Abstract
NOX 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 N2O or ammonia, formation. The current study focuses on possible N2O formation pathways over NOx storage/reduction (NSR) catalysts. More specifically, NH3 reactivity with catalyst surface species was investigated over a model Pt–Ba/γ-Al2O3 (1/20/100, w/w) NSR catalyst with and without NOx species stored (nitrites and nitrates). The NH3 originates from reduction of NOx in the regeneration phase. With NH3, two overall reactions can lead to N2 or N2O formation, namely 3NO + 2NH3 → 5/2 N2 + 3H2O and 4NO + NH3 → 5/2 N2O + 3/2H2O. These two are considered for catalyzed reaction between the entering NO and NH3. Surface nitrite and nitrate reduction reactions leading to N2 or N2O were also evaluated, all as a function of temperature and relative amount of NH3 in the gas phase. N2O was formed in the lower temperature range, and was more significant with lower NH3 concentrations. At higher temperatures, above 423 K, for NH3 concentrations higher than stoichiometric, only N2 was produced. In comparing the results from the samples with preformed nitrites/nitrates on the surface to those without, it is apparent that NH3 first reacts with gas-phase NO and then with pre-stored surface NOx species. Moreover, the reduction of surface nitrites/nitrates is complete only for high NH3/NO ratios, and when NH3 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, N2O was the dominant product at low temperature, when NO and NH3 conversions are low. At higher temperature, with increasing NO and NH3 conversions, N2 selectivity increases.
Highlights
Over the last few decades, stricter regulations on nitrogen oxide (NOx) emissions have been implemented, to mitigate its impact on human health and the environment [1,2,3,4]
Upon H2 admission to the reactor at t = 0 s, H 2 was completely consumed for about 180 s and the H2 outlet concentration gradually increased with time, reaching the inlet value at ca. 1400s. N2 was formed and after 350 s, NH3 was observed, reaching ca. 100 ppm
N2O is the dominant product at low temperature, and when NH3 concentrations are substoichiometric
Summary
Over the last few decades, stricter regulations on nitrogen oxide (NOx) emissions have been implemented, to mitigate its impact on human health and the environment [1,2,3,4]. There are two technologies that have emerged as solutions. For NOx reduction, NOx storage and reduction (NSR) catalysis and selective catalytic reduction (SCR) catalysis, as well as combining the two [5,6,7]. For SCR, the ammonia reactant is derived from hydrolysis of aqueous urea [8, 9]. An extra tank of aqueous urea is needed and solutions that avoid this extra feature are desired. NSR is challenged by cost and sulfur poisoning, but avoids the need for an external reductant. The focus of this study is the NSR catalyst
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