Catalyst systems for selective catalytic reduction + NO: X trapping: From fundamental understanding of the standard SCR reaction to practical applications for lean exhaust after-treatment

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 NOx storage material (BaO/Al2O3), which is able to trap and stabilize highly reactive NOx 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 NOx emission control during cold start transients of diesel vehicles. In fact, such systems (SCR catalyst + NOx storage material) are characterized by an intrinsic dual functionality, being able both to store NOx when urea cannot be injected (e.g. below 170 °C) and to reduce the stored NOx 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 NOx adsorbers and SCR catalysts both for fundamental understanding of the standard SCR mechanism and for abatement of cold start emissions.