Acidic Zirconia Mixed Oxides for NH ₃ -SCR Catalysts for PC and HD Applications 2011-01-1327

The reduction of NO_x 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/NO_x 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 NO_x 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/NO_x = 50%) 97% of NO_x were reduced to N₂ at only 200°C. The effect of several factors such as ageing temperature, NO₂/NO_x ratio, NH₃/NO_x ratio (i.e., α value) as well as the impact of SO₂, unburned hydrocarbons, CO and H₂ on the NO_x 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.