Showing papers on "Elementary reaction published in 2005"

Gas-phase catalysis by atomic and cluster metal ions: the ultimate single-site catalysts.

Abstract: Gas-phase experiments with state-of-the-art techniques of mass spectrometry provide detailed insights into numerous elementary processes. The focus of this Review is on elementary reactions of ions that achieve complete catalytic cycles under thermal conditions. The examples chosen cover aspects of catalysis pertinent to areas as diverse as atmospheric chemistry and surface chemistry. We describe how transfer of oxygen atoms, bond activation, and coupling of fragments can be mediated by atomic or cluster metal ions. In some cases truly unexpected analogies of the idealized gas-phase ion catalysis can be drawn with related chemical transformations in solution or the solid state, and so improve our understanding of the intrinsic operation of a practical catalyst at a strictly molecular level.

Modeling Elementary Heterogeneous Chemistry and Electrochemistry in Solid-Oxide Fuel Cells

Abstract: This paper presents a new computational framework for modeling chemically reacting flow in anode-supported solid-oxide fuel cells (SOFC). Depending on materials and operating conditions, SOFC anodes afford a possibility for internal reforming or catalytic partial oxidation of hydrocarbon fuels. An important new element of the model is the capability to represent elementary heterogeneous chemical kinetics in the form of multistep reaction mechanisms. Porous-media transport in the electrodes is represented with a dusty-gas model. Charge-transfer chemistry is represented in a modified Butler-Volmer setting that is derived from elementary reactions, but assuming a single rate-limiting step. The model is discussed in terms of systems with defined flow channels and planar membrane-electrode assemblies. However, the underlying theory is independent of the particular geometry. Examples are given to illustrate the model.

Unravelling combustion mechanisms through a quantitative understanding of elementary reactions

Abstract: This review of the role of reaction kinetics in combustion chemistry traces the historical evolution and present state of qualitative and quantitative understanding of a number of reaction systems. Starting from the H2–O2 system, in particular from the reaction between H and O2, mechanisms and key reactions for soot formation, for the appearance of NOx, and for processes of peroxy radicals in hydrocarbon oxidation are illustrated. The struggle for precise rate constants on the experimental and theoretical side is demonstrated for the example of the reaction H + O2 → OH + O. The intrinsic complexity of complex-forming bimolecular reactions, such as observed even in this reaction, also dominates most other key reactions of the systems considered and can be unravelled only with the help of quantum-chemical methods. The multi-channel character of these reactions often also requires the combination with master equation codes. Although kinetics provides an already impressive database for quantitative modelling of simple combustion systems, considerable effort is still required to quantitatively account for the complexities of more complicated fuel oxidation processes.

Elucidation of the mechanism of selenoprotein glutathione peroxidase (GPx)-catalyzed hydrogen peroxide reduction by two glutathione molecules: a density functional study.

Abstract: The mechanism of the hydrogen peroxide reduction by two molecules of glutathione catalyzed by the selenoprotein glutatione peroxidase (GPx) has been computationally studied. It has been shown that the first elementary reaction of this process, (E-SeH) + H2O2 → (E-SeOH) + H2O (1), proceeds via a stepwise pathway with the overall barrier of 17.1 kcal/mol, which is in good agreement with the experimental barrier of 14.9 kcal/mol. During reaction 1, the Gln83 residue has been found to play a key role as a proton acceptor, which is consistent with experiments. The second elementary reaction, (E-SeOH) + GSH → (E-Se-SG) + HOH (2), proceeds with the barrier of 17.9 kcal/mol. The last elementary reaction, (E-Se-SG) + GSH → (E-SeH) + GS-SG (3), is initiated with the coordination of the second glutathione molecule. The calculations clearly suggest that the amide backbone of the Gly50 residue directly participates in this reaction and the presence of two water molecules is absolutely vital for the reaction to occur. ...

Kinetic mechanism of methanol decomposition on Ni(111) surface: a theoretical study.

Abstract: The decomposition of methanol on the Ni(111) surface has been studied with the pseudopotential method of density functional theory−generalized gradient approximation (DFT−GGA) and with the repeated slab models. The adsorption energies of possible species and the activation energy barriers of the possible elementary reactions involved are obtained in the present work. The major reaction path on Ni surfaces involves the O−H bond breaking in CH3OH and the further decomposition of the resulting methoxy species to CO and H via stepwise hydrogen abstractions from CH3O. The abstraction of hydrogen from methoxy itself is the rate-limiting step. We also confirm that the C−O and C−H bond-breaking paths, which lead to the formation of surface methyl and hydroxyl and hydroxymethyl and atom hydrogen, respectively, have higher energy barriers. Therefore, the final products are the adsorbed CO and H atom.

On the origin of power-law kinetics in carbon oxidation

Abstract: Many experimental studies report high fractional orders for the global carbon/oxygen reaction, including several studies that cover wide ranges in oxygen partial pressure and observe power-law kinetics with a constant apparent order over the entire range. This persistent n th-order behavior is inconsistent with the simple Langmuir kinetic model and is also a challenge for more elaborate multi-step models of elementary reactions on ideal surfaces. The power law form is a simple and attractive rate law, but without a fundamental basis, it will remain empirical and ultimately controversial. The present paper evaluates the effect of surface or site heterogeneity as an explanation for the paradox of persistent power-law behavior. Simple models of intrinsic and induced heterogeneity are used to show that power-law kinetics are indeed expected when desorption or adsorption activation energy distributions are broad. Examination of experimental TPD data shows that desorption activation energy distributions are broad enough for global power-law kinetics to be generally expected for disordered carbons. The particular formulation of Haynes was evaluated as a candidate framework for describing the main features in the carbon oxidation database at temperatures below 1000 K and oxygen pressures above 0.01 bar. The Haynes turnover model predicts persistent power-law behavior and gives a promising description of the absolute reaction orders, activation energies, and near-atmospheric rates for several coal and polymer chars. It also predicts the low reaction order and its weak variation with pressure in the graphitized carbon black data of Tyler and coworkers. The origin of power-law kinetics is discussed in terms of the detailed behavior of three classes of active sites: reactive bare sites, partially covered sites, and stable oxide, and an approximate analytical expression is derived that relates global order features in the desorption activation energy distribution.

Chemistry of Ice Surfaces. Elementary Reaction Steps on Ice Studied by Reactive Ion Scattering

Abstract: This Account describes a recent study of reactions on ice surfaces with the emphasis on the mechanistic features of elementary reactions steps. Cs(+) reactive ion scattering (Cs(+) RIS) and low-energy sputtering (LES) techniques monitor the reactions by detecting the molecules and ions on the ice surface. The types of reactions include molecule diffusion and migration, proton transfer, and some simple reactions on frozen water and alcohol surfaces. Ice surface reactions exhibit unique behaviors due to a kinetic constraint, resulting in the isolation of reaction intermediates, preferential stabilization of charged species, and diversity of reaction products.

Lanthanide organometallic sandwich nanowires: formation mechanism.

Abstract: A molecular beam of europium-cyclooctatetraene sandwich nanowires Eu(n)()(COT)(m)() was produced by a laser vaporization synthesis method. The formation mechanism of the nanowires was quantitatively revealed by photoelectron and photoionization spectroscopies of the Eu-COT species, together with supporting theoretical calculations. From these results, it is confirmed that growth processes extending the length of Eu-COT nanowires involve a series of elementary reactions in which efficient charge transfer occurs at the terminal reaction sites. In every elementary step, the reaction proceeds between one reactant having low ionization energy and the other reactant having high electron affinity, probably via a "harpoon" mechanism.

Hydrogen Desorption Mechanism in a Li−N−H System by Means of the Isotopic Exchange Technique

Abstract: The hydrogen desorption mechanism in the reaction from LiH + LiNH2 to Li2NH + H2 was examined by thermal desorption mass spectrometry, thermogravimetric analysis, and Fourier transform IR analyses for the products replaced by LiD or LiND2 for LiH or LiNH2, respectively. The results obtained indicate that the hydrogen desorption reaction proceeds through the following two-step elementary reactions mediated by ammonia: 2LiNH2 → Li2NH + NH3 and LiH + NH3 → LiNH2 + H2, where hydrogen molecules are randomly formed from four equivalent hydrogen atoms in a hypothetical LiNH4 produced by the reaction between LiH and NH3 according to the laws of probability.

Acetylene coupling on Cu(111): formation of butadiene, benzene, and cyclooctatetraene.

Abstract: Acetylene trimerizes to benzene on the (111) face of copper, as it does on the (100) and (110) planes. However, Cu(111) also yields butadiene and cyclooctatetraene, the latter never previously found with Cu or any other material. No coverage threshold is observed for the onset of these coupling reactions, implying high adsorbate mobility: gaseous benzene is formed by a surface reaction rate-limited process, whereas butadiene and cyclooctatetraene are formed by desorption rate-limited processes. H/D isotope tracing shows that benzene formation proceeds via a statistically random associative mechanism, whereas butadiene formation is associated with strong kinetic isotope effects, probably associated with C-H cleavage. A pericyclic mechanism involving dimerization of C4H4 metallocycles is proposed to account for the formation of cyclooctatetraene. We also found that approximately 45 nm alpha-alumina supported copper particles operated under catalytic conditions at atmospheric pressure yield the same principal reaction products as those found with Cu(111) under vacuum conditions. It therefore seems likely that the elementary reaction steps that describe the surface chemistry of the model system are also important under practical conditions. Comparison of the structure, bonding, and reactivity of acetylene on Cu(111) and Pd(111) indicates that the effectiveness of copper in promoting C-H cleavage in adsorbed acetylene is associated with greater rehybridization of the C-C bond with concomitant weakening of the C-H bond.

Elementary reactions of formyl (HCO) radical studied by laser photolysis—transient absorption spectroscopy

Abstract: Several elementary reactions of formyl radical of combustion importance were studied using pulsed laser photolysis coupled to transient UV–Vis absorption spectroscopy: HCO → H + CO (1), HCO + HCO → products (2), and HCO + CH3 → products (3). One-pass UV absorption, multi-pass UV absorption as well as cavity ring-down spectroscopy in the red spectral region were used to monitor temporal profiles of HCO radical. Reaction (1) was studied over the buffer gas (He) pressure range 0.8–100 bar and the temperature range 498–769 K. Reactions (2a) , (2b) , (2c) , (3a) , (3b) as well as the UV absorption spectrum of HCO, were studied at 298 and 588 K, and the buffer gas (He) pressure of 1 bar. Pulsed laser photolysis (308, 320, and 193 nm) of acetaldehyde, propionaldehyde, and acetone was used to prepare mixtures of free radicals. The second-order rate constant of reaction (1) obtained from the data at 1 bar is: k1(He) = (0.8 ± 0.4) × 10−10exp(−(66.0 ± 3.4) kJ mol−1/RT) cm3 molecule−1 s−1. The HCO dissociation rate constants measured in this work are lower than those reported in the previous direct work. The difference is a factor of 2.2 at the highest temperature of the experiments and a factor of 3.5 at the low end. The experimental data indicate pressure dependence of the rate constant of dissociation of formyl radical 1, which was attributed to the early pressure fall-off expected based on the theory of isolated resonances. The UV absorption spectrum of HCO was revised. The maximum absorption cross-section of HCO is (7.3 ± 1.2) × 10−18 cm2 molecule−1 at 230 nm (temperature independent within the experimental error). The measured rate constants for reactions (2a) , (2b) , (2c) , (3a) , (3b) are: k2 = (3.6 ± 0.8) × 10−11 cm3 molecule−1 s−1 (298 K); k3 = (9.3 ± 2.3) × 10−11 cm3 molecule−1 s−1(298 and 588 K).

Elementary reaction mechanism of methylamine oxidation in supercritical water

Abstract: An elementary reaction model for the supercritical water oxidation (SCWO) of methylamine was proposed, based on a combustion mechanism that involved reactions relevant to NH2CH2O2. A comparison of the predicted results of our proposed model with the experimental data revealed that our model could predict the conversion via the SCWO of methylamine and the effects of temperature and oxygen concentrations on the NH3 selectivity quantitatively well, and also could qualitatively explain the trends of the measured product distribution, especially NH3 as the exclusive nitrogen-containing product of the SCWO of methylamine. This work clarified that the peroxy radical NH2CH2O2, which appears in low-temperature combustion through the reaction of O2 with CH2NH2, and the relevant elementary reactions and intermediate species had key roles in the SCWO of methylamine.

Cool flames in space: experimental and numerical studies of propane combustion

Abstract: New results from experiments on propane + oxygen cool flame and two-stage ignitions in microgravity are reported. These include the occurrence of a “self-stabilized” cool flame that survives for more than 10 s, before dying away. Then, by integration of reaction–diffusion equations in a one-dimensional model, using a reduced kinetic scheme including very recent kinetic data for propylperoxy radical reactions derived by DeSain et al., simulations of these phenomena are presented and discussed. A validation of the reduced mechanism is given first, by simulation of multiple stage ignitions in closed vessels under spatially uniform conditions. The model is used to obtain a detailed spatial structure for temperature and selected chemical species during the development of cool flames, simulated with heat and mass diffusion in 1-D. Also included are novel approaches to the simulation of pressure change in a spherical vessel and the light output from CH 2 O ∗ chemiluminescence. The onset and spatial growth of the cool flame, and its ability to stabilize for a reasonable interval, is traced to the interaction between the elementary reactions that govern the negative temperature coefficient of reaction rate and the temperature field that is controlled by thermal diffusion to the reaction vessel walls. The vessel surface was assumed to be chemically inactive.

Decomposition and ring expansion in methylcyclopentadiene: single-pulse shock tube and modeling study

Abstract: The thermal reactions of methylcyclopentadiene diluted in argon were studied behind reflected shock waves in a 2 in. i.d. pressurized driver single-pulse shock tube over the temperature range 1070–1270 K and overall densities of ∼3 × 10−5 mol/cm3. A plethora of products resulting from decompositions and ring expansion were found in the post-shock samples. They were, in order of decreasing abundance, cyclopentadiene, benzene, methane, ethane, naphthalene, acetylene, ethylene, C4H4, toluene, C3H4, and indene. Very minute yields of some other compounds were also observed. Production of all the products involves free radical reactions. The initiation of the free radical mechanisms in the decomposition of methylcyclopentadiene takes place via ejection of hydrogen atoms from SP3 carbons and dissociation of the methyl group attached to the ring. The H atoms and the methyl radicals initiate free radical reactions by abstraction of hydrogen atoms from SP3 carbons and by dissociative recombination of H atom and removal of a methyl group from the ring. In addition to these dissociations, there are several reactions that involve β-cleavage in the five-membered ring radical intermediates to produce open chain intermediates that then decompose to stable and unstable fragments. The ring expansion process that leads to the production of high yield of benzene takes place mainly from the methylene cyclopentadienyl intermediate. Ring expansion from methylcyclopentadiene itself does not take place. The total decomposition of methylcyclopentadiene in terms of a first-order rate constant is given by ktotal = 1011.31 exp (−46.6 × 103/RT) s−1. A reaction scheme containing 40 species and 105 elementary reactions was composed and computer simulation was performed over the temperature range 1050–1300 K at 25 K intervals. The agreement between the experimental results and the model prediction for most of the products is satisfactory.

The use of elementary reaction coordinates in the search for conical intersections

Abstract: A method to locate conical intersections between the ground-state potential surface and the first electronically excited states of polyatomic molecules is described. It is an extension of the Longuet-Higgins sign-change theorem and uses reaction coordinates of elementary reactions as the starting point of the analysis. It is shown that the complete molecular landscape 1 can be partitioned into 2-D domains, each bordered by a Longuet-Higgins loop formed from reaction coordinates of elementary reactions. A domain may contain a conical intersection and if it does, it contains only one (the uniqueness theorem), whose energy is higher than the neighboring minima or transition states. The method can be helped by symmetry, but applies also to systems having no symmetry elements. It is demonstrated for some simple cases. The presence of a conical intersection is manifested by the nature of ground-state thermal reactions, as shown for instance by the fact that the transition state in the ring opening of the cyclopropyl radical is nonsymmetric. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005

Oxidation of hydroxylamine by nitrous and nitric acids. Model development from first principle SCRF calculations.

Abstract: Ab initio molecular orbital calculations have been performed to develop an elementary reaction mechanism for the autocatalytic and scavenging reactions of hydroxylamine in an aqueous nitric acid medium. An improved understanding of the titled reactions is needed to determine the "stability boundary of hydroxylamine" for safe operations of the plutonium-uranium reduction extraction (PUREX) process. Under the operating conditions of the PUREX process, namely, 6 M nitric acid, the reactive forms of hydroxylamine are NH2OH, NH3OH+, and the complex NH3OH.NO3, and those of nitrous acid are NO+, H2ONO+, N2O4, N2O3, NO2, and NO. High-level CBSQB3/IEFPCM and CBSQB3/COSMO calculations were performed using GAUSSIAN03 to investigate the energy landscape and to explore a large number of possible ion-ion, ion-radical, ion-molecule, radical-radical, radical-molecule, and molecule-molecule pathways available to the reactive forms of the reactants in solution. It was found that in solution the autocatalytic generation of nitrous acid proceeds through free radical pathways at low-hydroxylamine concentrations from unprotonated NH2OH via hydrogen abstraction. At high [NH3OH+], we suggest a possible involvement of the NH3ONO+ intermediate via the reaction NH2ONO + NO2 --> HNO + HONO + NO. The NH3ONO+ intermediate, in turn, is formed favorably via the ion-ion reactions of NH3OH+ with NO+ and/or the reaction between NO+ and hydroxylammonium nitrate (HAN). The intermediates involved in the scavenging reaction of nitrous acid by hydroxylamine are NH3ONO+, NH2ONO, NH2(NO)O, NH(NO)OH, and HONNOH and the rate-determining step is the 1,2-NO migration in NH2ONO leading to NH2(NO)O. Reactions NH2ONO --> NH2(NO)O and NH2(NO)O --> NH(NO)OH were studied with two explicit water molecules and the results are discussed in the context of the importance of the explicit treatment of solvent in the determination of the energetics and mechanism of these processes. The rate constants for the reactions were estimated using transition-state theory and other traditional techniques. The kinetic parameters obtained at the B3LYP/CBSB7/IEFPCM level are in reasonable agreement with the limited experimental value. IEFPCM results on free energy of undelocalized polar ions such as NO3-, NO2-, and NH3OH+ are not very accurate and have difficulties in predicting the right direction of acid dissociation equilibrium of HONO2, HONO, and NH3OH+. Explicit incorporation of a solvation shell to these ions improves the theoretical descriptions of acid ionization equilibria as it captures some of the nonlocal effects of these ions. Additional work is needed to correctly describe the solvation shell and to introduce consistency in the theoretical treatment involving explicit solvent molecules. Nevertheless, this systematic exploration of reactions in solution and mechanism development for a solution phase process based on self-consistent reaction field (SCRF) results is likely to be one of the first of its kind.

The reaction of CH3 + O2: experimental determination of the rate coefficients for the product channels at high temperatures

Abstract: The reaction of methyl radicals (CH 3 ) with molecular oxygen (O 2 ) has been investigated in high-temperature shock tube experiments. The overall rate coefficient, k 1 = k 1a + k 1b , and individual rate coefficients for the two high-temperature product channels, (1a) producing CH 3 O + O and (1b) producing CH 2 O + OH, were determined using ultra-lean mixtures of CH 3 I and O 2 in Ar/He. Narrow-linewidth UV laser absorption at 306.7 nm was used to measure OH concentrations, for which the normalized rise time is sensitive to the overall rate coefficient k 1 but relatively insensitive to the branching ratio of the individual channels and to secondary reactions. Atomic resonance absorption spectroscopy measurements of O-atoms were used for a direct measurement of channel (1a) . Through the combination of measurements using the two different diagnostics, rate coefficient expressions for both channels were determined. Over the temperature range 1590–2430 K, k 1a = 6.08 × 10 7 T 1.54 exp (−14005/ T ) cm 3 mol −1 s −1 and k 1b = 68.6 T 2.86 exp (−4916/ T ) cm 3 mol −1 s −1 . The overall rate coefficient is in close agreement with a recent ab initio calculation and one other shock tube study, while comparison of k 1a and k 1b to these and other experimental studies yields mixed results. In contrast to one recent experimental study, reaction (1b) is found to be the dominant channel over the entire experimental temperature range.

Determination of the rate coefficient for the C3H3 + C3H3 reaction at high temperatures by shock-tube investigations

Abstract: The kinetics of the C 3 H 3 + C 3 H 3 reaction was investigated behind incident shock waves at temperatures ranging from 995 to 1440 K and at pressures between 600 and 1000 mbar with argon as bath gas. The C 3 H 3 radicals were generated by thermal decomposition of propargyl iodide with initial concentrations between 3 × 10 15 and 6 × 10 15 cm −3 corresponding to initial mole fractions between 750 and 800 ppm. Measurement of the UV absorption at 332.5 nm was used to monitor the C 3 H 3 concentration, and the rate coefficients for the C 3 H 3 + C 3 H 3 reaction were determined by fitting a second-order rate law to the absorption–time profiles. Values between 2 × 10 −11 cm 3 s −1 near 1000 K and 7.5 × 10 −12 cm 3 s −1 near 1400 K were obtained with no significant pressure dependence. The negative temperature dependence can be expressed in the form k 1 = 5.8 × 10 −13 exp (3534 K/ T ) cm 3 s −1 with an estimated maximum error of ±30% in k 1 . The reaction channel leading to C 6 H 5 + H is estimated to contribute with less than 10% to the overall reaction at temperatures between 1000 and 1300 K and, pressures ranging from 80 to 500 mbar.

The comparison of detailed chemical kinetic mechanisms; forward versus reverse rates with CHEMRev

Abstract: The computation of the reverse rate of an elementary reaction given the forward rate and requisite thermodynamic data is well established Nevertheless the procedure is extremely laborious and error prone, in particular, when large numbers of such calculations must be performed and the results fitted to traditional or extended forms of the Arrhenius rate constant equation We have developed a software application, CHEMRev, which automatically performs such computations for Chemkin-style reaction mechanisms © 2005 Wiley Periodicals, Inc Int J Chem Kinet 37: 119–125, 2005

Reaction Kinetics of Carbon Dioxide with Diethanolamine in Polar Organic Solvents

Abstract: The chemical absorption rate of carbon dioxide with diethanoamine was measured in such nonaqueous solvents as methanol, ethanol, n‐propanol, n‐butanol, ethylene glycol, propylene glycol, and propylene carbonate, and in water at 298 K and 101.3 kPa using a semibatch stirred tank with a plane gas‐liquid interface. The overall reaction rate constant obtained under the condition of fast reaction regime from the measured rate of absorption was used to get the elementary reaction rate constants in complicated reactions represented by reaction mechanism of carbamate formation and the order of overall reaction of CO2 with amine. Correlation between the elementary reaction rate constant and the solubility parameter of the solvent was presented.