Impact of unintentionally formed CH2O in oxygenated fuel exhausts on DeNOx-SCR at different NO2/NOx ratios under close to real conditions

Abstract

This study investigates for the first time the influence of the NO₂/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 NH₃ 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 NO₂/NO_x ratio. Especially at small NO₂/NO_x fractions, not only toxic products are formed, but also NO_x conversion efficiency is drastically deteriorated. In contrast, at high NO₂ fractions, the presence of formaldehyde prevents the formation of ammonium nitrate on the catalyst surface and thus prevents the release of N₂O, leading to a strong improvement in NO_x conversion efficiency compared to studies without formaldehyde.