Comparison of the effects of adding the isomers ethanol or dimethyl ether on ammonia oxidation chemistry

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This paper investigates the NH3 oxidation in the presence of the C2H6O-isomer additives ethanol (C2H5OH) and dimethyl ether (DME, CH3OCH3), across different temperature regimes. Observations were made by coupling a jet-stirred reactor with a molecular-beam mass spectrometer, covering a temperature range of 450–1180 K at atmospheric pressure, adding 10 %, 20 %, and 50 % C2H5OH or CH3OCH3 in the mixture, respectively, containing 95 % argon dilution, at three equivalence ratios (0.5/1.0/2.0), and a constant residence time of 1s. The proposed model, PTB-NH3/C2 1.1 mech, demonstrates satisfactory agreement with the data derived from this study. The results depict distinct impacts of the two isomers on ammonia oxidation. While three oxidation regimes (1st, 2nd, and 3rd) including an NTC behavior can be found in the DME case, only two regimes (2nd and 3rd) occur in the case of ethanol. The specific low-temperature kinetics of DME, e.g., the reactions CH2OCH2O2H + O2 = O2CH2OCH2O2H and CH3OCH2O2 = 2CH2O + OH, exhibit a distinctive role in the first oxidation regime of NH3 and subsequently NTC through their influence on OH radical formation. In the second oxidation regime, the role of DME in ammonia oxidation becomes critical as it competes with NH3-chemistry for OH radicals, which is less pronounced in the ethanol case. Nevertheless, NH3 consumptions with different isomer-blends follow a uniform reaction pathway, i.e., NH3 → NH2 → H2NO → HNO → NO → NO2 → N2O → N2. The third oxidation regime is characterized by rapid NH3 consumption primarily governed through N-chemistry but independent from the respective additive. Unlike the weak detection of N-C species in the first two regimes, HCN and HNCO become more important in the third regime because approximately 10 % NH2 proceeds the reaction pathway of CH3NH2 → CH2NH2 → CH2NH → H2CN → HCN → CH3CN → NCO → HNCO.