Stress-Alteration Enhancement of the Reactivity of Aluminum Nanoparticles in the Catalytic Decomposition of exo-Tetrahydrodicyclopentadiene (JP-10).

High-energy-density aluminum nanoparticles (AlNPs) upon thermal annealing followed by superquenching result in elevated stress levels in the metallic core and reduced surface energy at the core-shell interface. Isomer-selective vacuum ultraviolet-based photoionization mass spectrometry coupled to a high-temperature chemical microreactor reveals that these stress-altered AlNPs (SA-AlNPs) exhibit distinctive temperature-dependent reactivities toward catalytic decomposition of the hydrocarbon jet fuel exo-tetrahydrodicyclopentadiene (JP-10, C10H16) compared to untreated AlNPs (UN-AlNPs). SA-AlNPs show a delayed initiation of the decomposition for JP-10 by 200 K relative to the UN-AlNPs; however, the full decomposition is achieved at a 100 K lower temperature. Furthermore, there are fewer oxygenated products that are generated from the alumina surface-induced heterogeneous oxidation process and a larger fraction of closed- and open-shell hydrocarbons. Chemical insight bridging the reactivity order of SA-AlNPs at low and high temperatures, simultaneously, is obtained via a detailed examination of the product branching ratios obtained in this study.

Efficient Oxidative Decomposition of Jet-Fuel exo-Tetrahydrodicyclopentadiene (JP-10) by Aluminum Nanoparticles in a Catalytic Microreactor: An Online Vacuum Ultraviolet Photoionization Study.

The oxidation of gas-phase exo-tetrahydrodicyclopentadiene (JP-10, C10H16) over aluminum nanoparticles (AlNP) has been explored between a temperature range of 300 and 1250 K with a novel chemical microreactor. The results are compared with those obtained from chemical microreactor studies of helium-seeded JP-10 and of helium-oxygen-seeded JP-10 without AlNP to gauge the effects of molecular oxygen and AlNP, respectively. Vacuum ultraviolet (VUV) photoionization mass spectrometry reveals that oxidative decomposition of JP-10 in the presence of AlNP is lowered by 350 and 200 K with and without AlNP, respectively, in comparison with pyrolysis of the fuel. Overall, 63 nascent gas-phase products are identified through photoionization efficiency (PIE) curves; these can be categorized as oxygenated molecules and their radicals as well as closed-shell hydrocarbons along with hydrocarbon radicals. Quantitative branching ratios of the products reveal diminishing yields of oxidized species and enhanced branching ratios of hydrocarbon species with the increase in temperature. While in the low-temperature regime (300-1000 K), AlNP solely acts as an efficient heat transfer medium, in the higher-temperature regime (1000-1250 K), chemical reactivity is triggered, facilitating the primary decomposition of the parent JP-10 molecule. This enhanced reactivity of AlNP could plausibly be linked to the exposed reactive surface of the aluminum (Al) core generated upon the rupture of the alumina shell material above the melting point of the metal (Al).

Crossed-Beam Imaging of the Reaction of OH with Propanol Isomers.

We present a crossed-beam imaging study of the reactions of OH radicals with 1- and 2-propanol at a collision energy of 8 kcal mol-1 using 157 nm probe of the radical product. Our detection is selective for the α-H and ß-H abstraction in the 1-propanol case and for the α-H abstraction only in the 2-propanol case. The results show direct dynamics. A sharply peaked backscattered angular distribution is seen for the 2-propanol system and broader backward-sideways scattering for 1-propanol consistent with the different abstraction sites. The translational energy distributions peak at ∼35% of the collision energy, far from the heavy-light-heavy kinematic propensity. As this is ∼10% of the available energy, substantial vibrational excitation in the water product is inferred. The results are discussed in relation to analogous OH + butane and O(3P) + propanol reactions.

Multichannel Radical-Radical Reaction Dynamics of NO + Propargyl Probed by Broadband Rotational Spectroscopy.

Chirped-pulse rotational spectroscopy in a quasi-uniform flow has been used to investigate the reaction dynamics of a multichannel radical-radical reaction of relevance to planetary atmospheres and combustion. In this work, the NO + propargyl (C3H3) reaction was found to yield six product channels containing eight detected species. These products and their branching fractions (%), are as follows: HCN (50), HCNO (18), CH2CN (12), CH3CN (7.4), HC3N (6.2), HNC (2.3), CH2CO (1.3), HCO (1.8). The results are discussed in light of previous unimolecular photodissociation studies of isoxazole and prior potential energy surface calculations of the NO + C3H3 system. The results also show that the product branching is strongly influenced by the excess energy of the reactant radicals. The implications of the title reaction to the planetary atmospheres, particularly to Titan, are discussed.

Uniform supersonic flow sampling for detection by chirped-pulse rotational spectroscopy.

Chirped-pulse Fourier transform microwave (CP-FTMW) spectroscopy is a powerful near-universal detection method finding application in many areas. We have previously coupled it with supersonic flows (CPUF) to obtain product branching in reaction and photodissociation. Because chirped-pulse microwave detection requires monitoring the free induction decay on the timescale of microseconds, it cannot be employed with good sensitivity at the high densities achieved in some uniform supersonic flows. For application to low-temperature kinetics studies, a truly uniform flow is required to obtain reliable rate measurements and enjoy all the advantages that CP-FTMW has to offer. To this end, we present a new setup that combines sampling of uniform supersonic flows using an airfoil-shaped sampling device with chirped-pulse mmW detection. Density and temperature variations in the airfoil-sampled uniform flow were revealed using time-dependent rotational spectroscopy of pyridine and vinyl cyanide photoproducts, highlighting the use of UV photodissociation as a sensitive diagnostic tool for uniform flows. The performance of the new airfoil-equipped CPUF rotational spectrometer was validated using kinetics measurements of the CN + C2H6 reaction at 50 K with detection of the HCN product. Issues relating to product detection by rotational spectroscopy and airfoil sampling are discussed. We show that airfoil sampling enables direct measurements of low temperature reaction kinetics on a microsecond timescale, while rotational spectroscopic detection enables highly specific simultaneous detection of reactants and products.

Imaging the infrared multiphoton excitation and dissociation of propargyl chloride.

Infrared multiphoton excitation is combined with UV excitation and state-resolved probes of Cl(2P3/2), Cl*(2P1/2), and HCl to study the photochemistry of propargyl chloride. The results show evidence both of infrared multiphoton dissociation on the ground electronic state and infrared multiphoton excitation followed by UV dissociation. The results are interpreted with the aid of a full characterization of the stationary points on the ground state using ab initio methods, as well as our recent experimental and theoretical characterization of the UV photochemistry of the molecule. The data suggest elimination of HCl on the ground electronic state produces linear propadienylidene as a coproduct over a roaming-like transition state that accesses the Cl-H-C abstraction geometry. This identification is supported by separate chirped-pulse microwave studies in a quasi-uniform flow also reported here.

Isomer-specific detection in the UV photodissociation of the propargyl radical by chirped-pulse mm-wave spectroscopy in a pulsed quasi-uniform flow.

Isomer-specific detection and product branching fractions in the UV photodissociation of the propargyl radical is achieved through the use of chirped-pulse Fourier-transform mm-wave spectroscopy in a pulsed quasi-uniform flow (CPUF). Propargyl radicals are produced in the 193 nm photodissociation of 1,2-butadiene. Absorption of a second photon leads to H atom elimination giving three possible C3H2 isomers: singlets cyclopropenylidene (c-C3H2) and propadienylidene (l-C3H2), and triplet propargylene (3HCCCH). The singlet products and their appearance kinetics in the flow are directly determined by rotational spectroscopy, but due to the negligible dipole moment of propargylene, it is not directly monitored. However, we exploit the time-dependent kinetics of H-atom catalyzed isomerization to infer the branching to propargylene as well. We obtain the overall branching among H loss channels to be 2.9% (+1.1/-0.5) l-C3H2 + H, 16.8% (+3.2/-1.3) c-C3H2 + H, and 80.2 (+1.8/-4.2) 3HCCCH + H. Our findings are qualitatively consistent with earlier RRKM calculations in that the major channel in the photodissociation of the propargyl radical at 193 nm is to 3HCCCH + H; however, a greater contribution to the energetically most favorable isomer, c-C3H2 + H is observed in this work. We do not detect the predicted HCCC + H2 channel, but place an upper bound on its yield of 1%.

Direct versus Indirect Photodissociation of Isoxazole from Product Branching: A Chirped-Pulse Fourier Transform mm-Wave Spectroscopy/Pulsed Uniform Flow Investigation.

The UV photodissociation of isoxazole (c-C3H3NO) is studied in this work by chirped-pulse Fourier transform mm-wave spectroscopy in a pulsed uniform Laval flow. This approach offers a number of advantages over traditional spectroscopic detection methods due to its broadband, sub-MHz resolution, and fast-acquisition capabilities. In coupling this technique with a quasi-uniform Laval flow, we are able to obtain product branching fractions in the 193 nm photodissociation of isoxazole. Five dissociation channels are explored through direct detection of seven different photoproducts. These species and their respective branching fractions (%) include the following: HCN (53.8 ± 1.7), CH3CN (23.4 ± 6.8), HCO (9.5 ± 2.3), CH2CN (7.8 ± 2.9), CH2CO (3.8 ± 0.9), HCCCN (0.9 ± 0.2), and HNC (0.8 ± 0.2). Guided by previous electronic structure and dynamics simulations, we are able to elucidate the dissociation dynamics that govern the final product branching fractions observed in this work, which differ significantly from previous reports on the thermal decomposition of isoxazole. Interestingly, both direct and indirect dynamics contribute to its dissociation, and clear signatures of both are manifested in the relative branching ratios obtained. Consistent with previous studies on the unimolecular dissociation of isoxazole, our findings also suggest the importance of the open-shell singlet diradicaloid species vinylnitrene in the dissociation dynamics, regardless of the initially populated excited state. This work, taken together with previous investigations, provides a global picture of the complex dissociation pathways involved in the photodissociation of isoxazole.

Interfacial Nanostructure and Hydrogen Bond Networks of Choline Chloride and Glycerol Mixtures Probed with X-ray and Vibrational Spectroscopies.

The molecular distribution at the liquid-vapor interface and evolution of the hydrogen bond interactions in mixtures of glycerol and choline chloride are investigated using X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. Nanoscale depth profiles of supersaturated deep eutectic solvent (DES) mixtures up to ∼2 nm measured by ambient-pressure XPS show the enhancement of choline cation (Ch+) concentration by a factor of 2 at the liquid-vapor interface compared to the bulk. In addition, Raman spectral analysis of a wide range of DES mixtures reveals the conversion of gauche-conformer Ch+ into the anti-conformer in relatively lower ChCl concentrations. Finally, the depletion of Ch+ from the interface (probing depth = 0.4 nm) is demonstrated by aerosol-based velocity map imaging XPS measurements of glyceline and water mixtures. The nanostructure of liquid-vapor interfaces and structural rearrangement by hydration can provide critical insight into the molecular origin of the deep eutectic behavior and gas-capturing application of DESs.

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, C10H16) 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 (Al2O3) 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 (C6H5OH) and 2,4-cyclopentadiene-1-one (C5H4O)─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.

Radical-Radical Reaction Dynamics Probed Using Millimeterwave Spectroscopy: Propargyl + NH2/ND2.

We apply chirped-pulse uniform flow millimeterwave (CPUF-mmW) spectroscopy to study the complex multichannel reaction dynamics in the reaction between the propargyl and amino radicals (C3H3 + NH2/ND2), a radical-radical reaction of importance in the gas-phase chemistry of astrochemical environments and combustion systems. The photolytically generated radicals are allowed to react in a well-characterized quasi-uniform supersonic flow, and mmW rotational spectroscopy (70-93 GHz) is used for simultaneous detection of the reaction products: HCN, HNC, HC3N, DCN, DNC, and DC3N, while spectral intensities of the measured pure-rotational lines allow product branching to be quantified. High-level electronic structure calculations were used for theoretical prediction of the reaction pathways and branching. Experimentally deduced product branching fractions were compared with the results from statistical simulations based on the RRKM theory. Product branching was found to be strongly dependent on the excess internal energy of the C3H3 and NH2/ND2 reactants.