High-temperature Shock Waves Research Articles

Carbon-coordinated atomic cobalt directly embedded on carbon cloth for alkaline hydrogen evolution at high current density

Electrocatalysts for high current density hydrogen evolution reaction (HER) under alkaline conditions is critical for the widespread application of water electrolysis. Carbon-supported single-atom catalysts (SACs) stabilized by N atoms have been demonstrated as efficient electrocatalysts, but mostly for acidic HER. For alkaline HER, although defective-, edge-, and dual-sites have been found to improve the adsorption pathways of the intermediates, there is still a great challenge to precisely synthesize those complex configurations in experiments to maximize the synergistic effects. Since C atom has relatively weak electronegativity than N atom, it can induce different charge redistribution of the metal atom center. However, the reported C-coordinated SACs were derived from the N-containing precursors, which could not exclude the influence of heteroatoms on the activity completely. Herein, we reported the synthesis of carbon-coordinated CoC4 sites and ultra-small Co cluster directly embedded on pure carbon cloth (Co-SA/CC) via the high temperature shockwave (HTS) method. The C atoms on the first coordination shell can tune the local electronic structure and up-shift the d-band center of the Co atom, thus strengthening H2O molecule adsorption and promoting water dissociation. The self-supporting electrode with CoC4 can lower the interfacial contact resistance, leading to higher HER performance compared with that of Pt/C at high current density (e.g. 1000 mA/cm2). These results provide a new insight into the fundamental understanding of design and synthesis of C-coordinated atomically dispersed catalytic sites.

The long-lived reactive nitrogen species in the troposphere: DFTB model for atmospheric applications.

The longest lived reactive NO2 molecule formation in a dry and clean air environment under a high-temperature shock wave was investigated under three basic reactions (R2 for the O + NO system, R6 for the NO + NO3 system, and R7 for the NO + O3 system) in the atmospheric environment. With certain approaches, a DFTB3 model was used, which gave results close to the density functional theory. In the calculations, the related reactions up to 250 ps were examined at individual specific temperatures, and the temperature ranges that contributed to the formation of the NO2 molecule were determined. Moreover, a shock wave with both heating and cooling channels was applied only on R2 to see whether molecular concentrations were in good agreement with atmospheric information. The reaction products were examined under a shock wave of about 20 ps. At the end of the study, the applicability of the DFTB model to atmospheric systems was demonstrated by comparing it with experimental data and information. QCT approach was also used for the calculation of reaction rate constants of only O2-formation on the O + NO system. Here, all systems are focused on nitrogen species containing oxygen. In particular, the highest-population NO molecule that emerged in the lightning flash event was used as the reactant, while systems existing with the longest lived NO2 in the atmosphere after the lightning flash were focused in the product channel. As a result of the study, the hypothesis of geophysicists that almost all NO2 formed in the lightning flash event originates from the NO + O system was disproved. It has been proven that the presence of NO3 molecules that can withstand high temperatures in such systems should be evaluated.

Diabatic potential energy surfaces and semiclassical multi-state dynamics for fourteen coupled 3 A′ states of O3

Dissociation and energy transfer in high-energy collisions of O2 play important roles in simulating thermal energy content and heat flux in flows around hypersonic vehicles. Furthermore, atomic oxygen reactions on the vehicle surface are an important contributor to heat shield erosion. Molecular dynamics modeling is needed to better understand the relevant rate processes. Because it is necessary to model the gas flows in high-temperature shock waves, electronically excited states of O2 and O can be populated, and molecular dynamics simulations should include collisions of electronically excited species and electronically nonadiabatic collisions. This requires potential energy surfaces and state couplings for many energetically accessible electronic states. Here we report a systematic strategy to calculate such surfaces and couplings. We have applied this method to the fourteen lowest-energy potential energy surfaces in the 3 A′ manifold of O3, and we report a neural-network fit to diabatic potential energy matrix (DPEM). We illustrate the use of the resulting DPEM by carrying out semiclassical dynamics calculations of cross sections for excitation of O2 in 3 A′ collisions with O at two collision energies; these dynamics calculations are carried out by the curvature-driven coherent switching with decay of mixing method.

IO<sub>2</sub>-Supported Single-Atom Catalysts for Photocatalytic Reactions

Titania（TiO2） has been among the most widely investigated and used metal oxides over the past years, as it has various functional applications. Extensive research into TiO2 and industrial interest in this material have been triggered by its high abundance, excellent corrosion resistance, and low cost. To improve the activity of TiO2 in heterogeneous catalytic reactions, noble metals are used to accelerate the reactions. However, in the case of nanoparticles supported on TiO2, the active sites are usually limited to the peripheral sites of the noble metal particles or at the interface between the particle and the support. Thus, highly dispersed single metal atoms are desired for the effective utilization of precious noble metals. The study of oxide-supported isolated atoms, the so-called single-atom catalysts（SACs）, was pioneered by Zhang’s group. The high dispersion of precious noble metals results helps reduce the cost associated with catalyst preparation. Because of the presence of active centers as single atoms, the deactivation of metal atoms during the reaction, e.g., by coking for large agglomerates, is retarded. The unique coordination environment of the noble metal center provides special sites for the reaction, consequently increasing the selectivity of the reaction, including the enantioselectivity and stereoselectivity. Hence, supported SACs can bridge homogenous and heterogeneous reactions in solution as they provide selective reaction sites and are recyclable. Moreover, owing to the high site homogeneity of the isolated metal atoms, SACs are ideal models for establishing the structure-activity relationships. The present review provides an overview of recent works on the synthesis, characterization, and photocatalytic applications of SACs（Pt1, Pd1, Ir1, Rh1, Cu1, Ru1） supported on TiO2. The preparation of single atoms on TiO2 includes the creation of surface defective sites, surface modification, stabilization by high-temperature shockwave treatment, and metal-ligand self-assembly. Conventional characterization methods are categorized as microscopic imaging and spectroscopic methods, such as aberration-corrected scanning transmission electron microscopy（STEM）, scanning tunneling microscopy（STM）, extended X-ray absorption fine structure analysis（EXAFS）, and diffuse reflectance infrared Fourier transform spectroscopy（DRIFTS）. We attempted to address the critical factors that lead to the stabilization of single-metal atoms on TiO2, and elucidate the mechanism underlying the photocatalytic hydrogen evolution and CO2 reduction. Although many fascinating applications of TiO2-supported SACs in photocatalysis could only be addressed superficially and in a referencing manner, we hope to provide interested readers with guidelines based on the wide literature, and more specifically, to provide a comprehensive overview of TiO2-supported SACs.

High temperature shockwave stabilized single atoms.

The stability of single-atom catalysts is critical for their practical applications. Although a high temperature can promote the bond formation between metal atoms and the substrate with an enhanced stability, it often causes atom agglomeration and is incompatible with many temperature-sensitive substrates. Here, we report using controllable high-temperature shockwaves to synthesize and stabilize single atoms at very high temperatures (1,500-2,000 K), achieved by a periodic on-off heating that features a short on state (55 ms) and a ten-times longer off state. The high temperature provides the activation energy for atom dispersion by forming thermodynamically favourable metal-defect bonds and the off-state critically ensures the overall stability, especially for the substrate. The resultant high-temperature single atoms exhibit a superior thermal stability as durable catalysts. The reported shockwave method is facile, ultrafast and universal (for example, Pt, Ru and Co single atoms, and carbon, C3N4 and TiO2 substrates), which opens a general route for single-atom manufacturing that is conventionally challenging.

Dispersing nanoparticles into single atoms.

While heating a catalyst causes growth of nanoparticles through Ostwald ripening, repeated on/off high-temperature shockwaves can reverse the process, converting the nanoparticles into stable single-atom catalysts.

Effect of high-temperature shock-wave compression on few-layer MoS2, WS2 and MoSe2

Exposure of few-layer MoS2, WS2 and MoSe2 to high-temperature shock waves causes morphological changes and a significant decrease in the interlayer separation between the (002) planes, the decrease being greatest in MoSe2. Raman spectra show softening of both the A1g and the E2g1 modes initially, followed by a slightly stiffening. Using first-principles density functional theoretical analysis of the response of few-layer MoS2 to shock waves, we propose that a combination of shear and uniaxial compressive deformation leads to flattening of MoS2 sheets which is responsible for the changes in the vibrational spectra.

Temperature and Pressure Dependence of the Reaction 2CF3 (+ M) ⇔ C2F6 (+ M)

Limiting low- and high-pressure rate coefficients as well as full falloff curves have been modeled by unimolecular rate theory for the recombination reaction 2CF(3) (+ M) --> C(2)F(6) (+ M) and the reverse dissociation of C(2)F(6). The results are compared with experimental data from the literature. Although there are considerable discrepancies (up to a factor of 5) between various experimental data near 300 K and the database for high temperatures is still limited, we try to conclude on the temperature dependence of the high-pressure rate coefficient. We suggest that there is only a small and probably positive temperature coefficient of the latter quantity. The present theoretical modeling seems to be in agreement with this experimental result, but it is in disagreement with conclusions from earlier theoretical work. The difference is attributed to different empirical assumptions about the anisotropy of the potential. It is shown that nearly all previous experiments (except high-temperature shock wave and very low pressure pyrolysis/photolysis experiments) correspond to nearly limiting high-pressure conditions.

Nonequilibrium Dissociation Processes in Hyperbolic Atmospheric Entries

Physical-chemical processes encountered behind atmospheric entry shock waves are known to occur in extremely nonequilibrium conditions. Also, translational temperatures up to 100,000 K may be reached immediately behind the shock wave. Such challenging conditions require the development of adequate state-to-state models, which prevents using widespread first-order theories. A complete database for the simulation of state-resolved dissociation processes is presented in this paper. Rate coefficients valid up to very high temperatures have been obtained for diatom-diatom collisions, using the forced harmonic oscillator theory. The rate coefficients for atom-diatom collisions have been selected after a critical review of the existing data sets, as the forced harmonic oscillator theory proved inadequate for the simulation of such processes. Such a consistent state-to-state model has then been used for simulating nitrogen dissociation processes behind very high-temperature shock waves, and the obtained results have been compared with those obtained using popular one- and two-temperature models.

State-Resolved Dissociation Rates for Extremely Nonequilibrium Atmospheric Entries

This paper presents the application of the forced harmonic oscillator method to the simulation of state-resolved dissociation processes behind high-temperature shock waves typical of atmospheric reentries. Improvements have been brought to the model, considering a more precise method for the calculation of the different vibrational level energies,thereforeincreasingtheaccuracyofthepredictedtransitionprobabilitiesbetweenhighervibrationallevels close and above the dissociation limit. The model has been validated against data issued from recent experiments, as well as data issued from semiclassical trajectory calculations for collisions between different species. A good overall agreementisachievedagainstsuchotherdata.Adatabaseofreactionrateshasbeenconstructed withthepurposeof simulating shock-heated nitrogen flows. Dissociation processes behind a shock wave have been simulated for different postshock translational temperatures. At lower temperatures, the well-known ladder-climbing phenomenon is the main dissociation channel behind a shock wave, with dissociation occurring for transitions from the vibrational levels close to the dissociation limit. At higher temperatures, transitions between the different vibrational levels of nitrogen become roughly equiprobable, and the overall range of bound vibrational levels contributes to the dissociation.

Ethylene combustion studied over a wide temperature range in high-temperature shock waves

The initiation of ethylene combustion (at φ=1 and φ=1.5) was studied over a wide range of temperatures (∼800-1620 K) in shock waves. Ignition delay data showed the presence of two or three different chemically controlling regimes in the combustion. At low temperatures (∼900-1000 K) the time development of a combustion bubble was tracked via Schlieren techniques and was observed to grow slowly. Unburnt gas between the bubble and end wall that was compressed and heated by the latter exploded and gave rise to a strong combustion wave that traveled back through the partially burning gas. Emission measurements showed the presence of CH*, OH*, C 2 *, and a continuum emission attributed to CO 2 * flame bands. C 2 * was found to be only important in richer mixtures. CH* was formed only microseconds ahead of the other diatomics.

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Exposure of few-layer MoS2, WS2 and MoSe2 to high-temperature shock waves causes morphological changes and a significant decrease in the interlayer separation between the (0 0 2) planes, the decrease being greatest in MoSe2. Raman spectra show softening of both the A1g and the E2g1 modes initially, followed by a slightly stiffening. Using first-principles density functional theoretical analysis of the response of few-layer MoS2 to shock waves, we propose that a combination of shear and uniaxial compressive deformation leads to flattening of MoS2 sheets which is responsible for the changes in the vibrational spectra.

An upper limit to the abundance of lightning-produced amino acids in the Jovian water clouds

The effect of excess hydrogen on the synthesis of amino acids, by high-temperature shock waves in a hydrogen/methane/ammonia/water vapor mixture, was studied experimentally. The energy efficiency results, together with the best estimate of the lightning energy dissipation rate on Jupiter, from the Voyager data, were used to calculate an upper limit to the rate of amino acid production by lightning in the Jovian water clouds. Using reasonable values for the eddy diffusion coefficients within and below the water clouds, the column abundance of lightning-produced amino acids in the clouds was estimated to be 6.2 × 10 −6 cm-am. Hence, concentration of amino acids in water droplets would be 8 × 10 −8 mole liter −1.

Lightning synthesis of organic compounds on Jupiter

The production of distinctive disequilibrium species in high-temperature shock waves is considered from an energetic viewpoint in order to assess the production of CO, HCN, CH 2O, and hydrocarbons by lightning discharges on Jupiter. The maximum efficiency of production of these species is found to be ∼2 × 10 −14 g · erg −1 for shock to the CH 4/CO boundary (where p CO = p CH 4 ) at pressures below ∼100 b. Using Jupiter's entire internal heat source of 0.9 × 10 4 ergs · cm −2 · sec −1 to make lightning with the same efficiency as on Earth (10 −5 erg lightning per erg of total energy flux) produces ∼ 2 × 10 −15 g · cm −2 · sec −1 of CO, 2 × 10 −17 g · sec −1 of HCN, and ∼2 × 10 −19 g · cm −2 · sec −1 of organic matter. Photochemical and thermochemical sources of CO and N 2, the dominant disequilibrium shockwave products, are far larger than the lightning source. The steady-state mole fraction of the most abundant distinctive lightning product, HCN, near the 300°K level is predicted to be only ∼3 × 10 −17. Even if every erg of Jupiter's heat flux is used with perfect efficiency to shock gas to the optimum ( P, T) conditions for efficient HCN synthesis, X HCN from this source is only ∼3 × 10 −12. Lightning synthesis is of negligible importance on Jupiter.

The mechanisms of amino acids synthesis by high temperature shock-waves.

The mechanisms of amino acids synthesis behind high temperature shock-waves were elucidated and distinction was made between the steps occurring in the gas phase and those occurring in solution. In the presence of water vapor, aldehydes and HCN are formed separately in regions of different temperatures along the reacting gas. The aldehydes and ammonia condense to aldimines which add HCN to form alpha-amino nitriles, all in the gas phase. The hydrolysis to amino acids takes place in solution. In the absence of water vapor, aldimines and HCN are formed in the gas phase but condense to alpha-amino nitriles only in solution. A fair amount of oxygen only lowers the production of amino acids, which consequently could be still produced in the presence of oxygen in the Earth's primitive atmoshere. The waterless mechanism can operate in the Jovian atmosphere and supply it with ample amounts of amino acids, especially aspartic.

Pressure Probes for Research in Plasma Dynamics and Detonation

Three pressure transducers that can be used in high temperature gas dynamic studies are described and their performance discussed. It is shown that the Baganoff gauge can be usefully employed to study high temperature and pressure gaseous detonation wave phenomena, provided the gauge is constructed from glass. This modification results in greatly reduced sensitivity, but preserves the high frequency response and over-all characteristics of the original gauge. A second gauge, using the pressure bar principle, is shown to have wide application in studies of high temperature shock waves in the presence of large magnetic and electric fields, provided the pressure bar is constructed from a nonconductor having appropriate properties. The results of experiments with this gauge indicate that, even with poor acoustic matching of the gauge elements, it is possible to obtain an accurate estimate of the pressure behind a shock or blast wave with a time resolution of about 2 μsec. The third device described is designed for use where only the accurate arrival time of a pressure pulse is desired and has been used extensively at this Laboratory for plotting shock trajectories in magnetic shock tubes.

Theoretical Behavior of the Initial Supernova Light Emission.

view Abstract Citations References Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Theoretical Behavior of the Initial Supernova Light Emission. Colgate, Stirling A. ; White, Richard H. Abstract Hydrodynamic calculations of stellar unstable collapse have demonstrated explosions for all stars of mass greater than the Chandrasekhar limit M>1.SMo (Colgate, S. A., and White, R. H., "Cosmic Rays from Large Supernova," to be published in Proc. Intern. Conf. Cosmic Rays, Jaipur, India, 1963). The mechanism depends upon the emission and redeposition of an intense neutrino flux from the high-temperature shock wave of matter falling onto the dense neutron core. (T~1012 deg, p-10,4-10,5 g/cm3 r=10 km.) The resulting heating of matter at larger radius ejects a major fraction of the mass of the star first by an expansion close to the core, and further out by a strong shock wave. The internal energy following an adiabatic expansion of the envelope leads to a luminosity peak at 1 day and 1 % of the observed luminosity. The matter ejected from close to the core is initially exceedingly neutron rich and the subsequent beta decay following expansion to radiation transparency gives rise to a peak luminosity equal to the observed at a time of one week. This work was performed under the auspices of the U. S. Atomic Energy Commission. Publication: The Astronomical Journal Pub Date: 1964 DOI: 10.1086/109443 Bibcode: 1964AJ.....69R.537C full text sources ADS |