Nitrous oxide in diesel aftertreatment systems including DOC, DPF and urea‐SCR

• N 2 O has increased more than a dozen times after passing through the SCR systems. • A trajectory of N 2 O formation on an N 2 O and NO 2 /NOx map was introduced. • NO depletion played a crucial role in the start of N 2 O formation. • α at the start of N 2 O formation for Fe-zeolite earlier than Cu-zeolite catalyst. The formation of Nitrous oxide (N 2 O) over urea-based selective catalytic reduction (SCR) systems was scrutinized using exhaust gases from a 3.4-liter non-road diesel engine. Four catalysts were used for the SCR systems; Cu-zeolite (Cu/Fe = 4), Fe-zeolite (Fe only), vanadia and mixed (vanadia and Cu-zeolite) one. For all SCR systems, the N 2 O concentration at the SCR inlet (before SCR) was below 1 ppm, except for the low-load and low-speed engine conditions, where it was below 3 ppm. However, at the SCR outlet, the N 2 O concentration had risen to nearly 20 ppm, except for a few conditions. That is, N 2 O increased more than a dozen times by passing through the SCR systems. The amount of N 2 O formed within the SCR systems depended on the SCR catalysts; the vanadia catalyst had the lowest amount of N 2 O, while the Fe-zeolite catalyst as well as the Cu-zeolite catalyst had the highest. In addition, the formation of N 2 O was strongly involved in the ratio of nitrogen dioxide (NO 2 ) to nitrogen oxides (NOx) at the SCR inlet, which was coupled to SCR inlet temperatures. The higher ratio of NO 2 to NOx led to higher N 2 O at the SCR outlet. It was found that the start of N 2 O formation corresponded to the moment when nitric oxide (NO) was close to 0 ppm as the ratio of an ammonia (NH 3 ) in aqueous urea solution to NOx increased. In other words, NO depletion played a crucial role in the start of N 2 O formation when NO and NO 2 were simultaneously consumed. Therefore, the difference in N 2 O formation among the four catalysts for the SCR systems depended on how quickly NO was depleted. Results from α values of intersections of N 2 O and NO ( α @N₂O = NO ) for Cu- and Fe-zeolite catalysts showed that α @N₂O = NO for the Fe-zeolite catalyst are much smaller than that for the Cu-zeolite catalyst, and this implies that the Fe-zeolite catalyst can start N 2 O formation earlier or have faster NO depletion than the Cu-zeolite catalyst.