Formation pathways of HO 2 and OH changing as a function of temperature in photolytically initiated oxidation of dimethyl ether

Abstract

The time resolved product formation in oxidation of dimethyl ether (DME) has been studied between 298–625 K and 20–90 torr total pressure. Near-infrared frequency modulation spectroscopy (FMS) with Herriott type multi pass optics and UV absorption spectroscopy (UV) were conducted in the same cell. The reaction was initiated by pulsed photolysis in a mixture of Cl 2, O 2, and DME via CH 3OCH 2 radical formation. The reaction process was investigated through FMS measurement of HO 2 and OH, and UV measurement of CH 3OCH 2O 2. The yields of HO 2 and OH are obtained by comparison with reference mixtures, Cl 2, O 2, and CH 3OH for HO 2, and Cl 2, O 2, CH 3OH, and NO for OH, which convert 100% of initial Cl to HO 2 and OH. The CH 3OCH 2O 2 yield is also obtained. It was found that the HO 2 yield increases sharply over 500 K mainly with a longer time constant than that of R + O 2 reaction, while a prompt component exists throughout the temperature range at a few percent yield. OH was found to be produced promptly at a yield considerably larger than that known for the simplest alkanes. The CH 3OCH 2O 2 profile has a prompt rise followed by a gradual decay whose rate is consistent with the slow HO 2 formation. The species profiles were successfully predicted with a model constructed by modifying the existing one to suit the reduced pressure condition. After modification, it was inferred that the HO 2 formation over 500 K is secondary from HCHO + OH and HCO + O 2 and a part of HCO is formed directly from the O 2 adduct, whereas the HO 2 formation below 500 K is governed by CH 3OCH 2O 2 chemistry. The HCO forming pathway via isomerization–decomposition of the O 2 adduct, which was not included in the former models, was supported by our quantum-chemical calculations.