Mechanistic Aspect of N2O Formation Over Pt–Ba/γ-Al2O3 Catalysts

NO X reduction in lean-burn gasoline or diesel engines is challenging in the oxidizing environment, and depending on the after-treatment technology, can lead to by-product, such as N 2 O or ammonia, formation. The current study focuses on possible N 2 O formation pathways over NO x storage/reduction (NSR) catalysts. More specifically, NH 3 reactivity with catalyst surface species was investigated over a model Pt–Ba/γ-Al 2 O 3 (1/20/100, w/w) NSR catalyst with and without NO x species stored (nitrites and nitrates). The NH 3 originates from reduction of NO x in the regeneration phase. With NH 3 , two overall reactions can lead to N 2 or N 2 O formation, namely 3NO + 2NH 3 → 5/2 N 2 + 3H 2 O and 4NO + NH 3 → 5/2 N 2 O + 3/2H 2 O. These two are considered for catalyzed reaction between the entering NO and NH 3 . Surface nitrite and nitrate reduction reactions leading to N 2 or N 2 O were also evaluated, all as a function of temperature and relative amount of NH 3 in the gas phase. N 2 O was formed in the lower temperature range, and was more significant with lower NH 3 concentrations. At higher temperatures, above 423 K, for NH 3 concentrations higher than stoichiometric, only N 2 was produced. In comparing the results from the samples with preformed nitrites/nitrates on the surface to those without, it is apparent that NH 3 first reacts with gas-phase NO and then with pre-stored surface NO x species. Moreover, the reduction of surface nitrites/nitrates is complete only for high NH 3 /NO ratios, and when NH 3 is the limiting reactant, they remain on the catalyst surface unreacted until temperatures higher than 623 K, where they decompose. In general, for all performed experiments, N 2 O was the dominant product at low temperature, when NO and NH 3 conversions are low. At higher temperature, with increasing NO and NH 3 conversions, N 2 selectivity increases.