Aging of industrial Fe-zeolite based catalysts for nitrous oxide abatement in nitric acid production plants

The influence of hydrothermal aging on the structural and catalytic properties of two commercial Fe-exchanged zeolite catalysts (Fe-FER and Fe-ZSM-5) in the extruded form employed for the removal of N2O from the exhaust of nitric acid production plants has been investigated. Catalytic experiments showed that N2O conversion was retained on Fe-FER while it was totally lost on Fe-ZSM-5, which promoted N2O formation under reaction conditions of selective catalytic reduction of NOx with NH3 (SCR). Combining data from several characterization techniques, we were able to identify a unique difference in the aging behaviour of the two catalysts. Irrespective of the catalyst, Al-27 and Si-29-MAS NMR spectroscopy demonstrated significant loss of framework Al. NH3-TPD experiments corroborated the NMR results indicating a remarkable loss of NH3 storage capacity, especially at the expense of the Bronsted acidity. In contrast, UV-vis and X-ray absorption spectroscopy showed that the two catalysts experienced opposite behaviour with respect to the evolution of Fe species upon aging. Fe-ZSM-5 suffered from Fe agglomeration, while Fe species experienced re-dispersion in Fe-FER. These results indicate that the aging treatment of the Fe-exchanged zeolite catalysts does not inevitably lead to the agglomeration of isolated Fe sites. Hence, Fe-ZSM-5 develops into a less selective catalyst, promoting N2O formation through the undesired nonselective NH3 oxidation. Differently, this Fe-FER retains its catalytic activity and becomes less active towards NH3 oxidation. The results of our characterization and catalytic activity study could be of relevance for the selection of Fe-zeolite-based catalysts for this type of application. Furthermore, our investigation provides evidence that the fate of extra-framework Fe species under hydrothermal aging cannot be thoroughly described just by an agglomeration phenomenon.