Synthetic gas bench study of a 4-way catalytic converter: Catalytic oxidation, NOx storage/reduction and impact of soot loading and regeneration

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 on an engine bench test. Oxidation reactions were dominant in a lean environment: CO oxidation by NO(2) at low temperature followed by H(2), CO. NO and HC oxidation by O(2). NOx were stored on barium storage sites. In rich conditions H(2), CO and HC were used to reduce NOx. NH(3) production from H(2) 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 [1]. 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 NO(2) at low temperatures and also by HC. maybe through competition for storage sites with CO(2) produced during HC oxidation. Catalytic reactions were affected by the soot deposit. Continuous oxidation of soot by NO(2) also induced a slower NOx storage rate. (C) 2009 Elsevier B.V. All rights reserved.