Selective Catalytic Reduction for Treating the NOx Emissions from Lean-Burn Gasoline Engines: Performance Assessment

Selective catalytic reduction (SCR) with NH 3 provides an attractive alternative to lean NO x traps for controlling the NO x emissions from lean-burn gasoline engines. This paper summarizes a laboratory study to assess the effects of temperature, space velocity, and the concentrations of NO, NH 3 , and O 2 on the NO x conversion of an iron/zeolite SCR catalyst. A fresh sample was evaluated on slow temperature ramps with 5% O 2 and 250, 500, or 1000 ppm of NO and NH 3 . The NO x conversion at low temperatures decreased with increasing NO and NH 3 concentrations due to kinetic limitations. Conversely, the conversion at high temperatures increased with increasing NO and NH 3 concentrations because the portion of NH 3 oxidized by O 2 decreased with increasing NO concentration. Thermally aged samples were evaluated at different steady-state temperatures and at different space velocities where the NO concentration was held constant at 250, 500, or 1000 ppm and the NH 3 concentration was controlled at ten levels ranging from 143 to 1085 ppm. The NH 3 /NO ratios producing the minimum slip of NH 3 +NO x were approximately 1.0 or higher for most temperatures and NO concentrations when the space velocity was 25K hr −1 or below, allowing high NO x conversions to be achieved. When the space velocity was 47K hr −1 and higher, kinetic limitations and the need to limit the NH 3 slip resulted in optimum NH 3 /NO ratios less than 1.0, which limited the potential NO x conversion. The NO x conversion dropped with decreasing O 2 concentration, particularly at low temperatures, and the conversion was very low when the O 2 was removed from the feedgas.