Counterintuitive Catalytic Reactivity of the Aluminum Oxide "Passivation" Shell of Aluminum Nanoparticles Facilitating the Thermal Decomposition of exo -Tetrahydrodicyclopentadiene (JP-10).

High energy density aluminum nanoparticles (AlNPs) have been at the center of attention as additives to hydrocarbon jet fuels like exo -tetrahydrodicyclopentadiene (JP-10, C 10 H 16 ) aiming at the superior performance of volume-limited air-breathing propulsion systems. However, a fundamental understanding of the ignition and combustion chemistry of JP-10 in the presence of AlNPs has been elusive. Exploiting an isomer-selective comprehensive identification of the decomposition products in a newly designed high-temperature chemical microreactor coupled to vacuum ultraviolet photoionization, we reveal an active low-temperature heterogeneous surface chemistry commencing at 650 K involving the alumina (Al 2 O 3 ) shell. Contrary to textbook knowledge of an "inactive alumina surface", this unconventional reactivity, where oxygen is transferred from alumina to JP-10, leads to generating cyclic, oxygenated organics like phenol (C 6 H 5 OH) and 2,4-cyclopentadiene-1-one (C 5 H 4 O)─key tracers of an alumina-mediated interfacial chemistry. This counterintuitive reactivity transforms our knowledge of the (catalytic) processes of alumina-coated AlNPs on the molecular level.