Investigation of thermal decomposition of nitrobenzene: An energetic material

Oxidation and pyrolysis mechanisms of energetic materials shape their macro properties such as autoigni-tion and detonation. The high reaction-enthalpy, microsecond order reaction time, and toxicity necessitate investigation of their detailed chemical kinetics. A shock tube combined with laser absorption diagnos-tic provides time-resolved, high-sensitivity, and homogeneous environment to study thermal degrada-tion of energetic and hazardous materials. In this work, we investigated the thermal fragmentation of nitrobenzene (C6H5NO2), a prototypical energetic material and nitro-aromatic representative, behind re-flected shock waves. We studied its pyrolysis and decomposition branching channels over temperatures of 1155-1434 K and pressures of 0.82-1.78 bar. Channel-specific branching ratio was determined by em-ploying a two-color laser absorption diagnostic. Two decomposition pathways are found to be significant under our experimental conditions. The first channel ( k 1 ) produces phenyl radical (C6H5) and NO2 while the second channel ( k 2 ) generates phenoxy radical (C6H5O) and NO. We monitored the reaction progress by tracing channel-specific products, i.e., phenyl radical (C6H5) and NO, using ultraviolet-visible (UV-vis) and infrared (IR) absorption diagnostics. UV-vis light at 445 nm for phenyl radical detection came from the frequency doubling of 890 nm light, generated by a titanium-sapphire laser, which was pumped by a green (532 nm) Nd:YAG laser. IR light at 5.517 mu m for NO detection was generated by an external-cavity quantum cascade laser. We extracted the rate coefficients of the two decomposition channels and fitted them in Arrhenius form (units of s-1): k 1 = 8 . 49 x 10 14 exp ( -30476 TK ) k 2 = 2 . 31 x 10 13 exp (-27618 TK ) Rate coefficients of both channels show positive temperature dependence while we did not observe discernible pressure dependence over 0.82-1.78 bar. Our measurements agree with a previous theoret-ical work qualitatively that, under these high-temperature conditions, the decomposition channel pro-ducing phenyl radicals and NO2 ( k 1 ) prevails over the NO-producing channel. Quantitatively, however, the theoretical work overestimates the importance of k 1 channel, particularly at low temperatures. Our rate of production analysis shows that most of the phenyl radicals are consumed through their self -recombination while N atoms are mainly converted to NO2. This work benefits high-fidelity chemical modeling of the relevant safety and ignition aspects of nitrogen-containing energetic/hazardous materi-als.(c) 2023 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

Automated summary

This research investigated the thermal fragmentation and decomposition pathways of nitrobenzene (C6H5NO2) behind reflected shock waves. It was found that the decomposition channel producing phenyl radicals and NO2 (k1) prevails under high-temperature conditions. This study contributes to accurate chemical modeling of the safety and ignition aspects of nitrogen-containing energetic/hazardous materials.