Atmospheric fate of diketones and OH radical–kinetics,reaction force,ETS-NOCV analysis

In this work, we have focused on the atmospheric reaction chemistry of two α-diketones 2,3-Pentanedione (2,3-PTD) and 2,3-hexanedione (2,3-HEX) with OH radical. The full reaction pathway was studied theoretically under H-atom abstraction reaction using density functional theory and wave-function-based MP2 calculations. Single-point energy calculations were performed at CCSD(T) level of theory with 6–31+G(d,p) basis set. The H-atom abstraction from –CH₂ group is the most dominant channel in both 2,3-PTD and 2,3-HEX with OH radical. The fate of secondary reactions of peroxy and alkoxy radical is studied in detail. The reaction force analysis shows that abstraction process is dominated by structural rearrangement than electronic reordering. The ETS-NOCV-based reaction scheme is studied in order to find out the pair wise interaction energy of the chemical bonding. The ETS-NOCV method for all the transition states shows π-bonding nature for the bond breaking (C–H) and bond formation (O–H) due to the presence of hydrogen bond. The theoretical rate constant value matches well with the experimental rate constant value for both α-diketones. Normal linear Arrhenius behaviour for all the pathways is found in the range of 278–350 K. The short atmospheric lifetime indicates the removal process of diketones with OH radical.     

Mechanism and kinetics of the atmospheric degradation of 2-formylcinnamaldehyde with O3 and hydroxyl OH radicals – a theoretical study

In the present investigation, the reaction mechanism and kinetics of 2-formylcinnamaldehyde (2-FC) with O₃ and hydroxyl OH radicals were studied. The reaction of 2-FC with O₃ radical are initiated by the formation of primary ozonide, whereas the reaction of 2-FC with the hydroxyl OH radical are initiated by two different ways: (1). H-atom abstraction by hydroxyl OH radical from the –CHO and –CH = CHCHO group of 2-FC (2). Hydroxyl OH addition to the –CH = CHCHO group to the ring-opened 2-FC. These reactions lead to the formation of an alkyl radical. The reaction pathways corresponding to the reactions between 2-FC with O₃ and hydroxyl OH radicals have been analysed using density functionals of B3LYP and M06-2X level of methods with the 6-31+G(d,p) basis set. Single-point energy calculations for the most favourable reactive species are determined by B3LYP/6-311++G(d,p) and CCSD(T)/6-31+G(d,p) levels of theory. From the obtained results, the hydroxyl OH addition at C8 position of 2-FC are most favourable than the C9 position of 2-FC. The subsequent reactions of the alkyl radicals, formed from the hydroxyl OH addition at C8 position, are analysed in detail. The individual and overall rate constant for the most favourable reactions are calculated by canonical variational transition theory with small-curvature tunnelling corrections over the temperature range of 278–350 K. The calculated theoretical rate constants are in good agreement with the available experimental data. The Arrhenius plot of the rate constants with the temperature are fitted and the atmospheric lifetimes of the 2-FC with hydroxyl OH radical reaction in the troposphere calculate for the first time, which can be applied to the study on the atmospheric implications. The condensed Fukui function has been verified for the most favourable reaction sites. This study can be regarded as an attempt to investigate the O₃-initiated and hydroxyl OH-initiated reaction mechanisms of 2-FC in the atmosphere.     

Investigation of the kinetics of OH∗ and CH∗ chemiluminescence in hydrocarbon oxidation behind reflected shock waves

The temporal variation of chemiluminescence emission from OH^?(A² Σ ⁺) and CH^?(A² Δ) in reacting Ar-diluted H₂/O₂/CH₄, C₂H₂/O₂ and C₂H₂/N₂O mixtures was studied in a shock tube for a wide temperature range at atmospheric pressures and various equivalence ratios. Time-resolved emission measurements were used to evaluate the relative importance of different reaction pathways. The main formation channel for OH^? in hydrocarbon combustion was studied with CH₄ as benchmark fuel. Three reaction pathways leading to CH^? were studied with C₂H₂ as fuel. Based on well-validated ground-state chemistry models from literature, sub-mechanisms for OH^? and CH^? were developed. For the main OH^?-forming reaction CH+O₂=OH^?+CO, a rate coefficient of k ₂=(8.0±2.6)×10¹⁰ cm³?mol^?1?s^?1 was determined. For CH^? formation, best agreement was achieved when incorporating reactions C₂+OH=CH^?+CO (k ₅=2.0×10¹⁴ cm³?mol^?1?s^?1) and C₂H+O=CH^?+CO (k ₆=3.6×10¹²exp(?10.9 kJ?mol^?1/RT) cm³?mol^?1?s^?1) and neglecting the C₂H+O₂=CH^?+CO₂ reaction.     

Low-temperature combustion chemistry of biofuels: Pathways in the low-temperature (550–700 K) oxidation chemistry of isobutanol and tert-butanol

Butanol isomers are promising next-generation biofuels. Their use in internal combustion applications, especially those relying on low-temperature autoignition, requires an understanding of their low-temperature combustion chemistry. Whereas the high-temperature oxidation chemistry of all four butanol isomers has been the subject of substantial experimental and theoretical efforts, their low-temperature oxidation chemistry remains underexplored. In this work we report an experimental study on the fundamental low-temperature oxidation chemistry of two butanol isomers, tert-butanol and isobutanol, in low-pressure (4–5.1 Torr) experiments at 550 and 700 K. We use pulsed-photolytic chlorine atom initiation to generate hydroxyalkyl radicals derived from tert-butanol and isobutanol, and probe the chemistry of these radicals in the presence of an excess of O₂ by multiplexed time-resolved tunable synchrotron photoionization mass spectrometry. Isomer-resolved yields of stable products are determined, providing insight into the chemistry of the different hydroxyalkyl radicals. In isobutanol oxidation, we find that the reaction of the α-hydroxyalkyl radical with O₂ is predominantly linked to chain-terminating formation of HO₂. The Waddington mechanism, associated with chain-propagating formation of OH, is the main product channel in the reactions of O₂ with β-hydroxyalkyl radicals derived from both tert-butanol and isobutanol. In the tert-butanol case, direct HO₂ elimination is not possible in the β-hydroxyalkyl + O₂ reaction because of the absence of a beta C–H bond; this channel is available in the β-hydroxyalkyl + O₂ reaction for isobutanol, but we find that it is strongly suppressed. Observed evolution of the main products from 550 to 700 K can be qualitatively explained by an increasing role of hydroxyalkyl radical decomposition at 700 K.     

Single-shot imaging of OH radicals and simultaneous OH radical/acetone imaging with a tunable Nd : YAG laser

Single shot imaging capability for OH radical distributions in various atmospheric pressure methane flames upon excitation with a tunable frequency-quadrupled Nd:YAG laser is demonstrated. The laser wavelength can be tuned with an intra-cavity etalon to produce laser-induced fluorescence (LIF) signals from OH via absorption in the OH A–X (2,0) P ₁(10) line. Simultaneous single-shot imaging of the burnt and unburnt zones in laminar nonpremixed, premixed and turbulent flames is presented. The unburnt areas are visualized with LIF of acetone that is seeded to the methane fuel. Acetone levels are set to match signal intensities to that of the OH signals to allow imaging on a single intensified CCD camera. PACS 42.62.Fi; 33.50.Dq; 82.33.Vx     

Superconductivity,magnetism and crystal chemistry of Ba1−xKxFe2As2

BaFe₂As₂ is the parent compound of the ‘122’ iron arsenide superconductors and crystallizes with the tetragonal ThCr₂Si₂-type structure, space group I4/mmm. A spin-density-wave transition at 140 K is accompanied by a symmetry reduction to space group Fmmm and simultaneously by antiferromagnetic ordering. Hole-doping induces superconductivity in Ba_1?xK_xFe₂As₂ with a maximum T_c of 38 K at x ≈ 0.4. The upper critical fields approach 75 T with rather small anisotropy of H_c2. At low potassium concentrations (x ? 0.2), superconductivity apparently co-exists with the orthorhombically distorted and magnetically ordered phase. At doping levels x ? 0.3, the structural distortion and antiferromagnetic ordering is completely suppressed and the T_c is maximized. No magnetically ordered domains could be detected in optimally doped Ba_1?xK_xFe₂As₂ (x ? 0.3) by ⁵⁷Fe Mössbauer spectroscopy in contrast μSR results obtained with single crystals. The magnetic hyperfine interactions investigated by ⁵⁷Fe Mössbauer spectroscopy are discussed and compared to the ZrCuSiAs-type materials.     

Acoustic behaviour and equation of state of amorphous ethylene–vinyl acetate copolymer studied by means of high-pressure Brillouin scattering spectroscopy

The determination of the equation of state (EOS) of amorphous materials is very important for fundamental understanding of the glass transition and applications as well. Simultaneous observation of both longitudinal and transverse acoustic modes by Brillouin scattering spectroscopy has been one of the major methods to obtain EOS of amorphous materials. However, the transverse acoustic mode is hardly seen from some of the amorphous polymers, which makes it difficult to derive EOS. The temperature and pressure dependences of the acoustic properties of amorphous ethylene–vinyl acetate (EVA) copolymer were measured by using high-pressure Brillouin scattering spectroscopy. The temperature variation induced large changes in the frequency shift and linewidth of the longitudinal acoustic mode due to strong coupling between the structural relaxation process and the propagating density fluctuations. The residual linewidth in the glassy state was attributed to the remnant intramolecular motions of EVA, the activation energy of which was estimated to be ～3.30 ± 0.27 kcal/mol. The pressure–density relationship of EVA could be obtained for the first time by measuring the refractive index and using the Lorentz–Lorenz equation. The density and the refractive index exhibited monotonic increase up to approximately 12 GPa. The strong reduction of the acoustic damping at low pressures below ～3 GPa was attributed to the collapsing free volume in EVA. The present study clearly shows that measuring the refractive index by high-pressure Brillouin spectroscopy may be an alternative method to get the EOS of polymeric materials whose transverse acoustic mode is too weak to be observed.     

Effect of additives on structure and corrosion resistance of ceramic coatings on Mg–Li alloy by micro-arc oxidation

In order to improve the corrosion resistance of ceramic coatings formed on Mg–5mass%Li substrate by micro-arc oxidation (MAO) method, two kinds of additives (Na₂B₄O₇ and EDTA) were doped in Na₂SiO₃–Na₃PO₄ solution system. The surface and cross-section morphology feature, phase composition and elemental composition were examined by SEM, XRD and EDX, respectively. Corrosion resistance of ceramic coating was tested by electrochemical methods. It was revealed that all coatings were composed of MgO and Mg₂SiO₄, and had porous surface structure. Doping of additives had little effect on the elemental composition, while it influenced the morphological feature of the coating. The results of electrochemical tests showed that the coatings prepared in the solutions with additive had good corrosion resistance. The addition of EDTA to the solution made coatings thinner and more uniform which resulted in better general corrosion resistance. The addition of Na₂B₄O₇ to the solution made coatings much thicker and compacter, which improved the pitting corrosion resistance.     

Preparation of Pt–CeO2/MWNT nano-composites by reverse micellar method for methanol oxidation

We report the preparation of Pt–CeO₂ nanoparticles on the multi-walled carbon nanotubes (MWNTs) by a reverse micellar method. Transmission electron microscopy (TEM) analysis indicated that well-dispersed small Pt–CeO₂ nanoparticles were formed on the MWCNTs. X-ray diffraction (XRD) analysis confirmed the formation of the Pt–CeO₂ nanoparticles on the MWNTs. Cyclic voltammetry (CV) results demonstrated that the Pt–CeO₂/MWNT exhibited a higher methanol oxidation than did the Pt/MWNT catalyst. The CO stripping test showed that CeO₂ can make CO stripped at a lower potential, which is helpful for CO and methanol electro-oxidation.     

Laser ion kinetics: set-up and application of a single-shot femtosecond pump–probe technique

In this contribution the first measurements of a single-shot femtosecond laser pump–probe technique are reported. The technique is based on counter-propagating femtosecond laser pulses in a supersonic beam of a low density of sample molecules and simultaneous probe detection by ion or fragment-ion formation through a reflectron time-of-flight mass spectrometer. It will be shown that the range of the pump–probe delays covers the time span between 100 fs and 10 ps depending on the pulse width of the laser used and the stability of the volt ages of the mass spectrometer. The application of this technique to organometallic compounds as well as to medium-sized organic molecules reveals some insight into the electron-transfer process during ionization through a 1+1 multi-photon absorption procedure. Furthermore it is demonstrated that this technique is also applicable to the investigation of ultra-fast isomerization and fragmentation processes. As an example the results of the processes within iron pentacarbonyl and substituted benzalacetones are reported. Received: 22 October 1999 / Published online: 24 July 2000     

Formation and characterization of porous silicon–samarium/gadolinium nanocomposites: effect of substrate oxidation and biosynthesis process

Samarium and gadolinium nanoparticles synthesized by bioreduction process have been incorporated into nanostructured porous silicon template to form a nanocomposite. The structural and optical properties of PS–Gd and PS–Sm nanocomposites have been studied through TEM, SEM and UV–Vis spectroscopy. Extent of infiltration has been verified through reflectance interference Fourier transform spectroscopy as a function of substrate oxidation conditions. The substrates oxidized at 600 °C showed the maximum infiltration and the corresponding change of optical thickness due to nanoparticles. Such biodegradable nanocomposites in the form of particles can have potential applications in localized drug delivery and enhancement of the image contrast and optoelectronic devices. The results here reported open an energy-cheap procedure to take advantages of small rare earth nanoparticles and produced nanocomposites with their immersion in SiO₂ substrates, with the perspective to be replied in other similar substrates under controlled conditions.     

Mechanisms of the oxidation and combustion of normal paraffin hydrocarbons: Transition from C1–C10 to C11–C16

Recently, detailed kinetic mechanisms of the oxidation and combustion of higher hydrocarbons, composed of hundreds of components and thousands of elementary reactions, have been proposed. Despite the undoubtful advantages of such detailed mechanisms, their application to simulations of turbulent combustion and gas dynamic phenomena is difficult because of their complexity. At the same time, to some extent limited, they cannot be considered exhaustive. This work applies previously proposed algorithm for constructing an optimal mechanism of the high- and low-temperature oxidation and combustion of normal paraffin hydrocarbons, which takes into account the main processes determining the reaction rate and the formation of key intermediates and final products. The mechanism has the status of a nonempirical detailed mechanism, since all the constituent elementary reactions have a kinetic substantiation. The mechanism has two specific features: (1) it does not include reactions of so-called double oxygen addition (first to the peroxide radical, and then to its isomeric form), i.e., the first addition turns out to be sufficient; (2) it does not include isomeric compounds and their derivatives as intermediates, since this oxidation pathway is slower than the oxidation of molecules and radicals with normal structure. Application of the algorithm makes it possible to compile a compact mechanism, which is important for modeling chemical processes involving paraffin hydrocarbons C _n with large n. Previously, based on this algorithm, compact mechanisms of the oxidation and combustion of propane, n-butane, n-pentane, n-hexane, n-heptane, n-octane, n-nonane, and n-decane have been constructed. In this work, we constructed a nonempirical detailed mechanism of the oxidation and combustion of hydrocarbons from n-undecane to n-hexadecane. The most important feature of the new mechanism is its staged nature, which manifests itself through the emergence of cool and blue flames during low-temperature autoignition. The calculation results are compared with experimental data.     

Influence of turbulence–chemistry interaction for n-heptane spray combustion under diesel engine conditions with emphasis on soot formation and oxidation

The influence of the turbulence–chemistry interaction (TCI) for n-heptane sprays under diesel engine conditions has been investigated by means of computational fluid dynamics (CFD) simulations. The conditional moment closure approach, which has been previously validated thoroughly for such flows, and the homogeneous reactor (i.e. no turbulent combustion model) approach have been compared, in view of the recent resurgence of the latter approaches for diesel engine CFD. Experimental data available from a constant-volume combustion chamber have been used for model validation purposes for a broad range of conditions including variations in ambient oxygen (8?21% by vol.), ambient temperature (900 and 1000 K) and ambient density (14.8 and 30 kg/m³). The results from both numerical approaches have been compared to the experimental values of ignition delay (ID), flame lift-off length (LOL), and soot volume fraction distributions. TCI was found to have a weak influence on ignition delay for the conditions simulated, attributed to the low values of the scalar dissipation relative to the critical value above which auto-ignition does not occur. In contrast, the flame LOL was considerably affected, in particular at low oxygen concentrations. Quasi-steady soot formation was similar; however, pronounced differences in soot oxidation behaviour are reported. The differences were further emphasised for a case with short injection duration: in such conditions, TCI was found to play a major role concerning the soot oxidation behaviour because of the importance of soot-oxidiser structure in mixture fraction space. Neglecting TCI leads to a strong over-estimation of soot oxidation after the end of injection. The results suggest that for some engines, and for some phenomena, the neglect of turbulent fluctuations may lead to predictions of acceptable engineering accuracy, but that a proper turbulent combustion model is needed for more reliable results.     

Balance the oxidation resistance and mechanical properties of C/Si–C–N composite by a Si–O–C interphase

Carbon fiber reinforced Si–C–N matrix composite with a Si–O–C interphase (C/Si–O–C/Si–C–N) was fabricated via chemical vapor infiltration and polymer impregnation and pyrolysis process. The mechanical properties and oxidation behaviors of C/Si–O–C/Si–C–N were investigated using three-point-bending test and thermogravimetry. The results indicated that the oxidation resistance of C/Si–O–C/Si–C–N was improved as compared to C/Si–C–N with pyrolytic carbon (PyC) interphase (C/PyC/Si–C–N). The higher oxidation resistance of C/Si–O–C/Si–C–N attributed to the high inoxidizability of Si–O–C interlayer and low thermal stress in matrix. The flexural strength of C/Si–O–C/Si–C–N rivaled that of C/PyC/Si–C–N and the modulus was higher than that of C/PyC/Si–C–N. The suitable interphase and the optimized interface bonding can get the high oxidation resistance of the composites with the mechanical properties maintained.     

Atmospheric chemistry of HFE-7000 (i-C3F7OCH3) and isofluoro-propyl formate (i-C3F7OC(O)H): reactions with OH radicals,atmospheric lifetime and fate of alkoxy radical (i-C3F7OCH2O•) – a DFT study

ABSTRACT

The mechanism of hydrogen abstraction reaction between HFE-7000 (i-C₃F₇OCH₃) and OH radicals using M06-2X functional in conjunction with 6-31+G(d,p) basis set is investigated. The pre-reactive and post-reactive complexes from intrinsic reaction coordinate calculations are validated at entrance and exit channels, respectively. The standard enthalpies of formation for the species and bond dissociation energy for C–H bond are also estimated. The rate constants of the titled reactions over the temperature range of 250–450 K are reported. The OH-driven atmospheric life time of i-HFE-7000 is computed to be 3.19 years. The atmospheric fate of the alkoxy radical (i-C₃F₇OCH₂O^?) is also explored here for the first time. Three prominent plausible decomposition channels including oxidation are considered in detail. The thermochemical data reveal that reaction with O₂ is the dominant path for the decomposition of i-C₃F₇OCH₂O^? radical. Moreover, rate constant for the OH-initiated hydrogen abstraction of isofluoro-propyl formate (i-C₃F₇OC(O)H) is also reported.     

Study of the local order in LiNi1−yCoyO2 oxides synthesized by soft chemistry

Lattice dynamics of LiNi_1−yCo_yO₂ solid solution are investigated using Raman and FTIR measurements. Evolution of the vibrational spectra, i.e., frequency shift and band broadening of the stretching modes of either (Ni_1−yCo_yO₂)O₆ or LiO₆ octahedra, are studied as a function of the calcination temperature of the LiNi_1−yCo_yO₂ solid solution. Results show that the solid solution exhibits a one-mode behavior with an increase of the (Ni_1−yCo_yO₂)_n sheet covalency upon substitution of cobalt for nickel. The change in the covalency corresponds to an increase of the Madelung constant with decreasing the c/a value. The broadening of the vibrational bands for Ni-rich compounds is the result of the cation mixing in the crystal layers. The partially disordered cation distribution appearing in lithium nickelate materials can also explain the observed broadening of the Raman spectra. Paper presented at the 6th Euroconference on Solid State Ionics, Cetraro, Calabria, Italy, Sept. 12–19, 1999.     

The partial 1H NMR spectra of Al–OH and molecular H2O in hydrous aluminosilicate glasses: Component-Resolved analysis of 27Al–1H cross polarization and 1H spin-echo MAS NMR spectra

The Component-Resolved methodology was applied to ¹H spin-echo and ²⁷Al–¹H cross polarization (CP) MAS NMR data of aluminosilicate glasses. The method was able to resolve two components with different T2 relaxation rates, hydroxyl groups (OH) and molecular water (H₂O_mol), from the spin-echo data and to determine partial spectra and the relative abundances of OH and H₂O_mol. The algorithm resolved two to three components with different ²⁷Al–¹H CP dynamics from the ²⁷Al–¹H cross polarization data; the obtained partial NMR spectra for Al–OH are in excellent agreement with those obtained previously from the difference spectra between spectra with various contact times and confirm previous quantitative results and models for the Al–OH, Si–OH and H₂O_mol speciation (Malfait and Xue, 2010).     

Oxidation of Si(1 1 1) promoted by K multilayers: K and SiO2 islands

In the context of the alkali-metal promotion of Si oxidation, high-pressure oxidation of potassium multilayers on cooled Si(111) has been investigated using SEM, micro-AES and AFM. The oxidation process at high pressure turns the potassium islands observed at high coverage into potassium oxides islands. A subsequent potassium desorption at moderate temperature (900 K) yields SiO₂ islands (height of 20 nm and lateral dimensions of 10 m) surrounded by a thin continuous SiO₂ overlayer. Other conditions such as potassium multilayer coverage exposed to a low O₂ pressure, potassium monolayer or simultaneous potassium/oxygen adsorption yield a uniform SiO₂ overlayer.     

The keto–enol equilibrium and thermal conversion kinetics of 2- and 4-hydroxyacetophenone in the gas phase: a DFT study

Keto–enol tautomeric equilibrium and the mechanism of thermal conversion of 2- and 4-hydroxyacetophenone in gas phase have been studied by means of electronic structure calculations using density functional theory (DFT). A topological analysis of electron density evidence that the structure of keto and enol forms of 2-hydroxyacetophenone are stabilised by a relatively strong intramolecular hydrogen bond. 2- and 4-hydroxyacetophenone undergo deacetylation reactions yielding phenol and ketene. Two possible mechanisms are considered for these eliminations: the process takes place from the keto form (mechanism A), or occurs from the enolic form of the substrate (mechanism B). Quantum chemical calculations support the mechanism B, being found a good agreement with the experimental activation parameters. These results suggest that the rate-limiting step is the reaction of the enol through a concerted, non-synchronous, semi-polar, four-membered cyclic transition state (TS). The most advanced reaction coordinate in the TS is the rupture of O₁···H₁ bond, with an evolution in the order of 79.7%–80.9%. Theoretical results also suggest a three-step mechanism for the phenyl acetate formation from 2-hydroxyacetophenone.     

1+1′ resonant multiphoton ionisation of OH radicals via the A2Σ+ state: insights from direct comparison with A-X laser-induced fluorescence detection

A 1+1′ resonance-enhanced multiphoton ionisation (REMPI) scheme for OH X²Π radicals is characterised for a broad range of intermediate A²Σ⁺ (v = 1, J, F_i) levels. The intensities of OH A-X (1,0) transitions detected by subsequent fixed-frequency VUV ionisation are compared with those obtained by near simultaneous laser-induced fluorescence (LIF) measurements. The ratios of the 1+1′ REMPI to LIF signals are used to derive enhancement factors which reflect the VUV absorption to the OH A³Π, 3d, v = 0 Rydberg state and/or the fast autoionisation process that yields OH⁺ ions. The determination of the enhancement factors permits 1+1′ REMPI to be utilised as a quantitative state-specific probe of OH X²Π radicals.