Oxidation and selective reduction of NO over Fe-ZSM-5 – How related are these reactions?

Abstract

Fe-ZSM-5 catalysts were prepared by different techniques, including some with additional inert cations such as Na+ or Ca2+ blocking between 25% and 80% of the exchange capacity of the zeolite. Their catalytic behavior in NO oxidation, standard SCR, and fast SCR was studied, with their site structure in different catalyst states investigated by UV–vis and EPR spectroscopy. Their activity for oxidation of NO to NO2 was greatly boosted by previous contact with a feed containing a reductant, e.g. NH3, at elevated temperatures. Therefore, NO2 formation rates measured after mere calcination of freshly prepared samples are irrelevant for mechanistic discussions related to NOx abatement reactions. The rates of NO2 formation and standard SCR were demonstrated to be uncorrelated over a wide range of catalysts and reaction conditions. Depending on catalyst and reaction conditions, the rate of NO2 formation exceeded, equaled or fell short of the rate of standard SCR. Our results strongly suggest that NO2 formation is inhibited by NH3 in the reaction environment of standard SCR. As a result, NO2 formation is slower than standard SCR under many different reaction conditions, and therefore, it cannot be a part of the reaction mechanism of standard SCR. Our results favor earlier mechanistic concepts of standard SCR being initiated by oxidation of NO to nitrite, while oxidation to NO2 seems to require specific sites.