Kinetics Of Gas-phase Reactions Research Articles

Toward an Accurate Black-Box Tool for the Kinetics of Gas-Phase Reactions Involving Barrier-less Elementary Steps.

An enhanced computational protocol has been devised for the accurate characterization of gas-phase barrier-less reactions in the framework of the reaction-path (RP) and variable reaction coordinate variational transition-state theory. In particular, the synergistic combination of density functional theory and Monte Carlo sampling to optimize reactive fluxes led to a reliable yet effective computational workflow. A black-box strategy has been developed for selecting the most suited density functional with reference to a high-level one-dimensional reference potential. At the same time, different descriptions of hindered rotations are automatically selected, depending on the corresponding harmonic frequencies along the RP. The performance of the new tool is investigated by means of two prototypical reactions involving different degrees of static and dynamic correlation, namely, H2S + Cl and CH3 + CH3. The remarkable agreement of the computed kinetic parameters with the available experimental data confirms the accuracy and robustness of the proposed approach. Together with their intrinsic interest, these results also pave the way toward systematic investigations of gas-phase reactions involving barrier-less elementary steps by a reliable, user-friendly tool, which can be confidently used also by nonspecialists.

The Fate of Water in Hydrogen-Based Iron Oxide Reduction.

Gas-solid reactions are important for many redox processes that underpin the energy and sustainability transition. The specific case of hydrogen-based iron oxide reduction is the foundation to render the global steel industry fossil-free, an essential target as iron production is the largest single industrial emitter of carbon dioxide. This perception of gas-solid reactions has not only been limited by the availability of state-of-the-art techniques which can delve into the structure and chemistry of reacted solids, but one continues to miss an important reaction partner that defines the thermodynamics and kinetics of gas phase reactions: the gas molecules. In this investigation, cryogenic-atom probe tomography is used to study the quasi in situ evolution of iron oxide in the solid and gas phases of the direct reduction of iron oxide by deuterium gas at 700°C. So far several unknown atomic-scale characteristics are observed, including, D2 accumulation at the reaction interface; formation of a core (wüstite)-shell (iron) structure; inbound diffusion of D through the iron layer and partitioning of D among phases and defects; outbound diffusion of oxygen through the wüstite and/or through the iron to the next free available inner/outer surface; and the internal formation of heavy nano-water droplets at nano-pores.

A DFT evaluation of molecular reactivity of volatile organic compounds in support of chemical ionization mass spectrometry

Gas-phase molecular properties of volatile organic compounds (VOCs) play an important role in the selection of gas-phase reagent ions for chemical ionization mass spectrometry (CI-MS). Chemical ionization-based mass spectrometry techniques such as proton transfer reaction mass spectrometry (PTR-MS) and selected ion flow-tube mass spectrometry (SIFT-MS) provide real-time, rapid, and online detection and quantification of VOCs using thermodynamics and kinetics of ion-molecule gas-phase reactions. We apply hybrid density functional theory (DFT) to compute proton affinity (PA), ionization energy (IE), and global reactivity parameters for VOCs, which are widely regarded as the primary sources of taints and off-flavors in wine. Atomic polar tensor (APT) charges and total energies at the stationary point for neutral and protonated molecules are also computed. PA and IE values determine the CI-MS mode of reactions, either proton transfer or electron transfer from the reagent gas ions to VOCs. Global reactivity parameters, such as chemical potential (μ), chemical hardness (η), softness (σ), and electrophilic nature (ω) as obtained from frontier molecular orbitals, are considered useful in rationalizing the chemical reactivity patterns of the molecules. A benchmark calculation of indole molecule with MP2, B3LYP, and M06-2X DFT methods at thermodynamically and kinetically stable protonation sites further supports the applied DFT method. Since limited data are available on computed parameters, the reported values would support CI-MS quantification of trace-level VOCs not only in wine but also in various food products.

Computational investigation on mechanisms and kinetics of gas-phase reactions of 4-hydroxy-2-pentanone (4H2P) with hydroxyl radicals and subsequent reactions of CH3C(O)CH2C·(OH)CH3 radical

Computational investigation on mechanisms and kinetics of gas-phase reactions of 4-hydroxy-2-pentanone (4H2P) with hydroxyl radicals and subsequent reactions of CH3C(O)CH2C·(OH)CH3 radical

Theoretical justification of utilization of forest waste by incineration in a composition of bio-water-coal suspensions. Ignition stage

The results of the theoretical studies of the ignition processes of a completely new class of fuel for thermal power plants—Bio-water-coal fuel (Bio-WCF) based on coals of varying degrees of metamorphism, water and forest waste (forest combustible fuel – FCM) have been presented.A mathematical model of the processes of heat and mass transfer that occur during the induction period of time has been developed. The model differs from the known ones in a detailed description of the kinetics of the physicochemical processes occurring in the boundary layer and in the particle. The numerical simulation of the ignition process has been carried out and the ignition delay times (tign) have been calculated. According to the results of the theoretical studies, the influence of the kinetics of gas-phase reactions proceeding in the induction period of time has been established. It has been shown that OH radicals can be active centers-catalysts of the chemical reactions of ignition and combustion of hydrogen (H2), methane (CH4) and carbon monoxide (CO).The basic laws of the physicochemical processes occurring on the surface of a particle (after evaporation of moisture) of Bio-water-coal fuel have been established. It has been shown that secondary reactions of incomplete oxidation of carbon by atmospheric oxygen (with the formation of CO), water vapor (with the formation of CO (or CO2) and H2), hydrogen (with the formation of CH4) do not significantly affect the particles ignition characteristics and conditions (maximum deviations of the values ignition delay times do not exceed 6%).

Determination of kinetics and equilibria of heterogeneously catalyzed gas-phase reactions in gradientless autoclave reactors by using the total pressure method: Methanol synthesis

Rapid methods are very valuable in the determination of the kinetic and mass transfer effects for heterogeneously catalyzed reactions. The total pressure method is a classical tool in the measurement of the kinetics of gas-phase reactions, but it can be successfully applied to the kinetic measurements of gas-phase processes enhanced by solid catalysts. A general theory for the analysis of heterogeneously catalyzed gas-phase kinetics in gradientless batch reactors was presented for the case of intrinsic kinetic control and combined kinetic-diffusion control in porous catalysts. The concept was applied to gas-phase synthesis of methanol from carbon monoxide and hydrogen on a commercial copper-based catalyst (CuO/ZnO/Al2O3 R3-12 BASF). The reaction temperature was 180–210 °C and the initial total pressure was varied between 11 and 21 bar in a laboratory-scale autoclave reactor equipped with a rotating basket for the catalyst particles. The initial molar ratios CO-to-H2 were approximately 1:2, 1:3 and 1:4. The experimental data from methanol synthesis were compared with numerical simulations and a good agreement between the experiments and model simulations was achieved. The predicted equilibrium agrees with previously reported values.

Modeling of sonochemistry in water in the presence of dissolved carbon dioxide

CO2 capture and utilization (CCU) is a process that captures CO2 emissions from sources such as fossil fuel power plants and reuses them so that they will not enter the atmosphere. Among the various ways of recycling CO2, reduction reactions are extensively studied at lab-scale. However, CO2 reduction by standard methods is difficult. Sonochemistry may be used in CO2 gas mixtures bubbled through water subjected to ultrasound waves. Indeed, the sonochemical reduction of CO2 in water has been already investigated by some authors, showing that fuel species (CO and H2) are obtained in the final products. The aim of this work is to model, for a single bubble, the close coupling of the mechanisms of bubble dynamics with the kinetics of gas phase reactions in the bubble that can lead to CO2 reduction. An estimation of time-scales is used to define the controlling steps and consequently to solve a reduced model. The calculation of the concentration of free radicals and gases formed in the bubble is undertaken over many cycles to look at the effects of ultrasound frequency, pressure amplitude, initial bubble radius and bubble composition in CO2. The strong effect of bubble composition on the CO2 reduction rate is confirmed in accordance with experimental data from the literature. When the initial fraction of CO2 in the bubble is low, bubble growth and collapse are slightly modified with respect to simulation without CO2, and chemical reactions leading to CO2 reduction are promoted. However, the peak collapse temperature depends on the thermal properties of the CO2 and greatly decreases as the CO2 increases in the bubble. The model shows that initial bubble radius, ultrasound frequency and pressure amplitude play a critical role in CO2 reduction. Hence, in the case of a bubble with an initial radius of around 5 μm, CO2 reduction appears to be more favorable at a frequency around 300 kHz than at a low frequency of around 20 kHz. Finally, the industrial application of ultrasound to CO2 reduction in water would be largely dependent on sonochemical efficiency. Under the conditions tested, this process does not seem to be sufficiently efficient.

Is the gas-phase OH+H2CO reaction a source of HCO in interstellar cold dark clouds? A kinetic, dynamic and modelling study.

Chemical kinetics of neutral-neutral gas-phase reactions at ultralow temperatures is a fascinating research subject with important implications on the chemistry of complex organic molecules in the interstellar medium (T∼10-100K). Scarce kinetic information is currently available for this kind of reactions at T<200 K. In this work we use the CRESU (Cinétique de Réaction en Ecoulement Supersonique Uniforme, which means Reaction Kinetics in a Uniform Supersonic Flow) technique to measure for the first time the rate coefficients (k) of the gas-phase OH+H2CO reaction between 22 and 107 K. k values greatly increase from 2.1×10-11 cm3 s-1 at 107 K to 1.2×10-10 cm3 s-1 at 22 K. This is also confirmed by quasi-classical trajectories (QCT) at collision energies down to 0.1 meV performed using a new full dimension and ab initio potential energy surface, recently developed which generates highly accurate potential and includes long range dipole-dipole interactions. QCT calculations indicate that at low temperatures HCO is the exclusive product for the OH+H2CO reaction. In order to revisit the chemistry of HCO in cold dense clouds, k is reasonably extrapolated from the experimental results at 10K (2.6×10-10 cm3 s-1). The modeled abundances of HCO are in agreement with the observations in cold dark clouds for an evolving time of 105-106 yrs. The different sources of production of HCO are presented and the uncertainties in the chemical networks discussed. This reaction can be expected to be a competitive process in the chemistry of prestellar cores. The present reaction is shown to account for a few percent of the total HCO production rate. Extensions to photodissociation regions and diffuse clouds environments are also commented.

Reactions in silicon-nitrogen plasma.

Reaction mechanisms that lead to creation of silicon-nitrogen bonds are studied in detail. These reactions are of fundamental importance for silicon nitride synthesis by plasma enhanced chemical vapour deposition from the gas mixture of silane (SiH4) and ammonia (NH3). All reactions in SiH4-NH3 plasma can be categorised as some of the basic types of reactions: bond dissociation, neutral nucleophilic substitution, radical neutralisation, neutral-radical addition, silylene addition, silylene rearrangement, radical nucleophilic addition or hydrogen abstraction reaction. Energetics of these reactions is analysed in detail for a great number of reactions belonging to these categories, by using theoretical modelling. Geometry optimisations are carried out with the MP2/aug-cc-pVTZ level of theory and energetics is further determined with high level ab initio calculations at the CASPT2/aug-cc-pVTZ level, which enabled confirmation of relevance of several mechanisms as reactions that lead to silicon nitride growth from plasma enhanced chemical vapour deposition, as well as introduction of new, energetically favourable mechanisms. Besides amine radical assisted eliminative addition and proton transfer reactions, silylene addition reactions are thermodynamically and kinetically favourable since they lack energy barriers. A new reaction pathway for synthesis of silicon nitride from plasma is proposed. This pathway is enabled by the ability of silylene to create two weak dative bonds, which enables silylene-amine complexes to stick to the silicon nitride surface. Upon dissociation of amine from the surface-bound complex, silylene remains on the surface, available for reaction with other reactive species from plasma.

Energy and Technological Aspects of the Combustion of Ionized Gas-Dispersed Systems

This paper discusses the fl ame ionization effect on the combustion processes in gaseous suspensions of the dispersed fuels. It is shown that the two-phase fl ame ionization&lt;br /&gt;affects almost all the processes of the fuel conversion – kinetics of the dispersed fuel combustion, processes of the interphase heat and mass transfer, processes of the nucleation and coagulation in the fl ame, formation of nanoscale products of the metal particles combustion. It is shown that the electrostatic interaction between the charged nanoparticles and ions or polar molecules in the gaseous phase leads to the appearance of molecular “pumps” that can signifi cantly change the kinetics of the heterogeneous chemical reactions and the heat exchange between particles and gas. The infl uence of the thermal ionization of the fl ame on the nucleation mechanism&lt;br /&gt;and rate in gas-dispersed systems is discussed. The possibility of a barrier-free homogenous and heterogeneous nucleation in the dusty fl ame of metallic particles is shown. The effect of electrostatic and polarizing interactions of ions and molecules&lt;br /&gt;on the kinetics of the ternary gas-phase reactions is considered. The infl uence of the monodisperse aerosol ionization degree on the kinetics of its coagulation is analyzed. It is concluded that electrostatic interaction between the particles strongly affects the inhibition of the coagulation process in gas-phase combustion products and the possibility of very fast (explosive) charged particle coagulation of monodisperse aerosols. The possibility of the targeted size of metal oxides nanoparticles control, controlled ionizing of dusty fl ames and the role of ion particle entrainment, the dependency of their size of the fl ame ionization degree are discussed. Some effects arising in complex plasma of condensed combustion products under its own electric fi elds in fl ames, also when the burning dust is entrained into a constant electric field and their practical applications for diagnosis are considered.

From theoretical reaction dynamics to chemical modeling of combustion

The chemical modeling of combustion treats the chemical conversion of hundreds of species through thousands of reactions. Recent advances in theoretical methodologies and computational capabilities have transformed theoretical chemical kinetics from a largely empirical to a highly predictive science. As a result, theoretical chemistry is playing an increasingly significant role in the combustion modeling enterprise. The accurate prediction of the temperature and pressure dependence of gas phase reactions requires state-of-the-art implementations of a variety of theoretical methods: ab initio electronic structure theory, transition state theory, classical trajectory simulations, and the master equation. In this work, we illustrate the current state-of-the-art in predicting the kinetics of gas-phase reactions through sample calculations for some prototypical reactions central to combustion chemistry. These studies are used to highlight the success of theory, as well as its remaining challenges, through comparisons with experiments ranging from elementary reaction kinetics studies through to global observations such as flame speed measurements. The illustrations progress from the treatment of relatively simple abstraction and addition reactions, which proceed over a single transition state, through to the complexity of multiwell multichannel reactions that commonly occur in studies of the growth of polycyclic aromatic hydrocarbons. In addition to providing high quality rate prescriptions for combustion modelers, theory will be seen to indicate various shortcomings in the foundations of chemical modeling. Future progress in the fidelity of the chemical modeling of combustion will benefit from more widespread applications of theoretical chemical kinetics and from increasingly intimate couplings of theory, experiment, and modeling.

Theoretical Studies on Reactions of OH with H2SO4…NH3Complex and NH2with H2SO4in the Presence of Water

The OH + H2SO4…NH3, NH2 + H2SO4, and NH2 + H2SO4…H2O reactions have been theoretically investigated using high-accuracy quantum chemical methods and conventional transition state theory. The calculation results reveal that remarkable tunneling effects appear at 200 K in the OH + H2SO4…NH3 reaction, compared with obvious tunneling effects below 100 K in the OH + NH3 reaction. The calculated rate constants predict that the hydrogen atom of the free OH group in M1 (H2SO4…NH3) abstracted via OH can compete well with the H atom in H2SO4 abstracted by OH at 200–240 K. The reaction kinetic results also show that a single water molecule could play an important role in the NH2 + H2SO4…H2O reaction below 240 K. The present results provide new insights into the atmospheric oxidation of sulfuric acid, which could be of great use in understanding atmospheric nucleation processes. Additionally, the findings are tunneling effects enhanced by sulfuric acid, which may be wide applications in reaction kinetics of gas-phase reactions.

Laminar Burning Velocities of Atmospheric Coal Air Mixtures

The mechanism for laminar dust flame propagation can only be elucidated from a comprehensive mathematical model which incorporates conduction and radiation, as well as the chemical kinetics of particle devolatilization and gas phase and char reaction. The mathematical model for a flat, laminar, premixed coal-air flame is applied to the atmospheric coal-air mixtures studied by Smoot and co-workers, and comparisons are made with their measurements and predictions. Here the principal parameter for comparison is the laminar burning velocity. The studies of Smoot and co-workers are first reviewed and compared with those predicted by the present model. The effects of inlet temperature and devolatilization rate constants on the burning velocities are studied with the present model, and compared with their measurements and predictions. Their measured burning velocities are approximately predicted with the present model at relatively high coal concentrations, with a somewhat increased inlet temperature. From the comparisons, their model might over-estimate particle temperature and rates of devolatilization. This would enable coal-air mixtures to be burned without any form of preheat and would tend to increase their computed values of burning velocity.

A Theoretical Study on the Mechanism and Kinetics of the Reaction of Methylthiyl Radical with Ozone

Abstract Reaction mechanism and kinetics of gas-phase reaction of methylthiyl radical (CH3S (2A′)) with ozone (1A1) on the lowest doublet electronic state has been theoretically investigated. The composite method of G3MP2 was employed to obtain the optimized geometries of the stationary points and their energies. The most probable path for the title reaction is a barrier-less association channel to form a chemically activated intermediate of CH3SOOO*. The major products at lower temperatures are CH3SO and O2 while species like, CH3, SO, SO2, CH2S, CH2SOH, CH2SO, HO2, and OH are among the products of the probable secondary reactions. Direct hydrogen abstraction reaction of O3 with CH3S to form CH2S and HO3 is not an important channel at lower temperatures. Multichannel RRKM calculations along with the steady-state assumption for the energized intermediate and canonical variational transition-state theory have been used to predict the rate constants for the formation of the most important products. Rate constants for dissociation and isomerization of vibrationally “hot” CH3SO to form CH3 + SO and CH2SOH, respectively, are calculated by means of RRKM method, which are compared with those calculated for the vibrationally “cold” CH3SO. To comply with the previous experimental observations, the major product of the title reaction (CH3SO) should be produced as vibrationally “hot”.

Kinetics of gas-phase reactions of cyc-CF2CF2CF2CHFCH2[sbnd] and trans-cyc-CF2CF2CF2CHFCHF[sbnd] with OH radicals between 253 and 328 K

Rate constants for the reactions of cyc-CF2CF2CF2CHFCH2 (k1) and trans-cyc-CF2CF2CF2CHFCHF (k2) with OH radicals were assessed by a relative rate method. The values were determined as (9.35±5.83)×10−13exp[−(1197±180)/T] and (8.02±2.17)×10−13exp[−(1198±80)/T] between 253 and 328K; and (1.72±0.05)×10−14 and (1.43±0.03)×10−14cm3molecule−1s−1 at 298K, respectively. The atmospheric lifetimes were 2.8 and 3.2 years; the 100-year time horizon global warming potentials were estimated to be 211 and 241, respectively.

Full kinetics investigation of the formation reaction of phosphonate esters in the gas-phase: a theoretical study

In the present work, the proposed multiple-mechanisms have been investigated theoretically for the reaction between triphenyl phosphite and dimethyl acetylenedicarboxylate in the presence of N-H acid such as aniline for generation of phosphonate esters using ab initio molecular orbital theory in gas phase. The profile of the potential energy surface was constructed at the HF/6-311G(d,p) level of theory. The kinetics of the gas phase reaction was studied by evaluating the reaction path of various mechanisms. Between 12 speculative proposed mechanisms {step₁, step₂ (with four possibilities), step₃ (with three possibilities), and step₄} only the third speculative mechanism was recognized as a desirable mechanism. Theoretical kinetics data involving k and E(a), activation (ΔG(‡), ΔS(‡) and ΔH(‡)), and thermodynamic parameters (ΔG°, ΔS° and ΔH°) were calculated for each step of the various mechanisms. Step₁ of the desirable mechanism was identified as the rate determining step. Comparison of the theoretical desirable mechanism with the rate law that has been previously obtained by UV spectrophotometry experiments indicated that the results are in good agreement.

Experimental and modeling study of single coal particle combustion in O2/N2 and Oxy-fuel (O2/CO2) atmospheres

Coal particle combustion experiments were performed in a drop tube furnace (DTF) with oxygen concentration from 21% to 100%, in N2 and CO2 mixtures, under quiescent flow conditions. Small particles (75–90μm) of a high-volatile bituminous coal (PSOC-1451) and a lignite coal (DECS-11) are analyzed with particular attention to the particle burnout times and the particle surface temperatures. These experimental measurements are compared with the predictions of a comprehensive model of coal combustion. Combustion of coal particles is a multi-scale process where both chemical and physical phenomena are involved, thus it requires a coupled and accurate description of the kinetics as well as of the heat and mass transport phenomena. Important features of the model are a multistep kinetic scheme of coal volatilization and detailed kinetics of the successive gas-phase reactions and of the heterogeneous reactions of both char oxidation and gasification. The achieved overall agreement between the experimental data and the numerical predictions, in terms of particle temperature and burnout times, highlights the capability of the model to simulate the effect of different operating conditions in the coal combustion processes.

IR spectroscopy study of the kinetics of the gas-phase reaction between methanol and boron trichloride

We present the results of a Fourier transform IR spectroscopy study of the kinetics of the gas-phase reaction between methanol and boron trichloride. From spectral data on the kinetics of the process, we determined the rate constant for the reaction of CH3OH and BCl3. The studies were conducted on a setup consisting of a chemical reactor, optically and structurally combined with a commercial AF-3 Fourier transform IR spectrometer.

Kinetics of Gas-Phase Reaction of OH with Morpholine: An Experimental and Theoretical Study

Kinetics of reaction of OH radical with morpholine, a heterocyclic molecule with both oxygen and nitrogen atoms, has been investigated using laser photolysis-laser-induced fluorescence technique, in the temperature range of 298-363 K. The rate constant at room temperature (k(298)) is (8.0 +/- 0.1) x 10(-11) molecule(-1) cm(3) s(-1). The rate constant decreases with temperature in the range studied, with the approximate dependence given by k(T) = (1.1 +/- 0.1) x 10(-11) exp[(590 +/- 20)/T] cm(3) molecule(-1) s(-1). The rate constants are high compared with those of similar heterocyclic molecules with oxygen atom but comparable to those reported for aliphatic amines. Ab initio molecular orbital calculations show that prereactive complexes, 5-7 kcal mol(-1) lower in energy as compared with the reactants, are formed because of hydrogen bond interaction between OH and the N/O atom of morpholine. The stability of the complex involving the nitrogen atom is found to be more than that involving the oxygen atom. The optimized transition-state structures and energies for the different pathways of hydrogen abstraction from these prereactive complexes explain the observation of negative activation energy.

Kinetics and mechanism of gas-phase pyrolysis of ylides. Part 3.1Thermal reactivity ofα-carbonyl- and thiocarbonyl-stabilized methylenetriphenylphosphoranes

Fourteen ketone/thione-stabilized triphenylphosphonium methylides were subjected to conventional gas-phase and flash vacuum pyrolysis (FVP). The kinetics of the first-order thermal gas-phase reactions of all these compounds were investigated over 360–653 K temperature range. The values of the Arrhenius log A and energy of activation of these ylides averaged 11.52 ± 0.34 s−1 and 133.20 ± 3.14 kJ mol−1, respectively. The products of sealed-tube (static) and FVP were analyzed and compared. A mechanism is proposed to account for the products of reaction. The rate constants [k (s−1)] of the substrates at 500 K were calculated and used to substantiate the proposed mechanism of pyrolysis, and to rationalize the thermal gas-phase reactivities of the ylides under study. Copyright © 2010 John Wiley & Sons, Ltd.