Synthetic gas bench study of a 4-way catalytic converter: Catalytic oxidation, NOx storage/reduction

Patrick  Da Costa, Université Pierre et Marie Curie

The so-called 4-way catalytic converter (4WCC) has the ability to simultaneously convert CO, HC, NOx and particulate matter on a single support. It allows diesel vehicles to obey to increasingly stringent emission regulations while at the same time decreasing the space needed by the exhaust aftertreatment system.  It is combined with fine engine control strategies so as to ensure conversion of all pollutants. It is hence associated with a large number of catalytic reactions which interact with each other and compete for active sites. The behavior of a commercial 4WCC was characterized on a synthetic gas bench. Gas composition, temperatures and gas hourly space velocity were chosen close to real engine operating conditions. Samples were loaded with soot from differents fuels (Gasoil, B30 and Fisher Tropsch based one)on an engine bench test.  Oxidation reactions were dominant in a lean environment: CO oxidation by NO2 at low temperature followed by H2, CO, NO and HC oxidation by O2. NOx were stored on barium storage sites.  In rich conditions H2, CO and HC were used to reduce NOx. NH3 production from H2 was also observed. It could be used to further reduce NOx in lean conditions if stored on a downstream SCR system like in the Honda system. A further conversion of HC was obtained at high temperature due to steam reforming. Interactions and inhibitions were also found.
NOx storage appeared to be inhibited by CO oxidation with NO2 at low temperatures and also by HC, maybe through competition for storage sites with CO2 produced during HC oxidation.  Catalytic reactions were affected by the soot deposit and the origin of the soot.

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