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

Abstract

<div class="section abstract"><div class="htmlview paragraph">For the NO<sub>x</sub> removal from diesel exhaust, the selective catalytic reduction (SCR) and lean NO<sub>x</sub> 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<sub>x</sub> strategy implying the catalytic NO<sub>x</sub> reduction by hydrogen. For the investigations, a highly active H<sub>2</sub>-deNO<sub>x</sub> catalyst, originally engineered for lean H<sub>2</sub> combustion engines, was employed. This Pt-based catalyst reached peak NO<sub>x</sub> conversion of 95 % in synthetic diesel exhaust with N<sub>2</sub> selectivities up to 80 %. Additionally, driving cycle tests on a diesel engine test bench were also performed to evaluate the H<sub>2</sub>-deNO<sub>x</sub> performance under practical conditions. For this purpose, a diesel oxidation catalyst, a diesel particulate filter and a H<sub>2</sub> injection nozzle with mixing unit were placed upstream to the full size H<sub>2</sub>-deNO<sub>x</sub> 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<sub>x</sub> conversions up to 90 % at temperatures as low as 80 °C. However, outside the low-temperature region, H<sub>2</sub>-deNO<sub>x</sub> activity dropped significantly evidencing the need for an additional underfloor SCR system. Moreover, slight N<sub>2</sub>O formation was observed in the engine tests making further catalyst development necessary, since N<sub>2</sub>O is considered a critical component due to its global warming potential. Additionally, the H<sub>2</sub> demand for low-temperature deNO<sub>x</sub> in diesel passenger cars was estimated and a novel on-board H<sub>2</sub> production strategy based on DEF electrolysis was developed. This method provided both H<sub>2</sub> as well as gaseous NH<sub>3</sub>. Subsequent simulations of H<sub>2</sub> production demonstrate small size factors (≤ 525 cm<sup>3</sup>) and rather low energy consumption of the H<sub>2</sub> supply unit, e.g. 0.25 kWh for the TfL UIP driving cycle.</div></div>