Influence of NO Addition to n‑Butanal Oxidation: Experiments and Kinetic Modeling

The oxidation of pure n-butanal and the n-butanal/NO mixture was investigated in an atmospheric jet-stirred reactor at stoichiometric conditions and over the temperature range of 425–925 K. Mole fraction profiles of characteristic intermediates, like nitrogenous, carbonyl, and hydrocarbon intermediates were obtained using synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV–PIMS) via molecular beam sampling. A previous reported kinetic model of n-butanal oxidation was improved and a submechanism interpreting n-butanal/NO_x interaction kinetics was developed and validated against experimental measurements. Results show that NO addition results in a featured temperature dependence of n-butanal reactivity similar to that of hydrocarbon fuels, i.e., inhibiting reactivity at low temperatures while promoting at intermediate temperatures. However, the presence of the n-butanal carbonyl group introduces distinct kinetic effects on this system compared to previously reported hydrocarbon/NO systems. Model analysis suggests a pronounced interaction between NO and the n-butanal carbonyl group, particularly at low temperatures where NO notably accelerates the removal of the carbonyl group, yielding large amounts of propyl radicals. This interaction results in the suppression of peroxy radical chemistry on the alkyl chain and chain-branching reactions of the propyl radical, thus decreasing the low-temperature reactivity of n-butanal. At intermediate temperatures, the influence of the carbonyl group on reactivity is not as pronounced and the increased reactivity primarily depends upon the impact of NO_x on the active radical pool. For example, NO_x-related chemistry accelerated the formation of OH radicals, thus facilitating fuel decomposition and the further conversion of CO to CO₂. Besides, NO addition significantly affects the speciation of several products, such as methane, ethane, and formaldehyde, which was correlated to the additional competitive pathways introduced by NO_x.