Simultaneous improvement of ammonia mediated NOx SCR and soot oxidation for enhanced SCR-on-Filter application

Abstract

The integration of NOx reduction and catalytic soot oxidation was investigated for the SCRoF (Selective Catalytic Reduction on Filter) applications. By physically mixing a commercial SCR catalyst (either Fe-ZSM-5 and Cu-ZSM-5) with a soot oxidation catalyst (K/CeO2-PrO2), it was possible to lower the soot oxidation temperature by more than 150 degrees and, by optimizing the catalysts mass ratio in the mixture, NOx conversion simultaneously increased, because NO oxidation induced a fast SCR reaction pathway, unlike during standard SCR. Such an improvement in NOx conversion was more pronounced with the Fe-ZSM-5 than with the Cu-ZSM-5 zeolite, as the latter was more sensitive to the NO2/NOx ratio. In order to make the soot oxidation catalyst inactive towards ammonia oxidation, poisoning of the surface acid sites with 3.0 wt.% K2CO3 (corresponding to only 1.0 wt.% K) was performed. In the soot oxidation and SCR catalysts physical mixture, the soot was oxidized mainly by O2 and the contribution of NO2 to oxidation was negligible, as NO2 itself was a key reactant in the (kinetically much faster) SCR reaction.