Rapidly Pulsed Reductants for Diesel NO_x Reduction With Lean NO_x Traps: Comparison of Alkanes and Alkenes as the Reducing Agent

Lean NO_x traps (LNTs) are often used to reduce NO_x on smaller diesel passenger cars where urea-based selective catalytic reduction (SCR) systems may be difficult to package. However, the performance of LNTs at temperatures above 400 °C needs to be improved. Rapidly pulsed reductants (RPR) is a process in which hydrocarbons are injected in rapid pulses ahead of the LNT in order to improve its performance at higher temperatures and space velocities. This approach was developed by Toyota and was originally called Di-Air (diesel NO_x aftertreatment by adsorbed intermediate reductants) (Bisaiji et al., 2011, “Development of Di-Air—A New Diesel deNO_x System by Adsorbed Intermediate Reductants,” SAE Int. J. Fuels Lubr., 5(1), pp. 380–388). Four important parameters were identified to maximize NO_x conversion while minimizing fuel penalty associated with hydrocarbon injections in RPR operation: (1) flow field and reductant mixing uniformity, (2) pulsing parameters including the pulse frequency, duty cycle, and magnitude, (3) reductant type, and (4) catalyst composition, including the type and loading of precious metal and NO_x storage material, and the amount of oxygen storage capacity (OSC). In this study, RPR performance was assessed between 150 °C and 650 °C with several reductants including dodecane, propane, ethylene, propylene, H₂, and CO. Under RPR conditions, H₂, CO, C₁₂H₂₆, and C₂H₄ provided approximately 80% NO_x conversion at 500 °C; however, at 600 °C the conversions were significantly lower. The NO_x conversion with C₃H₈ was low across the entire temperature range. In contrast, C₃H₆ provided greater than 90% NO_x conversion over a broad range of 280–630 °C. This suggested that the high-temperature NO_x conversion with RPR improves as the reactivity of the hydrocarbon increases.