Low-Temperature NOx Reduction by H2 in Diesel Engine Exhaust

Abstract

For the NOx removal from diesel exhaust, the selective catalytic reduction (SCR) and lean NOx traps are established technologies. However, these procedures lack efficiency below 200 °C, which is of importance for city driving and cold start phases. Thus, the present paper deals with the development of a novel low-temperature deNOx strategy implying the catalytic NOx reduction by hydrogen. For the investigations, a highly active H2-deNOx catalyst, originally engineered for lean H2 combustion engines, was employed. This Pt-based catalyst reached peak NOx conversion of 95 % in synthetic diesel exhaust with N2 selectivities up to 80 %. Additionally, driving cycle tests on a diesel engine test bench were also performed to evaluate the H2-deNOx performance under practical conditions. For this purpose, a diesel oxidation catalyst, a diesel particulate filter and a H2 injection nozzle with mixing unit were placed upstream to the full size H2-deNOx catalyst. As a result, the Worldwide harmonized Light vehicles Test Cycle (WLTC), urban cycle segment of the Common Artemis Driving Cycle (CADC UC) and Transport for London Urban Inter Peak (TfL UIP) driving cycle revealed NOx conversions up to 90 % at temperatures as low as 80 °C. However, outside the low-temperature region, H2-deNOx activity dropped significantly evidencing the need for an additional underfloor SCR system. Moreover, slight N2O formation was observed in the engine tests making further catalyst development necessary, since N2O is considered a critical component due to its global warming potential. Additionally, the H2 demand for low-temperature deNOx in diesel passenger cars was estimated and a novel on-board H2 production strategy based on DEF electrolysis was developed. This method provided both H2 as well as gaseous NH3. Subsequent simulations of H2 production demonstrate small size factors (≤ 525 cm3) and rather low energy consumption of the H2 supply unit, e.g. 0.25 kWh for the TfL UIP driving cycle.