Key reaction steps in the simultaneous conversion of nitrous and nitric oxides on Fe-ZSM-5

Higher Fe dispersion lowers the temperature at which N₂O promotes the NO_x conversion and increases the extent of promotion of the NO_x conversion at the same temperature. Analysis of the normalized reaction rates demonstrates that the activity towards the NO-mediated N₂O decomposition reaction is at the base of the promotion of the NO_x conversion and confirms that the oxidative transformation of NO induced by N₂O is a fundamental step of the N₂O-NO-SCR reaction. The catalytic data were supported by operando DRIFTS and modulated excitation, which showed that the catalysts lost oxidation activity because the concentration of Fe atoms able to perform N₂O activation and NO adsorption decreased with an increasing agglomeration of Fe, thus proving the involvement of monomeric Fe sites in these processes.

Our combined catalytic and spectroscopic data allow us to propose that also in Fe-ZSM-5 the Fe sites involved in processes of N₂O activation and NO adsorption (and oxidation) are monomeric Fe²⁺ ions located in β- and γ-cationic exchange positions, respectively. In analogy with the results obtained previously on Fe-FER, the N₂O-NO-SCR reaction requires catalysts possessing homogeneous distribution of these two types of extra- framework Fe²⁺ sites.

Therefore, the speciation and location of the Fe cations within the zeolite framework are critical and need to be controlled to achieve an optimal simultaneous conversion of N₂O and NO.

For more information, the full study can be accessed here.