Using kinetics to characterize photoionization of key combustion intermediates

Printer-friendly Version

Home > View Article

The reactions of alkyl radicals, denoted by R, with molecular oxygen (O₂) are key components in the chemistry of autoignition, and in the atmospheric oxidation of hydrocarbons. These reactions proceed via formation, isomerization, and dissociation of alkylperoxy radicals, RO₂ (CRF News 23, No. 1). Recently, CRF researchers Giovanni Meloni, Peng Zou, Craig Taatjes, and David Osborn, in collaboration with Lawrence Berkeley National Laboratory (LBNL) scientists Musahid Ahmed and Stephen Leone, undertook experiments to characterize the photoionization of these important intermediate species. Calculations to analyze the experimental data were carried out in collaboration with Stephen Klippenstein, a former CRF researcher, now at Argonne National Laboratory.

Figure 1. Measurements of m/z = 29 ions formed from photoionization following photolysis of diethyl ketone (to produce ethyl radicals) in the presence of O₂. The traces on the right show the time behavior of the signal at low photon energy, which contains only direct ionization of ethyl radicals, and at high photon energy, which has contributions from direct ionization of ethyl radicals (that rapidly decay by reaction with oxygen) and from dissociative ionization of ethylperoxy radicals, which persist for longer times.

Photoionization is an important analytical tool for combustion and kinetics studies that can yield isomer-specific data about flame chemistry (CRF News 27, No. 4) and elementary reaction kinetics. However, photoionization of alkylperoxy radicals had been reported only for the simplest RO₂ species, methylperoxy (CH₃OO), and its ionization energy had never been measured. Detection of larger alkylperoxy radicals was difficult and was thought to be limited by low ionization cross sections. The work of the CRF researchers shows that it is rather the instability of the ROO⁺ cations that prevented detection of the larger alkylperoxy radicals by photoionization.

The experiments that allowed characterization of alkylperoxy ionization utilized the Multiplexed Chemical Kinetics Photoionization Mass Spectrometer (CRF News 28, No. 1), operating at LBNL's Advanced Light Source. In this apparatus, chemical reactions are initiated by laser photolysis of a suitable precursor in a quartz reactor. The reactor contents are then continuously sampled and analyzed by synchrotron photoionization mass spectrometry using a double-focusing mass spectrometer, yielding mass spectra of the reacting mixture as a function of time after the initiating laser pulse. This machine combines the power of photoionization by easily and widely tunable vacuum ultraviolet synchrotron radiation with simultaneous time-resolved detection of multiple masses.

The CRF scientists photolyzed symmetric ketones (e.g., acetone, diethyl ketone) in the presence of oxygen to create alkyl peroxy radicals. Because the mass spectrometer simultaneously probes all species in the reactor, the disappearance of the initially formed alkyl radicals through reaction with oxygen and the corresponding appearance of alkylperoxy radicals can both be monitored. Reacting methyl radicals, created from acetone photolysis with oxygen, created an easily discernable signal of methylperoxy CH₃OO at the parent mass, m/z = 47. The team monitored this signal as a function of the ionizing photon energy, and combined it with computation of Franck-Condon factors that model the shape of the ionization threshold, to make the first measurement of the adiabatic ionization energy of the methylperoxy radical.

However, similar experiments to follow the reaction of ethyl and propyl radicals with oxygen showed no signals at the masses of the corresponding alkylperoxy radicals. Nevertheless, because the full mass spectrum was measured as a function of time and ionizing photon energy, the signature of alkylperoxy ionization could still be discerned. Figure 1 shows the signal observed at m/z = 29, the mass of the ethyl radical, as a function of time and photon energy, in the reaction of ethyl radicals with oxygen. The spectrum shows two sources of C₂H₅⁺. The C₂H₅⁺ formed by direct ionization of ethyl radicals can be seen as the peak near t = 0, present at all photon energies in the figure. This signal rapidly decays to the baseline level for photon energies below about 10 eV, reflecting the removal of ethyl radicals by reaction with oxygen. At higher photon energies, however, m/z = 29 signal persists for long periods. This signal can be attributed to dissociative ionization of ethylperoxy radical, C₂H₅OO, to form C₂H₅⁺ andO₂. The instability of the ethylperoxy cation is related to the stability of the ethyl cation, which arises from its unusual three-center, two-electron bonding. Calculations carried out by the CRF researchers and their collaborators suggest that other linear alkyl cations that exhibit similar bonding will also not have stable alkylperoxy cations. Even so, time-resolved, tunable photoionization permits the contributions of direct ionization and dissociative ionization to be separated, as seen in Figure 1, enabling the concentration of the alkylperoxy radical to be monitored by photoionization detected at the alkyl cation mass.

Figure 2. Photon energy dependence of the dissociative ionization of methylperoxy radicals to give methyl cations and oxygen molecules. The fit yields the appearance energy of methyl cations from methylperoxy radicals.

Finally, measurement of similar dissociative ionization processes in the CH₃OO radical allowed the researchers to measure the CH₃– O₂ bond energy, an important thermodynamic quantity for combustion modeling. Figure 2 shows the photon energy dependence of the dissociative ionization of CH₃OO to form CH₃⁺ and O₂. The photon energy dependence is fit to account for the thermal energy (298 K) in the ion, yielding the appearance energy of CH₃⁺ from CH₃OO, which is related to the CH₃– O₂ bond energy via the very well-known ionization energy of CH₃. The derived bond energy of (127.6 ±5) kJ/Mol supports previous kinetic determinations of the CH₃ – O₂ bond energy and is slightly larger than more recent negative-ion measurements.

Future studies of peroxy radical photoionization will include measurements of peroxy radicals formed from unsaturated, resonance-stabilized, and aromatic hydrocarbon species to investigate the fundamental physics that governs the stability of alkylperoxy cations. The Multiplexed Chemical Kinetics Photoionization Mass Spectrometer is currently being employed by a team from multiple institutions including researchers from Howard University, Texas A&M University, and the Catholic University of America, in addition to the CRF and LBNL scientists, to investigate the chemistry of hydrocarbon oxidation (including alkylperoxy radical reactions), molecular weight growth and soot formation and hydrocarbon chemistry in the Earth's and extraterrestrial atmospheres.

Article taken from the Jan/Feb CRF News Volume 29 Number 1 (PDF - 1405K)


	© 2006 Sandia Corporation \| Questions and Comments \| Site Index \| Privacy and Security P.O. Box 969 * Livermore, California 94551