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Lean NOx Traps (LNTs) are one type of lean NOx reduction technology typically used in smaller diesel passenger cars where urea-based Selective Catalytic Reduction (SCR) systems may be difficult to package . However, the performance of lean NOx traps (LNT) at temperatures above 400 C needs to be improved. The use of Rapidly Pulsed Reductants (RPR) is a process in which hydrocarbons are injected in rapid pulses ahead of a LNT in order to expand its operating window to higher temperatures and space velocities. This approach has also been called Di-Air (diesel NOx aftertreatment by adsorbed intermediate reductants) by Toyota. There is a vast parameter space which could be explored to maximize RPR performance and reduce the fuel penalty associated with injecting hydrocarbons. In this study, the mixing uniformity of the injected pulses, the type of reductant, and the concentration of pulsed reductant in the main flow were investigated.

Low-cost lean NOx aftertreatment is one of the main challenges facing high-efficiency gasoline and diesel engines operating with lean mixtures. While there are many candidate technologies, they all offer tradeoffs. We have investigated a multi-component Dual SCR aftertreatment system that is capable of obtaining NOx reduction efficiencies of greater than 90% under lean conditions, without the use of precious metals or urea injection into the exhaust. The Dual SCR approach here uses an Ag HC-SCR catalyst followed by an NH3-SCR catalyst. In bench reactor studies from 150 °C to 500 °C, we have found, for modest C/N ratios, that NOx reacts over the first catalyst to predominantly form nitrogen. In addition, it also forms ammonia in sufficient quantities to react on the second NH3-SCR catalyst to improve system performance. The operational window and the formation of NH3 are improved in the presence of small quantities of hydrogen (0.1–1.0%).

The reactivity of NOx over two prototype catalysts has been measured in a new bench reactor for characterizing plasma–catalyst systems that allows for in–situ post–analysis of any species which may have adsorbed on the catalyst. In these initial studies without a plasma, NO2 was used to mimic the NOx output of a plasma reactor in a blended feedstream that mimics diesel exhaust. The baseline performance of the catalysts was measured as a function of temperature, hydrocarbon concentration, hydrocarbon type, and water content, usually at a space velocity of 29,000 h–1. Performance was assessed in terms of the percent conversion of the incoming NO2 to desirable non–NOx N–containing species. For the better of the two catalysts the conversion without water present peaked in the 30–40% range between 125°C and 175°C using a propene/propane mixture of hydrocarbons in a 10:1 C1:N ratio. Experiments with NO as the NOx component yielded very poor activities.