limited NH3 slip.

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.

Catalytic N2O Decomposition in fluidized bed combustion

Publisher Summary This chapter discusses the heterogeneous catalytic N 2 O decomposition into nitrogen (N 2 ) and oxygen (O 2 ) under fluidized bed combustion (FBC) flue gas conditions. Copper (Cu-), cobalt (Co-), and iron (Fe-) exchanged ZSM-5 zeolite and vacancies-containing manganese perovskite that are efficient catalysts for the decomposition of N 2 O into N 2 and O 2 at low temperatures, which can be ascribed to the high dispersion of these metals in the zeolite and to the presence of cation vacancies and mixed valence states of manganese in the perovskite system. The presence of O 2 inhibited the N 2 O decomposition for Cu-ZSM-5 and strongly decreased the N 2 C conversion in the case of Mn-perovsk. catalyst. SulfurSulfurSulfur dioxide (SO 2 ) increased the N 2 O decomposition for Fe-ZSM-5. N 2 O decomposition reaction is described by a two- or three-steps model with a rate-determining step depending on the catalyst system.