Analysis of the Coupling of HC–SCR by Ethanol and NH₃–SCR on Real Engine Emissions

Abstract

Selective catalytic reduction by ethanol on silver-based catalysts was proved to be very effective to abate the nitrogen oxides emitted at the exhaust of an automotive engine. Moreover, the selectivity to ammonia of this reaction may be exploited to further enhance the NOx reduction using a dedicated transition metal exchanged zeolite catalyst. This coupling between HC– and NH₃–SCR is called Dual SCR. In order to control the silver-based catalyst efficiency via ethanol injection, a NOx sensor is located downstream of it, as usually done for urea–SCR on series vehicles. Furthermore, based on the cross-sensitivity of this NOx sensor, large amounts of ammonia were estimated that would help to reduce the remaining NOx on the zeolite based catalyst. However, when measured by FTIR technique, the concentrations of ammonia produced by the HC–SCR catalyst were surprisingly not as high as expected, while large amounts of acetaldehyde were detected and, in a lesser extent, formaldehyde and hydrogen cyanide. NOx were partly reduced over the iron-exchanged zeolite catalyst, improving the overall deNOx efficiency by up to 15 points, while acetaldehyde to formaldehyde ratio reversed and ammonia concentration remains unchanged. The cross-sensitivity of the NOx sensor was further investigated on synthetic gas bench. If its partial dependence on the ammonia concentration is rather well known, the influence of aldehydes and hydrogen cyanide in presence of ammonia had not yet been investigated. The NOx sensor’s signal remains unchanged whatever the aldehydes concentration and a strong sensitivity to the hydrogen cyanide was highlighted.