Dual SCR Aftertreatment for Lean NOx Reduction

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 response of the system to other factors such as exhaust oxygen content and space velocity has been explored. The system does not appear to be poisoned by heavy hydrocarbons. It is shown that this approach to lean NOx reduction is well matched to low-temperature diesel combustion, and is a potential enabler to satisfy Tier2/Bin5 and Tier2/Bin2 emissions standards.