Labelled 15NO study on N2 and N2O formation over Pt-Ba/Al2O3 LNT catalysts

Mechanistic aspects involved in the formation of N2 and of N2O during the reduction of NO, stored nitrates and stored nitrates in the presence of NO are investigated in this work by means of isotopic labeling experiments over a model PtBa/Al2O3 NSR catalyst. The reduction of gaseous labeled NO with unlabelled NH3 leads to the formation of N2O at low temperature (below 180°C), and of N2 at high temperature. All N2 possible isotopes are observed, whereas only labeled molecules have been detected in the case of N2O. Hence the formation of nitrous oxide involves undissociated NO molecules, whereas that of N2 can be explained on the basis of the statistical coupling on Pt of 15N- and 14N-adatoms. However, due to a slight excess of the mixed 15N14N isotope, a SCR-like pathway likely operates as well. The reduction of the stored labelled nitrates is very selective to N2 and all isotopes are observed, confirming the occurrence of the recombination pathway. However also in this case a SCR-like pathway likely occurs and this explain the abundance of the 14N15N species. When the reduction of the stored nitrates is carried out in the presence of NO, NO is preferentially reduced pointing out the higher reactivity of gaseous NO if compared to the nitrates.