Ammonia blocking of the “Fast SCR” reactivity over a commercial Fe-zeolite catalyst for Diesel exhaust aftertreatment

The ammonia blocking effect on the Fast SCR catalytic mechanism at low temperature has been studied by means of dedicated transient reactivity runs performed over a state-of-the-art commercial Fe-zeolite catalyst. We show that the reduction of surface nitrates by NO is the key step in the mechanism, and is active already at 50 °C. However, in the presence of ammonia the reaction between NO and nitrates is stopped, and proceeds only on raising the temperature up to 140–160 °C, which thus represents an intrinsic lower bound to the Fast SCR activity. Evidence is provided that such a blocking effect is associated with a strong interaction between ammonia and surface nitrates, which prevents nitrates from reacting with NO: only upon increasing the temperature or decreasing the NH3 concentration nitrates are released due to dissociation of the ammonia–nitrate complex. The present data thus provide evidence that the blocking effect of NH3 on the Fast SCR activity at low temperature occurs not because of the ammonia competitive chemisorption on the catalytic sites, but because ammonia captures a key intermediate in an unreactive form.