Unexpected Low-Temperature deNOx Activity of AdSCR Systems for Cold Start NOx Abatement

Low-temperature operation of urea selective catalytic reduction (SCR) aftertreatment systems for the abatement of NOx from diesel engines presents new challenges related to poor catalytic activity and the urea injection temperature threshold. Physical mixtures of a NH3-SCR catalyst (e.g., Fe-/Cu-zeolite) and of a NOx storage material (e.g., BaO/Al2O3, CeO2/Al2O3) have shown promising performances in terms of overall NOx removal efficiency, as they can operate both as NOx adsorbers, trapping NOx during the cold start transient, and as SCR catalysts, reducing at higher temperatures the previously stored NOx with NH3. Herein, we extend the investigation of new Cu-CHA + BaO/Al2O3 AdSCR systems (AdSCR = adsorption + selective catalytic reduction) focusing on the reactivity between NOx and ammonia in the low-temperature window (from room temperature up to 170 °C). We find that the selective reduction of NO by NH3 over Cu-CHA is surprisingly enhanced by the presence of BaO/Al2O3 when ammonia is preadsorbed, leading to the onset of nitrogen formation already at 40 °C.