Extreme Low-Temperature Combustion Chemistry: Ozone-Initiated Oxidation of Methyl Hexanoate

Tracking the reaction networks of acetaldehyde oxide and glyoxal oxide Criegee intermediates in the ozone-assisted oxidation reaction of crotonaldehyde

The reaction of unsaturated compounds with ozone (O3) is recognized to lead to the formation of Criegee intermediates (CIs), which play a key role in controlling the atmospheric budget of hydroxyl radicals and secondary organic aerosols. The reaction network of two CIs with different functionality, i.e. acetaldehyde oxide (CH3CHOO) and glyoxal oxide (CHOCHOO) formed in the ozone-assisted oxidation reaction of crotanaldehyde (CA), is investigated over a temperature range between 390 K and 840 K in an atmospheric pressure jet-stirred reactor (JSR) at a residence time of 1.3 s, stoichiometry of 0.5 with a mixture of 1% crotonaldehyde, 10% O2, at an fixed ozone concentration of 1000 ppm and 89% Ar dilution. Molecular-beam mass spectrometry in conjunction with single photon tunable synchrotron vacuum-ultraviolet (VUV) radiation is used to identify elusive intermediates by means of experimental photoionization energy scans and ab initio threshold energy calculations for isomer identification. Addition of ozone (1000 ppm) is observed to trigger the oxidation of CA already at 390 K, which is below the temperature where the oxidation reaction of CA was observed in the absence of ozone. The observed CA + O3 product, C4H6O4, is found to be linked to a ketohydroperoxide (2-hydroperoxy-3-oxobutanal) resulting from the isomerization of the primary ozonide. Products corresponding to the CIs uni- and bi-molecular reactions were observed and identified. A network of CI reactions is identified in the temperature region below 600 K, characterized by CIs bimolecular reactions with species like aldehydes, i.e., formaldehyde, acetaldehyde, and crotonaldehyde and alkenes, i.e., ethene and propene. The region below 600 K is also characterized by the formation of important amounts of typical low-temperature oxidation products, such as hydrogen peroxide (H2O2), methyl hydroperoxide (CH3OOH), and ethyl hydroperoxide (C2H5OOH). Detection of additional oxygenated species such as alcohols, ketene, and aldehydes are indicative of multiple active oxidation routes. This study provides important information about the initial step involved in the CIs assisted oligomerization reactions in complex reactive environments where CIs with different functionalities are reacting simultaneously. It provides new mechanistic insights into ozone-assisted oxidation reactions of unsaturated aldehydes, which is critical for the development of improved atmospheric and combustion kinetics models.

1-Hexene Ozonolysis across Atmospheric and Combustion Temperatures via Synchrotron-Based Photoelectron Spectroscopy and Chemical Ionization Mass Spectrometry

This study investigates the complex interaction between ozone and the autoxidation of 1-hexene over a wide temperature range (300-800 K), overlapping atmospheric and combustion regimes. It is found that atmospheric molecular mechanisms initiate the oxidation of 1-hexene from room temperature up to combustion temperatures, leading to the formation of highly oxygenated organic molecules. As temperature rises, the highly oxygenated organic molecules contribute to radical-branching decomposition pathways inducing a high reactivity in the low-temperature combustion region, i.e., from 550 K. Above 650 K, the thermal decomposition of ozone into oxygen atoms becomes the dominant process, and a remarkable enhancement of the conversion is observed due to their diradical nature, counteracting the significant negative temperature coefficient behavior usually observed for 1-hexene. In order to better characterize the formation of heavy oxygenated organic molecules at the lowest temperatures, two analytical performance methods have been combined for the first time: synchrotron-based mass-selected photoelectron spectroscopy and orbitrap chemical ionization mass spectrometry. At the lowest studied temperatures (below 400 K), this analytical work has demonstrated the formation of the ketohydroperoxides usually found during the LTC oxidation of 1-hexene, as well as of molecules containing up to nine O atoms.

Exploring the Effect of Different Reactivity Promoters on the Oxidation of Ammonia in a Jet-Stirred Reactor

The low reactivity of ammonia (NH₃) is the main barrier to applying neat NH₃ as fuel in technical applications, such as internal combustion engines and gas turbines. Introducing combustion promoters as additives in NH₃-based fuel can be a feasible solution. In this work, the oxidation of ammonia by adding different reactivity promoters, i.e., hydrogen (H₂), methane (CH₄), and methanol (CH₃OH), was investigated in a jet-stirred reactor (JSR) at temperatures between 700 and 1200 K and at a pressure of 1 bar. The effect of ozone (O₃) was also studied, starting from an extremely low temperature (450 K). Species mole fraction profiles as a function of the temperature were measured by molecular-beam mass spectrometry (MBMS). With the help of the promoters, NH₃ consumption can be triggered at lower temperatures than in the neat NH₃ case. CH₃OH has the most prominent effect on enhancing the reactivity, followed by H₂ and CH₄. Furthermore, two-stage NH₃ consumption was observed in NH₃/CH₃OH blends, whereas no such phenomenon was found by adding H₂ or CH₄. The mechanism constructed in this work can reasonably reproduce the promoting effect of the additives on NH₃ oxidation. The cyanide chemistry is validated by the measurement of HCN and HNCO. The reaction CH₂O + NH₂ ⇄ HCO + NH₃ is responsible for the underestimation of CH₂O in NH₃/CH₄ fuel blends. The discrepancies observed in the modeling of NH₃ fuel blends are mainly due to the deviations in the neat NH₃ case. The total rate coefficient and the branching ratio of NH₂ + HO₂ are still controversial. The high branching fraction of the chain-propagating channel NH₂ + HO₂ ⇄ H₂NO + OH improves the model performance under low-pressure JSR conditions for neat NH₃ but overestimates the reactivity for NH₃ fuel blends. Based on this mechanism, the reaction pathway and rate of production analyses were conducted. The HONO-related reaction routine was found to be activated uniquely by adding CH₃OH, which enhances the reactivity most significantly. It was observed from the experiment that adding ozone to the oxidant can effectively initiate NH₃ consumption at temperatures below 450 K but unexpectedly inhibit the NH₃ consumption at temperatures higher than 900 K. The preliminary mechanism reveals that adding the elementary reactions between NH₃-related species and O₃ is effective for improving the model performance, but their rate coefficients have to be refined.

Plasma-Assisted Chemical-Looping Combustion: Low-Temperature Methane and Ethylene Oxidation with Nickel Oxide

The chemical reaction network of low-temperature plasma-assisted oxidation of methane (CH₄) and ethylene (C₂H₄) with nickel oxide (NiO) was investigated in a heated plasma reactor through time-dependent species measurements by electron-ionization molecular beam mass spectrometry (EI-MBMS). Methane (ethylene) oxidation by NiO was explored in temperature ranges from 300-700 °C (300-500 °C) and 300-800 °C (300-600 °C) for the plasma and nonplasma conditions. Significant enhancement of methane oxidation was observed with plasma between 400 and 500 °C, where no oxidation was observed under nonplasma conditions. For the oxidation of methane at higher temperatures, three different oxidation stages were observed: (I) a period of complete oxidation, (II) a period of incomplete CO oxidation, and (III) a period of carbon buildup. For the C₂H₄ experiments, and unlike the CH₄ experiments, the plasma resulted in a significant amount of new intermediate oxygenated species, such as CH₂O, CH₃OH, C₂H₄O, and C₂H₆O. Carbon deposits were observed under both methane and ethylene conditions and verified by X-ray photoelectron spectroscopy (XPS). ReaxFF (reactive force field) simulations were performed for the oxidation of CH₄ and C₂H₄ in a nonplasma environment. The simulated intermediates and products largely agree with the species measured in the experiments, though the predicted intermediate oxygenated species such as CH₂O and C₂H₆O were not observed in experiments under nonplasma conditions. A reaction pathway analysis for CH₄ and C₂H₄ reacting with NiO was created based on the observed species from the MBMS spectra along with ReaxFF simulations.