Reaction between soot and stored NOX over K-based LNT catalysts investigated by temperature programmed methods and labeling isotopic experiments

The interaction between NOX stored over K/Al2O3 and Pt-K/Al2O3 catalysts and soot is investigated in this work. TPD and TPO experiments are carried out where the reactivity/thermal decomposition of nitrates is investigated in the presence and in the absence of soot. It is found that the presence of soot favors the decomposition/reduction of the stored nitrates, while soot is being oxidized. Clear indication of the direct participation of the surface nitrates in soot oxidation is obtained, without the requirement of the preliminary thermal decomposition of nitrates. Such a mechanism occurs thanks to the mobility of the adsorbed NOX species and is not promoted by platinum. This surface reaction mechanism parallels the gas-phase soot oxidation by NO2 and O2 evolved upon decomposition of the stored NOX. K-based samples are more active than Ba-containing catalysts due to the higher mobility of the NOX species adsorbed on K with respect to Ba.