Showing papers on "Elementary reaction published in 2002"

Ionic reactions and pyrolysis of glycerol as competing reaction pathways in near- and supercritical water

Abstract: Experimental results of the decomposition of glycerol in near- and supercritical water are presented considering measurements in the temperature range of 622–748 K, at pressures of 25, 35, or 45 MPa, reaction times from 32 to 165 s, and different initial concentrations. The reaction was carried out in a tubular reactor and a conversion between 0.4 and 31% was observed. The main products of the glycerol degradation are methanol, acetaldehyde, propionaldehyde, acrolein, allyl alcohol, ethanol, formaldehyde, carbon monoxide, carbon dioxide, and hydrogen. The results are compared with the studies of other working groups. The non-Arrhenius behavior of the overall degradation, as well as the pressure dependence of the reaction rate, and furthermore, the product distribution indicates the occurrence of two competing reaction pathways. One pathway consists of ionic reaction steps, which are preferred at higher pressures and/or lower temperatures. The second reaction pathway is a free radical degradation and dominates at lower pressures and/or higher temperatures. For reaction modeling, both mechanisms, the ionic and the free radical reaction network are compiled into one reaction model. The computer software package chemkin was used for the model calculations. The reaction model and the kinetic parameters were optimized in order to describe the experimental results for glycerol and the main products at 450 bar and all temperatures. This reaction model, consisting of the ionic and the free radical sub-mechanism satisfactorily describes the complex reaction at 450 bar.

Methane transformation to carbon and hydrogen on Pd(100): Pathways and energetics from density functional theory calculations

Abstract: Density functional theory with gradient corrections has been employed to study the reaction pathways and the reaction energetics for the transformations of CH4 to C and H on a Pd(100) surface. On examination of transition state structures identified in each elementary reaction, a clear relationship between the valencies of the CHx fragments and the locations of the transition states emerges. The higher the valency of the CHx fragment, the higher the coordination number of the CHx with the surface atoms. The calculated reaction energetics are in good agreement with the experiments. In addition, calculation results are also used to illustrate an interesting issue concerning the CH3 stability on Pd surfaces.

Local flame structure in the well-stirred reactor regime

Abstract: Direct numerical simulations of hydrogen/air turbulent premixed flame progagating in the three-dimensional turbulence are conducted to investigate local flame structures in the well-stirred reactor regime. A detailed kinetic mechanism including 12 reactive species and 27 elementary reactions is used to represent the H 2 /air reaction in turbulence. Although the flame condition is classified into the well-stirred reactor regime, the geometry of the regions with high heat release rate shows thin sheetlike structure. The fluctuation of the heat release rate along the flame surface is relatively high, and the maximum heat release rate reaches up to 1,3 times the corresponding laminar flame. The heat release rate tends to be high in the regions convex toward the burned side. The flame structure in the case of the well-stirred reactor regime shows a double-layered feature. One may conclude that reaction zone becomes thick in the well-stirred reactor regimes only from temperature, H, and OH distributions, while the heat release rate, mass fraction of O atoms, and reaction rates of O atoms and OH radicals are fluctuating significantly in that region in fact. Specifically, reaction rates of O atoms and OH radicals show characteristic behaviors in the burned side due to their chemical characteristics. In the preheat zone, mass fraction and reaction rate of HO 2 show quite thin and smooth distributions compared to other properties such as the heat release rate. The distribution of H 2 O 2 reaction rate reflects the double-layered feature of the well-stirred reactor regime very well. It is shown that the double-layered feature can be explained by discussing the balances of the elementary reactions in detail. As the flame front can be defined even in the well-stirred reactor regime, statistics of the local flame elements are also discussed.

A DFT Study of CHx Chemisorption and Transition States for C−H Activation on the Ru(112̄0) Surface

Abstract: The thermodynamics of methane decomposition on the ruthenium (1120) surface has been investigated with ab initio periodic calculations. All surface intermediates are more stable than the gas-phase methane even if the last step of the decomposition path: CH → C + H, is highly endothermic. Among all of the surface species, CH2 appears to be the most stable. All of the surface species (CHx, x = 3−1 and H) adsorb on bridge-up sites, while atomic C prefers top-down sites. The transition states of the elementary reactions for the dissociation of methane on the ruthenium (1120) surface have been investigated with nudged elastic band method (NEB). The calculated barriers are 56 kJ mol-1 for methane decomposition, 11 kJ mol-1 for methyl decomposition, and 52 kJ mol-1 for methylene decomposition, respectively. The decomposition of CHads requires the highest activation energy from the series with 95 kJ mol-1.

Product State Resolved Dynamics of Elementary Reactions

Abstract: We describe how the photon-initiated reaction technique has been used to provide dynamical information about elementary gas-phase bimolecular reactions at the product quantum state-resolved level. ...

Quantum chemical study of the elementary reactions in zirconium oxide atomic layer deposition

Abstract: Elementary reactions in atomic layer deposition of zirconia using zirconium tetrachloride and water are investigated using the density functional theory. The atomistic mechanisms of the two deposition half cycles on the Zr–OH and Zr–Cl surface sites are investigated. Both half reactions proceed through the formation of stable intermediates, resulting in high barriers for HCl formation. We find that the intermediate stability is lowered as the surface temperature is raised. However, increasing temperature also increases the dissociation free-energy barrier, which in turn results in increased desorption of adsorbed precursors.

The acetic acid forming channel in the acetone + OH reaction: A combined experimental and theoretical investigation

Abstract: In a flow reactor–molecular beam sampling mass spectrometry investigation of the elementary reaction of acetone with OH at 290 K, no significant production of acetic acid could be measured; absolute calibrations result in a branching fraction of the OH-addition/CH3-elimination channel of at most ≈5%. In a theoretical study of the acetone+OH reaction, the potential energy profiles of the OH-addition/CH3-elimination channel, the direct H-abstraction channel, and the indirect H-abstraction path via a hydrogen-bonded OH–acetone complex, were characterized at the B3LYP-DFT/6-31G(d,p) and B3LYP-DFT/6-311++G(d,p) levels of theory, with single-point CCSD(T)/6-311++G(2d,2p) energy calculations. At all levels, the barrier for OH-addition is found to be 6±0.5 kcal mol−1, and at least 2.5 kcal mol−1 higher than that for the H-abstraction channels. Transition state theory and RRKM - master equation calculations indicate that the OH-addition channel is negligible at all relevant atmospheric temperatures. These results are in disagreement with recent reports that the OH-addition/CH3-elimination channel contributes about 50% at room temperature.

Chemical kinetics and mechanism

Abstract: Part 1: Chemical Kinetics Introduction A Closer Look at Chemical Reactions Rate in Chemical Kinetics Factors Determining the Rate of a Chemical Reaction Determining Experimental Rate Equations at a Fixed Temperature The Effect of Temperature on the Rate of a Chemical Reaction Elementary Reactions Reaction Mechanism Part 2: The Mechanism of Substitution Organic Reactions Reaction Mechanisms Ionic Substitution Reactions SN2 and SN1 Reaction Mechanisms SN2 Versus SN1 Part 3: Elimination: Pathways and Products Introduction: ss-Elimination Reactions The E2 Mechanism The E1 Mechanism Elimination Versus Substitution Other Useful Elimination Reactions Case Study: Shape-Selective Catalysis Using Zeolites.

Accuracy of the centrifugal sudden approximation in the H+H2O reaction and accurate integral cross sections for the H+H2O→H2+OH abstraction reaction

Abstract: The initial state selected time-dependent wave packet method has been extended to calculate the total reaction probability for atom-triatom reactions with total angular momentum J>0 by treating both bonds in the triatom reagent reactively. The total exchange and abstraction reaction probabilities for the title reaction with J=15 calculated with 2 K-blocks (the projection of the total angular momentum on the body-fixed axis) show that one has to treat both OH bonds in the H2O reagent reactively for the exchange reaction, but for the abstraction reaction one can treat one OH bond as a spectator bond to get accurate results. This is in accord with what had been found for the total reaction probabilities for J=0 [Phys. Rev. Lett. 89, 103201 (2002)]. The J=15 reaction probabilities also show that the CS (centrifugal sudden) approximation is inadequate for the title reaction, in particular for the abstraction reaction. The integral cross sections for the abstraction reaction, calculated without the CS approxima...

The reaction of F + D2 at ultra-low temperatures: the effect of rotational excitation

Abstract: We report an ab initio study of the dynamics of the reaction F + D2→DF + D, where D2 is initially in a rotationally excited state (j = 2). The possibility of obtaining ultra-cold molecules and of investigating reaction dynamics at ultra-low temperature relies on the production of molecules in a well defined quantum state and it is important to know the relative efficiency of the rotational quenching and of the chemical reaction. We examine here a reaction with an activation barrier, the reaction of F with D2, and we find that quenching dominates the reaction when the initial rotational level lies energetically below the barrier, so severe trap loss may occur before the reaction can take place.

1,3-Dipolar additions involving allenes: A density functional study of concerted and stepwise mechanisms

Abstract: Structures and energetics of reactants, transition structures, diradical intermediates and products of the 1,3-dipolar cycloadditions of allene with diazomethane, nitrile oxide and nitrone, have been investigated using DFT calculations at B3LYP/6-31G(d) level and the entire reaction surface has been explored. Two pathways are open for these reactions leading to two regiochemical products and in each way, the reaction can proceed in concerted or stepwise manner. In the stepwise process, there are two modes of attack, one leading to an allylic intermediate and the other an anti intermediate. A thorough search and analysis of these reaction paths show that stepwise modes are preferred over concerted modes, and regio 1 reaction prefers a stepwise path that involves a gauche intermediate whereas regio 2 reaction follows a stepwise path where an allylic intermediate is formed. Mainly the heteroatomic influence on the stability of diradical intermediates and other species in the reaction path and the cumulenic strain on allene determine the mechanism. Above factors also explain greater stability of allylic TS of regio 2 reaction compared to anti TS of regio 1 reaction, and this in turn explains the observed regioselectivity of the allene–diazomethane reaction. Computed deformation energies and bond orders establish that the favoured transition states are reactant-like and hence involve lower activation energies.

Stochastic model of reaction rate oscillations in the CO oxidation on nm-sized palladium particles

Abstract: A mesoscopic stochastic model of the catalytic reaction 2CO+O2→2CO2 on the surface of a metal particle is considered. The model is a Markovian chain of elementary reaction steps, which mimics the catalytic oxidation of CO on a nm-sized Pd particle. The model takes into account the effect of the particle size on the reaction rate and the role of temporal fluctuations of the concentrations of the reactants. The main goal of the paper is the comparison of the dynamics produced by the stochastic model and the deterministic model obtained via averaging of the master equation, while the catalyst particle size is reduced. Intrinsic fluctuations during the reaction are shown to change the reaction kinetics drastically for small metal particles with only several hundreds of surface atoms.

Measurements of hydrogen cyanide and its chemical production rate in a laminar methane/air, non-premixed flame using cw cavity ringdown spectroscopy

Abstract: Cavity ringdown spectroscopy using an external cavity tunable diode laser is coupled with microprobe sampling to quantitatively determine hydrogen cyanide concentrations in a methane/air non-premixed flame. HCN concentration data were combined with concentration, velocity, and temperature data previously collected in this flame system to perform a net production rate analysis from a solution of the species conservation equation. This net rate profile exhibits two dominant features: a production feature in a flame region just rich of the stoichiometric surface and a destruction feature at the stoichiometric surface. A reaction path analysis was performed for a series of HCN elementary reactions. The net rate of HCN formation and destruction calculated from this chemical approach agreed well in peak locations with those calculated using the transport rates described above. Analysis of the individual reaction data suggests that destruction of HCN is dominated by oxygen-atom reactions producing several radical species that are likely to oxidize further to NO. Thus, the magnitude of this destruction feature may be an indirect measure of the local NO formation rate through the prompt mechanism.

Initial Reaction Steps in the Condensed-Phase Decomposition of Propellants

Abstract: Understanding the reaction mechanisms for the decomposition of NO2-containing energetic materialsin the condensed phase is critical to our development of detailed kinetic models of these energetic materials in propellant combustion. To date, the reaction mechanisms in the condensed phase have been represented by global reactions. The detailed elementary reactions subsequent to the initial NO2 bond scissioning are not known. Using quantum chemical calculations, we have investigated the possible early steps in the decomposition of energetic materials that can occur in the condensed phase. We have used methylnitrate, methylnitramine, and nitroethane as prototypes for O−NO2, N−NO2 and C−NO2 nitro compounds. We find the energetic radials formed from the initial NO2 bond scissioning can be converted to unsaturated non-radical intermediates as an alternative to the unzipping of the energetic radical. This reaction pathway is caused by the cage effect, which prevents the NO2 molecule from diffusing away before it can react with the energetic radical to form HONO. We propose a new, prompt oxidation mechanism in which the HONO, while still trapped within the cage, can add back onto the energetic molecule. This produces oxidation products in the condensed phase that normally would not be produced until much later in the flame. We propose that this prompt oxidation mechanism may be a general feature of both nitramines and nitrate esters. The resulting HONO formed by the H-atom abstraction will be strongly influenced by the physical properties of the condensed phase. The applicability of this mechanism is demonstrated for decomposition of ethylnitrate, illustrating the importance of the cage effect in enabling this mechanism to occur at low temperatures.

High temperature oxidation of iron atoms by CO2

Abstract: The reaction of Fe-atoms with CO2 was studied in the temperature range of 1330 K⩽T⩽2650 K at pressures between 1.0 and 1.6 bar in a shock wave reactor. Resonance absorption spectroscopy was applied for time-resolved measurement of Fe-atoms, O-atoms, and CO-molecules in initial gas mixtures containing Fe(CO)5 and CO2, highly diluted in argon. The experiments showed at early reaction time an Fe-consumption and a simultaneous CO formation which was interpreted by reaction R2 (explained below): The rate coefficient was determined to be k2=1014.51±0.10 exp(−15000±400 K/T) cm3 mol−1 s−1. At higher temperatures and longer reaction times a quasi stationary Fe-atom concentration level was observed which could not be explained by (R2) only. Therefore secondary reactions were considered in a simplified reaction mechanism, enabling the simulation of the complete Fe, O, and CO concentration profiles. For the major secondary reaction (R3, explained below): a rate coefficient of k3=1015.59±0.46 exp(−23000±1700 K/T) cm3 mol−1 s−1 was determined, which is regarded more like a fitting parameter than an elementary reaction.

Quasiclassical Rate Coefficients for the H2+H2 Reaction and Dissociation

Abstract: Vibrational state-to-state quasiclassical rate coefficients of the H2+H2 reaction summed over product rotational states for thermalized reactants’ rotations and translations are given at various values of the temperature in the range 1000–4000 K. Values are given for both reactive and nonreactive processes. Separate values are also given for processes involving dissociation.

MECHGEN: computer aided generation and reduction of reaction mechanisms.

Abstract: The paper describes selection rules implemented in a software generating “possible reaction mechanism”, i.e. a set of elementary reactions chosen from all stoichiometrically possible reactions. The novelty of the approach lies in the fact that the user has to define all species involved (reactants, intermediates, products), and the rules applied with user-set limits reduce the resulting mechanism to a reasonable set of possible elementary reactions. The computer code consists of five parts: (i) definition of species, and introducing its characteristics (structure and thermodynamic data); (ii) definition of the reacting system and generation of all stoichiometrically possible reactions; (iii) reduction of the mechanisms using complexity and thermodynamic constraints based on user-set limits; (iv) calculation of the resulting pathways (routes of the various atoms or groups of atoms transferred from one species to another); and (v) tools to help visualization of the process by finding those elementary proce...

The Influence of Catalytic Surfaces on the Barriers to Elementary Surface Reaction Steps

Abstract: The transition states to elementary reaction steps on surfaces have received little attention because of the lack of experimental probes of their structure and properties. This lack of understanding of the transition states for surface reactions places severe constraints on our ability to predict the kinetics of catalytic reactions and other surface chemical processes. The use of substituent effects has provided one approach to probe the nature of transition states and a means for determining whether such transition states can be considered to occur early or late in the reaction coordinate. This has been applied to several well-defined elementary surface reactions. As examples, the transition state for ‚-hydride elimination in adsorbed alkyl and alkoxy groups is believed to occur late in the reaction coordinate while the transition state for dehalogenation reactions on surfaces is believed to occur early in the reaction coordinate. Combining this knowledge with a comprehensive review of the barriers to these reactions on a wide variety of surfaces has suggested a simple proposition for considering the effects of surfaces on the barriers to elementary reactions. The barriers to elementary reaction steps with late transition states are expected to be sensitive to the nature of the surface while the barriers to reactions with early transition states are expected to be relatively insensitive to the nature of the surface. This proposition is illustrated by first considering the trivial examples of molecular adsorption and desorption on surfaces and then by discussion of surface activated ‚-hydride elimination and dehalogenation reactions.

Formation of CO in the Reaction of Oxygen Atoms with CH3: Reaction over a Barrier but Not through a Saddle Point

Abstract: A type of chemical reaction characterized by a reaction path proceeding over an energy barrier (a ridge of the potential energy surface) but not through a saddle point is described. A method of computation of rate constants for such “over-the-ridge” reactions is developed on the basis of RRKM and transition state theories. Formulas and computational procedures are presented for the cases of microscopic energy-dependent rates of unimolecular reactions and the thermally averaged rates of unimolecular or bimolecular reactions. This method is applied to compute the branching fraction of the CO-producing channel (channel 1c) of the reaction of O atoms with CH3 radicals. A quantum chemical computational study of the potential energy surface (PES) of the decomposition of CH3O was performed using UHF, UMP2, QCISD, and B3LYP methods as well as single-point G2 and CBS-Q energy calculations. The results demonstrate that the products of the reaction channel 1c are formed in a process characterized by trajectories in ...

The Application of the Concept of Extent of Reaction

Abstract: The concept of extent of reaction permits the definition of the related variable, degree of advancement. The expression of the concentrations in the reaction rate equation as a function of one degree of advancement variable leads to considerable simplification of the mathematical problem and yields a single differential equation to solve. However, the misuse of these concepts leads to wrong results, which have survived for 40 years even in textbooks on chemical kinetics. In fact, the definition of one degree of advancement variable for each elementary reaction step is required in the case of multistep reactions.

Development and Validation of a Mathematical Model for the Chemical Vapor Deposition of Alumina from Mixtures of Aluminum Trichloride, Carbon Dioxide, and Hydrogen

Abstract: Detailed homogeneous and heterogeneous chemistry models are formulated for the chemical vapor deposition (CVD) of alumina from mixtures of aluminum trichloride, hydrogen, and carbon dioxide. Since formation of the species that are required for the incorporation of oxygen atoms in the film (primarily H 2 O and OH) takes place through the pathways of the water-gas-shift reaction, a complete kinetic model for this process is incorporated as a submodel in the homogeneous chemistry model. Information obtained from the analysis of the thermodynamic equilibrium in the gas phase of the reacting mixture and from past experimental and theoretical studies is employed to determine which elementary reaction steps play an important role in the overall deposition process The kinetic model is introduced into the transport and reaction model of a tubular, hot-wall CVD reactor, and it is employed to obtain results on the effects of the operating conditions on the variation of the gas-phase composition, deposition rate, and surface species coverages with residence time, under conditions typically encountered in aluminum oxide chemical vapor deposition. Experimental data obtained in a tubular, hot-wall CVD reactor in our laboratory are used to validate the predictions of the overall model. The model is found to he capable of reproducing successfully the experimental data and the various behavior patterns observed in the experiments, such as the occurrence of a maximum in the variation of the deposition rate with parameters that affect strongly the residence time of the mixture in the reactor.

A Theoretical Study of the Bifurcation Reaction II : Acetic Acid

Abstract: The competing reactions in the thermal unimolecular decomposition of acetic acid were examined theoretically. Calculations of the transition state structures, the activation energies, and the intrinsic reaction coordinates (IRC) for the two competing reaction paths were performed by using the molecular orbital and the density functional theory. The potential barrier heights were mostly the same for the two competing reaction paths, which were consistent with previous studies. An examination of the mode coupling between the IRC and vibrational modes predicted no significant difference in the rates for the corresponding paths in agreement with experimental results. This is different from the case of formic acid which was reported in our previous paper of this series. The unimolecular reaction rate seems to be controlled by the number of effective vibrational modes which couple strongly with the IRC before the reactant arrives at the transition state.

Phase Separation Dynamics and Reaction Kinetics of Ternary Mixture Coupled with Interfacial Chemical Reaction

Abstract: In this paper, computer simulation results of phase separation dynamics of ternary mixture (A, B, and C) coupled with an interfacial chemical reaction A + BdC in two-dimension are presented. The effect of reduction of interfacial free energy due to the presence of species C along the interface is taken into consideration in our study. In the case of fixed domain size, it is shown in simulations that for both reversible and irreversible reactions the generation of species C is not affected by the reaction rate constants, and it is a diffusioncontrolled process. Also, the simulation reveals that for reversible chemical reaction in the case of fixed domain size, the equilibrium of concentrations is established. In the cases of coupling between domain growth and reversible reaction kinetics, it is shown in simulations that the domain growth eventually freezes regardless of the magnitude of the reaction rate constants. The higher the reaction rate constants are, the slower the evolution to reach the reach steady state is, the larger the steady state domain size is, and the smaller the average concentration of species C is.

Theoretical Study of the Kolbe-Schmitt Reaction Mechanism

Abstract: A theoretical study of the Kolbe-Schmitt reaction mechanism, performed using a DFT method, reveals that the reaction between sodium phenoxide and carbon dioxide proceeds with the formation of three transition states and three intermediates. In the first step of the reaction, a polarized O-Na bond of sodium phenoxide is attacked by the carbon dioxide molecule, and the intermediate NaPh-CO 2 complex is formed. In the next step of the reaction the electrophilic carbon atom attacks the ring primarily at the ortho position, thus forming two new intermediates. The final product, sodium salicylate, is formed by a 1,3-proton shift from C to O atom. The mechanism agrees with the experimental data related to the Kolbe-Schmitt reaction.