Catalyst systems for selective catalytic reduction + NOx trapping: from fundamental understanding of the standard SCR reaction to practical applications for lean exhaust after-treatment

Abstract

We have developed chemical trapping techniques as a novel tool to assess the nature of unstable reaction intermediates in the standard SCR reaction at low temperatures (120–200 °C). For this purpose, we have conducted transient response experiments over mechanical mixtures of an SCR catalyst (Fe-ZSM-5 or Cu-CHA) and a NO_x storage material (BaO/Al₂O₃), which is able to trap and stabilize highly reactive NO_x species. The results conclusively confirm that NO oxidative activation forms a gaseous intermediate, which acts like a nitrite precursor (e.g. HONO/N₂O₃) and is eventually stored on BaO/Al₂O₃. Such a species is also able to react with ammonia to produce N₂, and is therefore proposed as a key intermediate of the standard SCR mechanism. We have further demonstrated that the capability of chemical trapping mechanical mixtures to capture NO in O₂ at low temperature can also be exploited in practical applications for NO_x emission control during cold start transients of diesel vehicles. In fact, such systems (SCR catalyst + NO_x storage material) are characterized by an intrinsic dual functionality, being able both to store NO_x when urea cannot be injected (e.g. below 170 °C) and to reduce the stored NO_x with ammonia at higher temperatures in a single device. Accordingly, these mixtures have been renamed adsorption + selective catalytic reduction (AdSCR) systems. This review will summarize the main results achieved when implementing mechanical mixtures of NO_x adsorbers and SCR catalysts both for fundamental understanding of the standard SCR mechanism and for abatement of cold start emissions.