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Summary This study investigated the reduction of nitrogen oxides (NOx) and N2O emissions by selective catalytic oxidation (SCO) of an ammonia oxidation catalyst (AOC). This was conducted using a diesel engine with various oxidation catalysts, such as a diesel oxidation catalyst and a diesel particulate filter. The after‐treatment system was equipped with urea‐selective catalytic reduction (SCR) using a commercially available V2O5/TiO2 catalyst and an AOC to prevent ammonia slip. The performance of the system, after treatment, was tested under various engine loads and urea injection conditions. The operating conditions that were applied included abnormal operation of urea‐SCR in the presence of abundant NH3 due to excessive spraying of urea. At 400°C to 450°C, and a high load, the diesel engine produced undesirable reactions at V2O5/TiO2 based urea‐SCR, causing emission of excess N2O. Additionally, under identical operating conditions, the AOC with Pt metal increased the emissions of NO, NO2, and N2O via oxidation of NH3. Excess emission of NO increases the total amount of NOx, including NO2. However, undesirable oxidation of NH3 at the AOC was significantly decreased at 460°C.

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