Selective Catalytic Reduction for Treating the NO _x Emissions from Lean-Burn Gasoline Engines: Performance Assessment

Selective catalytic reduction (SCR) with NH₃ 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, NH₃, and O₂ on the NO_x 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 NO_x 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+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 NH₃ slip resulted in optimum NH₃/NO ratios less than 1.0, which limited the potential NO_x conversion. The NO_x conversion dropped with decreasing O₂ concentration, particularly at low temperatures, and the conversion was very low when the O₂ was removed from the feedgas.