Combined Spectroscopic and Computational Investigation on the Oxidation of exo-Tetrahydrodicyclopentadiene (JP-10; C10H16) Doped with Titanium-Aluminum-Boron Reactive Metal Nanopowder

Stephen J Brotton; Sahan D Perera; Anupam Misra; N Fabian Kleimeier; Andrew M Turner; Ralf I Kaiser; Mark Palenik; Matthew T Finn; Albert Epshteyn; Bing-Jian Sun; Li-Jie Zhang; Agnes H H Chang

doi:10.1021/acs.jpca.1c08335

Combined Spectroscopic and Computational Investigation on the Oxidation of exo-Tetrahydrodicyclopentadiene (JP-10; C₁₀H₁₆) Doped with Titanium-Aluminum-Boron Reactive Metal Nanopowder

J Phys Chem A. 2022 Jan 13;126(1):125-144. doi: 10.1021/acs.jpca.1c08335. Epub 2021 Dec 22.

Affiliations

¹ Department of Chemistry, University of Hawaii at Manoa, Honolulu, Hawaii 96822, United States.
² U.S. Naval Research Laboratory, Washington, D.C., Washington, D.C. 20375, United States.
³ Department of Chemistry, National Dong Hwa University, Shoufeng, Hualien 974, Taiwan.

PMID: 34935392
DOI: 10.1021/acs.jpca.1c08335

Abstract

We report the results on the combustion of single, levitated droplets of exo-tetrahydrodicyclopentadiene (JP-10) doped with titanium-aluminum-boron (Ti-Al-B) reactive metal nanopowders (RMNPs) in an oxygen (60%)-argon (40%) atmosphere by exploiting an ultrasonic levitator with droplets ignited by a carbon dioxide laser. Ultraviolet-visible (UV-vis) emission spectroscopy revealed the presence of gas-phase aluminum (Al) and titanium (Ti) atoms. These atoms can be oxidized in the gas phase by molecular oxygen to form spectroscopically detected aluminum monoxide (AlO) and titanium monoxide (TiO) transients. Analysis of the optical ignition videos supports that the nanoparticles are ignited before JP-10. The detection of boron monoxide (BO) further proposes an active surface chemistry through the oxidation of the RMNPs and the release of at least BO into the gas phase. The oxidation of gas-phase BO by molecular oxygen to boron dioxide (BO₂) plus atomic oxygen might operate in the gas phase, although the involvement of surface oxidation processes of RMNPs to BO₂ cannot be discounted. The UV-vis emission spectra also revealed the key reactive intermediates (OH, CH, C₂, and HCO) of the oxidation of JP-10. Electronic structure calculations reveal that the presence of reactive radicals has a profound impact on the oxidation of JP-10. Although titanium monoxide (TiO) reacts to produce titanium dioxide (TiO₂), it does not engage in an active JP-10 chemistry as all abstraction pathways are endoergic by more than 217 kJ mol^-1. This is similar for atomic aluminum and titanium, whose hydrogen abstraction reactions from JP-10 were revealed to be endoergic by at least 77 kJ mol^-1. Therefore, aluminum and titanium react preferentially with molecular oxygen to produce their monoxides. However, the formation of BO, AlO, and BO₂ supplies a pool of highly reactive radicals, which can abstract hydrogen from JP-10 via transition states ranging from only 1 to 5 kJ mol^-1 above the separated reactants, forming JP-10 radicals along with the hydrogen abstraction products (boron hydride oxide, aluminum monohydroxide, and metaboric acid) in the overall exoergic reactions. These abstraction barriers are well below the barriers of abstractions for ground-state atomic oxygen and molecular oxygen. In this sense, gas-phase BO, AlO, and BO₂ catalyze the oxidation of gas-phase JP-10 via hydrogen abstraction, forming highly reactive JP-10 radicals. Overall, the addition of RMNPs to JP-10 not only provides a higher energy density fuel but is also expected to lead to shorter ignition delays compared to pure JP-10 due to the highly reactive pool of radicals (BO, AlO, and BO₂) formed in the initial stage of the oxidation process.