Pathways for N ₂ and N ₂O formation during the reduction of NO _x over Pt-Ba/Al ₂O ₃ LNT catalysts investigated by labeling isotopic experiments

Mechanistic aspects involved in the formation of N ₂ and of N ₂O during the reduction of gas-phase NO and of NO _x stored over a model PtBa/Al ₂O ₃ NSR catalyst are investigated using unlabeled ammonia and labeled NO. The reduction of the stored NO _x species (labeled nitrites and nitrates) with NH ₃ leads to the selective formation of N ₂ as a major product and of small amounts of nitrous oxide. Based on the nitrogen isotopic distribution, it appears that N ₂ formation occurs primarily through the statistical coupling of N-atoms formed by dissociation of NO _x- and NH ₃-related surface intermediates, although an SCR-pathway (involving the coupling of NH ₃- and NO-derived ad-species) is also likely to occur. It appears as well that the formation of nitrous oxide involves either the coupling of two adsorbed NO molecules or the recombination of an adsorbed NO molecule with an adsorbed NH _x fragment.