Impact of unintentionally formed CH 2 O in oxygenated fuel exhausts on DeNO x -SCR at different NO 2 /NO x ratios under close to real conditions

This study investigates for the first time the influence of the NO 2 /NO x ratio on formaldehyde-related effects under close to real conditions using a copper exchanged chabazite type zeolite (Cu-CHA) series production catalyst. This study demonstrates that under the chosen reaction conditions, formaldehyde reacts preferentially with NH 3 to form a yet to be determined intermediate, which subsequently leads either to its catalytic decomposition without the formation of hazardous pollutants, or to thermal decomposition with the release of toxic HCN, depending on the NO 2 /NO x ratio. Especially at small NO 2 /NO x fractions, not only toxic products are formed, but also NO x conversion efficiency is drastically deteriorated. In contrast, at high NO 2 fractions, the presence of formaldehyde prevents the formation of ammonium nitrate on the catalyst surface and thus prevents the release of N 2 O, leading to a strong improvement in NO x conversion efficiency compared to studies without formaldehyde.