Low-Temperature NO <sub>x</sub> Reduction by H <sub>2</sub> in Diesel Engine Exhaust

<div class="section abstract"><div class="htmlview paragraph">For the NO_x removal from diesel exhaust, the selective catalytic reduction (SCR) and lean NO_x 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 deNO_x strategy implying the catalytic NO_x reduction by hydrogen. For the investigations, a highly active H₂-deNO_x catalyst, originally engineered for lean H₂ combustion engines, was employed. This Pt-based catalyst reached peak NO_x 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-deNO_x 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-deNO_x 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 NO_x conversions up to 90 % at temperatures as low as 80 °C. However, outside the low-temperature region, H₂-deNO_x 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 deNO_x 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.</div></div>