Research Group Prof. Dr. G. Friedrichs

HO2 Kinetics

Reactions of hydroperoxy radicals (HO2) are known to play decisive roles in atmospheric chemistry, low-temperature combustion, flame propagation, and fuel self ignition, which causes engine knocking. However, the investigation of HO2 chemistry is rather challenging, partly due to a lack of reliable HO2 radical sources and partly due to the difficulties involved in a time-resolved HO2 detection.
We were able to measure the overall rate constant of the reaction HO2C2H5 => products for the first time. Forming C2H5O + OH as the main reaction products, this reaction constitutes an important chain branching source. For example, the reaction was shown to be among the most sensitive reactions for the observed total ignition delay in the NTC region of n-heptane/air ignition (Kazakov et al. Chemical and Physical Processes in Combustion (2005), 46).
hyperoxid1Concentration-vs.-time profiles of HO2 and C2H5 were directly measured by means of time-resolved mass spectrometry in a laser photolysis/flow reactor combination. H2O2 was produced by the thermal decomposition of a urea hydrogenperoxide adduct as a new and reliable H2O2 source. Excimer laser photolysis of reaction mixtures containing C2H6H2O2 and oxalyl chloride provided experimental conditions such the the reaction HO2C2H5 could be studied under nearly pseudo first-order conditions with HO2 as the excess component (see Figure).
The need for a simultaneous generation of two different radical species constitutes a major problem for measurements of rate constants of radical-radical cross reactions. Moreover the possibility of competing radical self-reactions or reactions of the radicals with the precursor or product molecules requires a subtle control of the experimental conditions. As already mentioned above, the radicals C2H52 were generated by excimer laser photolysis (λ = 193 nm) of mixtures of C2H6H2O2 and oxalyl chloride. The C2H5 and HO2 radicals were thus generated by the fast H abstraction reactions of the primarily formed Cl atoms and OH radicals with C2H6 and H2O2, respectively. Here, a kinetic separation of the C2H5 and HO2 formation pathways (see mechanism below) allowed for a straightforward control of the absolute radical concentrations by simply adjusting the relative initial concentrations of C2H6 and (COCl)2 versus that of H2O2


[1] W. Ludwig, B. Brandt, G. Friedrichs, and F. Temps, "Kinetics of the Reaction C2H5 + HO2 by Time-Resolved Mass Spectrometry", J. Phys. Chem. A 110 (2006) 3330-3337.

Contributing researchers: G. Friedrichs, J. Gripp, F. Temps and (formerly) W. Ludwig, B. Brandt