Selective Catalytic Reduction for Treating the NO
Emissions from Lean-Burn Gasoline Engines: Performance Assessment
Selective catalytic reduction (SCR) with NH3 provides an attractive alternative to lean NOx traps for controlling the NOx 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, NH3, and O2 on the NOx conversion of an iron/zeolite SCR catalyst. A fresh sample was evaluated on slow temperature ramps with 5% O2 and 250, 500, or 1000 ppm of NO and NH3. The NOx conversion at low temperatures decreased with increasing NO and NH3 concentrations due to kinetic limitations. Conversely, the conversion at high temperatures increased with increasing NO and NH3 concentrations because the portion of NH3 oxidized by O2 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 NH3 concentration was controlled at ten levels ranging from 143 to 1085 ppm. The NH3/NO ratios producing the minimum slip of NH3+NOx were approximately 1.0 or higher for most temperatures and NO concentrations when the space velocity was 25K hr-1 or below, allowing high NOx conversions to be achieved. When the space velocity was 47K hr-1 and higher, kinetic limitations and the need to limit the NH3 slip resulted in optimum NH3/NO ratios less than 1.0, which limited the potential NOx conversion. The NOx conversion dropped with decreasing O2 concentration, particularly at low temperatures, and the conversion was very low when the O2 was removed from the feedgas.