Copper exchanged ZSM-5 zeolite was extensively investigated in order to highlight the knowledge of the singular features of copper sites in this catalytic system and to improve its hydrothermal stability. Concerning the first point the mutual interaction between NO and N2O on Cu-ZSM5 has been studied under experimental conditions determining the occurrence of both decomposition reactions. The oscillating behaviour observed in N2O decomposition is strongly influenced by the presence of NO, as the latter quenches the oscillations stabilising the highest active state of the catalyst. A reaction mechanism was proposed to explain the experimental behaviour, based on the assumption that NO reduces the catalyst sites from Cu+2 to Cu+, via the formation of adsorbed nitro-species and gaseous NO2. The role of both nitro-species and nitrates in the desorption of oxygen from the catalyst under reaction conditions was subsequently investigate by means of adsorption and temperature programmed desorption of NO. During NO ad¬sorption the unsteady-state forma¬tion of gaseous N2O on He pre-treated zeolite and gaseous NO2 on the oxidised sample has been observed. These phenomena were associated respec¬tively to the re-¬oxidation from Cu+ to Cu+2, or to the reduction from Cu+2 to Cu+. Finally, the simulta¬neous desorption of both NO and O2 in 1:1 ratio at 400°C during TPD in He flow has been related to the nitrate species decomposition. The amount of this species is strongly af¬fected by the time of exposition during NO adsorption while both the pre-treatment and the NO inlet concentration play a less significant role. The second purpose of this work was to study preparation methods of rare earth (RE) containing Cu-ZSM5 catalysts in order to increase their stability in the presence of steam. Two different kinds of catalyst have been synthesised: CuRE-Z (RE=La, Sm or Ce) samples have been prepared by simultaneous ion-exchanging; RECu-Z samples have been obtained by exchanging into two steps the rare earth cation and then the copper ion. All rare earth containing catalysts are more active than the corresponding Cu-ZSM5. Moreover, the preparation method seems to play a significant role: the samples synthesised by a two steps ion-exchange (RECu-Z) exhibiting the best performances. After steam ageing treatment the activity of all catalysts decreased, quite completely for Cu-ZSM5, only weakly for rare earth containing samples due to the high hydrothermal resistance induced by RE cations. The modifications inferred to catalysts by the ageing treatments were characterised by means of BET analysis and NO adsorption. The loss of activity was mainly attributed to a lower reducibility of copper sites in the aged catalysts since the values of BET surface area remain quite unchanged.
Processi catalitici innovativi per l'abbattimento dell'ossido di azoto / Russo, Gennaro. - (2001).
Processi catalitici innovativi per l'abbattimento dell'ossido di azoto
RUSSO, GENNARO
2001
Abstract
Copper exchanged ZSM-5 zeolite was extensively investigated in order to highlight the knowledge of the singular features of copper sites in this catalytic system and to improve its hydrothermal stability. Concerning the first point the mutual interaction between NO and N2O on Cu-ZSM5 has been studied under experimental conditions determining the occurrence of both decomposition reactions. The oscillating behaviour observed in N2O decomposition is strongly influenced by the presence of NO, as the latter quenches the oscillations stabilising the highest active state of the catalyst. A reaction mechanism was proposed to explain the experimental behaviour, based on the assumption that NO reduces the catalyst sites from Cu+2 to Cu+, via the formation of adsorbed nitro-species and gaseous NO2. The role of both nitro-species and nitrates in the desorption of oxygen from the catalyst under reaction conditions was subsequently investigate by means of adsorption and temperature programmed desorption of NO. During NO ad¬sorption the unsteady-state forma¬tion of gaseous N2O on He pre-treated zeolite and gaseous NO2 on the oxidised sample has been observed. These phenomena were associated respec¬tively to the re-¬oxidation from Cu+ to Cu+2, or to the reduction from Cu+2 to Cu+. Finally, the simulta¬neous desorption of both NO and O2 in 1:1 ratio at 400°C during TPD in He flow has been related to the nitrate species decomposition. The amount of this species is strongly af¬fected by the time of exposition during NO adsorption while both the pre-treatment and the NO inlet concentration play a less significant role. The second purpose of this work was to study preparation methods of rare earth (RE) containing Cu-ZSM5 catalysts in order to increase their stability in the presence of steam. Two different kinds of catalyst have been synthesised: CuRE-Z (RE=La, Sm or Ce) samples have been prepared by simultaneous ion-exchanging; RECu-Z samples have been obtained by exchanging into two steps the rare earth cation and then the copper ion. All rare earth containing catalysts are more active than the corresponding Cu-ZSM5. Moreover, the preparation method seems to play a significant role: the samples synthesised by a two steps ion-exchange (RECu-Z) exhibiting the best performances. After steam ageing treatment the activity of all catalysts decreased, quite completely for Cu-ZSM5, only weakly for rare earth containing samples due to the high hydrothermal resistance induced by RE cations. The modifications inferred to catalysts by the ageing treatments were characterised by means of BET analysis and NO adsorption. The loss of activity was mainly attributed to a lower reducibility of copper sites in the aged catalysts since the values of BET surface area remain quite unchanged.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
