Labeled ¹⁵NO Study on N₂ and N₂O Formation Over Pt–Ba/Al₂O₃ NSR Catalysts

Mechanistic aspects involved in the formation of N₂ and of N₂O during the reduction of NO, stored nitrites 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 of ¹⁵N- and ¹⁴N-adatoms on Pt. 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 explains the abundance of the ¹⁴N¹⁵N species. When the reduction of the stored nitrates is carried out in the presence of NO, this species is preferentially reduced pointing out the higher reactivity of gaseous NO if compared to the nitrates.