Aftertreatment System for NOx and Soot Removal - Evaluation of an Integrated System

The two major problems of diesel emission control are the reduction of nitrogen oxides and particulates. This paper describes experimental investigations to achieve both a separation of soot particles as well as a catalytic NO_x reduction with hydrocarbons under lean diesel exhaust gas conditions. For that purpose a diesel particle trap is coated with a catalyst based on a Pt containing zeolite. Preliminary studies have been performed on the catalytic NO_x reduction to evaluate the efficiency of a Pt/zeolite system as well as to establish the impact of operation conditions on the catalyst performance. The activity of the prepared samples (catalytic coating on particle trap) has been determined under model gas test conditions. Much attention has been focussed on the steady-state kinetics of the surface processes. Another aspect considered is the N₂O formation which can be reduced, when alkali-earth or rare-earth oxides are added to the catalyst system. Additionally, the prepared samples have been tested on an engine test bench under steady state conditions and finally by vehicle tests. A fuel additive containing an organic-sodium compound is used to regenerate the particulate trap. The examined Pt/zeolite system on a particle trap exhibits NO_x conversions up to 60 % under model gas test conditions. Furthermore, the addition of CeO₂ to the catalyst reduces the N₂O formation. The engine test bench experiments result in a maximum NO_x conversion of about 30 %, whereas during the vehicle dynamometer tests only NO_x conversions up to 10 % could be attained.