The effect of soot on ammonium nitrate species and NO2selective catalytic reduction over Cu–zeolite catalyst-coated particulate filter

Abstract

A selective catalytic reduction (SCR)-coated particulate filter was evaluated by means of dynamic tests performed using NH₃, NO₂, O₂and H₂O. The reactions were examined both prior to and after soot removal in order to study the effect of soot on ammonium nitrate formation and decomposition, ammonia storage and NO₂SCR. A slightly larger ammonia storage capacity was observed when soot was present in the sample, which indicated that small amounts of ammonia can adsorb on the soot. Feeding of NO₂and NH₃in the presence of O₂and H₂O at low temperature (150, 175 and 200°C) leads to a large formation of ammonium nitrate species and during the subsequent temperature ramp using H₂O and argon, a production of nitrous oxides was observed. The N₂O formation is often related to ammonium nitrate decomposition, and our results showed that the N₂O formation was clearly decreased by the presence of soot. We therefore propose that in the presence of soot, there are fewer ammonium nitrate species on the surface due to the interactions with the soot. Indeed, we do observe CO₂production during the reaction conditions also at 150°C, which shows that there is a reaction with these species and soot. In addition, the conversion of NO_xdue to NO₂SCR was significantly enhanced in the presence of soot; we attribute this to the smaller amount of ammonium nitrate species present in the experiments where soot is available since it is well known that ammonium nitrate formation is a major problem at low temperature due to the blocking of the catalytic sites. Further, a scanning electron microscopy analysis of the soot particles shows that they are about 30–40 nm and are therefore too large to enter the pores of the zeolites. There are likely Cu_xO_yor other copper species available on the outside of the zeolite crystallites, which could have been enhanced due to the hydrothermal treatment at 850°C of the SCR-coated filter prior to the soot loading. We therefore propose that soot is interacting with the ammonium nitrate species on the Cu_xO_yor other copper species on the surface of the zeolite particles, which reduces the ammonium nitrate blocking of the catalyst and thereby results in higher NO₂SCR activity.