NOx reduction by urea in the presence of NO2 on metal substrated SCR catalysts for heavy-duty vehicles

The emission limitations for heavy-duty vehicles are coming stricter between 2005 - 2008 in Europe, Japan and United States. In addition to engine, fuel and control modifications, efficient exhaust gas after treatments like oxidation/deNO_x catalysts and particulate filters are needed. In mobile truck applications the system should operate at low (<300°C) and stand high temperatures (500-650°C) in transient driving conditions. Coated V₂O₅/TiO₂-WO₃ based SCR catalysts on thin metal foil substrates have been studied here in laboratory and engine experiments. The open-coating method enables the high volumetric amount of SCR catalyst evenly coated on high cell density substrates (e.g. 600 cpsi). A new washcoat composition with platinum loading has been used in pre-oxidation catalyst to reach the NO₂ concentrations, which initiate the SCR reaction clearly below 300°C. Dip-coated new catalyst structures with lower cell densities and mixing channel shapes improved the efficiencies in mass transfer controlled region. The coated mixer structure confirmed uniform urea hydrolysis and ammonia generation particularly at low temperatures. The NH₃ slip was cut down by the optimization of urea injection and a small post-oxidation catalyst. The combinations of different metallic catalysts match well into modular truck silencer structures.