Abstract
AbstractThe formation of typical low‐temperature oxidation products is observed in laminar premixed low‐pressure flames investigated by photoionization molecular‐beam mass spectrometry at the Swiss Light Source. The C1–C4 alkyl hydroperoxides can be identified in n‐butane‐ and 2‐butene‐doped hydrogen flames by their photoionization efficiency spectra at m/z 48, 62, 76, and 90. C1–C3 alkyl hydroperoxides are also observed in a propane‐doped hydrogen flame and in a neat propane flame. In addition, threshold photoelectron spectra reveal the presence of the alkyl hydroperoxides. In the 2‐butene/H2 flame, the photoionization spectrum at m/z 88 also enables the identification of butenyl hydroperoxides by comparison with calculated ionization energies of the alkenyl hydroperoxides and a literature spectrum. The low‐temperature species are formed close to the burner surface with maximum mole fractions at 0.25–0.75 mm above the burner. At 0.5 mm, even the methylperoxy radical (CH3OO) is measured for the first time in a laminar premixed flame. The rate of production analyses show that consumption of the hydroperoxyalkyl radicals results in the formation of cyclic ethers. In the n‐butane/H2 flame, ethylene oxide, oxetane, and methyloxirane are identified. Besides expected small oxygenated species, for example, formaldehyde or acetaldehyde, the larger C4 oxygenates butanone (C2H5COCH3) and 2,3‐butanedione (C4H6O2) are formed in the two C4 hydrocarbon‐doped hydrogen flames. Quantification of alkyl hydroperoxides with estimated photoionization cross sections based on the corresponding alcohols, which have similar photoelectron structures to the alkyl hydroperoxides, shows that mole fractions are on the order of 10−5–10−6 in the n‐butane/H2 flame. Measurements are corroborated by simulations, which also predict the presence of some peroxides in detectable concentrations, that is, mole fractions larger than 10−7, under the investigated conditions. The observation of peroxide species and cyclic ethers in the investigated laminar premixed flames give new insights into the contribution of low‐temperature combustion chemistry in a flame.
Highlights
Advanced combustion strategies and novel fuels are a key factor in future energy supply and transportation as they promise improved efficiencies and reductions of carbon dioxide and air pollutant emissions
Bourgalais et al.[30] coupled a jet-stirred reactor (JSR) for the first time to a double imaging photoelectron photoion coincidence (i2PEPICO) spectrometer at the synchrotron SOLEIL to obtain threshold photoelectron (TPE) spectra for species generated upon low-temperature oxidation of n-pentane
The n-butane-doped hydrogen flame was measured with the single-imaging photoelectron photoion coincidence setup as described by Oßwald et al.[38] for flame-sampling molecular-beam mass spectrometry
Summary
Advanced combustion strategies and novel fuels are a key factor in future energy supply and transportation as they promise improved efficiencies and reductions of carbon dioxide and air pollutant emissions. Ethyl, and butyl hydroperoxides were identified based on their ionization energies by photoionization efficiency (PIE) spectra in the photon energy range from 8.5 to 11.5 eV.[20,21] Further progress in the detection of low-temperature species and in understanding the LTC of hydrocarbons was reported for the oxidation of propane,[22,23] n-butane,[24,25,26] n-pentane,[27] n-heptane,[28] or 1hexene.[29] Recently, Bourgalais et al.[30] coupled a jet-stirred reactor (JSR) for the first time to a double imaging photoelectron photoion coincidence (i2PEPICO) spectrometer at the synchrotron SOLEIL to obtain threshold photoelectron (TPE) spectra for species generated upon low-temperature oxidation of n-pentane They measured photoelectron spectra of the smallest alkyl hydroperoxides, that is, methyl and ethyl hydroperoxide, and were able to resolve isomers at mass-to-charge ratios of 72, 84, 86, and 88.30. For the n-butane/H2 flame, all detected species were quantified so that a comprehensive dataset including low-temperature species is available
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