Brotton Stephen J, Perera Sahan D, Misra Anupam, Kleimeier N Fabian, Turner Andrew M, Kaiser Ralf I, Palenik Mark, Finn Matthew T, Epshteyn Albert, Sun Bing-Jian, Zhang Li-Jie, Chang Agnes H H
Department of Chemistry, University of Hawaii at Manoa, Honolulu, Hawaii 96822, United States.
U.S. Naval Research Laboratory, Washington, D.C., Washington, D.C. 20375, United States.
J Phys Chem A. 2022 Jan 13;126(1):125-144. doi: 10.1021/acs.jpca.1c08335. Epub 2021 Dec 22.
We report the results on the combustion of single, levitated droplets of -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 (BO) plus atomic oxygen might operate in the gas phase, although the involvement of surface oxidation processes of RMNPs to BO cannot be discounted. The UV-vis emission spectra also revealed the key reactive intermediates (OH, CH, C, 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 (TiO), it does not engage in an active JP-10 chemistry as all abstraction pathways are endoergic by more than 217 kJ mol. 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. Therefore, aluminum and titanium react preferentially with molecular oxygen to produce their monoxides. However, the formation of BO, AlO, and BO 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 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 BO 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 BO) formed in the initial stage of the oxidation process.
我们报告了在氧气（60%）-氩气（40%）气氛中，利用超声波悬浮器使掺杂有钛铝硼（Ti-Al-B）活性金属纳米粉末（RMNPs）的单滴悬浮的 - 四氢二环戊二烯（JP - 10）液滴燃烧的结果，液滴由二氧化碳激光点燃。紫外 - 可见（UV - vis）发射光谱揭示了气相铝（Al）和钛（Ti）原子的存在。这些原子可在气相中被分子氧氧化，形成光谱检测到的一氧化铝（AlO）和一氧化钛（TiO）瞬态。对光学点火视频的分析支持纳米颗粒在JP - 10之前被点燃。一氧化硼（BO）的检测进一步表明通过RMNPs的氧化以及至少BO释放到气相中存在活跃的表面化学过程。尽管不能排除RMNPs表面氧化过程对BO的参与，但分子氧将气相BO氧化为二氧化硼（BO）加原子氧的过程可能在气相中发生。UV - vis发射光谱还揭示了JP - 10氧化的关键反应中间体（OH、CH、C和HCO）。电子结构计算表明，反应性自由基的存在对JP - 10的氧化有深远影响。尽管一氧化钛（TiO）反应生成二氧化钛（TiO），但它不参与活跃的JP - 10化学反应，因为所有抽象途径的吸能都超过217 kJ/mol。原子铝和钛的情况类似，它们从JP - 10的氢提取反应被发现至少吸能77 kJ/mol。因此，铝和钛优先与分子氧反应生成它们的一氧化物。然而，BO、AlO和BO的形成提供了一组高反应性自由基，它们可以通过仅比分离的反应物高1至5 kJ/mol的过渡态从JP - 10中提取氢，在整体放能反应中形成JP - 10自由基以及氢提取产物（硼氢氧化物、氢氧化铝和偏硼酸）。这些提取势垒远低于基态原子氧和分子氧的提取势垒。从这个意义上说，气相BO、AlO和BO通过氢提取催化气相JP - 10的氧化，形成高反应性的JP - 10自由基。总体而言，向JP - 10中添加RMNPs不仅提供了更高能量密度的燃料，而且由于在氧化过程初始阶段形成的高反应性自由基池（BO、AlO和BO），预计与纯JP - 10相比点火延迟更短。
