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

Abstract

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 (1^st, 2^nd, and 3^rd) including an NTC behavior can be found in the DME case, only two regimes (2^nd and 3^rd) 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.