High-temperature measurements of the rates of the reactions CH2O + Ar → Products and CH2O + O2 → Products

The two-channel thermal decomposition of formaldehyde [CH₂O], (1a) CH₂O + Ar → HCO + H + Ar, and (1b) CH₂O + Ar → H₂ + CO + Ar, was studied in shock tube experiments in the 2258-2687 K temperature range, at an average total pressure of 1.6 atm. OH radicals, generated on shock heating trioxane-O₂-Ar mixtures, were monitored behind the reflected shock front using narrow-linewidth laser absorption. 1,3,5 trioxane [C₃H₆O₃] was used as the CH₂O precursor in the current experiments. H-atoms formed upon CH₂O and HCO decomposition rapidly react with O₂ to produce OH via H + O₂ → O + OH. The recorded OH time-histories show dominant sensitivity to the formaldehyde decomposition pathways. The second-order reaction rate coefficients were inferred by matching measured and modeled OH profiles behind the reflected shock. Two-parameter fits for k_1a and k_1b, applicable in this temperature range, are:

     

Mechanisms of radical removal by SO2

It is well established from experiments in premixed, laminar flames, jet-stirred reactors, flow reactors, and batch reactors that SO₂ acts to catalyze hydrogen atom removal at stoichiometric and reducing conditions. However, the commonly accepted mechanism for radical removal, SO₂ + H(+M) ? HOSO(+M), HOSO + H/OH ? SO₂ + H₂/H₂O, has been challenged by recent theoretical and experimental results. Based on ab initio calculations for key reactions, we update the kinetic model for this chemistry and re-examine the mechanism of fuel/SO₂ interactions. We find that the interaction of SO₂ with the radical pool is more complex than previously assumed, involving HOSO and SO, as well as, at high temperatures also HSO, SH, and S. The revised mechanism with a high rate constant for H + SO₂ recombination and with SO + H₂O, rather than SO₂ + H₂, as major products of the HOSO + H reaction is in agreement with a range of experimental results from batch and flow reactors, as well as laminar flames.     

Theoretical study on the kinetics for OH reactions with CH3OH and C2H5OH

Kinetics and mechanisms for reactions of OH with methanol and ethanol have been investigated at the CCSD(T)/6-311 + G(3df, 2p)//MP2/6-311 + G(3df, 2p) level of theory. The total and individual rate constants, and product branching ratios for the reactions have been computed in the temperature range 200-3000 K with variational transition state theory by including the effects of multiple reflections above the wells of their pre-reaction complexes, quantum-mechanical tunneling and hindered internal rotations. The predicted results can be represented by the expressions k₁ = 4.65 × 10⁻²⁰ × T^2.68 exp(414/T) and k₂ = 9.11 × 10⁻²⁰ × T^2.58 exp(748/T) cm³ molecule⁻¹ s⁻¹ for the CH₃OH and C₂H₅OH reactions, respectively. These results are in reasonable agreements with available experimental data except that of OH + C₂H₅OH in the high temperature range. The former reaction produces 96-89% of the H₂O + CH₂OH products, whereas the latter process produces 98-70% of H₂O + CH₃CHOH and 2-21% of the H₂O + CH₂CH₂OH products in the temperature range computed (200-3000 K).     

Theoretical rate coefficients for the reaction of methyl radical with hydroperoxyl radical and for methylhydroperoxide decomposition

The kinetics of the CH₃ + HO₂ bimolecular reaction and the thermal decomposition of CH₃OOH are studied theoretically. Direct variable reaction coordinate transition state theory (VRC-TST), coupled with high level multireference electronic structure calculations, is used to compute capture rates for the CH₃ + HO₂ reaction and to characterize the transition state of the barrierless CH₃O + OH product channel. The CH₂O + H₂O product channel and the CH₃ + HO₂ → CH₄ + O₂ reaction are treated using variational transition state theory and the harmonic oscillator and rigid rotor approximations. Pressure dependence and product branching in the bimolecular and decomposition reactions are modeled using master equation simulations. The predicted rate coefficients for the major products channels of the bimolecular reaction, CH₃O + OH and CH₄ + O₂, are found to be in excellent agreement with values obtained in two recent modeling studies. The present calculations are also used to obtain rate coefficients for the CH₃O + OH association/decomposition reaction.     

Mixed water/OH structures on Pd(1 1 1)

Chemisorbed O and water react on Pd(1 1 1) at low temperatures to form a mixed OH/H₂O layer with a (√3 × √3)R30° registry. Reaction requires at least two water molecules to each O before the (2 × 2)O islands are consumed, the most stable OH/water structure being a (OH + H₂O) layer containing 0.67 ML of oxygen, formed by the reaction 3H₂O + O → 2(H₂O + OH). This structure is stabilised compared to pure water structures, decomposing at 190 K as OH recombines and water desorbs. The (√3 × √3)R30° − (OH + H₂O) phase cannot be formed by O/H reaction and is distinct from the (√3 × √3)R30° structure formed by O/H coadsorption below 200 K. Mixed OH/water structures do not react with coadsorbed H below 190 K on Pd(1 1 1), preventing this phase catalyzing the low temperature H₂/O₂ reaction which only occurs at higher temperatures.     

Kinetic modeling of the benzyl + HO2 reaction

Benzyl is a resonantly stabilized radical that commonly occurs as an intermediate in the combustion of aromatic compounds. The bimolecular reaction of benzyl with HO₂ is important in the oxidation of toluene, especially at low to moderate temperatures, where unimolecular decomposition of the benzyl radical is slow. We show that the addition of HO₂ to the methylene site in benzyl produces a vibrationally excited benzylhydroperoxide adduct, with over 60 kcal mol⁻¹ (251 kJ mol⁻¹) of excess energy above the ground state. RRKM simulations are performed on the benzyl + HO₂ reaction, using thermochemical and kinetic parameters obtained from ab initio calculations, with variational transition state theory (VTST) for treatment of barrierless radical + radical reaction kinetics. Our results reveal that the benzyl + HO₂ reaction proceeds predominantly to the benzoxyl radical + OH at temperatures of around 800 K and above, with the production of stabilized benzylhydroperoxide molecules dominating at lower temperatures. The heat of formation of the benzyl radical is calculated as 52.5 kcal mol⁻¹ (219.7 kJ mol⁻¹) at the G3B3 level of theory, in relative agreement with other recent determinations of this value.     

Towards XUV lasers with charge transfer pumping

Detailed studies of charge exchange pumping of ions in femtosecond laser-produced plasmas colliding with a pulsed gas jet are presented. Strong selective excitation of XUV ionic transitions in the reaction C⁴⁺ + H → C³⁺ + H⁺ is observed. Dependences of line intensities on various experimental parameters are reported which are in good agreement with the theory of charge transfer processes. Analyses of experimental data provide evidence that an efficient charge exchange pumping is realized at densities of reagents well in excess of 10¹⁶ cm⁻³, which is essential for the realization of XUV lasers. In preliminary investigations of the reaction C⁶⁺ + H → C⁵⁺ (n = 3, 4) + H⁺ a strong increase of line intensities at 13.5 and 18.2 nm is reported. Analysis of lasing in Na-like ions with an example for Chlorine promises even more efficient pumping as compared with the before analyzed hydrogen like ions.     

Kinetics of the H + NCO reaction

Ab initio transition state theory (TST) based master equation simulations are used to predict the temperature and pressure dependence of the H + NCO reaction rate and product branching. The barrierless entrance channels to form singlet HNCO and NCOH are studied with variable reaction coordinate TST employing a potential energy surface obtained from multi-reference configuration interaction ab initio calculations. The remaining channels, including reactions on the triplet surface, are studied with standard TST methods employing high level electronic structure results. The energy transfer parameters for the master equation simulations arise from a fit to the experimentally observed HNCO dissociation rate. The lowest energy threshold to formation of bimolecular products, ³NH + CO, lies well below the reactants. The bottleneck for intersystem crossing, which precedes the formation of ³NH + CO from the singlet adducts, becomes the dominant bottleneck for that channel at quite low energies relative to reactants. The effect of this bottleneck is studied with model calculations designed to reproduce detailed experimental observations of photolysis branching ratios. This bottleneck greatly reduces the flux from H + NCO to ³NH + CO via the singlet adducts. As a result, stabilization and reaction on solely the triplet surface are significant components of the overall rate. The present predictions for the high pressure and collisionless limit rate coefficients are accurately reproduced over the 200-2500 K range by the expressions, 1.53 × 10⁻⁵T^−1.86exp(−399/T) + 1.07 × 10³T^−3.15exp(−15219/T) and 5.62 × 10⁻¹²T^0.493exp(148/T) cm³ molecule⁻¹ s⁻¹, respectively, where T is in K. These predictions are in reasonably satisfactory agreement with the somewhat discordant experimental rate measurements.     

Rate coefficients measurement for the energy-pooling collisions Cs(5D) + Cs(5D) → Cs(6S) + Cs(nl = 9D, 11S, 7F)

We report experimental rate coefficients for the energy-pooling collisions Cs(5D) + Cs(5D) → Cs(6S) + Cs(nl = 9D, 11S, 7F). In the experiment the Cs(5D) state was populated via photodissociation of Cs₂ molecules using an argon-ion laser at wavelength 488.0 nm. We also consider the competing process 6P_1/2 + 7S → 6S + (nl = 9D, 11S, 7F) that might also populate 9D, 11S and 7F. An intermodulation technique was used to select the fluorescence contributions due only to the process 6P_1/2 + 7S → 6S + (nl = 9D, 11S, 7F). The excited atom (nl_J) density and spatial distribution were mapped by monitoring the absorption of a counterpropagating probe laser beam tuned to various transitions. The measured excited atom densities are combined with measured fluorescence ratios to yield rate coefficients for the energy-pooling collisions Cs(5D) + Cs(5D) → Cs(6S) + Cs(nl = 9D, 11S, 7F). The rate coefficients for nl = 9D, 11S, 7F are (4.1 ± 2.0) × 10⁻¹⁰ cm³ s⁻¹, (1.6 ± 0.8) × 10⁻¹⁰ cm³ s⁻¹ and (3.6 ± 1.8) × 10⁻¹⁰ cm³ s⁻¹, respectively. The contributions to the rate coefficients from other energy transfer processes are also discussed.     

Ultrahigh-channel-count fiber Bragg grating based on the triple sampling method

A triple sampling method to have enabled excellent channel uniformity and high in-band energy efficiency is firstly proposed for the design of an ultrahigh-channel-count fiber Bragg grating (FBG), which is based on the simultaneous utilization of two amplitude-assisted phase sampling (AAPS) functions and a phase-only sampling (POS) function. As an example, one linearly chirped FBG with consecutive 1215 channels enabling to cover all fiber telecom bands (O + E + S + C + L + U) is numerically demonstrated, which has a length of 9 cm, a dispersion of − 1360 ps/nm, and a channel spacing of 50 GHz. The maximum index-change required for 10 dB strength of the FBG is less than 6 × 10^− 3.     

A theoretical study of the water gas shift reaction mechanism on Cu(1 1 1) model system

The water gas shift (WGS) reaction is an important reaction system and has wide applications in several processes. However, the mechanism of the reaction is still in dispute. In this paper we have investigated the reaction mechanism on the model Cu(1 1 1) system using the density functional method and slab models. We have characterized the kinetics and the thermodynamics of the four reaction pathways containing 24 elementary steps and computed the reaction potential energy surfaces. Calculations show that the formate (HCOO) intermediate mechanism (CO + OH → HCOO → CO₂ + H) and the associative mechanism (CO + OH → CO₂ + H) are kinetically unlikely because of the high formation barrier. On the other hand, the carboxyl (HOCO) intermediate mechanism (CO + OH → HOCO → CO₂ + H) and the redox mechanism (CO + O → CO₂) are demonstrated to be feasible. Our calculations also indicate that surface oxygen atoms can reduce the barriers of both water dissociation and HOCO decomposition significantly. The calculated potential energy surfaces show that the water dissociation which produces OH groups is the rate-determining step at the initial stage of the reaction or in the absence of surface oxygen atoms. With the development of the reaction or in the presence of oxygen atoms on the surface, CO + OH → HOCO and CO + O → CO₂ become the rate-limiting step for the carboxyl and redox mechanisms, respectively.     

The bΣ(b0) → XΣ(X10,X21) and aΔ(a2) → X21 transitions of AsBr

Emission spectra of the b¹Σ⁺(b0⁺) → X³Σ⁻(X₁0⁺,X₂1) and a¹Δ(a2) → X₂1 transitions of AsBr have been measured in the near-infrared spectral region with a Fourier-transform spectrometer. The arsenic bromide radicals were generated in fast-flow systems by reaction of arsenic vapor (As_x) with bromine and were excited by microwave-discharged oxygen. The most prominent features in the spectrum are the Δv = +1,0,−1, and −2 band sequences of the b¹Σ⁺(b0⁺) → X³Σ⁻(X₁0⁺) transition in the range 11 700-12 700 cm⁻¹. With lower intensities, the Δv = 0 and −1 sequences of the b¹Σ⁺(b0⁺) → X³Σ⁻(X₂1) sub-system show up in the same range. Further to the red, between 6000 and 6700 cm⁻¹, the Δv = 0, +1, and −1 sequences of the hitherto unknown a¹Δ(a2) → X₂1 transition are observed. Analyses of medium- and high-resolution spectra have yielded improved molecular constants for the X₁0⁺, X₂1, and b0⁺ states and first values of the electronic energy and the vibrational constants of the a2 state.     

A density functional theory study on the adsorption and dissociation of N2O on Cu2O(1 1 1) surface

Density functional theory has been employed to investigate the adsorption and the dissociation of an N₂O at different sites on perfect and defective Cu₂O(1 1 1) surfaces. The calculations are performed on periodic systems using slab model. The Lewis acid site, Cu_CUS, and Lewis base site, O_SUF are considered for adsorption. Adsorption energies and the energies of the dissociation reaction N₂O → N₂ + O(s) at different sites are calculated. The calculations show that adsorption of N₂O is more favorable on Cu_CUS adsorption site energetically. Cu_CUS site exhibits a very high activity. The Cu_CUS-N₂O reaction is exothermic with a reaction energy of 77.45 kJ mol⁻¹ and an activation energy of 88.82 kJ mol⁻¹, whereas the O_SUF-N₂O reaction is endothermic with a reaction energy of 205.21 kJ mol⁻¹ and an activation energy of 256.19 kJ mol⁻¹. The calculations for defective surface indicate that O vacancy cannot obviously improve the catalytic activity of Cu₂O.     

High-pressure laminar flame speeds and kinetic modeling of carbon monoxide/hydrogen combustion

Laminar flame speeds were accurately measured for CO/H₂/air and CO/H₂/O₂/helium mixtures at different equivalence ratios and mixing ratios by the constant-pressure spherical flame technique for pressures up to 40 atmospheres. A kinetic mechanism based on recently published reaction rate constants is presented to model these measured laminar flame speeds as well as a limited set of other experimental data. The reaction rate constant of CO + HO₂ → CO₂ + OH was determined to be k = 1.15 × 10⁵T^2.278 exp(−17.55 kcal/RT) cm³ mol⁻¹ s⁻¹ at 300-2500 K by ab initio calculations. The kinetic model accurately predicts our measured flame speeds and the non-premixed counterflow ignition temperatures determined in our previous study, as well as homogeneous system data from literature, such as concentration profiles from flow reactor and ignition delay time from shock tube experiments.     

Kinetic study of chlorine behavior in the waste incineration process

The waste incineration atmosphere was simulated as HCl/H₂O/O₂/CO₂/N₂ in order to experimentally study chlorine behavior as temperature ranges from 1173 to 1473 K and residence time varies. The results show that Cl radicals, produced by the decomposition of HCl at high temperature, mainly recombine to form Cl₂ and HCl at the quenching section. It was found that temperature, residence time, cooling rate and feeding gas composition influence Cl₂ concentration. To thoroughly understand this reaction system, a kinetic model was developed and validated against experimental results. The key reactions and main pathway were found out with the use of sensitivity and rate of production analysis (ROP). The reaction HCl + O₂ → Cl + HO₂ was shown to initiate the reaction system, and it was found that a significant amount of Cl₂ was simultaneously produced by the following high temperature reaction: Cl + HOCl → Cl₂ + OH. In the cooling process, the main consumption reactions of Cl radicals were H₂O + Cl → HCl + OH, OH + Cl → HCl + O and Cl + Cl + M → Cl₂ + M. Among these, the first two reactions can be used to explain the effect of H₂O on the concentration of Cl radical at high temperature. In addition, the influence of the quenching rate on the distribution of chlorine was found to occur because of the varying effects that temperature change causes to the different Cl radical consumption reactions.     

Shock tube measurements of ignition delay times and OH time-histories in dimethyl ether oxidation

Ignition delay times and OH concentration time-histories were measured in DME/O₂/Ar mixtures behind reflected shock waves. Initial reflected shock conditions covered temperatures (T₅) from 1175 to 1900 K, pressures (P₅) from 1.6 to 6.6 bar, and equivalence ratios (?) from 0.5 to 3.0. Ignition delay times were measured by collecting OH^∗ emission near 307 nm, while OH time-histories were measured using laser absorption of the R₁(5) line of the A-X(0,0) transition at 306.7 nm. The ignition delay times extended the available experimental database of DME to a greater range of equivalence ratios and pressures. Measured ignition delay times were compared to simulations based on DME oxidation mechanisms by Fischer et al. [7] and Zhao et al. [9]. Both mechanisms predict the magnitude of ignition delay times well. OH time-histories were also compared to simulations based on both mechanisms. Despite predicting ignition delay times well, neither mechanism agrees with the measured OH time-histories. OH Sensitivity analysis was applied and the reactions DME ↔ CH₃O + CH₃ and H + O₂ ↔ OH + O were found to be most important. Previous measurements of DME ↔ CH₃O + CH₃ are not available above 1220 K, so the rate was directly measured in this work using the OH diagnostic. The rate expression k[1/s] =  1.61 × 10⁷⁹T^−18.4 exp(−58600/T), valid at pressures near 1.5 bar, was inferred based on previous pyrolysis measurements and the current study. This rate accurately describes a broad range of experimental work at temperatures from 680 to 1750 K, but is most accurate near the temperature range of the study, 1350-1750 K. When this rate is used in both the Fischer et al. and Zhao et al. mechanisms, agreement between measured OH and the model predictions is significantly improved at all temperatures.     

Characterization of hybrid electrolytes prepared from the Li2S-P2S5 glasses and alkanediols

Inorganic-organic hybrid electrolytes were prepared by the mechanochemical method using the Li⁺ ion conductive 70Li₂S·30P₂S₅ glass and various alkanediols. Local structure of the prepared electrolytes was analyzed by FT-IR and Raman spectroscopy. The effects of the proportion and chain length of alkanediols on conductivity of the hybrid electrolytes were investigated. The hybrid electrolyte with 2 mol.% of 1,4-butanediol exhibited the conductivity of 9.7 × 10^− 5 S cm^− 1 at room temperature and the unity of lithium ion transference number. The use of alkanediols with shorter chain length was effective in increasing conductivity of hybrid electrolytes. The electrolyte using ethyleneglycol showed the highest conductivity of 1.1 × 10^− 4 S cm^− 1 at room temperature. Lowering glass transition temperature by incorporation of alkanediols is responsible for the enhancement of conductivity of hybrid electrolytes.     

N2O emission in the 4.5 μM region: high excitation of the bending mode transitions v1v2v3→v1v2(v3−1) with (2v1+v2)=5

Eight emission spectra of pure N₂O and N₂O + N₂ + He mixtures excited by a radio frequency discharge were recorded by Fourier Transform Spectroscopy at a resolution of 0.005 and 0.004 cm⁻¹ in the 4.5 μm region. Results (wavenumbers, band centers, and spectroscopic constants) concerning nine new vibrational transitions which have not been observed before, and which occur between highly excited levels of the bending mode are reported. The derived spectroscopic parameters allow us to reproduce the experimental wavenumbers with an RMS error lower than 4.5 × 10⁻⁴ cm⁻¹.     

The reaction of allyl radicals with oxygen atoms—rate coefficient and product branching

The primary product formation of the C₃H₅ + O reaction in the gas phase has been studied at room temperature. Allyl radicals (C₃H₅) and O atoms were generated by laser flash photolysis at λ = 193 nm of the precursors C₃H₅Cl, C₃H₅Br, C₆H₁₀ (1,5-hexadiene), and SO₂, respectively. The educts and the products were detected by using quantitative FTIR spectroscopy. The combined product analysis of the experiments with the different precursors leads to the following relative branching fractions: C₃H₅ + O → C₃H₄O + H (47%), C₂H₄ + H + CO (41%), H₂CO + C₂H₂ + H (7%), CH₃CCH + OH and CH₂CCH₂ + OH (<5%). The rate of reaction has been studied relative to CH₃OCH₂ + O and C₂H₅ + O in the temperature range from 300 to 623 K. Here, the radicals were produced via the fast reactions of propene, dimethyl ether, and ethane, respectively, with atomic fluorine. Laser-induced multiphoton ionization combined with TOF mass spectrometry and molecular beam sampling from a flow reactor was used for the specific and sensitive detection of the C₃H₅, C₂H₅, and CH₃COCH₂ radicals. The rate coefficient of the reaction C₃H₅ + O was derived with reference to the reaction C₂H₅ + O leading to k(C₃H₅ + O) = (1.11 ± 0.2) × 10¹⁴ cm³/(mol s) in the temperature range 300-623 K. The C₃H₅ + O rate and channel branching, when incorporated in a suitable detailed reaction mechanism, have a large influence on benzene and allyl concentration profiles in fuel-rich propene flames, on the propene flame speed, and on propene ignition delay times.     

Quasifission and difference in formation of evaporation residues in the O + W and F + Ta reactions

The role of the entrance channel has been studied to ascertain a cause of the observed difference between the evaporation residue cross sections normalized to the fusion cross sections in the ¹⁹F + ¹⁸¹Ta and ¹⁶O + ¹⁸⁴W reactions at high excitation energies. The theoretical analysis performed in the framework of the dinuclear system and advanced statistical models showed that the more intense yield of evaporation residues in the ¹⁶O + ¹⁸⁴W reaction in comparison with that in the ¹⁹F + ¹⁸¹Ta reaction was explained by the large capture and fusion cross sections in the former reaction, which is in agreement with the experimental data. The observed decrease in the evaporation residue cross section normalized to the fusion cross section in the ¹⁹F + ¹⁸¹Ta reaction, in comparison with one in the ¹⁶O + ¹⁸⁴W reaction at large excitation energies, is caused by the unintentional inclusion of the quasifission and fast fission contributions in the fissionlike fragment yields that were used in reconstructing the experimental fusion cross section in the normalizing procedure. The range of the angular momentum distribution for both systems was similar, but the partial cross sections are different, showing the presence of a difference in the hindrance to complete fusion in both reactions.