Mechanism of propene poisoning on Fe-ZSM-5 for selective catalytic reduction of NO_x with ammonia

Application of Fe-zeolites for urea-SCR of NO_x in diesel engine is limited by catalyst deactivation with hydrocarbons. In this work, we investigated the effect of propene on the activity of Fe-ZSM-5 for selective catalytic reduction of NO_x with ammonia (NH₃-SCR), and proposed a deactivation mechanism of Fe³⁺ active site blockage by propene residue. The NO conversion decreased in the presence of propene at various temperatures, while the effect was not significant when NO was replaced by NO₂ in the feed, especially at low temperatures (<300 °C). The surface area and pore volume were decreased due to carbonaceous deposition. The site blockage was mainly on Fe³⁺ sites on which NO was to be oxidized to NO₂. The activity for NO oxidation to NO₂ was significantly inhibited on a propene poisoned catalyst below 400 °C. The adsorption of NH₃ on the Brönsted acid sites to form NH ₄⁺ was not hindered even on the propene poisoned catalyst and the amount of absorbed NH₃ was still abundant and enough to react with NO₂ to generate N₂. The hydrocarbon oxygenates such as formate, acetate and containing nitrogen organic compounds were observed on catalyst surface, however, no graphitic carbonaceous deposit was formed.