Measuring hydroperoxide chain-branching agents during n-pentane low-temperature oxidation

Abstract The reactions of chain-branching agents, such as H 2 O 2 and hydroperoxides, have a decisive role in the occurrence of autoignition. The formation of these agents has been investigated in an atmospheric-pressure jet-stirred reactor during the low-temperature oxidation of n- pentane (initial fuel mole fraction of 0.01, residence time of 2 s) using three different diagnostics: time-of-flight mass spectrometry combined with tunable synchrotron photoionization, time-of-flight mass spectrometry combined with laser photoionization, and cw- cavity ring-down spectroscopy. These three diagnostics enable a combined analysis of H 2 O 2 , C 1 –C 2 , and C 5 alkylhydroperoxides, C 3 –C 5 alkenylhydroperoxides, and C 5 alkylhydroperoxides including a carbonyl function (ketohydroperoxides). Results using both types of mass spectrometry are compared for the stoichiometric mixture. Formation data are presented at equivalence ratios from 0.5 to 2 for these peroxides and of two oxygenated products, ketene and pentanediones, which are not usually analyzed during jet-stirred reactor oxidation. The formation of alkenylhydroperoxides during alkane oxidation is followed for the first time. A recently developed model of n- pentane oxidation aids discussion of the kinetics of these products and of proposed pathways for C 3 –C 5 alkenylhydroperoxides and the pentanediones.