Acidic Zirconia Mixed Oxides for NH
-SCR Catalysts for PC and HD Applications
The reduction of NOx using NH₃ has been successfully practiced for stationary sources and more recently, for passenger cars and heavy-duty applications in the US and Europe. Due to the up-coming Euro 6 regulations (2014) for passenger cars and light-duty trucks, NH₃-SCR catalyst technologies are extensively studied by OEMs, catalysts manufacturers and raw materials suppliers, all looking for technologies very efficient at low temperatures, thermally stable up to 800°C and with a limited impact of the NO₂/NOx ratio on activity. Among the new emerging SCR catalytic systems, non-zeolitic zirconia-based mixed oxides were introduced recently in SAE 2008 as promising new SCR catalysts. This paper reports the progress made on these materials. "Synthetic Gas Bench Tests" performed on powder catalysts show a significant increase of the NOx conversion, in comparison with that previously reported. This behavior was confirmed also by laboratory tests with catalyst-coated cordierite monoliths. 50% NO conversion was attained at 250°C by the standard SCR process. By applying the fast SCR conditions (NO₂/NOx = 50%) 97% of NOx were reduced to N₂ at only 200°C. The effect of several factors such as ageing temperature, NO₂/NOx ratio, NH₃/NOx ratio (i.e., α value) as well as the impact of SO₂, unburned hydrocarbons, CO and H₂ on the NOx conversion were also investigated. Preliminary results on the testing of a full-size SCR prototype at an engine bench will be discussed as well. The outcome confirmed the potential of these materials for the formulation of Euro 6 SCR catalysts.