SCR Catalyst Systems Optimized for Lightoff and Steady-State Performance

A laboratory study was performed to optimize a zoned configuration of an iron (Fe) SCR catalyst and a copper (Cu) SCR catalyst in order to provide high NO_x conversion at lean A/F ratios over a broad range of temperature for diesel and lean-burn gasoline applications. With an optimized space velocity of 8,300 hr^-1, a 67% (by volume) Fe section followed by a 33% Cu section provided at least 80% NO_x conversion from approximately 230°C to 640°C when evaluated with 500 ppm NO and NH₃. To improve the lean lightoff performance of the SCR catalyst system during a cold start, a Cu SCR catalyst that was 1/4 as long as the rear Cu SCR catalyst was placed in front of the Fe SCR catalyst. When evaluated with an excess of NH₃ (NH₃/NO ratio of 2.2), the Cu+Fe+Cu SCR system had significantly improved lightoff performance relative to the Fe+Cu SCR system, although the front Cu SCR catalyst did decrease the NO_x conversion at temperatures above 475°C by oxidizing some of the NH₃ to N₂ or NO.